Fuel injection plugs

[Greg Hermann]

Hi Howard--

Had to finally jump in here--

>    I didn't say that very well..... What I meant to say was that due
>to fuel charge stratification the area in which the spark plug is
>located where combustion is initiated is at normal ratio, but the
>surrounding area may be extremely lean.
'
This is correct, and now well said. The lean overall mixture in these
engines at part throttle allows better economy by reducing PUMPING losses
significantly. Basically--throttling is an irreversible process, and all
the air that passes through the engine when it is throttled must be pumped
from manifold pressure back up to atmospheric pressure in order to get out
of the exhaust pipe.

>    The ideal engine (in terms of efficiency) is like a diesel

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!

where
>power output is controlled by fuel supplied not by air fuel mixture
>being supplied under a vacuum (throttling) which results in lower max
>pressure prior combustion.  Basic thermodynamic theory shows that for
>a given amount of thermal energy the pressures will be higher if a
>full charge of air is available for expansion.

This is incorrect. 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. For a given compression
ratio, you will always get the highest possible peak temperature for the
cycle at a stoichiometric a/f ratio. Diesels cannot run at stoich--the fuel
can never find all the air!!  There are some things which modify this to a
degree, such as selective burning of hydrogen (with a higher heat of
combustion) rather than carbon at slightly rich a/f ratios, and more
complete combustion of the fuel at slightly lean a/f ratios--but these
factoids are not that relevant to this discussion.

We are after all
>developing power from the pressure,

What you are really developing power from is the MEAN EFFECTIVE PRESSURE
during the cycle. A higher peak means nothing (except higher stress on the
engine) if the mean pressure is lower!

so any way we can achieve more
>pressure from a given amount of fuel will improve our efficiency.

As stated above, it is temperature that matters most!

Where a diesel gains is in two places:

At part throttle--they have no pumping losses.

At any throttle--they generally have a higher 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. )

A much more efficient thermodynamic cycle.

The ability to use virtually all of the air they breathe in for combustion.

The ability to burn their fuel efficiently at much higher engine speeds.

Regards, Greg