Battery and engine based "inverter-generator"?

FlyFishn · November 2021

All,

I came across this forum doing some online searching. From what I've seen in other threads there seems to be a wealth of knowledge here.

I have done some studying of off-grid systems for the past few years and have been looking at the higher wattage Signineer inverters - their 15kw and 18kw units. However, an older thread here I found pointed to the fact that the THD on these was pretty high - on the order of 10% on the high end.

I could discuss power requirements, solutions, generators, solar/wind, etc all day long, but I'm sure I would loose everyone's attention at the end of the "book".

What I am spinning ideas around on at the moment is using an inverter around the 6kw running wattage size, pairing that with an engine spinning an alternator like that of a wind turbine (think Hugh Piggot 3 phase, but not limited to 3-phase), and having an appropriately sized battery bank.

The numbers I would chase after are as follows:

- Battery bank C sized to meet the peak draw of the inverter without tanking the supply voltage to the inverter too much over a short period of time, maybe a minute

- Alternator output that with the engine at idle (lowest RPM) the base power production could account for average energy consumption (kwh), perhaps enough that when the battery bank is fully charged circuitry could be used to shut engine off

- Alternator output that with the engine at full speed it can maintain system voltage under heavy/peak starting loads for a short period of time

(maybe keeping the voltage from dropping under a certain voltage in 60 seconds of the highest power draw - this would be a combination of the bank C and full engine speed alternator output - if the full speed engine output supplies, say, 1/2 of the draw and the bank C provides the other 1/2 - the bank voltage will drop slower than if the engine/alternator supplied 1/4 at full speed and the battery bank supplied 3/4).

Some additional ideas I have are:

- Use analog current and voltage sensing to regulate engine throttle (no computers, like a Raspberry Pi, to program or fail and spare parts in case a transistor or relay goes it can be easily fixed)

- Use an electric start engine and build a control circuit (based off the above also - not sure if the analog sensing should be on the AC side or the DC side, or possibly both - battery voltage would be DC, obviously, but load current sensing could be AC) that will allow the engine to shut off under low load demands if the bank is fully charged

- With a conventional inverter/battery set up this system could be also powered from wind and/or solar for more fixed temporary set ups (camping, cabin stays). From a truly "portable" perspective, though - it wouldn't be feasible to set up a solar panel array or a wind turbine - too much work to use for, say, an afternoon working out in the field.

Questions:

- Is there a way to do this using capacitors instead of batteries? I know some DC power supplies use capacitor banks (Gamma Research HPS-1A is one example - http://gammaresearch.net/hps-1a.html). However, they are still "short term" storage (see below reference on inverter generators also) in the case of these such power supplies.

Frame of reference here: The "inverter generators" (like the popular Honda EU2200i, one of the ones I have) are this way. However, their operational theory is quite a bit different than the theory I am trying to get at - they are entirely reliant on the alternator power to run their inverters. There is no "stored" power to draw from. As the load surpasses what power the alternator provides at idle the engine throttle is increased so the alternator power output comes up to meet the load demand, then as the load drops so does the engine RPM. I would imagine the capacitor capacity they have gone with is something to allow the initial "hit" of a start up load so the control circuitry can get the engine RPM up high enough to supply the higher load without over-current protection shutting off the inverter output. However, on my EU2200i you have to take it out of "eco mode" to get to the higher end of the start up wattage range - and that mode increases the "idle" RPM substantially, and thus increases fuel consumption. And there is no way to have the engine automatically shut off and restart between low/no load periods.

Capacitors would likely be able to provide a significantly more compact package that would allow the initial "hit" of a start up load, compared to, say, LiFePO4 batteries for the same given discharge/C rate. However, the longer term storage would seem the limitation - and batteries might still be the best option.

I suppose by "longer term storage" I should try to define that. I am not saying storage for days, more like maybe a few hours or over-night. A sample use-case would be car camping. With low load requirements for some lights, fans, and charging cell phones, for example - there would really be no need for an engine to continuously run (like the EU2200i example) until the bank voltage sagged enough to warrant starting the engine to recharge, then shut off. Especially over-night when a lot of things are ff (except maybe a fan or two and phones charging) - the draw may be so low we could get the whole night on just the battery power. If it was hot and we needed to run an air conditioner - when the air conditioner kicked on, maybe then the draw/battery voltage sag could trip the engine to start for the period of time it needed to then when the battery voltage got high enough it could either shut off or drop to idle, depending on what the running draw was.

- Are there any split-phase inverters (output both 120v and 240v, preferably with the ability to run all 120v also) in the 6kw range (running wattage, starting wattage higher) that have low THD for sensitive electronics? Under 5% THD is acceptable, but lower is better.

- Has anyone ever done something similar with their off-grid wind/solar system during periods of low power production? I would imagine engine driven generators are extremely common back-up power options in case of low production or if a break-down occurs and you need to take the alternative system out of line for work. However, a conventional (either rotary or inverter) engine driven generator doesn't operate on the same "theory" - more of a controlled engine-driven charger than a "generator".

mike95490 · November 2021

Most big inverters have an internal bank of filter caps to supply the 120hz DC for the inverter to work.   The battery bank is a large portion of the "filter" and it's not it's voltage sagging in 90seconds, but that the bank has low enough impedance to be able to supply the peak requirement of the 60Hz signal the inverter produces.   If you want to fake some more with capacitors, use
low impedance / low ESR series . Often these should be wired with low impedance braid or bus bar, and fuses are suggested. Also a pre-charge circuit becomes necessary. I'd not try it with super caps, unless they have low ESR versions now.
120hz is your target freq, not 1,000hz
for long life, power caps should be 2x working voltage, and they self-heat, so aim for the higher temp versions.
The most efficacious may be a cluster of mid size caps, not a single giant one. ( and better cooling )

https://www.nichiconcapacitors.com/product-category/nichicon-low-impedance-capacitors/
https://hyperelectronic.net/wiki/capacitor/capacitor-impedance/
https://www.badcaps.net/forum/showthread.php?t=32066   ( first post has terms bass-ackwards)
https://www.pa4tim.nl/?p=3775   ( I like this fellows lab bench, I used to do that stuff for +30 years )

( inverters often have a battery spec of 100A per 1kw of utilized output, so a 20Kw inverter wants to see a VERY large battery bank or equivalent. )
And always check the life spec : Life: 8000 Hour ( 8760 hours in a year, then capacity slowly degrades ) over spec and you might get 2 years.
That's why I replaced my inverter at 11 years, I know the input caps are getting real soft. Add in the ordering and shipping delays, and getting an installer to help me lift a 120# inverter, I didn't want to run of generator for a week.

FlyFishn · November 2021

mike95490 said:

Most big inverters have an internal bank of filter caps to supply the 120hz DC for the inverter to work. The battery bank is a large portion of the "filter" and it's not it's voltage sagging in 90seconds, but that the bank has low enough impedance to be able to supply the peak requirement of the 60Hz signal the inverter produces.

Interesting comment. I am trying to wrap my mind around what you said. DC is DC because it is just that - direct current. Are you saying the modulation of the inverter creating the 60hz AC causes the load/amperage to pulse at your claimed 120hz? I am not sure what else would pulse as there isn't anything driving the battery power to modulate, say, a square wave voltage pulse - essentially turning the power on and off at said frequency. The battery power itself is straight DC.

I have a 800-1000w or so Xantrex inverter (ProWatt SW I think) that runs on 12v. I suppose I could put an amp/watt meter on it and watch the voltage on an oscilloscope to see what its doing. I am not sure if that would resemble a "large power inverter" though.

I have heard of there being a "start up" procedure with some devices. I don't recall if they are charge controllers or inverters, but the issue was directly connecting to a battery bank could blow some components inside. The resolution is to apply power to the device via a resistor so that there isn't the initial surge of the full bank voltage/power hitting the device at start up then after a few seconds switch to directly connecting. That almost sounds like charging capacitors?

BB. · November 2021

Yes, as Mike says, the current draw from the DC battery bus to a single phase AC inverter is a 120 Hz "sine squared" pulse train (for 60 Hz inverter).

The AC output of the inverter is a sign wave from +170 volts to zero volts to -170 volts to zero volts... 60x per second.

One of the power equations is Power = Voltage * Current ... So the AC inverter pulls peak power at +170 and -170 volts peak, and zero power at zero volts... That is why the DC input current is a pulse train vs "smooth" DC current flow.

