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  • Query about Diesel Engine

    PRATIK DOKANIA

    Member

    Updated: Oct 27, 2024
    Views: 1.0K
    why do we use diesel externally in a compression engine unlike fuel air mixture in petrol engine??
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Replies

  • zaveri

    Member • Jul 22, 2012

    maybe it has got something to do with the low ignition temperature of diesel
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  • white_hat

    Member • Jul 22, 2012

    Density is the reason.
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  • ARISTOTLEUC

    Member • Jul 22, 2012

    I think it is due to it high cetane number. It can ignite and burn even in the limited oxygen supply, unlike petrol.
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  • Ramani Aswath

    Member • Jul 22, 2012

    In a diesel engine the compression ratio is about three times that in a petrol engine. Because of this, in adiabatic compression the the temperature of air increases to above the ignition point for diesel. At the point of maximum compression diesel injected in the form of fine droplets catches fire. leading to an increase in pressure that pushes the piston down driving the crank shaft. Diesel engines are able to extract more power for the same amount of fuel because of the higher compression ratio. The downside is higher wear and tear.
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  • mechky

    Member • Jul 22, 2012

    PRATIK DOKANIA
    why do we use diesel externally in a compression engine unlike fuel air mixture in petrol engine??
    Flash point of diesel is about 62 degree celsius which is much higher than petrol and its auto-ignition temperature is 210 degree celsius which is lower than petrol. For fuel to form an ignitable mixture with air, it must reach its flash point.

    In petrol engines due to low compression ratio of engine and high auto-ignition temperature of petrol, it is safe to induct air-petrol mixture during suction stroke and compress it above flash point temperature and then spark plug ignites the fuel. The chances of pre-ignition are less.

    In diesel engines, the compression ratio is high and the temperature of air at the end of compression is much higher than the flash point as well as auto-ignition temperature of diesel. If diesel-air mixture is compressed then problem of pre-ignition will occur. To avoid this diesel is injected at the end of compression stroke in form of high pressure sprays.

    This is my point of view and I may be wrong.
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  • PRATIK DOKANIA

    Member • Jul 22, 2012

    if adiabatic compression of air increases temperature too high to burn diesel than the required temperature, then why we use such high compression ratio.
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  • mechky

    Member • Jul 22, 2012

    PRATIK DOKANIA
    if adiabatic compression of air increases temperature too high to burn diesel than the required temperature, then why we use such high compression ratio.
    A high compression ratio is desirable because it results in high thermal efficiency of engine which in turn means that the engine can extract more mechanical energy from a given mass of air-fuel mixture. Higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

    For petrol there is an upper limit on compression ratio due to the problem of knocking.
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  • white_hat

    Member • Jul 23, 2012

    ARISTOTLEUC
    I think it is due to it high cetane number. It can ignite and burn even in the limited oxygen supply, unlike petrol.
    Dude, cetane number is just a way for expressing diesel knocking.
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  • white_hat

    Member • Jul 23, 2012

    PRATIK DOKANIA
    why do we use diesel externally in a compression engine unlike fuel air mixture in petrol engine??
    Petrol is more volatile than diesel when it is sprayed onto the air flow it forms a uniform air-fuel mixture.
    Diesel is heavier,less volatile it will not form a homogeneous mixture.The density of diesel is 0.86 gm/cc and that of petrol is 0.72 gm/cc.
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  • ARISTOTLEUC

    Member • Jul 25, 2012

    white_hat
    Dude, cetane number is just a way for expressing diesel knocking.
    Cetane number is a measure of
    a fuel's ignition delay; the time period
    between the start of injection and the
    first identifiable pressure increase
    during combustion of the fuel. In a
    particular diesel engine, higher cetane
    fuels will have shorter ignition delay
    periods than lower cetane fuels. Cetane
    numbers are only used for the
    relatively light distillate diesel oils. (Wikipedia) while knocking is the metallic sound heard in engine due to the explosion of packets air-fuel mixture outside the envelope of normal combustion face when the combustion with responds to ignition starts off correctly in a S.I engine. They are two different thing I guess, Dude.
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  • white_hat

    Member • Jul 26, 2012

    ARISTOTLEUC
    Cetane number is a measure of
    a fuel's ignition delay; the time period
    between the start of injection and the
    first identifiable pressure increase
    during combustion of the fuel. In a
    particular diesel engine, higher cetane
    fuels will have shorter ignition delay
    periods than lower cetane fuels. Cetane
    numbers are only used for the
    relatively light distillate diesel oils. (Wikipedia) while knocking is the metallic sound heard in engine due to the explosion of packets air-fuel mixture outside the envelope of normal combustion face when the combustion with responds to ignition starts off correctly in a S.I engine. They are two different thing I guess, Dude.
    There is knocking in both SI and CI engines.Cetane number and Octane number is measurement of expressing fuel quality.
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  • white_hat

    Member • Jul 26, 2012

    Low cetane number diesel means more knocking.
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