When gas is compressed, the temperature of it will rise,
with diesel engines using this very property to ignite
the fuel.  Air is then drawn into the cylinder and
compressed by the rising piston at a much high
compression ratio than gas engines, up to 25:1, with
the air temperature reaching 700 - 900 degrees C.

At the top of the piston stroke, the diesel fuel is
injected into the combustion chamber at high pressure,
then through an atomizing nozzle, it mixes with the
hot high pressured air.  The resulting mixture will
ignite and burn very rapidly. This combustion will
cause the gas in the chamber to heat up rapidly,
which increases the pressure and forces the piston
downwards.

The connecting rod will transmit this motion to the
crankshaft.  The scavenging of the engine is either
done by ports or valves.  To get the most out of
a diesel engine, use of a turbocharger to compress
the intake of air is vital.  You can also use an
aftercooler or intercooler to cool the intake air
after compression by the turbocharger to further
increase your efficiency.

An important part of older diesel engines was the
govenor, which limited the speed of the engine by
controlling the rate of fuel that was delivered. 
Unlike gas engines, the air that comes in is not
throttled, so the engine would overspeed if this
wasn't done.  Older style injection systems were
driven by a gear system that came from the engine.

The diesel engine is truly an advancement to vehicles
as we know it and I hope a honda civic hybrid has this
quality which I am planning to have one this year. 
As technology gets better, you can expect the diesel
engine to get better as well,possibly even proving just
how much better it is to the gasoline engine.