Fuel injection plugs

[Howard Wilkinson]

Greg:
    I must point out that this statement is erroneous:

> 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.

Actually on a weight basis they are quite close..... by volumn diesel
has considerably more energy.  There is actually a fairly close
correlation between fuel energy and fuel weight.         H.W.


-----Original Message-----
From: Greg Hermann <bearbvd at sni.net>
To: diy_efi at efi332.eng.ohio-state.edu
<diy_efi at efi332.eng.ohio-state.edu>
Date: Wednesday, April 28, 1999 10:26 PM
Subject: Re: Fuel injection plugs


>>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
>
>
>