The batteries are expected to provide (most) of those peak currents... The capacitors are not (typically) large enough to "filter" or smooth the DC current draw.

Because there are usually capacitors on the DC side of solar charge controllers and AC inverters--They typically do pull some current when initially connected to the DC bus. And it will cause a bit of a spark--If connecting bare wire, don't let the arc surprise you and cause the wire to be shorted to something else.

Usually, we like to suggest using a DC rated circuit breaker for these connections. They both protect wiring in case of failures and short circuits, and provide a handy on/off switch. Vs fuses and a separate power switch.

You can "pre-charge" the capacitors with some sort of power resistor and switch--But most folks do not bother. Will the capacitors last longer if "pre-charged"? In theory, yes. In practice probably not so that you would notice...

Just as an aside, I used to work for a voice mail systems company... And it was not unheard of to have a system run for years, but when taken down for normal service, for the AC (or DC) input main power supply to fail on power up when trying to place back in service.

-Bill

mike95490 · November 2021

Yes, the battery bank has to supply surges 120 times a second to the inverter. a half ohm resistor in the series DC to the inverter, should generate enough voltage to see it on a scope. ( put this on the - battery lead, scope ground to - battery scope tip to inverter - side of the resistor. )

Careful with DC measurements with scope leads, the scope ground could short the Bat + or AC 120V to ground through the scope, causing bad things to happen to the scope.

Because of my NiFe bank high resistance, I needed to go with a higher Ah bank than load demand was, to be able to power a pump for 4 hours and not cook anything, XW inverters have a Capacitor board thermal sensor and shut down when the caps overheat from insufficient battery capacity.

FlyFishn · November 2021

Are there any inverters in a small cube package that are low THD and around 6-7kw? The Sigineer in the link below is a good example of the form-factor I am after. This is a lot different than a wall-mount cabinet that I see some of the XW, and similar, solar system inverters seem to be.

http://store.evtv.me/proddetail.php?prod=Sigineer6kw

I will have to figure up the battery requirement, or battery + capacitor requirement (not sure I could get away with just capacitors) and see what kind of space that is going to take.

My idea is to package everything on a cart of sorts, perhaps an enclosure for sound reduction. The engine, alternator, and a cube inverter would be a pretty compact package, all things considered. However, the batteries could be a challenge on space requirements.

For what it is worth - I looked up the manual for the 6kw Sigineer and I don't see any mention of battery capacity requirements that would get down to the reactance. All they mention is voltages. This unit is both an inverter and charger, which is interesting. They have a few nifty features that make it pretty attractive for off-grid use. One of those is they have an output that will enable the remote operation of a generator when battery voltage goes too low. That is part of what I want to incorporate in to my system so the fact it is already there is cool. However, with the high THD the unit won't work for my application. I am not sure if there are other inverters that fit the form factor that can do all of this?

BB. · November 2021

Looking at an inverter-charger, like your link, is probably the way to go--Otherwise, just an AC inverter--You have the whole issue of AC to DC power conversion (i.e., Battery Charger or similar) for your application.

We have used rules of thumb for sizing battery banks... More or less, for Flooded Cell Lead Acid, 250 Watt inverter rating per 100 AH @ 12 volts; 500 Watt watt inverter rating per 100 AH @ 24 volts; and 1,000 Watt inverter rating per 100 AH @ 48 volts... AGM, Li Ion, "super caps", etc. can run smaller battery/storage capacity because of their higher surge capabilities.

For batteries, you need to keep the battery bus voltage over the charge/discharge point of the battery--Or you end up with micro cycling. Such as, a 12 volt battery bank, the terminal voltage must remain >~12.8 VDC with that 120 Hz DC inverter load. If you let the cycling voltage fall below ~12.7 volts, then the battery is charging/discharging 120 times a second and "wearing out" over time by cycling. (by how much, I really cannot guess).

We can also size the battery bank for "optimal" surge performance and charging times... For example, Flooded Cell Lead Acid batteries we suggest around 4x the daily WH/AH usage with solar... You get good surge performance and, on average, a 25% discharge can be recharged by solar power only (hours of sun per day--Vs the 24 hour per day charging available from utility or genset power). If you discharge the battery bank to 50% SoC (as an example), it usually takes something like 2 sunny days to completely recharge the bank (like 5 hours of bulk charging + upwards of 6 hours of "absorb" charging = 11 hours of "enough sun" -- Usually >1 day of sun in sky).

li Ion batteries are "better" for smaller battery banks (vs loads). Because they can supply C/5 to C/1 (1 hour discharge rate) for discharging and recharging... And they do not have the "absorb" time that Lead Acid require. Of course, you have to pick your batteries (and Battery Management System) based on your needs and the cell specifications.

Other things to think about... Lead Acid batteries work "better" than Li Ion batteries in below ~40F temperatures (cannot recharge Li Ion batteries below freezing). And Li Ion batteries work better than Lead Acid it hot conditions. Other issues such as temperature cycling (do batteries get "hot" from genset cooling) and vibration (Diesels and 1 cylinder motors tend to shake themselves apart unless well balanced and good vibration isolation).

Regarding THD (total harmonic distortion)... Typically 5% is the maximum THD allowed/accepted for "utility grade" power.

10% THD is not great, but much better than the ~30%+ THD that MSW inverters output (MSW will work with something like 80% of AC appliances vs 10% that can fail in hours on MSW). Of course, it is up to you to figure which is the 80% that works, and the rest to avoid MSW.

In general, heavily loaded induction motors (compressors, pumps, etc.) do not "like" MSW (modified sine/square wave) power. The 30% THD goes into heating the motor windings/components. Many "cheap" power supplies (such as battery chargers for hand tools) did not like MSW either (they overheated with the "square wave")--Although some mfg of tool chargers have redesigned their chargers to accept MSW power.

Many folks believe that computers are very power sensitive devices... If you get Power Factor Corrected (PFC) based DC power supplies, they are not really sensitive to AC power at all... And in fact may be able to even take DC voltage too (as always, details matter).

You might look at hybrid type construction/stadium lights for some ideas... There are units that have both battery banks and fuel (diesel/etc.) generators combined into one unit:

https://www.constructioncomplete.com/media/downloadable/brochure/GHSGreen-B.pdf

More or less like a Prius Hybrid system.

I am not sure what a super cap + genset buys you... Gensets (standard/non-inverter type) have pretty good surge causalities anyway. Adding an inverter+super cap does not seem to buy much.

Now, something with larger battery storage (AH/WH) can make really good sense. Many modern Inverter-Chargers have a mode call generator support. This, more or less, allows the use of a smaller/fuel efficient genset for "average power needs" and the inverter-charger will support high surge current (and "quiet time" power) for loads that exceed the genset rated output (i.e. 3 kWatts from genset, 3 kWatts from battery bank = 6 kWatts to load).

For highly variable load throughout the day--A "hybrid" generator+battery bank system can save lots of fuel... You operate the generator under optimum conditions (such as 50% to 80% of rated output power) and get the best fuel efficiency. And use the battery bank to supply off peak loading (AM and Evening for cooking, lighting, computers--And off peak middle of day and overnight quiet times).

Running diesels below ~40-50% loading is not great for motor life. And running gasoline/propane gensets below ~50% becomes fuel inefficient. Diesels have better fuel efficiency than Otto Cycle engines (gas/propane), but don't like light loading. Inverter-generators can be "relatively" fuel efficient down to 25% loading.

I could see the advantage, to a degree to use an "inverter-generator" with something like a super cap to support high power surges when running with "eco throttle on" (I have stalled Honda eu2000i gensets with the starting surge of a couple of refrigerators running eco throttle).

But I would want to characterize the loads and surge power requirements vs cost of fuel+inverter+Capacitor bank (and maintenance--Super caps may last 5 years??? Time, Temperature, current, and Voltage all have effects on life).

And when you add significant energy storage (battery banks), size, weight, and costs become big factors too. Fuel is just so much more energy storage efficient. Some quick examples:

2x golf cart batteries => 12 volts @ 200 AH
12 volts * 200 AH = 2,400 Watt*Hours
2,400 WH * 0.5 capacity derating (longer life/surge current) = 1,200 WH
1,200 WH / 3,500 WH per gallon of gasoline (typical small inverter-genset) = 0.34 gallons of gasoline
120 lbs of battery vs 2 lbs of gasoline

Can run similar numbers for different mixes of battery and genset wattage/type and fuel types.

