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When I see electrical generators specified, they usually give the fuel consumption as at "rated load". So,for example, a 5 kilowatt generator might consume 0.6 gallons per hour.

However, what happens if the load is less than 5 kilowatts? Does the generator still run at the fixed rate and any excess electricity is just shorted to ground?

Or does the engine run slower and just generator less energy? How does it know what rate to run at? Does it have to manually adjusted?

If the engine is run at less than its maximum load, what would be a typical loss of efficiency? Can someone provide an example curve that shows how a generator loses efficiency at sub-optimal loads?

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  • $\begingroup$ When you say generators, are inverter generators also part of the category. Since common inverter generators do use variable speed based on load. $\endgroup$
    – Netduke
    Nov 16, 2016 at 13:32

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Generators don't use fuel at all. You are apparently asking about a generator coupled to some sort of chemical engine, like a "genset".

Usually these engine/generator combinations have a regulator that tries to keep the speed reasonably constant. This supplies as much fuel to the engine as necessary to maintain that speed. With lower electrical load on the generator, it puts less mechanical load on the engine, which then requires less fuel to maintain the same speed.

This is no different from the cruise control in your car. It maintains 55 miles/hour, for example, on a highway as you go up and down hills. However, it opens the throttle more, and delivers more fuel to the engine, when going up hills than when going down. Your milage (miles/gallon) is quite different going up a hill than down it, even though both are at the same speed.

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  • $\begingroup$ That is a good analogy. Like going up a hill. So, I guess, the engine just needs a rotation speed detector. If it starts to slow down, it increases the fuel rate. $\endgroup$ Nov 16, 2016 at 14:51
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At a lower load they run at the same speed but the generator becomes easier to turn which means the throttle to the engine driving it can be reduced to save fuel. The throttle is controlled by a system designed to maintain the correct speed no matter the load.

The efficiency Vs load curve is going to be generator specific.

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If you are talking about gas electric generators designed to supply household ac power, there are two basic types. These generators must supply a reasonably accurate 60hz frequency power, and there are two basic ways to achieve that.

most commonly the generator is wound to produce 60hz at a specific speed and the generator must be run at that constant speed regardless of load (as noted the throttle is closed to reduce air/fuel under reduced load.)

the alternate is to use an electronic device called an inverter to regulate the ac frequency. This permits the actual generator to be run at variable speed. (This is typical of small camping type generators). Inverter type gensets do indeed drop speed to a minimal level under light load. Note this has as much to do with making them quiet as saving fuel. Note also the inverter itself has an efficiency cost to it so these units may be less fuel efficient at high load than comparable fixed speed units.

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Most home gasoline-engine generators have a "2-pole" rotor which is spun at 3600 RPM by a governor on the engine. This results in a pair of 60 Hz sine-wave volatges coming out of the stator winding...180 degrees out-of-phase with each other. The losses in a typical gasoline engine at 3600 RPM are quite significant. If you happen to have an automobile with a tachometer, you might note that the RPM rarely goes over 3600. If the electrical load on the generator increases, the rotor current is increased to maintain 120 volts (normal line voltage). That puts a higher load on the gasoline engine and the governor opens the throttle to maintain speed. Very simple and inexpensive.

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Most generators will attempt to maintain a constant sped/frequency regardless of load. As far as the engine is concerned the load is the torque on the alternator which is itself dependant on the current being drawn.

Engine RPM is controlled by some sort of electronic or mechanical regulator ie some sort of feedback control system, which detects engine speed and adjusts the throttle to attempt to maintain a preset rpm.

In practice this regulation is imperfect and there will often be a some lag with the engine speed slowing noticeably when a large load is applied, for example starting a large electric motor.

Note also that the power generated is proportional to the electrical impedance across the alternator ie the current that what ever is connected to it is trying to draw so less load simply means that the engine needs to deliver less torque to maintain the same rpm. For example if the engine is running with nothing connected to the electrical output then no electrical power is generated and the engine only has to overcome its own internal friction, as soon as a load is connected this creates a back emf in the alternator which imposes a torque on the engine which needs to be matched by opening the throttle to maintain a constant speed.

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  • $\begingroup$ This is interesting, I had no idea that there was actually a mechanical force imposed by higher load, but this makes a lot of sense. A motor literally operating backwards, basically. $\endgroup$
    – trpt4him
    Nov 24, 2018 at 17:24
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There are two types of generator sets of interest: regular generator sets and inverter generator sets.

