Ed Huntress wrote:
{..]

Ed,

I'm wondering if some of the problem here is with the nature of the
combustion in the 2 engines. My understanding is that the SI engine burns
a vapourised fuel air mix which burns like any gaseous fuel air mix but in
the diesel combustion the fuel droplets burn at the surface of the
droplet in the combustion air, the lighter fractions burning first leaving
the heavier fractions, hence the particulates left behind. The modern
diesels in small vehicles, at least in Europe as I think regulations have
constrained small diesels in the US, have much finer atomisation of the
fuel due to the high pressures used which allows shorter burn times and
higher resultant RPM but the nature of the combustion still differs from
that of a petrol engine.


The ignition and combustion processes definitely are very different. It
happens that gasoline is only partly vaporized when it enters a cylinder --
there was a lot of research on this in the '70s and they learned that
complete vaporization is not the best thing -- but the essential difference
is something else.

The big thing is that the fuel burns as it's injected, and the period of
injection stretches out the combustion so there is a minimal amount of shock
from the sudden high pressure. Unlike a SI engine, a diesel doesn't depend
upon the mixture to control the combustion rate. The ideal situation is to
have a fuel that burns quickly -- that's why you want a high cetane
rating -- and to control the combustion rate by the rate of injection. Some
of the newest engines divide the injection into several distinct periods.

Yes I'm aware of some of the modern developments as a mate of mine works
for Ricardo and gets involved in diesel design. He has mentioned that
some of the modern automotive engine management systems for diesel
engines are more sophisticated than those for SI engines. One of the
things he mentioned recently that was mentioned to him was a limitation
of a diesel is that the fuel has to be injected when the air is hot
enough to ignite it. I wonder what happens if it is injected too early
and the air charge isn't hot enough.
All of which suggests why gasoline, with its low cetane equivalent and slow
burning, can foul up the process. If you burn gasoline in a diesel that's
set up for conventional diesel fuel, fuel is injected faster than it burns,
so you wind up with regions of unburned fuel/air mix in the cylinder, at
extremely high compression, that are invitations to detonation. That was
what I was guessing before, but I did some digging today and learned that is
apparently what happens.

I went through a lot of SAE paper abstracts today, to try to catch up, and I
see that there are all kinds of experiments going on with different fuels
and mixes, including gasoline, in diesels. The primary purpose of the
gasoline is to slow down combustion and thereby to reduce NOx emissions.
They have to advance injection by quite a bit and, apparently, use complex
strategies for injection, including multiple shots and different kinds of
aiming of the injectors. The most interesting research is on something
called HCCI (homogeneous charge/compression ignition) engines that are
basically diesels, but some of which have "spark assist." Some of these run
at compression ratios that are intermediate between conventional SI engines
and conventional diesels.

There is a lot going on. It's pretty interesting, especially since they now
have better tools to analyze combustion in an engine.

--
Ed Huntress