From an engineering point of view--One can do almost anything. From a bankers' point of view--It should make economic sense too.

-Bill

mike95490 · November 2021

That Sigineer inverter is expecting a tesla S battery bank @ 24V.

And the weight difference is staggering, the 6.8kw XW at 120+ lbs, with a honking big transformer. And the price is a lot less than I'd expect. i didn't see any specs, efficiency, THD ___

At half power ( 3kw) it will pull 125A, but you have to size your cables to handle your surge load, or the cable resistance will cause a low voltage shutdown. That's a good reason for a 48V system, lower DC amp cable issues.

FlyFishn · November 2021

Thanks for the detailed info from those that have replied. Great food for thought.

Re: THD - the Sigineer 6kw inverter is under 10% THD. I have a small welding machine that requires under 5% and that is part of the idea for the portable power system here - adequate power for the welder for the upper end of the range I may use it (not the upper range of the machine - if I'm needing to get that high I may not even be using the particular machine and have a 15kw generator to handle the heavy loads should I need it).

As to the numbers with the voltage drop under load and the resistance of the leads between the bank and the inverter - yep. I understand all those numbers.

As to the charger/inverter combination - I don't particularly need an inverter that charges because my thought with the engine/alternator combination is to use the type of alternator that a wind turbine uses (think Hugh Piggot 3 phase designs, not necessarily limited to 3 phase, though). That would be a DC (post-rectification) power source. I would imagine that I would need a separate charge controller that could handle the power/voltage that the alternator sends to it. The alternator would have to be designed (think coil windings) to output the correct voltage so at idle there is still some current, but with the throttle going up the coils don't burn up, either, from over-current.

As to batteries and weight - my thoughts were to use lithium batteries. I will have to do the numbers to see how much Ah capacity I need and how many I will need and see what that bulk/weight comes to.

BB. · November 2021

You probably don't want to use a simple rectifier and capacitors by themselves. You probably would get down to a power factor of 0.6 or 0.5 on that sort of input. Or almost 40-50% of the output current of the genset is "wasted" because of how diode/capacitor front ends of power supplies is just not very efficient. A PFC (power factor corrected) AC input power supply is better than 0.95 PF (1.0 is "perfect").

The other issue is the rectified AC is typically around 170 or 340 VDC peak (from 120/240 VAC input with or without voltage doubling of the diode/cap input). So you either need to use a transformer to drop the AC voltage, or some sort of buck mode DC converter to get your XX VDC charging voltage.

There are DC genset available for battery charging/DC.

http://www.portacharger.com/24v-dc-portable-generators/portacharger-2440.php
http://www.centralmainediesel.com/order/Kohler-Diesel-DC-Generator.asp?page=Kohler_Diesel_DC
https://www.auroragenerators.com/product-page/24-volt-dc-generator
http://www.pmgenerators.com/dcgenerators (48 vdc generator head only)

You linked to an inverter-charger... And if you are in that realm--Then look for a inverter-charger that has generator support--Everything you need in one box (excluding batteries and genset/charging source). I thought that was where you were aiming for with your power cart--Genset power, battery power, inverter-genset function, small to large battery banks, genset support.

Yes, for this system, Li Ion may be the best fit--Assuming you do not need to operate at or below freezing (without battery heating).

-Bill

mike95490 · November 2021

I would avoid the complexities of designing your own charging system, both the 6 phase source and the charge controller. Remember, most commercial MPPT charge controllers are only suitable for solar, not wind, as they cannot manipulate the MPPT point for wind and wind needs a Diversion Load of 2x the rated turbine output. Same problem with a engine driven generator, the controller cannot regulate it like it can a PV panel. Stick with the integral charger in the inverter and a ordinary generator large enough to feed it.
You can go the exotic route, but you will pay for it.

checkthisout · November 2021

What are you guys talking about?

BB. · November 2021

More or less, as I understand (not putting words in the orginal poster's posts)--Wants to design/build a "hybrid inverter-generator" system that can use a small battery bank or capacitor to carry AC loads for a short period of time before starting the genset...

We get lots of folks asking about how to minimize the cost of off grid power systems... Whether a smaller battery bank, more efficient fuel use for a genset, etc...

The short answer is nothing is "free". When you try to cost reduce one part of a system, you end up with costs (and/or limitations) in another part of system.

To get an idea of what something like this entails... Loot at a Toyota Prius or similar hybrid vehicle. The standard Prius as a (relatively) small battery bank (can only have enough energy for a couple/few miles of electric only driving). The computer will control the engine (start/stop), and the "transmission" to match engine power/RPM + the three phase motors that make up part of the transmission, and based on road conditions and driver's foot on gas/brake will try to (for example) braking energy into the small battery bank, and use that energy to help propel the car (low speed, additional power for passing, etc.)...

And now think about how you would design and build a hybrid car that was similar in performance to what Toyota (and others) have already done.

It is not an easy job. Same thing with "rolling your own" genset+alternator+inverter-charger+battery bank.

Almost anything can be done with electronics these days... But it is not easy or cheap to start from ground zero.

More or less, most of the time you are better off to get a "standard" (relatively large) battery bank of some sort, connect it to several chargers (i.e., solar panel charging, a Hybrid AC inverter-charger--does both, and usually some sort of genset/backup genset for bad weather or if something else fails).

People have and continue to design and build their own systems (whether e-cars or wind turbines or even inverter-chargers). There is lots of information out there and some do it your self kits (i.e., pre-made circuit boards, bills of materials, software source code, etc.). But that is a big chunk to bite off at first attempt.

The modern inverter-chargers out there, integrated with generators, solar charge controllers, and now even Battery Management Systems for Lithium Ion battery banks--And their capabilities are amazing (one function that still is amazing to me--"generator support" mode for AC inverter-charger--You can have a "small genset" connected to an inverter-charger and a large/variable AC load... If the AC load is low, most of the genset power goes to charging the battery bank... When the AC load "jumps" (electric stove, well pump, A/C system, etc.), the generator will supply (for example) 2,000 Watts and the AC inverter-charger will "support" the AC output with another 2,000 Watts from the battery bank. Allows you to use a small and relatively efficient genset for general charging/backup, and the inverter-charger+battery bank to handle large/short term AC loads. This allows you to run the small genset efficiently (at 50%-80% of rated output) and the inverter-charger to manage charging/loads transparently to the genset.... It is a really cool function (and something that modern electronics/designs are good at--Mixing/matching power flows to create something that is better than the individual components).

-Bill

checkthisout · November 2021

I'm still lost because something like the victron multiplus does exactly that.

With inverter generators like the yamaha 3000 and honda 3000 and victron multiplus what else would an off-gridder need?

BB. · November 2021

Inverter-generators are nice (I have a couple, like them quiet and fuel efficient at lower output power vs "standard" gensets). They do (usually) have very accurate frequency and voltage control--But inverter-generators tend to have less surge current support and will (usually) drop the AC power immediately if surge rating is exceeded (vs standard generators that usually just "ride through" high surge current (RPM drop and voltage sag--But for many loads, after the surge, the genset comes back up to speed and voltage).

However, the above I was (mostly) talking about a separate Hybrid AC inverter-charger and its functions (such as "generator support").

An inverter-generator was not really the issue in the above Q&A above.

-Bill

RCinFLA · December 2021

Inverter-generator three phase permanent magnet alternator V-I curve is somewhat complicated. For 120 vac inverter-generators, the alternator output rectified DC level varies from about 175 vdc to over 300 vdc depending on electrical load and engine rpm. The HV DC filter caps are allowed to slump in voltage with peak load but should stay above 170 vdc. When in reduced rpm 'ECO' mode the sinewave peak is clipped when a surge load happens until engine rpm increases to meet the new demand.

Filter capacitors in inverters and inverter-generators are for supplying high frequency PWM current pulses, not for filtering 120 Hz ripple current. Filtering 120 Hz ripple current for 3-8kW inverter would take many Farads of capacitance.

Having a generator or inverter-generator capable of supplying peak loads surge is not the most efficient way. Low frequency (heavy iron) inverters provides the most cost effective and efficient way to supply peak surge requirement. Just have to be sure batteries are capable of the required surge current without voltage collapse. LFP batteries are good for this and have very high round trip discharge-recharge efficiency in the mid to high 90%. LFP battery weakness is low temperature operation, but does not take too much heater power to keep them warm in the winter with insulated battery box.

