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[Michael T Casey]

I'm not much on an engine guy. I was wondering, will the amperage load on a generator affect the rate of fuel consumption? Assuming any auto-throttle or idle functions are turned off. I'm going to do a runtime test but not sure if an electrical load is necessary to get accurate results.

The context:
I regularly use a large quantity of generators to power audio/lighting along the course of marathon road races.  Typically use Honda EU3000 and EU6500. Several of the setups are just two EONs with an iPod for music. I'd like to downsize those to Honda EU2000's, but I want to test the runtime of a tank of gas first.  Do I need to plug in some EONs and play some music to get a true reading, or will letting the gennie run with no load suffice?

[Tim McCulloch]

I'm not much on an engine guy. I was wondering, will the amperage load on a generator affect the rate of fuel consumption? Assuming any auto-throttle or idle functions are turned off. I'm going to do a runtime test but not sure if an electrical load is necessary to get accurate results.

The context:
I regularly use a large quantity of generators to power audio/lighting along the course of marathon road races.  Typically use Honda EU3000 and EU6500. Several of the setups are just two EONs with an iPod for music. I'd like to downsize those to Honda EU2000's, but I want to test the runtime of a tank of gas first.  Do I need to plug in some EONs and play some music to get a true reading, or will letting the gennie run with no load suffice?

What does Honda say?  Most manufacturers have runtime data based on loading available.  It's not terribly granular but should give you a decent idea of how 25% -50%-100% loading affects fuel consumption.  You might find that running a smaller genny harder will use more fuel than running a bigger genny with a lighter load... or not.

[Jonathan Johnson]

Short answer: yes.

Long answer: aside from generators with inverters, a generator must maintain a specific speed in order to maintain frequency, whether at no load or full load. As you add more load, it requires more energy to turn the generator. This would have the effect of slowing down the engine, so to compensate the engine governor opens the throttle more to increase fuel flow in order to maintain speed.

Even the inverter generators will use more fuel with higher load, but they do moderate the engine speed according to load to maximize fuel efficiency.

EDIT: The auto-throttle/idle functions on a non-inverter generator will reduce the engine speed when no load is present. It takes more energy/fuel to spin an unloaded engine at a higher speed than a lower speed; even with "no load" there is still a load internal to the engine: spinning the flywheel, pushing the piston up and down, drawing fuel/air mixture in and pushing exhaust out. The faster the engine turns, the greater the internal load is.

[frank kayser]

<snip>
 Do I need to plug in some EONs and play some music to get a true reading, or will letting the gennie run with no load suffice?

Yes.  To get a realistic load on the gennie, you need to plug in some EONs and play some music at roughly the same volume.  Eons use an amp design that takes less power at idle and low signal than at full-tilt boogie.


Read Mac's answer carefully - smaller gennie may actually use more fuel than what is currently in use.  Honda's load tables will give you a fairly good idea of the fuel vs. load proportions over time.


frank

[Bob Leonard]

What does Honda say?  Most manufacturers have runtime data based on loading available.  It's not terribly granular but should give you a decent idea of how 25% -50%-100% loading affects fuel consumption.  You might find that running a smaller genny harder will use more fuel than running a bigger genny with a lighter load... or not.

This, plus, a generator is dependent on constant rpm to maintain the proper output voltage. This constant rpm is regulated through the use of a governor that may allow slight fluctuations in rpm, but seldom much more than 100 rpm high/low. The governor has full control of the generators throttle and will open the throttle as needed if the engine load reduces engine rpm, or conversely will shut the throttle should rpm increase. After the engine most generators will control minor fluctuations in voltage through the use of a regulator circuit. This circuit combined with the speed of the engine are what maintains stable voltage. The higher the load, the more strain on the engine, the more the governor calls for fuel as the engine rpm slows, hence the shorter run time. Larger generators actually tend to be more economical to run because the larger horsepower generators are not as susceptible to heavy loads or changes in load which will cause the governor to call for more fuel to keep the engine running at the same rpm. Also notice your generator probably doesn't have a throttle. This is by design as the manufacturer will set the throttle to run the engine within it's peak power band, usually around 3200 rpm for most small engines.

A good comparison may be your snow blower or lawn mower when they come up against heavy snow or higher grass. The engine has to create more horsepower and that means more fuel. The generator senses the load as the engine slows, opens the throttle, and feeds more fuel to the engine. You can hear this when it happens, small engines 101.

The manufacturer will have ballpark specs you can use, but the only real way to estimate fuel consumption will be to run your generator under load and check the fuel at the end of the day.

[Richard Turner]

At a guess the 6500 model is based on the gx390 39cc 13 hp engine Have used them on various machines, hydraulic power packs and pumos etc, the 1.3 gallon tank lasts couple hours under moderate to heavy load


the newer i series gennies are more efficient with the invertor

An old antique diesel petter (lister) single cylinder 5000w gen set would run for 24 hours on 4 gallon of diesel at 60% load, of course they also weight over 600 lbs with all connected peripherals....

[Bob Leonard]

That's tremendous horse power from an engine that size. Did you mean 13hp from 390cc?

[Frank Koenig]

Spark ignition (Otto cycle) internal combustion engines achieve their highest efficiency at full power so, in general, a small engine running full tilt boogie will be more efficient than a larger engine running at part power. This is because the pressure of the mixture entering the combustion chamber is higher at wide open throttle (or full boost in the case of turbocharging) resulting in a larger pressure ratio and higher thermodynamic efficiency.

Diesels have higher part load efficiency than spark ignition engines since the combustion air is not throttled, thereby maintaining a constant pressure ratio. Just the size of the fuel shot is reduced to reduce power.

Inverter based generators partially get around the poor part load efficiency of spark ignition engines by operating the engine at the lowest speed necessary to achieve the required power, allowing for less throttling. The output frequency is determined by the inverter and the efficiency of the electrical machine is high (compared with the IC engine) and independent of rotational speed over a wide range. This is just like getting better milage in a car by selecting the highest gear that can provide the required power without lugging.

Now it may be that very small, cheap generators have poor efficiency but that would be due to an inferior combustion chamber design, such as a flathead, or poor mixture control requiring a rich of stochiometric mixture to get smooth operation, or an inefficient electrical machine. I bet the small Honda inverters are pretty good though, and they all publish tank capacity and run time at various loads, so shop and compare.

--Frank

[Jonathan Johnson]

Spark ignition (Otto cycle) internal combustion engines achieve their highest efficiency at full power so, in general, a small engine running full tilt boogie will be more efficient than a larger engine running at part power.

To clarify, an internal combustion gasoline engine will be capable of producing more horsepower-hours (watthours) per unit of fuel consumed at full speed versus a lower speed.

However, given the same apparent load, at full speed it will consume more fuel per unit of time than at half speed, since actual load tends to increase with speed. (What I mean by the difference between apparent load and actual load can be explained this way: at half-speed, a generator will not light a given incandescent bulb to its full brightness, whereas at full speed it will. The "apparent load" is represented by the wattage listed on the light bulb's spec sheet; connected to the generator; the "actual load" is the wattage that is actually consumed.)

At full speed, the ratio of fuel used to energy released by the aforementioned light bulb will be lower than at half-speed. The inefficiency at lower speed is due to the energy lost by the motor and generator itself (represented as acoustic and thermal energy). At lower speeds, the ratio of energy lost due to inefficiency compared to energy spent on desired work is higher than at higher speeds.

(Not sure if I succeeded in clarifying.)

[Stephen Swaffer]

Simplification:

More load requires more fuel

Generator/engine efficiencies vary

Actual field test with actual load=real, usable answers

[ProSoundWeb Community]