# What is the efficiency of running an electric motor from a gas generator?

If you were to run an electric motor from a gasoline or diesel generator, how big would the generator have to be to run something like the Siemens electric engine? Is this a viable model for replacing a heavy piston engine or is it too inefficient?

The link below says standard portable generators are capable of 6.13 kWh per gallon of gas.

The Siemens only weighs 50kg and delivers a continuous output of 260kw. Does that mean I'd need a generator 42.4 times as powerful than most portable generators and be burning a gallon of gas a minute?

I know a lot of small aircraft engines weigh around 150kg and several use gear boxes as well.

So Does this mean you could save the weight to make the plane electric but not battery powered or is the generator simply going to be too inefficient?

• My aircraft burns 9 gallons per hour, in your scenario, you'd be burning 60 gallons per hour, doesn't seem like a win to me. – Ron Beyer Jun 2 '17 at 3:22
• Valid, but could a larger generator make it more efficient, especially if it were closer if it's closer to a Beechcraft bonanza than a Cessna 172? – oeste Jun 2 '17 at 3:24
• The conversion isn't free, so why not just hook a propeller to it and cut out the middle-man? The issue with the larger generator is that you also have a larger stator, meaning more (very heavy) weight. Even if it was 100% efficient you are reducing the useful load and space. A small generator can extend the range of batteries, but really isn't going to replace them. I know somebody here has the math. – Ron Beyer Jun 2 '17 at 3:28
• A scenario like this would work in cars, due to weight being a less important factor, and due to the car accelerating and stopping often and the electrical circuit is able to store the power from deceleration of the car and reuse it for acceleration. In city traffic that is. Airplane engines work at a constant load that is much higher than the load on a car engine on the freeway, and there is no deceleration energy to be reused. – Koyovis Jun 2 '17 at 3:46
• There are some experimental airplanes flying with a hybrid system like that. E.g. the e-Genius (ifb.uni-stuttgart.de/egenius) has a small Wankel diesel engine for range extension (>300km on batteries, 1000km with the wankel). The results are propably good- in the next version they will be using a larger diesel engine. – Gypaets Jun 2 '17 at 17:31

I know a lot of small aircraft engines weigh around 150kg and several use gear boxes as well.

Almost no piston aircraft use gearboxes. The only one that I know of that ever did was the Porsche Powered Mooney PFM which was largely a failure overall, with the gearbox being often cited as an issue. Smaller turbo-prop planes do use gearboxes due to the super high RPM range of a turbine but those are generally larger than the ones that seem to be in question here.

So Does this mean you could save the weight to make the plane electric but not battery powered or is the generator simply going to be too inefficient?

As mentioned the issue here is simply weight and line loss. You are taking mechanical energy converting it to electrical, bussing it somewhere then converting it back to mechanical. There will always be line loss in a process like that when you don't really need it (since you need to spin the prop to begin with). Gas-Electric (be it diesel or gasoline) is advantageous as you can drive full torque at 0-RPM which is useful when getting a giant freight train rolling but does not provide a ton of benefit to light aircraft. You are also introducing at least 3 major points of failure (engine -> generator -> motor) to a system that previously only had one.

• The compare/contrast with diesel-electric locomotives is especially instructive... such systems do very well in that field meeting their needs & working within their constraints, where volume is large and weight is actually considered a good thing. The completely different needs & constraints in aviation drive entirely different solutions. Jet propulsion is great, but not for locomotives! – Ralph J Jun 3 '17 at 11:28

Analogous to train powertrains :). The great characteristic of electric motors is the large torque generation at standstill, compared to combustion engines. We all tend to look at power output for engine comparison, but it is Newton's second law that provides the acceleration to achieve the end velocity: force (or torque) does that. Power equations just make the computations easier, especially if torque is a function of rpm and if there are gearboxes with selectable gears.

The four stroke petrol (gasoline) engines provide zero torque at standstill and a pesky amount of torque at idle. Two-stroke engines, diesel pistons, gas turbines, electric motors all provide more torque at standstill or idle. Ships have 2-stroke diesels for all the torque that the propellor must provide. Trains indeed use the combustion engine-torque motor combination for better torque control for heavy trains, the alternative is a slipping clutch which has obvious drawbacks. Cars now have combustion - torque combination with batteries to regenerate energy that would otherwise be lost from deceleration.

So if we treat your question as a design question, we can see the reasons why electric motors were successfully applied in other vehicles: to overcome a dominant design issue. In aircraft there are three dominant design issues: weight, weight and weight. And safety. An aircraft that is too heavy simply will not fly. Sure a B747 freighter can lift a tank, but the skin is made from aluminium, the engines are twin axle gas turbines (high torque, low weight!), and everything in the aircraft has been designed for low weight first.

Aircraft don't need slip clutches, the air itself does that already. They just need a light way to create a lot of torque and output power. They also need redundancy: if you have one gas turbine providing electricity for four motors, and the turbine fails, all four motors stop. If you have 4 separate gas turbines, failure of one of them is not catastrophic. For preventing failures aircraft need simplicity. Converting kinetic energy to electrical to aerodynamic propulsion is too complicated.

All of this will be a different story once we can generate electricity in huge amounts from light weight fuel cells, but we haven't cracked that nut yet.

What you describe sounds like a gas-electric transmission (gas/diesel engine drives generator, generator drives one or multiple electric engines that propel the device).

Such systems are sufficiently efficient and very common in trains, ships and other big, heavy and powerful machinery.

Seems that nobody ever attempted to build a plane like that, but, to my opinion, it should fly. There may not be enough benefits to justify the additional weight of the generator and powerful electric engine. For instance the possibility to place the larger and heavier gas engines anywhere in the plane and distribute their power evenly between all (multiple) propellers may be an advantage.

• Nobody attempted that indeed, and that may tell you something. What they did successfully attempt is to transfer high required power via hydraulics. Hydraulics! The leaky noisy stepdaughter of power generation, and the aeroplane version uses oil that does not burn but will happily dissolve your arm if given a chance. Simply because of the weight. Unfortunately, weight may prevent aircraft from flying. – Koyovis Jun 2 '17 at 23:21
• @Koyovis Nobody attempted that: In the last aeronautic trade fair I visited there were at least four companies offering such systems. In the past there were no lightweight electric motors, but nowadays experimental airplanes using a gas generator and electric motor(s) do exist. – Gypaets Jun 3 '17 at 6:31
• @Gypaets Well that is good news, and it means that in 20 years time there may be very mature aeroplanes flying around in that configuration. What advantages were their sales teams promoting? – Koyovis Jun 3 '17 at 6:39
• @Koyovis I didn't speak with the sales team, but it is difficult to power multirotor/distributed propulsion systems like the Volocopter with a piston engine attached to each propeller. In the e-Genius case the propeller is placed in a really efficient place were only a small lightweight motor fits (try attaching a piston engine to the top of your vertical stabilizer). The same 2m diameter propeller at the front connected directly to a piston engine would require a much heavier landing gear. It probably pays off. – Gypaets Jun 3 '17 at 7:11
• I understand. Was just curious how the Volocopter would compare to a similar platform with one big propellor. – Koyovis Jun 3 '17 at 7:51