Fuel injection plugs

[Greg Hermann]

>> > An Otto cycle engine burns its fuel at a nearly constant VOLUME, and at an
>> > approximately stoich a/f ratio--therefore its peak pressure and temperature
>> > go quite a bit higher than a diesel's do!!!
>>
>> This can't be right, "If the volume of a system (such as a gas) is held
>>constant,
>> that system can do no work."  That being said, a constant volume
>>reaction would do
>> no net work.

Thanks for the answer, Steve. I must have missed this post earlier!

What I said is most certainly correct, and your have described why rather
well. I never said "constant volume expansion, I said constant volume
combustion. It is mechanically impossible to inject fuel into a diesel
quickly enough to approach the burn rate that an Otto (spark) cycle engine
has.
>
>The spark engine burns it's fuel much faster than the diesel engine.  It
>burns fast enough that it's simplified to a constant volume compression
>because it burns while the size of the combustion chamber is not
>changing size that much (right around TDC).  The volume then changes of
>course, when the piston moves down under the pressure of the hot burned
>mixture.  That's where the work comes from.  The whole cycle isn't
>constant volume, just the part where the heat is added.
>
>The diesel engine injects and burns fuel during the whole power stroke.
>While the fuel is burning and trying to raise pressure, the piston is
>moving down and releasing pressure.  It's modeled as a constant pressure
>expansion.
>
>--steve
>
>>
>> > These facts are exactly why an Otto cycle is inherently more efficient than
>> > a diesel cycle!!
>>
>> I think I am missing something here, maybe just my flimsy grasp of the
>>1st law of
>> thermodynics.  But my automotive books do indicate that a diesel has greater
>> thermal efficiency, by virtue of maintaining more of the heat produced.

The diesel cycle is less efficient than the Otto--their higher compression
(really expansion) ratio brings some of this back. The fact that they have
no pumping losses at part throttle is the big one behind why they get
better mileage in everyday driving.

Regards, Greg