Regular generator sets create 50 Hz or 60 Hz electricity directly all the time. Thus, the rotation rate of the generator always has to be 3000 RPM or 3600 RPM. However, this doesn't mean the engine is running at 3000 RPM or 3600 RPM. The engine may be connected to the generator by a belt, by a chain or by using gears. Nevertheless, the engine is all the time running at constant speed, whatever it may be. In directly connected generators (no gears, belts or chains), obviously the running speed of the engine has to be 3000 RPM or 3600 RPM.

Because the engine is running at constant speed, it is never "idling" if you don't have any load. The load on the engine decreases, but its speed doesn't. If it's a gasoline engine, the throttle plate will be nearly closed with no load, creating lots of pumping losses. Friction losses also don't decrease. Therefore, the fuel consumption will be reduced only very little if you disconnect all load. Diesel generators would be slightly better because they have no pumping losses, but they would still suffer from the constant friction in the engine. I don't have any figures for regular generator sets, but expect them to be poor below the rated load.

Inverter generator sets are different. They create high-voltage direct current, and then use pulse width modulation to synthesize sine wave, or for some early or really cheap (not common anymore nowadays) generator sets, modified square wave with three levels: positive, zero, negative.

Because the frequency of the output sine wave is synthesized, the engine may be running at any speed provided that the engine can supply enough power. Usually there's an "economy switch". Some cheap Chinese generator sets may say in the manual that "economy switch" must be off for high loads, better generator sets can even provide full load with "economy switch" on. What the "economy switch" does is it allow free RPMs. With "economy switch" on, RPM is typically variable, very low at no power output, very high at full power output. With "economy switch" off, RPM is always at the maximum power RPM. In better generators (not those cheap Chinese ones), you only need the "economy switch" off if you are planning to start a really large tool or appliance such as a large circular saw, that can require a high inrush current when starting, creating a momentary high load. Typically these inverter generator sets are designed to be lightweight, able to be carried with one hand, so the flywheel is lightweight, and given that the engine is single cylinder, it can stall if there's a huge inrush current. When you turn the "economy switch" off, you increase the RPM of the engine, allowing starting huge tools or appliances. Then you can immediately turn the switch on after the inrush current is gone and the engine stays at optimal RPM.

Even inverter generator sets that run on gasoline suffer from pumping losses. At very low load, the throttle plate is nearly closed, creating an intake vacuum that causes those pumping losses. Diesel generators would be better but heavier and more expensive. The main benefits of inverter generators are that (1) if RPM decreases, friction losses decrease, and (2) if RPM decreases, throttle plate can be opened more for same power output, which reduces but does not eliminate pumping losses. Also noise reduces if engine speed reduces.

Here's an example of fuel consumption of Honda EU10i (1000 watt inverter generator set): https://hsaoy.com/wp-content/uploads/PAKM_Honda_EU10i.pdf

In case the link goes stale, here are the figures:

0 W,    0.25 l/h
215 W,  0.29 l/h, 1.35 l/kWh
430 W,  0.38 l/h, 0.88 l/kWh
630 W,  0.47 l/h, 0.74 l/kWh
829 W,  0.59 l/h, 0.71 l/kWh
1029 W, 0.77 l/h, 0.75 l/kWh

So you can see that at full load, it's consuming about 3x the gasoline it would be consuming at no load. That's hardly optimal. A car cruising at 120 km/h is consuming maybe 7 liters / 100 km, or 8.4 liters per hour whereas a car idling would be consuming 0.5 liters per hour, 16.8x difference. And the "cruising at 120 km/h" wouldn't even be full load for the car. Whereas for these small inverter generators, full load consumes 3x the idle consumption.

Gasoline contains about 32 MJ/l or 8.89 kWh/l, so the best efficiency this generator set (Honda EU10i) can attain is below 16%. Typical efficiency at low load is maybe 10%. As you can see, most of the energy goes into keeping the engine running, and very little goes into the produced electrical power output, except at high loads where produced electrical power output exceeds the constant consumption.

A larger generator would be less efficient at same load. At 386W average load, my Champion 92001i (1900 watt unit) consumes 0.44 liters per hour of small engine gasoline (which would correspond to 0.42 liters per hour of ethanol-less regular gasoline -- the small engine gasoline has low energy density). The EU10i at same load would consume about 0.36 liters per hour (interpolated from the figures), not sure if the EU10i test was done with ethanol-less regular gasoline, with E10 gasoline or with small engine gasoline.

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