In an internal combustion engine why must the gas & air be compressed before ignition

  1. I have been told that compressing the mixture would make the explosion "more powerful". However I cannot wrap my head around why it would be more powerful. Is it because by compressing the mixture potential energy is formed and thus igniting the mixture would release the potential energy from the gasoline plus the potential energy of the compressed mixture? In this sense we do get a "more powerful" explosion but I don't think it makes much of a difference because the internal combustion chamber is so small.
     
  2. jcsd
  3. SteamKing

    SteamKing 8,625
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    Re: In an internal combustion engine why must the gas & air be compressed before igni

    The first internal combustion engines were known as 'atmospheric' engines' because they did not compress the fuel-air mixture before combustion occurred. Although these engines worked, they were not very efficient. Otto's contribution was to compress the fuel-air mixture before ignition, thus greatly increasing the efficiency of the cycle, since efficiency depends to a great extent on the compression ratio of the engine.
     
  4. Re: In an internal combustion engine why must the gas & air be compressed before igni

    Ah okay, I am currently reading about compression ratios on wikipedia and it says:
    "A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel"

    I don't understand why higher compression ratios permit the same temperature to be reached with less fuel. I understand that compressing the mixture would heat it up since we are doing work to it but how would that allow for combustion with less fuel? Doesn't combustion depend on a set temperature and does not depend on how much fuel there is? In other words even without a high compression ratio, a tiny drop of fuel should still combust at that set temperature.
     
  5. Re: In an internal combustion engine why must the gas & air be compressed before igni

    I've heard/ been told/ thought that it's because the "molecules are closer together" and therefore they combine easier or the energy gets transferred easier or something. But, upon further scrutiny this is not a fully satisfactory explanation.
     
  6. Nugatory

    Staff: Mentor

    Re: In an internal combustion engine why must the gas & air be compressed before igni

    The amount of useful mechanical work done by an expanding gas pushing on a piston is given by W=PΔV, where P is the pressure and ΔV is the change in volume (calculate the force on the piston and the distance it moves to see why). All the rest of the energy from burning the fuel ends up as waste heat.

    Compressing the mixture gives us both high P (lots of heat in a small volume means high pressures) and a large ΔV, hence high efficiency.
     
  7. Re: In an internal combustion engine why must the gas & air be compressed before igni

    Working from the original post. The reason a higher compression ratio results in a bigger bang can be visualised by thinking of a small amount of gun powder sitting in a small pile on a bench. You light it, and it burns with a small flash and a bit of smoke. If you take that same quantity of gun powder and wrap it up in some paper, stick a fuse in it and light it, you get an explosion. The energy that is released is the same, but the amount of energy released v's time changes dramatically.
    With the higher compression comes a faster mixture burn, and that faster mixture burn results in higher temperature and peak cylinder pressure. Because there is only a limited amount of time to effectively burn the combustion charge, the higher compression ratio allows for (generally) a higher working rev range. And therefor, an engine with higher compression has a tendency to be more efficient than a lower compression engine.

    Damo
     
  8. Re: In an internal combustion engine why must the gas & air be compressed before igni

    Well it's bit late to do it afterwards, since you want to extract mechanical work by expansion.

    As to the Physics.

    No the bang is not bigger. The energy available from combustion is set by the bonds between the molecules.

    What is different is that the temperature of combustion increases with pressure.
    The temperature of the environment remains the same.

    Thus there is a greater temperature difference between the combustion products and the environment, following prior compression.

    It is a thermodynamic theorem that the greater the temperature difference between the hot and the cold working temperatures the greater the proportion of the energy that can be extracted as mechanical work.

    So this is the most efficient way.

    A further comment.

    Compression also increases the gas temperature and diesel engines use this fact to raise the temperature to ignition temp.
     
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