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Originally Posted by mechman600
Is this true will all engines? Four stroke/two stroke? Gas/Diesel? etc...
Just curious. I have never heard the 14 degree ATDC figure before.
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I don't know about gasoline engines, but it's not necessarily true for commone rail diesels. I've seen the same engine have peak pressure locations everywhere from TDC up to 20 dATDC. It all depends on the operating condition.
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...something to remember, ignition STARTS ahead of TDC, takes a finite amount of time (combustion flame rate) to happen, all while the piston rapidly slows its upward motion until it passes through TDC, and then begins to rapidly increase its downward motion...in coincidence with the peak combustion pressure (~14º ATDC for gasoline; other fuels alcohol & methanol, etc. are different -- does NOT apply to diesel).
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Yes, ignition doesn't necessarily start before TDC in a common rail diesel. This is particularly true for some of the newer ones that have special operating modes intended to create heat for the aftertreatment (DPFs SCRs, etc.). In these modes, the combustion is intentionally delayed so that less heat gets extracted during the power stroke and more heat gets sent into the exhaust. In some cases the injection doesn't even begin until after TDC. As long as there's enough compression, there will still be enough heat to cause ignition. I've seen some operating conditions where combustion doesn't even start until 10 deg after TDC.