Direct Injection

[Gary Derian]

I am aware of Orbital's work.  Their system is a great way to make a 2
stroke meet emission requirements.  The air assisted injection is necessary
to achieve fuel air mixing in a limited time since injection occurs after
the exhaust port is closed.

Bosch direct injection used a mechanical pump timed to END the injection
cycle at 60 deg ATDC on the intake stroke.  This gives time for mixing.  To
inject after the intake valve closes greatly reduces the mixing time and may
result in unwanted stratification.  Wanted stratification is another matter.

The main benefits to direct injection explained here seem to center on the
ability to create a stratified charge, a combustible mixture near the spark
with excess air elsewhere.  This improves part throttle economy.  It also
reduces HC and CO emissions.  NOx are also reduced but not enough to suit
current emissions standards.  For that, reducing catalysts are needed but
they will not reduce NOx in the presence of excess oxygen.  With a
stochiometric mix, stratified charge becomes moot.  I recognize that it is
possible to have with a lean mixture in certain operating regimes and still
meet NOx.  Honda and GM did this but I believe these were considered
emission defeat devices and are no longer used.

For economy, modern cars have 5 and 6 speed transmissions with very tall
cruise gears.  These techniques reduce pumping loses without the complex
engine controls.

So now we have stochiometric fuel mix and economy gearing.  So where does
direct injection help?  I'm not saying we should stop research on this
matter, but this list, it seems to me, is primarily concerned with improving
the performance of our cars by modifying the factory injection systems.  For
these purposes, direct injection has no benefit.

Gary Derian <gderian at oh.verio.com>