[Diy_efi] Intro, and q's about fuel economy

[Greg Hermann]

At 12:06 PM 12/15/02, James wrote:
>Hello everybody.
>
>My name's James.  I'm a junior in EE.
>
>I've joined an extracurricular vehicle team to have some fun in college.
>I love cars and I love electronics, so it works out well.  :-)
>
>The task at hand is to build a fuel injection system for a lawnmower
>engine, single cylinder, probably L-head (possibly custom ohc head).
>
>Most of y'all seem pretty friendly and knowledgeable, so I thought I'd
>go out on a limb, admit I'm trying to become an engineer, and see whether
>anyone with experience had any suggestions.

Tough to get great economy out of an L head engine because of the high
surface area of the chamber (through which heat can be conducted to the
head/block). But, STILL, probably better to stay with what you have than to
try to build a head from scratch !

First thing to do is use coatings--on ALL chamber surfaces: head, block,
valve heads, and piston crown. KEEP all possible heat IN the chamber !! You
might even look at using the "Bore Tech" treatment on the cylinder
bore--not only does this make the bore harder than a lady of the night's
heart, it helps to retain heat.

Also, check out the idea of using a teflon impregnated hard anodizing on
the piston skirt (to reduce friction). Nimet Industries, Gary, Indiana, is
one place to get such a coating applied.

Second is to make a cusom piston that comes as close as possible to the
head--and gives LOTS of squish area. By close, I mean you should hand fit
things so that the piston crown only misses the head by about .015" to
.020" !! (Assume you are NOT revving the engine particularly high.) Higher
compression gives higher efficiency, and well designed squish area gives
much more efficient, faster combustion. Don't lose sight of the fact that
faster combustion ALSO yields more efficiency in the cycle itself.

Playing with the shape of the piston crown OR the head over it--as in
making a couple of grooves that will cause the mixture to "JET" out of the
squish space in a manner that will cause it to form a couple of
counter-rotating vortices in the chamber space over the two valves.

If this engine has an updraft carby, STAY WITH THAT CONFIGURATION !! Aim
the injector right AT the area where the edge of the throttle butterfly is
as it begins to open--there is sonic flow there, which will help greatly in
atomizing the fuel. An exhaust hot spot in the intake manifold above the
butterfly is another VERY helpful thing. One could also aim the injector
directly AT such a hot spot.

Pre heating the fuel (like to 200 or 250 F) before it goes to the injecor
can be a great help. Takes some clever pipe routing to avoid getting vapor
to the injector, though.

Using an AIR SHROUDED injector--either from a BMW OR from a  Bosch CIS
system would also help GREATLY. Quite plausible to control the fuel
pressure to a CIS (consant flow) injector electronically, and thus keep
things REAL simple !!

Either way, you are gonna need a REALLY small injector , and likely pretty
low fuel pressure in order to get the fuel delivery low enough for such a
small engine. Atomization of the fuel is a SERIOUS problem at low fuel
pressures, so be ready to pay careful attention to it. A too big injector
with a short duty cycle is NOT a good way to get accurate control of the
fueling rate !!!

With LOTS of squish area, you might well be able to push the A/F ratio as
far as 17.5 or 18 to one.

Take a peek at the archives on efi-332 for info on "Breakdown Discharge
Ignition" (BDI). This has the potential to not only light a lean mixture
more effectively, but also to give you a MUCH quicker burn with less
ignition advance--and thus more efficiency.

Last of all--water injection CAN be used to improve efficiency !! (Shhhh!!!)

Greg (Sorry, but most of this is the ME side of what is needed !!)


>
>The curious can see what I've accomplished so far at
>http://splorg.org/~jester/smv/  Sorry about the load times, I have slow
>dsl.  And if you can't connect, the rain pro'lly knocked the power out
>(again).
>
>The only real purpose of the project is to get the best gas millage
>possible.  We'd like to use EFI to get precise control over the
>stoichiometry of the fuel mixture.
>
>I've read that the most efficient ratio is 17:1.  Does anyone know whether
>this number is applicable to all engines, and how much (quant or
>qualitatively) economy you lose when you stray from this?
>
>Also, I think we need a lean burn oxygen sensor to accomplish this...
>anyone agree or disagree?  Any parts recommendations?
>
>Cheers,
>
>-james
>
>
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