"Noddy" <me@[EMAIL PROTECTED]
> wrote:
>"Bernd Felsche" <bernie@[EMAIL PROTECTED]
> wrote:
>> How much _heavier_?? How many _grams_?
>Lots, and very distinctly so.
>Your average diesel engine piston is close to twice the weight of
>that of a SI engine of around the same capacity,
Nope. Not for engines designed in the past 30 years:
<http://www.maesco.com/products/vw_engines/vw_spec/vw_spec.html>
130/135 kg for a 1.9-litre Diesel vs 111 kg for a 1.8-litre petrol.
Nowhere near "double".
>and things like connecting rods and crankshafts are also
>considerably heavier.
Slightly heavier. VW list the substantial changes in converting the
Golf's 1.5-litre gasoline engine to diesel in 1976 being a more
massive: the head (incor****ating swirl chamber), stiffer piston with
larger gudgeon pin, wider and stronger toothed wheels for the crank,
camshaft and injection pump drive ... and obviously the injection
pump. That's it.
>They also almost always run a very heavy flywheel compared to a
>petrol engine.
Not double the mass.
>> Other than the cylinder head, there's not much gain in mass in any
>> component.
>You must be looking at different diesel engines to the ones I've seen.
When did you close your eyes? :-)
>> respective shortcomings. Notably that SI (Otto cycle) would have to
>> provide all the torque over a shorter angle of operating cycle.
>> All th fuel is burnt "at once".
>It is, but then the net effect of that burning is what's im****tant,
>not the manner in which the burning occurs.
Once more around the dance floor?
Having the peak pressure close to TDC puts the majority of the
piston's thrust RADIAL to the crank. All that does is increase the
radial pressure on the bearings; it doesn't do much to TURN the
crank.
It increases the cutoff ratio in a CI engine to put more
fuel in later and hence the thermodynamic efficiency reduces; but
the mechanical efficiency is greater. Nett overall efficiency
increases up to a point; largely determined by the geometry of the
engine and how long complete burning can be maintained.
>> The diesel _can_ burn for a lot longer.
>It can
>> Reciprocating components are how much heavier?
>900kg :)
>With respects, I can't give you *specifics* off the top of my head,
>but it's clear to anyone that's had a small capacity diesel engine
>apart that the internal components are heavier and more robust than
>those you'll find in any petrol engine and that weight penalty
>affects performance.
It only adds up in grams; not kilograms when you're dealing with the
internals. The *really* heavier bits are the head and the flywheel.
Neither of them reciprocates; unless you have real problems.
>> The speed being limited by the practical limits of fuel burning in
>> the Diesel cycle, is any greater mass in the reciprocating
>> components to cope with higher stresses due to forces in the
>> reciprocating parts; or to cope with higher cylinder pressures?
>A combination of both I expect.
Both less than the temperature and the need to maintain rigidity for
compression.
>There is an *incredible* amount of stress on a piston/rod/crank/bearing
cap
>when compression ratio's get in the order of 20:1.
Demonstrably not incredible: I believe it.
Peak combustion pressures are much more significant than the
pressure from compression. And they are similar in SI engines.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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