Why are inverter generators so much more compact than regular generators?

The small 900 - 5000 watt inverter generators that Honda and other manufactures have made popular throughout the last 10 years. Does the use of an inverter allow for a smaller ICE and generator that can make equivalent power by spinning faster than 3600rpm?

The engine will be the same size for any given output. A 1500 Watt generator will always have at least a 2 HP motor, as 2 HP is equivalent to 1500 watts. The noise and space reduction come from the fact that the inverter gensets do not have to run at 3600 / 1800 RPM (3000/1500 for 50 Hz systems) constantly to drive a relatively bulky AC generator.

Different manufacturers use different electrical conversions, but the most efficient one is using the engine to drive a three (or more) phase high voltage generator, which is much more compact then the typical 120/240 VAC Single phase generator, then rectify that output to DC, and feed it to a sine-wave generating inverter.

The speed you mentioned of 3600 RPM is only needed if you are trying to derive 60 HZ output from a two pole motor. By using either a generator where you don’t care about the output frequency or voltage, you can use essentially any speed for the prime mover.

What I meant about the size of the engine was actually displacement. Since the engine does not have to run at synchronous speed it can do an equivalent amount of work with less displacement by spinning faster… right?

Still a little confused on this if anyone has more time to post a little more detail? I’m not an electrical engineer. Just what I remember from HS physics

displacement is a red herring; for a given output power, you’re going to need an engine of a certain horsepower to drive the generator regardless of how the actual electricity generation is done.

what Khendrask was saying is that if you have a traditional genset, the engine drives an alternator (a term for an AC generator) and has to run at a fixed speed to provide the correct AC frequency. For the US this would be 60 Hz and means the alternator has to spin at 3600 for a two-pole design, 1800 rpm for a four-pole design, etc.

an “inverter” generator is more like the alternator in a car; it generates polyphase AC which is then rectified into DC (and maybe filtered) which is then fed to an electronic inverter (like those things you can buy at the store which plug into a lighter socket and give you AC) which creates the 60 Hz AC output. The advantage to this is that it no longer relies on a fixed rotational speed, so in theory you can use any old engine as long as it has sufficient horsepower.

An engine is just a set of chambers of a given volume where fuel is detonated. Displacement is related to engine size and weight. Displacement essentially determines horsepower for a naturally aspirated engine because

  1. There is a practical limit to how fast the engine can run at for a prolonged period, as higher RPMs mean more mechanical wear and less fuel efficiency

  2. Since there’s a practical limit to RPMs, then with each cycle of the engine, if it is naturally aspirated, there is a given volume of air that can fit inside the cylinder. You add only enough fuel to react all of that oxygen. This chemical reaction then produces a given amount of power, and, while the implementation details can nudge the power output up or down a little, that’s your horsepower.

For a naturally aspirated engine limited to a certain RPM, then, displacement is directly proportional to horsepower.

Another factor here is that if you have to run at a fixed RPM, you have to have enough torque to run at that RPM no matter the load condition. This means an engine with a lot of displacement. If you can run a smaller engine and develop enough horsepower at an arbitrary RPM, you can support a bigger electrical load.

hm. so, why did a 5.0 liter V8 only make 135 horsepower in 1980 while a 5.0 liter V8 can make 400+ horsepower today?

He did specify ‘naturally aspirated’ the modern engines make more horse power because they are using fuel injected engines that force a stronger fuel air mixtures. Nothing about an engine is ‘natural’ but a carburetor and unassisted air intake is more natural in terms of engines.

nope. “naturally aspirated” means the engine does not have forced induction. it opens the intake valve(s) and the air:fuel charge is drawn in by the piston moving downward in the cylinder. the stoichiometric air:fuel ratio for gasoline is 14.7:1 regardless of whether fuel is metered by a carburetor or fuel injection.

in my example, the 135 hp 5.0 liter V8 and the 400+hp 5.0 liter V8 are both naturally aspirated.

Evolution. Aluminum cylinder heads vs iron, higher compression, 4 valves per cylinder, more sophisticated fuel injection. Etc

I’m not even sure what you’re asking at this point.

Fundamentally, there are only 2 possible explanations :

  1. The modern engine gets more energy out of the same combustion of a charge of fuel and air.

This is probably true, but the difference cannot be large enough to explain it - 1980 technology understood the need for adiabatic mixtures of fuel and air, etc.

  1. The modern engine has more charges of fuel and air combusted per unit of time. Since the displacement is supposed to be the same, the only explanation is the modern engine has a higher RPM.

Frankly, it’s probably a combination of factors :
a. Higher RPM on the modern engine
b. More accurate fuel/air mixtures with real time electronic controls
b. The way “displacement” is measured has changed, and the 1980 engine had inflated numbers

You can’t cheat thermodynamics, folks. Combustion efficiency can’t be increased past a certain point, and so for more power from the same engine, there’s gotta be more fuel and more air going into it.

nope. it’s airflow. the modern 5.0 liter engine wastes less energy on just trying to draw the air:fuel charge into the cylinders. it has less restrictive ports, more advantageous valve angles, variable cam timing, and a few other things.

