If gasoline combustion is just a heating method for the air to expand and cause pressure then why can't we find another way to heat the air in the engine during the "combustion" stage? Have people tried?

  • 4
    Actually, most of air is not oxygen but rather nitrogen. So, nitrogen expanding is doing most of the work. During combustion, the oxygen is actually combined with the fuel to form water vapor (the oxide of hydrogen) and carbon dioxide. So, strictly speaking, what is expanding is water vapor, carbon dioxide and nitrogen, not oxygen.
    – juhist
    Feb 4, 2017 at 17:42
  • We did, well not air as such but a mixture of air and water; the steam engine was a precursor to the petrol engine which used a superheater to create a gas used to move the pistons. Feb 6, 2017 at 12:06

4 Answers 4


As juhist said, just about every remotely feasible fuel has been tried in the history of motorized transport. Petrol and diesel became popular because:

  1. they're easy to use
  2. they're reasonably safe
  3. they're available in enormous quantities, enough to fuel close to a billion cars.
  4. they can be made cheaply,
  5. they have high enough energy density to not take up large amounts of space.

Ever since the first oil crisis in the early 1970s, people have been trying to find an alternative to petrol and diesel. Lots of potential solutions have been tried, but they all failed at at least one of the above criteria. The electric car revolution of the past few years is the first time an alternative has been found that satisfies 1-3 and can get close enough on criteria 4 and 5.

  • May I suggest: 5. High energy density. Feb 4, 2017 at 20:56
  • Good one, added,
    – Hobbes
    Feb 4, 2017 at 21:00

Yes, people have tried. For example, in addition to gasoline, you can use diesel. You can use alcohols (ethanol, methanol). The fuel doesn't even have to be a liquid: you can use natural gas. All of these fuels have been used in cars. Probably the most exotic fuel is wood gas, which requires a separate unit to convert wood into syngas.

The fuel doesn't even have to be injected inside the engine like what is done in internal combustion engines. In that case, this is called external combustion engine. Probably the most notable external combustion engine is the Stirling engine. However, external combustion engines have a low power to weight ratio, making it unlikely that external combustion engines would be used in passenger cars.

  • 2
    External combustion engines were used before internal ones came about. This was at the beginning of the automotive era . See this Wiki article on it. Feb 4, 2017 at 18:34
  • For that matter, there have been coal-fired and wood-burning cars.
    – Mark
    Feb 4, 2017 at 19:56
  • 1
    I suspect the most notable external combustion engine is actually a steam engine.
    – Random832
    Feb 5, 2017 at 0:16

"If gas is just a heating method for the air to expand and cause pressure" - it is not.

The combustion products also increase the pressure in the enclosed volume, see the Ideal gas law:

pV = nRT

Both n and T are increased by the combustion; V remains the same at the moment of combustion, so p has to increase.

  • 2 C8H18 + 25 O2 = 16 CO2 + 18 H2O // so that's (assuming octane is a typically sized molecule of gasoline - the end result ends up about half a percentage point higher for pentane) 34 output molecules for every 25 input oxygen molecules, with 30% of air being oxygen, an (up to) 11% increase in the number of molecules. Doesn't seem impressive vs the temperature-based increase.
    – Random832
    Feb 5, 2017 at 0:10
  • The OP's premise is that the only reason to combust the mixture is to expand it through heating. Feb 5, 2017 at 0:19

It is a balance. You attempt to keep combustion chamber temperatures higher and increase efficiency of the gas. In a typical engine around 40% of that potential is lost due to cooling alone. You loose over 30% right out the exhaust which is why you are seeing turbochargers used to increase efficiency. The other 10% is frictional losses leaving you with the magic 20% (give or take a few percent) efficiency we currently have.
If you go to high then the piston rings have trouble lubricating the cylinder walls and you have increased wear.

Now the fuel type out of it you still need to lubricate the cylinder walls. There have been ring designs to increase the efficiency of the gas but none has yielded any reliable results.

Once again it is a balance. There have been attempts at exotic rings and cylinder selves to increase the thermal efficiency of many types of volatile fuels. But the end result is that the combustion chamber perfers a temperature level that we currently run at.

Running a engine "hot" provides increased thermal efficiency. Recovering the exhaust through turbochargers increases the efficiency. Both have been done already getting us a few ticks above 20%.



The next step is to eliminate frictional losses further. That is currently being done(in F1/racing circuits) and should become available in high end cars in a few years trickling down to mainstream cars later with the use of free valve/ solenoid computer controlled actuated valves. . . They will allow for increased control of the exhaust timing and tuning increasing the efficiency of turbochargers. The electronic valves will also allow for more controlled air fuel intake. The days of oil actuated valves at varying rpm ranges will be gone replaced by millisecond precision as the engine and transmission demand it for the condition. While the heat inside the cylinder will not increase any time soon having greater control of the valves themselves will lead to increased compression within the cylinder and increased pressure providing better use of whatever fuel is filled into the tank.

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