On 24/08/15 18:43, NY wrote:
"Tim Streater" wrote in message
.. .
No sorry Try as I might I can't find any ignition on my diesel.

Try looking inside the cylinders. Without ignition how the **** do you
think it runs at all?

Compression of the mixture?

No, just compression of the impounded air. There is no 'fuel/air
mixture' as such. The fuel is sprayed into the cylinder at very high
pressure to overcome the 'back pressure' of the compressed and heated
air where it is immediately ignited on contact with the air, burning
continuously in a manner analogous to the flame you get by igniting
the spray from a can of aerosol hair lacquer spray (or the way fuel
is burned in a jet engine's combustion chamber).

I don't know the exact details for typical high speed diesel engine
injection timings but, afaicr, the injection can start in advance of
TDC at higher revs and continues spraying for something like 50% of
the power stroke, give or take 25% or so.

Ah, I'd assumed that the fuel/air mixture was ingested on the
downstroke, followed by the upward compression stroke which by
compressing, heated the air until it reached ignition temperature. Was
that never the case then - did diesels always have injectors?

To the best of my knowledge, diesel engines have "always" (*) used
injectors to define the timing of the ignition.


Nope. Tractors often use(d) manifold injection.

So to put the issue of timing to bed:

- petrol engines draw fuel-and-air mixture into the cylinder on the
induction stroke and ignite it by a spark which is timed to occur just
before top dead centre; I believe some engines have a second spark
during the power stroke to ignite any unburnt fuel; the fuel-and-air
mixture used to be mixed in a carburettor and the fuel is now injected
into the inlet manifold; they always have a precisely controlled
proportion of fuel to air.

- diesel engines draw air into the cylinder, compress it to about 30:1
compression ratio (unlike about 7:1 for petrol) which causes it to heat
up to a temperature at which fuel will ignite on contact; shortly before
TDC the fuel is injected and this injection may continue for part of the
power stroke; they always have an excess of air relative to fuel.

So the spark of the petrol engine and the injection of the fuel of the
diesel engine both determine the "timing" of the engine in the same way.

Except that fuel injection will be a short while before actual ignition

For both types of engine, the air may be sucked in at atmospheric
pressure (normally-aspirated) or may be blown in at more than
atmospheric pressure by a turbo- or supercharger. Turbo/super chargers
fit more than the rated capacity of air into the cylinder, so allowing
more fuel to be injected and thus giving an increase in the effective
capacity of the engine.



(*) Ignoring possible variations in early development engines.


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