# How does the energy and weight efficiency compare between a battery/electrical propulsion and combustion engine?

For the same total energy and power output would a battery powered vehicle be lighter than one which uses a fuel burning engine (Currently)?

This question is intended for helicopters.

• Downvote because the answer is so obvious. – Peter A. Schneider Feb 16 '16 at 11:21
• @PeterA.Schneider Not fair. Answers are only obvious when you know them. How many pilots and engineers, let alone folks not in aviation, understand energy densities? – Simon Feb 16 '16 at 12:06
• @Simon Well, there are many questions (and their answers) on the Aviation stack which I truly enjoy. There are aviation experts here which provide insights and information not readily available to the general public. But the energy density of batteries vs. fossil fuels is not one of them. First of all the answer is trivially googleable. Second, the answer is visible in everyday life, starting with the limits of electric cars (which don't even attempt to fly!). Did the author never change a car battery which is as heavy as all the gasoline in the tank but sometimes can't even start the car? – Peter A. Schneider Feb 16 '16 at 12:37
• @PeterA.Schneider Well, if Googlability was a criteria, then I guess somewhere between a third and a half of all questions on this site are not welcome. – Simon Feb 16 '16 at 18:01
• @PeterA.Schneider 's statement is not so obvious to me, an electric powered vehicle can be lighter than a vehicle using fuel with same power. Indeed fuel (45 MJ/kg) is more efficient than Li-ion battery (0.5 MJ/kg). But this is for fuel only. Compare also the weight of the engine, and the rest of the infrastructure. There is a limit, for small power, where this statement is not true anymore. If a minimum power is specified, the statement may be true, but the question doesn't specify (now) a limit. So +1 to this question. – mins Feb 16 '16 at 20:37

Poorly. That is a reason why we don't see battery powered aircraft beyond some experiments. The specific power is still much worse for batteries.

## Energy storage:

• Jet fuel has specific energy around 43 MJ/kg and energy density around 35 GJ/m³[1].
• The best mass-produced rechargeable batteries to this day, Lithium-ion polymer, the best kind has specific energy 0.95 MJ/kg and energy density at most 2.6 GJ/m³[2].

That is more than an order of magnitude difference in favour of hydrocarbon fuel (almost 2 orders by weight, one by volume). Even taking into account the 2-3 times higher efficiency of electric engines (which do not suffer the theoretical efficiency limits associated with heat engines) hydrocarbon fuel is an order of magnitude better by weight.

## Engines:

• According to [3], the power-to-weight ratios for electric engines and gas turbines are comparable (in the range of 5-10 kW/kg).

Electric engine won't give any advantage to electricity.

Well, the difference in energy density is so huge that even large difference in engine weight wouldn't help much anyway. For models, electricity is OK as due to the square-cube law they have a lot of power and lift and small-scale heat engines are difficult. But for aircraft carrying humans, batteries are very far from usable.

• @Federico, 43/0.95 = 45, which is more than $10^{1.5}$. – Jan Hudec Feb 16 '16 at 9:52
• And it gets worse with flight time. While fuel is consumed and the aircraft gets lighter, batteries will have the same mass, regardless how much energy is left. – Peter Kämpf Feb 16 '16 at 10:28
• Everything is limited by the second law of thermodynamics. That's why it's a law. Perhaps you're thinking of Carnot efficiency? – J... Feb 16 '16 at 12:12
• @JanHudec No, the second law says that entropy in a closed system increases - this applies to electric motors as well as to combustion engines. Anything that isn't limited by the second law is either a perpetual motion machine or a free energy source. The efficiency limit of a combustion engine is best described by the Carnot cycle. – J... Feb 16 '16 at 13:06
• @IanRingrose If you put sixteen 777-200s in a square of 4x4 (wingtip to wingtip, nose-to-tail) and drew an unbroken square around all sixteen aircraft, you would have a solar cell that, if it was 20% efficient (typical for a good cell), would produce about half the power of one Trent-800 engine (in full sunshine, pointed at the sun). – J... Feb 16 '16 at 14:00