[Vwdiesel] New to list

[Roger Brown]

Bill Truesdell wrote:
>
> Anton Largiader wrote:
>
> >Actually I don't think it's accurate to say that it runs by detonation.
> >Detonation is the premature auto-ignition of fuel/air mixture that's already
> >in the combustion chamber, which is sort of an explosion in the case of
> >gasoline.  Diesel is compression-ignited, but the fuel is only added when
> >combustion is wanted, and has a more controlled burn.
> >
> >
> Deflagration is probably the correct term for both. Deflagration is a
> more controlled burn. Can be very fast, so it seems like detonation
> (both can be called an explosion, which is where the confusion starts),
> but detonation implies a shock wave propagation of the explosive front
> and is more chemical in nature while deflagration implies a burning
> front. Detonation fronts, since they are shock waves, can move in the
> thousand of feet per second and all the material is in transition at
> approximately the same time, so the effect is deadly.
> For example, gunpowder deflagrates and does not detonate. Burn it in the
> open and it does little but encase it in a rifle bullet and, because of
> pressure and temperature, it burns more quickly and an explosion
> (deflagration contained) results. But no detonation. If it did detonate,
> it would shatter the bullet chamber.
>
> Either can be started at a specific point or points, blasting cap for
> detonation and spark plug or hot spot for deflagration. Deflagration can
> transform to detonation under the right circumstances and material. You
> can burn HMX (a military explosive) but in the process, it might detonate.

The main difference in spark ignition and compression ignition engines is that in SI engines, the burning is a constant volume
process, since all the air/fuel mixing is done when the spark plug fires so you have a rapid pressure rise/shock wave that
expands out from the spark to quickly burn the fuel.  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, since the piston is moving down in the cylinder increasing the volume
as the burning happens.

Flame fronts propogate about 50 times slower in a constant pressure environment as compared to a constant volume environment
which is the main reason you'll probably never see a decent sized diesel engine that revs much over 5000 RPM.  And its not a
diesel vs. gasoline thing, if you burn gasoline in constant pressure it burns slow, if you burn diesel in a constant volume, it
burns fast.