Sure. But there’s only so far you can go to boost performance with that without pressurizing the air intake. Even if the airflow was such that each cylinder fills with air at atmospheric pressure instantly, if you aren’t willing to go past 6k or so RPM (heavy wear, need expensive parts if you go higher), then you get 6k * displacement charges of air to combust per minute. Multiply the combustion energy by your combustion efficiency, correct for Carnot, and there’s your maximum possible power output.

Brake Mean Effective Pressure (BMEP) coupled with displacement is what predicts an engine’s theoretical maximum output in torque. the theoretical maximum torque from a 5 liter engine is around 400 lb-ft. doesn’t matter if it’s from a Mustang (412 hp, 390 lb-ft) or an F-150 (360 hp, 380 lb-ft,) or a Ferrari 458 (562 hp, 398 lb-ft.)

I presumed you were talking about the ford 302 windsor engine and the 5.0 coyote engine. The things I listed are what helps the newer motor make more power from the same displacement.

I think that your trying to look at this too theoretically. Engines in the mid-late 70’s and 80’s were very “choked down” on horsepower from those of the late 60’s and early 70’s. Catalytic converters were introduced, lower octane unleaded fuels were substituted, and fuel economy became a large concern due to gas prices. Fast forward to today and we’ve made it possible to have your cake and eat it too. (Power and efficiency) Computer controlled port fuel injection helps ensure that each cylinder will always receive the ideal fuel ratio, something a carbeurator could only dream of. Variable valve timing helps to improve volumetric efficiency throughout the entire rpm range. Some naturally aspirated engines are even able to exceed 100% volumetric efficiency.

This. For a given displacement, modern naturally-aspirated engines breathe better (more torque and more horsepower), rev higher (more horsepower), and operate at higher compression ratios (more efficient, thus more torque and more horsepower) than their older versions.

Back to the OP: the use of inverters allows the engine to operate at a modest RPM, reducing noise output. If anything, this means an increase in the required displacement, which should mean a larger engine. But as Khendrask notes, the electrical hardware for an inverter genset is much more compact than for a simple single-phase alternator genset - so overall, you end up with a more compact genset.

Thanks for clearing that up and getting us back on topic machine elf. I will have to research three phase generators now to understand why they are more compact.

Check out a diagram of a 3-phase alternator.

A single-phase alternator has only one field (stator) coil, so as the engine-driven rotor spins, peak current only occurs twice per revolution (one is a +peak, the other is a -peak). In between, the current flow in the stator coil is something less, i.e. the rotor is making less than peak power most of the time, even making zero instantaneous power twice per revolution. Your shaft torque varies from max to zero twice per revolution, so you need some robust hardware, and a durable shaft coupling that can live for a long time with those big torque fluctuations. But you’ve got single-phase AC power at the output, which is good: if you size things right and spin it at exactly 3600RPM, you’ll have 60-hz 110AC power available, and you can plug your beer cooler directly into it.

Now take that single-phase alternator and add two more stator coils 120 degrees apart from the first, and you have made a three-phase alternator. Now you get peak current occurring six times per revolution; the shaft is loaded more evenly over time (you can make it lighter, thinner, with a less rugged coupling to the engine), and the system as a whole is generating a more constant power output. If you can supply enough mechanical power to the shaft to keep it spinning, you’ll get three times the electrical power out of this alternator. The problem is that now you’ll need a device that can make use of three-phase power, like a three-phase motor, which is something the average joe hardly ever sees in person. Since three-phase power isn’t really useful when you’re camping or trying to keep your sump pump running, you take the outputs of the three coils on your alternator and attach them to an inverter that performs some solid-state magic and spits out single-phase 110VAC for whatever you care to plug in.

The added benefit of using the inverter is that the frequency of your alternator’s output no longer needs to be exactly 60 hertz; you can spin the alternator at whatever speed you want (usually the goal is a slower, quieter engine RPM), and the inverter will take whatever comes out, pass it through its solid-state electronic sausage grinder and spit out delicious 60-hertz, 110AC power.

Since you didn’t need three times the power, now you can redesign your 3-phase alternator to use smaller magnets, fewer turns on each coil, and whatever else you want to do to make it smaller, until you get the power capacity back down to your original goal. Presto: a smaller, quieter genset for the power capacity you wanted.

An additional benefit is that gasoline, spark-ignited engines are more efficient at higher loads. So for a light electrical demand, instead of running at 3600 RPM and 10% throttle, the inverter lets you run at (for example) 1800 RPM and 20% throttle, giving you more kilowatt-hours per gallon. If you’re drawing the max rated electrical power, the engine can still spin up to 3600 RPM and WOT to meet that need, so despite what I said upthread, the engine shouldn’t need to be any bigger. But since it runs at part load most of the time (and is more efficient there than a single-phase system), maybe the designers can install a smaller fuel tank while achieving the same elapsed time between refills.