Fuel injection plugs

[James Ballenger]

Greg Hermann wrote:

> An Otto cycle is actually MUCH more efficient at full throttle than a
> diesel, especially if you only consider the "air efficiency"--adding fuel
> to the mix brings it closer to a diesel, but not down that faR!

    It is my understanding that a 4 stroke gasoline engine is roughly 25%
efficient, where a diesel is roughly 35% effecient.  It is also my
understanding that effeciency is defined as the (amount of energy
produced)/(amount of energy used).  Gasoline also has more energy per unit
volume than diesel fuel, right?  So given the same volume of fuel used and
roughly equal power output, the gasoline engine will be less efficient.  I
know, I know, they don't have the same power output.  A diesel engine does use
more fuel than a gas engine because of it doesn't mix well right?  It seems to
me that the increased pressure of a diesel further tips the scale in diesels
favor though.  For a given volume of fuel, a diesel produces roughly twice the
pressure as a gas engine right?  So using   PV=nRT, which simplifies to PV~T,
we can say that a diesel has the potential for twice the thermal output for a
given volume of fuel.  I realize that frictional losses, and the flow
characteristics of these two systems are detrimental to the diesel.  I realize
too that gas engines will produce more horsepower at high rpm and conversely a
diesel will produce more torque at low rpm.  I guess the only way to compare
effeciency is to find how much fuel each system uses at a given rpm and compare
it to power output.  Feel free to tell me where I goofed, I'm just an
engineering student learning....





> Where spark engines gain--
>
> Lower internal friction (the cross-over point where efficiency gains from
> higher compression/expansion ratio are actually CANCELLED OUT  by increases
> in internal friction happens somewhere in the 11.5 to 13.5 :1 compression
> ratio range. )

If this were the case, then why would we use diesels at all?  Surely this must
be the case at a given rpm, where the frictional losses overcome increased
power?



> A much more efficient thermodynamic cycle.

You said above "Thermal efficiency depends upon the PEAK TEMPERATURE
reached during th cycle and upon the effective EXPANSION RATIO which is
available during the power stroke of the cycle."  Being that a diesel sees
greater pressure and temperature, wouldn't that neccessarily mean that it has a
more efficient thermodynamic cycle?

James Ballenger