I would stay away from high frequency hybrid inverters as their first stage DC-HV DC converter is more delicate and not capable of supplying much peak surge power. When the high frequency ferrite transformer of battery to HV DC converter stage hard saturates the MOSFET current shoots through the roof unless control safeguards shut down their gate drive. Safeguard sensing at 60 Hz with 25-50 KHz switching on MOSFET is often not fast enough to save MOSFET's from destruction. Split-phase 120/240vac, HF inverters and inverter-generators, are just two 120vac HF inverters in series. A LF inverter has the low freq transformer that can help level the power demand on each 120vac phase. A HF split-phase 240/120vac inverter cannot do this, and would require an extra low freq transformer attached to its output to balance 120vac phase power.

AC power factor for charging is important when doing charging from generators. LF hybrid inverters have good power factor for AC input charging. Some HF inverters just use AC-DC rectifiers to HV-DC downconverter yielding a poor AC charging power factor.

Running a generator or inverter-generator most of time is inefficient, even if inverter-generator is operating at reduced rpm. With LFP battery efficiency, it is better to run gen at electrical load that yields optimum fuel to electrical kWH's efficiency to charge battery then shut gen down and run on batteries.

Most LF hybrid inverters and some HF hybrid inverters allow generator and inverter power to be summed allowing more AC output power than either device alone. If needing this then an inverter generator would be good choice to use, If inverter supplies all the needed power then no benefit is yielded from inverter-generator for fuel efficiency to just charge battery at optimum generator load. Inverter-generators have superior freq stability which is important for hybrid inverter staying phase locked to generator but a synchronous generator with stepper motor throttle and electronic rpm control does a good job. If generator noise is a factor then inverter-generators have the advantage but they are not always so quiet when operating at peak fuel efficiency loading.

Problem with an oversized inverter is higher power inverter has higher no-load power demand. A 12-15kW inverter may have over 250 watts of no-load idle power. Some high power inverters shut down some of paralleled MOSFET's to reduce idle power but this can create problems for delivering peak surge currents without stressing the active subset of MOSFET's.

checkthisout · December 2021

So if an inverter generator was run at 90% load and an equal non-inverter unit was run the same (let's say a Honda EU7000is Vs a GX6500, both with 390 CC engines, which one would use more fuel and why?

If both were run at 50% load, which one would use more fuel and why?

If we then substituted a 3k standard generator in place of our 7000is inverter chugging away at half load producing 3kw, what would the difference in fuel consumption be?

FYI, the Honda EU7000is and EX6500 have 5.1 and 6.2 gallon tanks. The EU7000is runs the same amount of time at full load as the EX6500.

It would appear the inverter model is more efficient by a fairly large margin.

checkthisout · December 2021

Also, my Yamaha 3000ISEB generator pulls an extra 500 watts out of the starting battery for starting hard loads and also helps the Gen pick up a sudden load like a saw when in economy.

Is that what you're thinking of OP?

BB. · December 2021

You have to have made measurements of those operating conditions with each genset...

HOWEVER, in general, a standard gasoline or propane genset will give better fuel economy (kWH per gallon of fuel) at 90% loading vs an inverter genset (the inverter system has 5-10% conversion losses.

At 50% loading, both gensets would probably be similar fuel usage. At 25% loading, the inverter-generator will usually have much better fuel economy as it will slow down the engine RPM at lower electrical loading. The standard genset, roughly, will consume 100% of rated fuel flow at 100% loading, 50% of rated fuel flow at 50% loading, and 50% of rated fuel flow at 0% loading (Otto cycle engines are not that efficient at lower loadings).

Diesel cycle engines are more (thermally) efficient combustion cycle. And Diesel fuel has more energy per gallon. So they tend to be much more fuel efficient.

And size does matter... Small engines are simply less fuel efficient than large (and slower turning) engines.

However, when you need electricity, a 25 kWatt genset, while more efficient at the 5-25 kWatt loading, simply will still use more fuel than a 2-3 kWatt inverter-generator supplying 300-500 Watts.

I did a spread sheet just doing that--Taking Honda generator specifications and figuring out their fuel efficiency (kWH per gallon) at different loadings and fuel flow (gallons per hour).

Note: I was interested specifically in the Honda euxxxx family of inverter-generators--Both because they are quiet (I live in the suburbs) and I did not want to store 50 gallon drums of fuel.

Here is a small 6.5 kWatt diesel genset with a nice fuel flow vs % loading chart. I will add this to my spread sheet.

http://www.centralmainediesel.com/order/Kohler-Diesel-6500-Watt-Diesel-Generator.asp?page=Y6875

Model	Peak Watts	100% running W	25% running W	Full load time H	25% load time	Tank gallons	100% WH per Gallon	25% WH per gallon	25% gph	100% gph
eu1000i	1000	900	225	3.2	7.1	0.6	4,800	2,663	0.085	0.188
eu2000i	2000	1600	400	3.4	8.1	0.95	5,726	3,411	0.117	0.279
eu2200i	2200	1800	450	3.2	8.1	0.95	6,063	3,837	0.117	0.297
eu3000i	3000	2600	650	3.5	7.7	1.6	5,688	3,128	0.208	0.457
eu3000iS	3000	2800	700	7.1	20	3.4	5,847	4,118	0.170	0.479
eu7000is	7000	5500	1375	6.4	16	5.1	6,902	4,314	0.319	0.797

Non-Inverter			50% loading		50% load time			1/2 WH/gallon	1/2 gph	100% gph
Champion 3500	4375	3500	1750		12	4.7		4,468	0.392

Guesses below	Peak Watts	100% Running Watts	25% running Watts	Full load Hours	25% load Hours	Tank gallons	100% WH/gallon	25% WH/Gallons	25% gph	100% gph
Camp @ 100%/25% guess	4375	3500	875	6	12	4.7	4,468	2,234	0.392	0.783

6.5 kWatt Kohler Diesel generator (note: no fuel tank--Just pick 5 gallon for comparison to other gensets here)
Kohler 6.5 kW	6500	6120	1530	8.6	17.9	5	10,526	5,477	0.279	0.581

Note the Champion 3,500 Watt 100%/25% numbers are pure guesses and probably the real numbers would be higher fuel efficiency (10-20% better?)--But I could not find a standard gasoline genset that gave 100%/25% fuel usage numbers.

More or less, the larger the genset, the more fuel efficient.

And inverter-generators are more fuel efficient than standard gensets at 25% usage (again, the standard genset fuel usage is a rough estimate here).

Contractors have used both inverter-gensets and then bought a standard genset when the inverter-genset failed/wore out. And quickly went and purchased a new inverter-genset because of the high fuel usage of the standard genset.

For construction sites and homes, we probably use 25% loading most of the time (fridge/freezers run at 50% or less duty cycle, only cook during meal time, etc.). If you had heavy constant loads (lighting for a stadium or A/C or for off grid folks--charging the battery bank for 5 hours during stormy weather)--Then you can load your genset at 50%+ of rated load, and get good fuel efficiency out of standard or inverter gensets.

And even if I could justify (noise, smell, and no vehicles that use diesel) a small Kohler 6.5 kWatt genset, it still would use more fuel to power my home at 25% rated load vs a 2-3 kWatt inverter genset. (the 0% loaded Kohler still uses ~0.2 gph of fuel vs the 25% loaded eu2x00i at 0.117 gph).

Here is a copy of the nice 6.5 kWatt Kohler Diesel chart from Central Main Diesel:

And a 4 kWatt standard genset:

http://www.centralmainediesel.com/order/Honda-4kw-Gasoline-Generator.asp?page=H04580

Using un-auditioned manufacturer specifications and guesses at fuel usage--Only can take you so far. The last chart of a 4 kWatt gasoline genset is much more complete--And could be used to update/replace the Champion genset estimate... But it is getting late here.

And a happy coincidence... The 4 kWatt standard genset from CMD uses exactly the same engine as the Honda eu3000is -- A Honda GX 200 motor. So you can make direct comparisons between the same engine on both a standard 4 kWatt genset vs the Honda eu3000iS inverter-genset...

For example, using 700 Watt load (eu3000is-25% of 2,800 Watt rated output) and 22% loading of the 4 kWatt genset (3,250 Watt continuous rated)--The inverter generator uses 0.170 gph and the standard genset uses about 0.33 gph---The inverter genset uses much less fuel than the same engine with standard alternator at 25%/17.5% electrical output. Or the standard genset uses about 1.9x more fuel for same power at ~25% loading vs the inverter-genset.

