"Noddy" <me@[EMAIL PROTECTED]
> wrote:
>"the_dawggie" <the_dawggie@[EMAIL PROTECTED]
> wrote:
>> I have an issue for Node about how a diesel engine works.
>> They are more a plunger effect than hit with hammer SI
>> effect. I know that, and it's more than obvious. Athol too
>> should know that. It's one of the most basic facts of diesel
>> engines. Petrol bangs, diesel burns.
>If that *really* was the case, then there'd be no need to make
>internal engine components for diesel engines any stronger than
>petrol ones.
Eh? VW diesel engines of the same caapcity are not much heavier than
their petrol-fuelled base engines. vis original Golf Diesel.
Other than higher compression ratio and fuel pressures, the
Diesel cycle is a "gentler" cycle on mechanical components.
Of course the crankshaft has to be a little tougher, given that it's
used to more effectively when there's the greatest mechanical
advantage upon it.
>The fact that they do tells you that a diesel engine
>is under *way* more stress during it's operation than a petrol
>engine is on average, and that "stress" comes from compressing and
>firing the mixture of air & fuel.
A great deal more stress can be taken care of with little extra
mass. That's what VW learnt when they first "dieseled" the 1.5-litre
SI engine from the Golf.
You don't need twice the mass to control twice the cylinder
pressure.
>A diesel engine is, in effect, similar to a petrol engine which is
>detonating, and that's why they're noisy. Firing the engine by
>compressing the air & fuel to the point where it gets hot enough to
>fire is *far* more severe an action than firing it with a spark.
The difference is that all the fuel isn't compressed; it's
progressively injected; a little bit at first to get the combustion
going (pilot combustion) and then more and more to maintain a high
cylinder pressure for more efficient conversion to torque at the
crank.
>The "coffee plunger" theory is funny though :)
Not funny. Fun.
Fuel can and usually *is* during the power stroke, after TDC so that
the cylinder pressure can have the best mechanical advantage on the
crankshaft; producing torque over a wider range of crank angles.
Torque is greater overall because there is a "flow" of torque during
a crank rotation than with an SI engine.
It is the "slow burn" that is the fundamental reason why CI engines
won't spin as quickly as SI in practice.
Article in furlongs/fortnight for your enjoyment:
<http://www.bankspower.com/Tech_todaysdiesel.cfm>
Those with a copy of Rogers & Mayhew (dusty or otherwise) might find
it worthwhile to review sections 12.6 for the theory and 17.3 for
some of the practical aspects.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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