[Vwdiesel] New to list

[Roger Brown]

LBaird119 at aol.com wrote:
>
> > In a CI engine, it takes time to inject the diesel fuel into the cylinder
> >  and it burns in more of a constant pressure environment
>
>   I'm no engineer and I believe Roger is but.. (correct me if I'm wrong)
> Doesn't
> the traditional "knock" that we all know and love so well, come from a
> rapid pressure rise in the cylinder?  The TDI is quieter because it injects
> the fuel at two points rather than just the one so it's pressure rise is
> more controlled.  According to your description it seems more likely
> for a gas engine to knock than a diesel.  Much of the burning in a gas
> engine can happen in the upper exhaust and that's why in the 70's they
> pumped air into the exhaust manifolds.  They reduced emissions by
> improving the burn that was happening in the manifold and downpipe.
> Not to mention that gas EGT's seem to be capable of running about
> 50% higher than diesel.  Of course that's heat rather than pressure
> but it's due to later burn I believe.

A spark ignition gasoline engine doesn't knock because you run a high enough octane fuel to avoid that problem, octane rating
measures the fuel's resistance to detonation.  In a diesel, you get knocking because diesel is supposed to burn, cetane rating
measures its ability to ignite.  So you will get a shock wave when you inject a spray of diesel into a cylinder full of high
temperature air.  However, the flame front doesn't move the same speed as the shock wave and you keep feeding fuel in over a
finite period of time, since its impossible to pump all the fuel in at one time.  I imagine the IDI pre-chamber has a little to
do with the kncoking and if you can get more injection points you can spread out the flame and pressure variations.