Pretty neat what you can find if you hunt around on the Internet.

-Bill

checkthisout · December 2021

Why does the Honda EU7000 Inverter consume less fuel at rated output (6500 watts) than the GX6500 (6500 watts) even though both run the same engine displacement if Inverter generators are less efficient at full load than standard generators?

I think the assertion is that our EU7000 is less efficient running half load producing 3000 watts than a 3000 watt generator running full load.

Is that what you are talking about?

I think there is one thing you're missing and that's the inverter generator can set a lower RPM at 3000 Watts than the 3000 Watt standard generator. Engines are more thermally efficient with higher throttle settings (higher manifold pressure) and when they are turning low RPM's because of the energy needed to for the piston and connecting rod to change directions with every RPM. This negates the losses that would normally be experienced with running a large engine at light loads. It's the reason overdrive increases fuel efficiency in cars and why highway economy is so closely matched between same model cars with different engine options. I also propose the Inverter unit runs a smaller alternator than a standard generator. (look at the fuel efficiency of a honda 3k "cycloconverter" VS other 3k models.

Conversely, it's also worth noting the EU7000 can run a lower full output RPM than the 6500 at the same rated MAXIMUM output. Considering the EU7000 might only turn 3200RPM and open the throttle more while the 6500 must run 3600 RPM. The inverter generator can then also turn 4500 RPM if needed in order to boost output. The 6500 cannot do this and must rely only "reserve throttle" alone which means the engine has to be larger than needed to support the rated output.

BB. · December 2021

I cannot really make detailed statements that the designers (and marketing, mfg, etc.) and the tradeoffs they may have made... Noise insulation/enclosure raises costs, possibly impedes cooling airflow, etc... On the other hand, the enclosure may improve cooling airflow. What are the mechanical and electrical limits of the "alternator+inverter system"...

And there is "specsmanship"... The eu2000i went from 10 hours of run time and 1+ gallon fuel tank to 9 hours of runtime and a 0.95 gallon fuel tank all on 25% rated power output. Where there actual changes over time (hopefully mechanical improvements and possible manufacturability concerns addressed). Possibly pollution control and safety requirements.--Know that they now have a catalytic+muffler if they want to sell in California these days. New Honda's are starting to add carbon monoxide detectors (not sure I like this--Something that shuts the genset down that also has a life and possible failures over time).

A 10-20% change in fuel economy. Engine/carburettor/fuel injection/electronic ignition, alternator improvements, etc. are usually within the realm of "tweaking" a design.

Cutting fuel use in 1/2 at 25% loading between two gensets (inverter generator vs standard generator with same engine)--That usually requires a "change in thinking"... That is the inverter-generator that made the huge difference.

Just like transmissions from 3 speed to 4/5/8/10 speed to variable transmission (mechanical belt or the "electrically modulated type" such as in a Prius).

Single speed bikes vs multi-geared bikes... Just designers trying to match the energy source "power curves" to the needs (speed/weight/road conditions/etc.) of vehicle or that of electrical loads and make a product that consumers will purchase.

The original spreadsheet I made was just to help me model my needs (low noise, low fuel consumption, at relatively low output power--A fridge+freezer, koi pond, and a few LED lights) and what genset would make sense. "Your needs" will, most likely, be different vs mine. And your solutions could be different too.

Now that there is a "market" for inverter-generators... There have been a large number of units available these days that are similar (or possibly even more features) than the Hondas at 1/2 the price. The "early" generation of inverter-gensets, had very poor reliability and MSW inverters (at least a few I looked at). The newer ones seem to be getting better and are PSW inverters.

-Bill

checkthisout · December 2021

I think the inverter generator from a reputable manufacturer is the way to go, assuming you're running gasoline. I have seen no evidence they are less efficient than standard generators under any operating load unless grossly underloaded of course. In other words and as I know you know, your 7kw inverter only putting out 500 watts is wasting fuel compared to a 1000 watt generator putting out 500 watts.

In regards to the OP, my Yamaha 3000 ISEB generator takes electrical energy from the starting battery in order to augment boost for starting a load. 500 watts more than the non-boost model for 20 seconds. https://www.yamahagenerators.com/EF3000iSEB-p/ef3000iseb.htm

For this reason, this generator can run a smaller engine than the Honda EU3000is and thus consumes less fuel but is able to handle even greater loads.

I know of no other generators that do this and I think it's what the OP is asking about. Kinda neat. When you open it up there are two 8 Ga cables running from the starting battery to the inverter that provide the boost. If the starting battery is going bad the generator struggles run to run chop saws and such.

BB. · December 2021

Regarding:

In regards to the OP, my Yamaha 3000 ISEB generator takes electrical energy from the starting battery in order to augment boost for starting a load. 500 watts more than the non-boost model for 20 seconds. https://www.yamahagenerators.com/EF3000iSEB-p/ef3000iseb.htm

I am not sure how to compare this vs the Honda eu3000is ... The battery boosted inverter is pretty neat. But saving on fuel consumption is difficult to prove--And the difference of a few percent is probably meaningless in real life--But Marketing does take a hold here too.

From your link:

Long run time ‐ Continuous operation for up to 18.6 hours without refueling (at 1/4 rated load with “Smart Throttle” function on).

But from the "specs" tab:

Continuous operation for up to 19 hours without refueling (at 1/4 rated load with “Smart Throttle” function on).

3.4 Gallons

So we already have a mismatch in specifications in the same website... Albeit a small one. And for the Yamaha, I could not find its 100% runtime rating on its internal fuel tank.

The Honda eu3000is:
https://powerequipment.honda.com/generators/models/eu3000is

Fuel Tank Capacity 3.4 gal.

Run Time per Tankful 6.9 hr @ rated load 19.6 hr @ 1/4 load

So here, we have a 5% longer runtime for the Honda over the Yamaha "3000" gensets(19.6/18.6 hours @ 3.4 gallons of fuel)... Hand waving that 5% "better" fuel efficiency is noticeable by the average user... I could not find a 100% rated load fuel consumption for the Yamaha. So cannot compare full power fuel efficiency.

But, since I am interested in data and comparisons... The "2000" class of Yamaha vs Honda is interesting. The Yamaha has a significantly smaller engine vs the Honda--What do those numbers look like? This link has all the specs in one place:
https://generatorpick.com/yamaha-vs-honda-generator/

Yamaha EF2200iS vs Honda EU2200i YAMAHA EF2200IS HONDA EU2200I
Yamaha vs Honda numbers below:
Peak watts 2200W 2200W
Rated watts 1800W 1800W
Engine OHV, air-https://www.centralmainediesel.com/order/Yamaha-EF3000iSEB-3000-Watt-Inverter-Generator-with-Boost-Technology.asp?page=EF3000iSEB, single cylinder, 4-stroke OHC, 4-stroke, single cylinder Displacement 79cc 121cc
Fuel tank 1.24 gal 0.95 gal.
Run time 10.5 hrs (1/4 load) 8.1 hrs (1/4 load)

So, using Watt*Hours per gallon of fuel for efficiency:

1800 Watts * 0.25 loading = 450 Watts
Yamaha = 450 Watts * 10.5 hours * 1/1.24 gallons = 3,810 WH per gallon
Honda - 450 Watts * 8.1 hours * 1/0.95 gallons = 3,836 WH per gallon

Or something like 0.6% difference between Honda and Yamaha here... So at 25% loading, the smaller engine of the Yamaha does not seem to offer significant fuel savings with the "smaller" Yamaha motor vs the Honda engine (121/79=1.53) even though the Honda engine is 53% larger volume.

On the 100% efficiency--Again, for the Yamaha, I could not find a 100% rated runtime... So cannot make that comparison here either on engine size vs efficiency (or Y vs H overall efficiency).

For the average user--I still suggest that most people that choose smaller gensets on average random loads will probably be averaging around 25% loading vs (picked out of the air) 80% loading.

For off grid folks--Battery charging, running a backup electric heater--Yes, it is very possible to run these gensets at 80% loading--But we don't have any numbers here to make a Y vs H decision.

Realistically, probably at best, only a small difference in the 100% loading fuel efficiency between the two gensets--And using the Honda fuel flow here for the Yamaha (planning purposes) is more than "close enough".

......

