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Differences between otto and diesel

  1. May 13, 2014 #1
    Why do we want self-ignition in diesel engines but do not want it in otto engines or in other words why does
    self-ignition cause detonation in otto engines but do not cause in diesel engines?
    Last edited: May 13, 2014
  2. jcsd
  3. May 13, 2014 #2
    not really sure about this, but it may be because petrol burns much much faster than diesel.
    that is why diesel could only rev about 4-5k rpm. petrol burn so fast engines could rev 15'000+

    so if diesel burns slowly, the explosion has not much power.
    petrol will explode much faster and because of that creating much more stress on engine internals.

    I'm not really sure about this, so if I'm wrong please correct me.
  4. May 13, 2014 #3


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    A diesel engine is carefully designed so that the fuel ignites and burns after the piston has produced maximum compression in the cylinder, i.e., the piston is now moving down w.r.t. the cylinder head. As the piston moves downward, the combustion of the fuel acts momentarily to offset the loss of pressure in the cylinder which results from the increase in volume due to the movement of the piston.

    In a spark-ignition engine, the self-ignition can occur at inopportune moments before or after the spark is produced. If self-ignition occurs before the piston reaches top dead center (TDC), there will be a loss of power, since the engine is working against itself briefly. If the abrupt increase in pressure is severe enough, damage to the piston, piston rod, or crankshaft may occur. In general, spark ignition engines are not as robustly designed as a diesel engine of comparable output, and thus the internal parts of the spark ignition engine are not able to withstand the additional forces caused by self-ignition which occurs at inopportune times.


  5. May 13, 2014 #4
    Diesel-cylinders have a maximum cylinder-compression before detonation of appx 19 bar, petrols 10 bar.

    In a diesel, fuel is introduced into the burn-chamber at high presssure at the detonation-time(s) desired, into air allready compressed by the piston (and turbo, usually). Fuel ingnites on introduction, by intended self ignition. The high-pressure fuel-valves decide the detonation time in a diesel-engine. Normally several fuel-injections are made through one detonation, to soften operation, and to lower NOx-emissions.

    A petrol-engine usually introduces fuel to air before the compression-stroke, and the mix is not ignited by self iginition, because of the lower gas pressure/temperature on the time it is ignited by the spark plug. The spark-plug decides the start of detonation on a petrol.

    If a petrol-engine was intended to self ignite, it would vary in performance , as the fuel-air-mix would reach detonation-point at different degrees of crankdue to engine temperature.

    Self detonation in a petrol-engine may be due to low octane fuel (ignites on lower temperature) or high compression engine (higher temperature in burn-chamber), both allowing fuel-air to ignite before spark ignition. Typically, high performance petrol cars require high octane fuel. Some alchohols allows higher pre-spark comp/temp than petrol, and are well thought of for certain otto-motorsport purposes.
  6. May 15, 2014 #5


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    A diesel only injects the fuel into the cylinder when it is needed to burn.
    An Otto must compress a stoichiometric air fuel mixture, without ignition before the spark.
    Efficiency is proportional to compression ratio, so diesel is both more efficient and less fuel critical.
  7. May 15, 2014 #6


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    I agree with most of what you said, but this part isn't quite right - the engines do have a compression ratio (V2/V1) of approximately 19 and 10 for diesel and gasoline respectively (with some variance depending on engine design specifics), but peak pre-ignition cylinder pressure is much higher. For 10:1 compression, P2/P1 will be about 25, assuming adiabatic compression and a gamma of about 1.4 (which isn't quite accurate, but it'll do for an approximation). For 19:1 compression, P2/P1 is almost 62. The reason the pressure ratio is much larger than the volume ratio is because the air heats as it is compressed, which significantly increases the pressure.
  8. May 15, 2014 #7
    Dear cjl and mech-eng, I accept error.

    I was thinking compression ratio 19:1 and 10:1 (poorly remembered figures from car-magazines read 20 years ago). With heat from compression, and off cylinder-walls/head, pressures are wrong in my approximation. Apologies to you both, and to anyone who's wasted otherwise useful time to counter this fault.
  9. May 15, 2014 #8


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    Maximum compression ratio in diesels is not related to fuel ignition. It is usually limited by the piston burning/collapse/melting temperature. It is rare to find a diesel with a compression ratio much over about 21 as then the piston will fail when in continuous operation at higher RPM.

    Minimum compression ratios below about 15 do not generate sufficient heat to immediately ignite diesel fuel as it is injected. Diesel knock is normal, but damaging detonation in diesels only occurs when there is a significant delay in the ignition of the injected fuel. Detonation occurs when the engine is cold, the compression ratio is low in a worn engine, or the fuel has a cetane rating below 40.
  10. May 16, 2014 #9
    modern diesel engines have CR as low as 14:1 (mazda)
    15-16 is common...

    petrol engines go as high as 14:1. also mazda engines...
  11. May 16, 2014 #10


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    Low volumetric compression ratios are typical with turbo charged engines where the pressure boost provides a multiplying factor.

    That is possible with knock detection and intelligent fuel injection.
    What fuel are they designed to use? Ethanol has an effective octane number of 113.
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