Fuel injection plugs

[Greg Hermann]

>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.

The difference MIGHT be almost this big at part throttle--but the
difference is due mostly to pumping losses, not necessarily to
thermodynamic superiority

 It is also my
>understanding that effeciency is defined as the (amount of energy
>produced)/(amount of energy used).

This is correct.

 Gasoline also has more energy per unit
>volume than diesel fuel, right?

They are really quite close on a volume basis! within 2% or so, with
gasoline a bit ahead.

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?

No--more like it cannot burn all of the oxygen it breathes in, so it cannot
make as much power.

 It seems to
>me that the increased pressure of a diesel further tips the scale in diesels
>favor though.

See below about which engine has higher peak pressures.

 For a given volume of fuel, a diesel produces roughly twice the
>pressure as a gas engine right?

Not at all--only place the pressure is higher is at the top of the
compression stroke, prior to ignition/injection.

 Feel free to tell me where I goofed, I'm just an
>engineering student learning....

Where you goofed is by just applying pv=nrt.

The power that you can (ideally) get out of a given amount of gas will
always be limited by the (useful) delta T times the specific heat times the
mass flow. The "useful" delta T is limited by the available expansion
ratio. If the gas is losing heat to the water jacket as it expands, you
will get less than the ideal amount of work out of it.

T1/T2= (P1/P2) exp (0.283) = (V2/V1) exp (0.4) ----These exponents are for
air, and would change somewhat for different gasses. But plenty close
enough to figure out how things work. Likewise--specifc heat varies with
both pressure and temperature, but holding it constant is close enough to
get a GOOD feel for how things work.

The ideal work out of a power stroke, W = (T1-T2) x (specific heat) x (mass
flow).

As you can see from the first equation,  (V2/V1) ---the expansion ratio--
determines how much delta T is possible with a given  engine design.
But--since we are talking about RATIOS, a higher absolute T1  allows more
work from a given mass flow at any given expansion ratio.
>
>
>
>
>
>> 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,

No, it does not!!

A diesel sees a higher pressure at the top of its compression stroke than a
spark (Otto) engine does, but then burns its fuel at a nearly CONSTANT
pressure. Because it cannot burn at stoichiometric, and because it burns at
constant pressure, its peak temps are not as high!

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!!!

These facts are exactly why an Otto cycle is inherently more efficient than
a diesel cycle!!

 wouldn't that neccessarily mean that it has a
>more efficient thermodynamic cycle?

See above. The difference between constant pressure and constant volume
burning is the reason for the difference.

Regards, Greg
>
>James Ballenger