I just remembered a website that does (usually) offer more fuel usage numbers for the models they carry. In this case for the Yamaha EF3000iSEB, they do have the 100% loading fuel usage (I don't know how they obtained these numbers--Hopefully they are accurate):

https://www.centralmainediesel.com/order/Yamaha-EF3000iSEB-3000-Watt-Inverter-Generator-with-Boost-Technology.asp?page=EF3000iSEB

Model Number			EF3000iSEB
Consumption at 1/2 load			0.221 GPH
Consumption at 3/4 load			0.297 GPH
Consumption at full load			0.39 GPH

From their chart, there is also a zero load fuel consumption of 0.132 gph and 25% load of 0.1675 gph and a 3.4 gallon tank...

2,800 Watt * 1 hour * 0.25 loading * 1/0.1675 gph = 4,179 WH per gallon @ 25% loading
2,800 Watt * 1 hour * 1.00 loading * 1/0.3900 gph = 7,170 WH per gallon @ 100% loading

The Honda fuel efficiency (from Honda specs, my spread sheet above):
25% loading = 4,118 WH per gallon
100% loading = 5,847 WH per gallon

So, using the Central Main Diesel website--The 100% loading for the Yamaha 3000 is much better than the Honda 3000 (7,170/5,847=1.23) or ~23% more fuel efficient (more energy per gallon of fuel) at full load... Assuming the numbers are accurate--7kW per gallon (gasoline) is very impressive...

Just for the heck of it, looked up a 4 cylinder water cooled 20 kWatt gasoline genset fuel flow numbers:

https://www.centralmainediesel.com/order/Ford-20-kW-Gasoline-Generator.asp?page=F01709

Engine
Type	Ford MSG-425
Cylinders	4
Operating RPM	1800 RPM
Horsepower	40 HP
Cylinder Block	Aluminum Cylinder Block w/ Cast Iron Sleeve
Bore & Stroke	3.5" x 3.93"
Displacement	2.5 Liters

Consumption
Consumption at 1/2 load	1.27 gal/hr
Consumption at 3/4 load	1.94 gal/hr
Consumption at full load	2.2 gal/hr

20,000 Watts * 1 hour * 1.00 loading * 1/2.20 gph = 9,091 WH per gallon (100%)
20,000 Watts * 1 hour * 0.75 loading * 1/1.94 gph = 7,732 WH per gallon (75%)
20,000 Watts * 1 hour * 0.50 loading * 1/1.27 gph = 7,874 WH per gallon (50%)

So, in general, larger gensets, more fuel efficient. Running at higher loading (%), more fuel efficient (in general?).

The Yamaha @ 100% loading (from CMD webset)--Darn good fuel efficiency at full power... For the smaller gensets, engine size vs fuel efficiency--Difficult to find any generalities in fuel efficiency for "same rated" gensets.

-Bill "I do enjoy modeling for decision making help" B.

checkthisout · December 2021

FlyFishn said:

Are there any inverters in a small cube package that are low THD and around 6-7kw? The Sigineer in the link below is a good example of the form-factor I am after. This is a lot different than a wall-mount cabinet that I see some of the XW, and similar, solar system inverters seem to be.
http://store.evtv.me/proddetail.php?prod=Sigineer6kw
I will have to figure up the battery requirement, or battery + capacitor requirement (not sure I could get away with just capacitors) and see what kind of space that is going to take.
My idea is to package everything on a cart of sorts, perhaps an enclosure for sound reduction. The engine, alternator, and a cube inverter would be a pretty compact package, all things considered. However, the batteries could be a challenge on space requirements.

For what it is worth - I looked up the manual for the 6kw Sigineer and I don't see any mention of battery capacity requirements that would get down to the reactance. All they mention is voltages. This unit is both an inverter and charger, which is interesting. They have a few nifty features that make it pretty attractive for off-grid use. One of those is they have an output that will enable the remote operation of a generator when battery voltage goes too low. That is part of what I want to incorporate in to my system so the fact it is already there is cool. However, with the high THD the unit won't work for my application. I am not sure if there are other inverters that fit the form factor that can do all of this?

AIMS Power PICOGLF60W24V240VS 24 Volt Pure Sine Inverter Charger, 6000 Watt Low Frequency Inverter 110/220Vac Split Phase, 18000 Watt Surge, Battery Priority Selector, Terminal Block, GFCI https://www.amazon.com/dp/B00HHNCQ00/ref=cm_sw_r_apan_glt_fabc_ECP6YQ67PBX6FFSR55PH

That signineer looks to be the twin of three above Aims model.

Is there 1 company in China that makes everything of everything or something??

checkthisout · December 2021

BB. said:

Regarding:
In regards to the OP, my Yamaha 3000 ISEB generator takes electrical energy from the starting battery in order to augment boost for starting a load. 500 watts more than the non-boost model for 20 seconds. https://www.yamahagenerators.com/EF3000iSEB-p/ef3000iseb.htm
I am not sure how to compare this vs the Honda eu3000is ... The battery boosted inverter is pretty neat. But saving on fuel consumption is difficult to prove--And the difference of a few percent is probably meaningless in real life--But Marketing does take a hold here too.

From your link:
Long run time ‐ Continuous operation for up to 18.6 hours without refueling (at 1/4 rated load with “Smart Throttle” function on).
But from the "specs" tab:

Continuous operation for up to 19 hours without refueling (at 1/4 rated load with “Smart Throttle” function on).

3.4 Gallons

So we already have a mismatch in specifications in the same website... Albeit a small one. And for the Yamaha, I could not find its 100% runtime rating on its internal fuel tank.

The Honda eu3000is:
https://powerequipment.honda.com/generators/models/eu3000is

Fuel Tank Capacity 3.4 gal.
Run Time per Tankful 6.9 hr @ rated load 19.6 hr @ 1/4 load

So here, we have a 5% longer runtime for the Honda over the Yamaha "3000" gensets(19.6/18.6 hours @ 3.4 gallons of fuel)... Hand waving that 5% "better" fuel efficiency is noticeable by the average user... I could not find a 100% rated load fuel consumption for the Yamaha. So cannot compare full power fuel efficiency.

But, since I am interested in data and comparisons... The "2000" class of Yamaha vs Honda is interesting. The Yamaha has a significantly smaller engine vs the Honda--What do those numbers look like? This link has all the specs in one place:
https://generatorpick.com/yamaha-vs-honda-generator/

Yamaha EF2200iS vs Honda EU2200i YAMAHA EF2200IS HONDA EU2200I
Yamaha vs Honda numbers below:
Peak watts 2200W 2200W
Rated watts 1800W 1800W
Engine OHV, air-https://www.centralmainediesel.com/order/Yamaha-EF3000iSEB-3000-Watt-Inverter-Generator-with-Boost-Technology.asp?page=EF3000iSEB, single cylinder, 4-stroke OHC, 4-stroke, single cylinder Displacement 79cc 121cc
Fuel tank 1.24 gal 0.95 gal.
Run time 10.5 hrs (1/4 load) 8.1 hrs (1/4 load)

So, using Watt*Hours per gallon of fuel for efficiency:

1800 Watts * 0.25 loading = 450 Watts
Yamaha = 450 Watts * 10.5 hours * 1/1.24 gallons = 3,810 WH per gallon
Honda - 450 Watts * 8.1 hours * 1/0.95 gallons = 3,836 WH per gallon

Or something like 0.6% difference between Honda and Yamaha here... So at 25% loading, the smaller engine of the Yamaha does not seem to offer significant fuel savings with the "smaller" Yamaha motor vs the Honda engine (121/79=1.53) even though the Honda engine is 53% larger volume.

On the 100% efficiency--Again, for the Yamaha, I could not find a 100% rated runtime... So cannot make that comparison here either on engine size vs efficiency (or Y vs H overall efficiency).

For the average user--I still suggest that most people that choose smaller gensets on average random loads will probably be averaging around 25% loading vs (picked out of the air) 80% loading.

For off grid folks--Battery charging, running a backup electric heater--Yes, it is very possible to run these gensets at 80% loading--But we don't have any numbers here to make a Y vs H decision.

Realistically, probably at best, only a small difference in the 100% loading fuel efficiency between the two gensets--And using the Honda fuel flow here for the Yamaha (planning purposes) is more than "close enough".

......

