Direct Injection - Stratified

[Jim Zurlo]

Almost all large (>6 liters/cylinder) lean burn natural gas engines are
stratified charge.  They have a prechamber where fuel only is admitted.
The spark plug is located in the prechamber.  This allows the engine to run
much leaner (lambda = 2) than an open chamber engine.  For a given lambda
stratified charge does produce more NOx, but the prechamber allows the
engine to be run leaner where NOx emissions are lower.  Prechamber engines
have the best efficiency, topping out around 43% thermal efficiency now.
For the ultimate in low NOx emissions nothing beats a stoichiometric engine
with a three way converter.  There has been a renewed interest in direct
injection gasoline engines recently for improved fuel econonmy.  The only
sticking point is reducing the exhaust NOx emissions with a lean NOx
catalyst.   Lot's of people are working on that and there are solutions
(Selective Catalytic Reduction) but they are really expensive and require a
source of ammonia.  Not something you want to carry around in a vehicle.
Jim Zurlo
zurlo at execpc.com


At 02:30 PM 5/3/99 GMT, you wrote:
>Contrary to public myth, there actually is more than one stratified charge
>design - but then anyone who was not blindly prejudiced would have know that.
>
>For Example - the Texaco /MANN design where the fuel is injected past a
>sparking plug that is firing repeatedly.  Gets the best of both diesel for
>economy and otto cause it can burn rich of stoic and great economy because it
>can run WOT always and no denotation.  
>
>Then there is the unscavenged "blind"prechamber  also known as a torch cell,
>which is not involved in air flow.
>
>The fuel scavenged prechamber ( the more common ) variant - which has fuel
>shot thru it - kinda like the spark plug injector thingee that started this
>thread.  Its not involved in air flow.
>
>The air scavenged pre chamber ala Honda CVCC - which has a separate intake
>valve ( very small ) and port and runner - again not interfering with flow -
>Honda continued this design with a three valve head that had two intakes, one
>exhaust and a cvcc valve.  Didn't seem to slow them down much.  Didn't
seem to
>have any negative impact on air flow or power or economy.
>
>In fact, excepting some obscure diesel designs, none of the popular
>pre-chambers is a device that is involved in the main air flow so they can't
>possible introduce a restriction into the path of fuel or air.
>
>The minor loss of theoretical peak thermal efficiency is greatly offset by
the
>vast increase in range of combustion, quality of fuel, and faster burning.
>
>But hey - I must be a dumbshit - I only researched it extensively instead of
>simply anally extracting some spurious so there comment.
>
>------------------------------
>
>Date: Sun, 02 May 1999 22:31:00 -0500
>From: dave.williams at chaos.lrk.ar.us (Dave Williams)
>Subject: Re: Direct Injection - Stratified
>
>- -> Any time a device that restricts flow is introduced into the path of
>- -> air and fuel, power is reduced.  A pre-chamber is such a device and
>- -> such an engine will lose efficiency compared to an open chamber
>- -> design.
>
> A prechamber doesn't necessarily have to restrict flow.  It can be *in
>addition* to normal flow, as was used in various Japanese automobile
>engines a few years ago.
>
>==dave.williams at chaos.lrk.ar.us======================================
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