Fuel injection plugs

[James Ballenger]

Greg Hermann wrote:

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

    This was a figure from an automotive engines book I had from an autoshop
class.  The figure seems to be confirmed by other texts/sources, but they don't
detail the conditions under which the engines see this effeciency numbers.


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

You got me there, according to this site (below) it is exactly 3%.
http://www.cs.ruu.nl/wais/html/na-dir/autos/gasoline-faq/part1.html

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

    As long as the fuel is completely burned, it shouldnt matter should it?  Seems
like you could have as much oxygen just sitting there as you want, it would just
not participate in the reaction.

>   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.

    But this is where the reaction occurs and where the temperature and pressure
are relevant.  In the other 3 strokes, the only thing affecting thermal effeciency
is thermal losses.  I was speaking in terms of the combustion, a diesel can see
20:1 where a gas engine might only see 10:1.  The diesel generates enough pressure
and heat to self-ignite the mixture!  Do you mean that after/during ignition, the
force/pressure generated by a gasoline reaction is greater than a diesel reaction?
Why would a diesel produce so much more torque then?

> 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.

    Assuming the same types of thermal losses between the two, the diesel will
still retain twice as much heat because of the nearly doubled pressure.  Most of
the heat of these systems is lost to the water jacket and  exhaust system, but at
least they can compared given proportianate thermal losses.  I used pv=nrt just to
find temperature with regard to the volume and pressure of the system, I don't see
how it is not applicable (except that we are not dealing with an *ideal* gas).

> 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.

    Q=C(Tf-Ti).  I think we need to know the temperatures of the reactions and the
heat capacity of gas and diesel to be able to make and assumption.  According to
the equation, it could vary quite a bit assuming a large temperature difference.

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

My physics book defines work as the integral of p*dv, so the work done will be
proportional to the expansion ratio.

> 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.

Right, the diesel sees a far greater t1 being that it self-ignites the fuel at this
point right?



> 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!

If it didnt see a higher peak temp, then why would diesel self-ignite?  By virtue
of the doubled pressure, due to cr, at tdc combined with the combustion reaction
the temp would have to be higher.  Why would the reaction be a constant pressure
reaction?  In any case, the pressure would increase during the reaction.


> 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.


> 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.

James Ballenger

btw, what do you guys have your word wrap set to?  Every message I write now seems
to come out with the wrong word wrap.