I just remembered a website that does (usually) offer more fuel usage numbers for the models they carry. In this case for the Yamaha EF3000iSEB, they do have the 100% loading fuel usage (I don't know how they obtained these numbers--Hopefully they are accurate):

https://www.centralmainediesel.com/order/Yamaha-EF3000iSEB-3000-Watt-Inverter-Generator-with-Boost-Technology.asp?page=EF3000iSEB

Model Number EF3000iSEB
Consumption at 1/2 load 0.221 GPH
Consumption at 3/4 load 0.297 GPH
Consumption at full load 0.39 GPH

From their chart, there is also a zero load fuel consumption of 0.132 gph and 25% load of 0.1675 gph and a 3.4 gallon tank...

2,800 Watt * 1 hour * 0.25 loading * 1/0.1675 gph = 4,179 WH per gallon @ 25% loading
2,800 Watt * 1 hour * 1.00 loading * 1/0.3900 gph = 7,170 WH per gallon @ 100% loading

The Honda fuel efficiency (from Honda specs, my spread sheet above):
25% loading = 4,118 WH per gallon
100% loading = 5,847 WH per gallon

So, using the Central Main Diesel website--The 100% loading for the Yamaha 3000 is much better than the Honda 3000 (7,170/5,847=1.23) or ~23% more fuel efficient (more energy per gallon of fuel) at full load... Assuming the numbers are accurate--7kW per gallon (gasoline) is very impressive...

Just for the heck of it, looked up a 4 cylinder water cooled 20 kWatt gasoline genset fuel flow numbers:

https://www.centralmainediesel.com/order/Ford-20-kW-Gasoline-Generator.asp?page=F01709

Engine
Type Ford MSG-425
Cylinders 4
Operating RPM 1800 RPM
Horsepower 40 HP
Cylinder Block Aluminum Cylinder Block w/ Cast Iron Sleeve
Bore & Stroke 3.5" x 3.93"
Displacement 2.5 Liters

Consumption
Consumption at 1/2 load 1.27 gal/hr
Consumption at 3/4 load 1.94 gal/hr
Consumption at full load 2.2 gal/hr

20,000 Watts * 1 hour * 1.00 loading * 1/2.20 gph = 9,091 WH per gallon (100%)
20,000 Watts * 1 hour * 0.75 loading * 1/1.94 gph = 7,732 WH per gallon (75%)
20,000 Watts * 1 hour * 0.50 loading * 1/1.27 gph = 7,874 WH per gallon (50%)

So, in general, larger gensets, more fuel efficient. Running at higher loading (%), more fuel efficient (in general?).

The Yamaha @ 100% loading (from CMD webset)--Darn good fuel efficiency at full power... For the smaller gensets, engine size vs fuel efficiency--Difficult to find any generalities in fuel efficiency for "same rated" gensets.

-Bill "I do enjoy modeling for decision making help" B.

Good info. Yikes!

Here are the basic principles I know:

1) Lower engine RPM is more efficient-It takes more fuel to spin the engine faster.
2) Higher throttle settings are more efficient, throttle meaning manifold pressure-Engines are more efficient under load because higher throttle openings increase thermal efficiency.
3) Liquid cooling is better-Air-Cooled motors are designed to stay cool at full load while mowing the lawn in death valley heat. Anything less than that means the engine is running cooler than it should and is losing thermal efficiency.

Our inverter generator decouples engine speed from Hz. Our 6500 watt standard generator Honda up against our 7000 watt inverter model with the same engine and rated output means this:

Our 6500 watt standard generator must spin 3600 RPM. 3600 RPM is not necessary to achieve the horsepower output needed but is necessary for 60Hz.(We are wasting fuel on unnecessary engine speed) In order to have leftover surge capability, our standard generator must also run a larger engine than what is necessary at full output so the throttle can be opened more to maintain engine speed and thus 60hz when a surge in power is needed. We are wasting fuel at anything less than full throttle. (We are wasting fuel due to lower manifold pressure)

Our inverter generator with the same engine does not need to spin 3600 RPM at rated output. It can run say 3000 RPM with a greater throttle opening (higher manifold pressure) which gets us to the horsepower we need to make 6500 watts but the greater throttle opening means better thermal efficiency as well the lower RPM means less fuel wasted unnecessarily spinning a reciprocating mass.

Larger gensets are more efficient because mass and volume are not linear. Larger engines (larger cylinders) are thermally efficient than smaller ones. Larger gensets are liquid cooled and probably have a better engine design (fuel injection, cam, tumble flow intake and such) than your average air-cooled job.

Diesels are more efficient than gas motors, obviously.

It's worth noting that our large gensets would also benefit from decoupling engine speed from HZ. They would see all the same efficiency benefits as our puny little homeowner gensets. They could turn lower RPM at greater load etc.

The most efficient setup would be a turbocharged, diesel, inverter generator. You drop engine RPM a bunch and keep increasing boost.

I think maybe some people think inverter generators are less efficient at full load because of theoretical losses in the inverter. Is that the case? I wouldn't be able to dispute this because I don't know if that's the case or not. I also am not 100% sure if alternator in an inverter/generator is more efficient than a regular generator. Time to read.....

BB. · December 2021

Regarding:

Larger gensets are more efficient because mass and volume are not linear. Larger engines (larger cylinders) are thermally efficient than smaller ones. Larger gensets are liquid cooled and probably have a better engine design (fuel injection, cam, tumble flow intake and such) than your average air-cooled job.

Specifically, very roughly. if you have a 1" cube and a 2" cube--Or 2x larger linear enlargement. Surface area and volume/mass between the two cubes:

1" cube
1" * 1" * 6 sides = 6 square inches surface area
1" * 1" * 1" = 1 Cubic Inch volume
2" cube (2x larger)
2 * 2" * 6 sides = 24 square inches
2" * 2" * 2" = 8 cubic inch volume

The volume/mass expands at the cube of the size (linear). And surface area expands at the square of size.

More or less, engines (and refrigerators)--The larger the "thing", the the volume/mass increases faster that the surface area--Less thermal losses to the walls of the engine cylinder or walls of a refrigerator (vs a refrigerated warehouse).

Yes, there are pumping losses for a Otto cycle engine (throttle plate creates a restriction on input air flow--The engine is now a "vacuum pump). Diesel cycle--No throttle plate, no (or greatly reduced) pumping losses at lower power output.

There are so many variables (for example, in older carburettor car engines, they would increase the fuel air ratio--Lower ratio (less fuel per unit air) is more efficient. Higher fuel to air ratio, better power output. Ignition timing (cars with variable timing). Water pump is also a "loss" for engine power...

Just to play the game further... Back to the CMD website and look for a ~20 kWatt diesel genset to compare with the gasoline 20 kWatt genset. What does it look like in kWH/Gallon of fuel:

A 20 kWatt 1,800 RPM Diesel genset, 4 cylinder, water cooled... Very similar configuration to the 20 kWatt Ford genset above:

http://www.centralmainediesel.com/order/2000TS.asp?page=2000TS

Model Number			01226
Engine			-
Model			Isuzu 4LE1
Type			Vertical In-Line, Indirect Injection
Horsepower			32 HP
Oil Capacity (w/ Oil Filter)			Approx. 6.4 qts
RPM			1800
Cylinders			4
Cylinder Block			Aluminum w/ Cast Iron Sleeves
Bore & Stroke			3.35 x 3.78 in.
Cooling			Liquid-Cooling, Pushor Fan
Coolant Capacity			Approx. 6.8 Qts
Fuel			No. 2 Diesel
Electric Fuel Pump			12 V - Standard
Cold Weather Starting Aid			Glow Plugs
Starting System			Electric

Model Number			01226
Consumption			-
Consumption at 1/2 load			0.98 gallons/hour
Consumption at 3/4 load			1.41 gallons/hour
Consumption at full load			1.81 gallons/hour

20,000 Watts * 1/1 power * 1 hour * 1/1.81 gph diesel = 11,050 WH/Gallon (diesel) (100%)
20,000 Watts * 3/4 power * 1 hour * 1/1.41 gph diesel = 10,638 WH / Gallon (diesel) (75%)
20,000 Watts * 1/2 power * 1 hour * 1/0.98 gph diesel = 10,204 WH / Gallon (diesel) (50%)

Fuel specific heat/energy (numbers are average values):

https://www.engineeringtoolbox.com/fossil-fuels-energy-content-d_1298.html

Gasoline = 124,340 BTU/Gallon (non-California standard formulation)
Diesel = 138,490 BTI/Gallon (low sulfur diesel)

If we take the 20 kWatt gasoline genset numbers and take into account the greater BTU content per gallon for diesel:

138,490 diesel / 124,340 gasoline = 1.114 x more energy per gallon of diesel

20kWatt gasoline engine fuel eff:
9,091 WH per gallon (100%) * 1.114 = 10,127 WH / Gallon (100%) corrected for diesel fuel higher energy
7,732 WH per gallon (75%) * 1.114 = 8,613 WH / Gallon (75%)
7,874 WH per gallon (50%) * 1.114 = 8.771 WH / Gallon (50%)

Diesel efficiency advantage excluding higher specific heat per gallon of Diesel vs Gasoline:
11,050 "Diesel cycle" / 10,127 "Otto cycle " = 1.09 "Diesel" advanage
10,638 "D" / 8,613 "O" = 1.24 "Diesel" advantage
10,204 "D" / 8.771 "O" = 1.16 "Diesel" advantage

At least based on these numbers--A "larger" genset to genset comparison shows (excluding higher energy content per gallon of diesel fuel) we are looking at the "Diesel" cycle being 9% to 24% more energy efficient comparing these two gensets....

