EFI and Two-Strokes

[jb24 at chrysler.com]

>So, any boost pressure above the naturally occurring crankcase
>compression will only serve to shoot air-fuel straight out the exhaust
>port, wasting fuel and losing boost pressure.
> The key to avoiding this (and this is where my memory gets hazy) is
>to modify the porting so that the exhaust opens after the transfer.
>There had to be something else involved - otherwise you'd have a
>cylinder full of burning gas exhausting down the transfer ports into
>a crankcase full of air-fuel mixture - ouch. Maybe it was direct
>injection.. how do two-stroke turbo diesels work?
> Good luck with it. If you can solve these problems and double a
>two-stroke's power, you could just about go into orbit.
>
>Stuart.

Stuart's right, fuel injection and turbocharging are two separate
issues.
The idea was to get all the associated hardware for a turbo, and
concentrate
on getting the little stroker running right with the option to expand
later.
The MAF should help with misfire, cutting back on fuel when the pipe
isn't
working and keeping mixture right when there is excess air (and fuel)
in the
exhaust (keeping a HEGO from reading right).  Most racing two-strokes
have
something on the order of 120% volumetric efficiency, but can only keep
60-70%
of that in the cylinder, thus something like a little less than twice
the
specific power of a four-stroke.  A turbo ought to give me that
(hopefully
much better) volumetric efficiency over a broader rpm range.

To get a turbo to stuff 2 bar into a two-stroke - and keep it there -
requires
some serious redesign.  Turbo-diesels have poppet-valves in the head
for
exhaust, letting you close those ports before the transfer ports, but
limiting
rpms (crank-speed camshafts).  Direct-injection keeps the fuel out of
the
exhaust, which is why the automakers use it on their two-stroke
programs.

Once you get into the problem of closing the exhaust before the
transfer ports,
you find there is no simple answer.  Blowback into the crankcase
shouldn't be
a problem, as you need a positive pressure ratio to get flow into the
cylinder
with naturally aspirated engines today - a supercharger will just up
the ratio.
My other project is to figure out how to close ports in the cylinder
wall other
than with the piston.  If that works, naturally aspirated two-strokes
ought to
be able to keep more than 70% of their mixture, and turbos with similar
volumes,
just at higher pressure.  I really would like to double power output,
but that
would mean learning how to keep already fragile engines from grenading
- most
likely getting into the design of the whole motor, not just the fueling
like I
am now.

John Bucknell
jb24 at chrysler.com (not in powertrain, doing this on my own)