Have no idea how accuratly the fuel flow data was gathered/measured--But does give us some numbers to think about.

-Bill

BB. · December 2021

Finally found the hours per tank @ 100% loading from an old manual for the Yamaha EF3000iSEB:

https://www.yamahamotorsports.com/assets/service/manuals/0/LIT-19626-01-71_2173.pdf (page 43)

7.8 hours * 2,800 Watts * 1/3.43 gallon tank = 6,376 WH per gallon (vs Honda eu3000is @ 5,688 WH/Gallon)
6,376 Y / 5,688 H = 1.12
and not the 7,170 WH per gallon from the CMD data

So, with this data, the Yamaha seems to be 12% more efficient than the Honda @ 100% rated output (for their "3000 Watt inverter-generator" families).

-Bill

checkthisout · December 2021

BB. said:

Finally found the hours per tank @ 100% loading from an old manual for the Yamaha EF3000iSEB:

https://www.yamahamotorsports.com/assets/service/manuals/0/LIT-19626-01-71_2173.pdf (page 43)
7.8 hours * 2,800 Watts * 1/3.43 gallon tank = 6,376 WH per gallon (vs Honda eu3000is @ 5,688 WH/Gallon)
6,376 Y / 5,688 H = 1.12
and not the 7,170 WH per gallon from the CMD data
So, with this data, the Yamaha seems to be 12% more efficient than the Honda @ 100% rated output (for their "3000 Watt inverter-generator" families).

-Bill

And 500 watts higher surge output due to running a boost setup like the OP wants!

checkthisout · December 2021

BB. said:

Regarding:
Larger gensets are more efficient because mass and volume are not linear. Larger engines (larger cylinders) are thermally efficient than smaller ones. Larger gensets are liquid cooled and probably have a better engine design (fuel injection, cam, tumble flow intake and such) than your average air-cooled job.
Specifically, very roughly. if you have a 1" cube and a 2" cube--Or 2x larger linear enlargement. Surface area and volume/mass between the two cubes:
1" cube
1" * 1" * 6 sides = 6 square inches surface area
1" * 1" * 1" = 1 Cubic Inch volume
2" cube (2x larger)
2 * 2" * 6 sides = 24 square inches
2" * 2" * 2" = 8 cubic inch volume
The volume/mass expands at the cube of the size (linear). And surface area expands at the square of size.

More or less, engines (and refrigerators)--The larger the "thing", the the volume/mass increases faster that the surface area--Less thermal losses to the walls of the engine cylinder or walls of a refrigerator (vs a refrigerated warehouse).

Yes, there are pumping losses for a Otto cycle engine (throttle plate creates a restriction on input air flow--The engine is now a "vacuum pump). Diesel cycle--No throttle plate, no (or greatly reduced) pumping losses at lower power output.

There are so many variables (for example, in older carburettor car engines, they would increase the fuel air ratio--Lower ratio (less fuel per unit air) is more efficient. Higher fuel to air ratio, better power output. Ignition timing (cars with variable timing). Water pump is also a "loss" for engine power...

Just to play the game further... Back to the CMD website and look for a ~20 kWatt diesel genset to compare with the gasoline 20 kWatt genset. What does it look like in kWH/Gallon of fuel:

A 20 kWatt 1,800 RPM Diesel genset, 4 cylinder, water cooled... Very similar configuration to the 20 kWatt Ford genset above:

http://www.centralmainediesel.com/order/2000TS.asp?page=2000TS

Model Number 01226
Engine -
Model Isuzu 4LE1
Type Vertical In-Line, Indirect Injection
Horsepower 32 HP
Oil Capacity (w/ Oil Filter) Approx. 6.4 qts
RPM 1800
Cylinders 4
Cylinder Block Aluminum w/ Cast Iron Sleeves
Bore & Stroke 3.35 x 3.78 in.
Cooling Liquid-Cooling, Pushor Fan
Coolant Capacity Approx. 6.8 Qts
Fuel No. 2 Diesel
Electric Fuel Pump 12 V - Standard
Cold Weather Starting Aid Glow Plugs
Starting System Electric

Model Number 01226
Consumption -
Consumption at 1/2 load 0.98 gallons/hour
Consumption at 3/4 load 1.41 gallons/hour
Consumption at full load 1.81 gallons/hour

20,000 Watts * 1/1 power * 1 hour * 1/1.81 gph diesel = 11,050 WH/Gallon (diesel) (100%)
20,000 Watts * 3/4 power * 1 hour * 1/1.41 gph diesel = 10,638 WH / Gallon (diesel) (75%)
20,000 Watts * 1/2 power * 1 hour * 1/0.98 gph diesel = 10,204 WH / Gallon (diesel) (50%)

Fuel specific heat/energy (numbers are average values):

https://www.engineeringtoolbox.com/fossil-fuels-energy-content-d_1298.html

Gasoline = 124,340 BTU/Gallon (non-California standard formulation)
Diesel = 138,490 BTI/Gallon (low sulfur diesel)

If we take the 20 kWatt gasoline genset numbers and take into account the greater BTU content per gallon for diesel:

138,490 diesel / 124,340 gasoline = 1.114 x more energy per gallon of diesel

20kWatt gasoline engine fuel eff:
9,091 WH per gallon (100%) * 1.114 = 10,127 WH / Gallon (100%) corrected for diesel fuel higher energy
7,732 WH per gallon (75%) * 1.114 = 8,613 WH / Gallon (75%)
7,874 WH per gallon (50%) * 1.114 = 8.771 WH / Gallon (50%)

Diesel efficiency advantage excluding higher specific heat per gallon of Diesel vs Gasoline:
11,050 "Diesel cycle" / 10,127 "Otto cycle " = 1.09 "Diesel" advanage
10,638 "D" / 8,613 "O" = 1.24 "Diesel" advantage
10,204 "D" / 8.771 "O" = 1.16 "Diesel" advantage

At least based on these numbers--A "larger" genset to genset comparison shows (excluding higher energy content per gallon of diesel fuel) we are looking at the "Diesel" cycle being 9% to 24% more energy efficient comparing these two gensets....

Have no idea how accuratly the fuel flow data was gathered/measured--But does give us some numbers to think about.

-Bill

My very bad. I meant "volume" not "mass". My finger speed far exceeds my proofreading skills. Although if you were dealing with like materials the same rule applies for mass of course.

Less surface area to volume means less heat rejection to the water jacket or air which means better thermal efficiency as well as less quench areas and boundary layers......better combustion efficiency.

The throttle plate restricts which zaps some horsepower (parasitic loss) but also thermal efficiency is lowered because the effective compression ratio is lowered. The lower the operating pressure in the cylinder the less heat that gets turned into work because the temperature difference between TDC and BDC becomes less.

Engines with a higher compression ration are more efficient that those with lower compression ratios. Diesels run both a higher fixed ratio (mechanical engine design) and a higher effective ratio due to lack of throttle plate.

Cool to see these basics pencil out on the Yamaha gasser unit. It looks like it would behoove all generator manufacturers to go with a setup that steals boost wattage from the starting battery or dedicated battery and go with a slightly smaller engine option. Of course then you are into which is more expensive....the fuel or rube goldberg to get that extra 12% under some conditions.

Battery and engine based "inverter-generator"?

Comments

Categories