Alternative to O2 injection, EGR? (Re: DIY_EFI Digest V5 #34)

[Greg Hermann]

>Hunt David wrote:
>
>>
>> Since the spend gas is added after the throttle plate the total gas in the
>> combustion chamber is increased as compared to no recirculation. Thus the
>> "dynamic" pressure (effective compression ratio) is increased.  At part
>> throttle the chamber certainly doesn't fill due to the restriction of the
>> throttle and the dynamic compression ratio is quite small, so even a small
>> amount of recirculated gas increases the dynamic compression ratio.  It also
>> increases the preignition temperature (promoting better vaporization
>> (mixing)), and provides a (relatively) cooling effect for pockets of
>> "leanness" reducing the possiblity of detonation.  REDUCING.

First of all, EGR does _NOT_ affect either static or effective compression
ratio OR expansion ratio.
 It provides a diluent to the combustion gasses--which effective lowers
peak combustion temperatures, thus lowering NOX formation during
combustion. Also--lowering theoretical cycle efficiency to some extent.
(Mr. Carnot strikes again!)

EGR, at part throttle, because it is a diluent, reduces intake vacuum
downstream of the throttle without any increase in power output, and thus
reduces part throttle pumping losses. This can constitute a net gain in
efficiency _ONLY_ at part throttle, despite the loss in cycle
efficiency--BUT--usually can only do so _IF _the timing is compensated
(advance further, back to optimum when the EGR is on) for the fact that the
EGR diluent slows down the rate of flame travel.

Net effect of EGR at part throttle is probably to reduce detonation at part
throttle--with slower burn, higher chamber pressure and temp and more
advance being bad factors, charge dilution, more chamber turbulence (due to
the denser charge) and a chemical effect on the combustion process being
good factors.

Use of an EGR cooler  would be a plus all around, in any case.
>
>My question is, can EGR be used as a substitute to O2 injection for knock
>suppression?  My mechanic friend warned me about O2 injection, that it
>will tend
>to remove the oil from the cylinder walls and on the valve stems.
>Supposedly it
>will cause rapid engine wear, i.e. cylinder taper and stem wear.

>
>I want a Win - Win situation with detonation suppression.

You will not be able get a win-win situation with EGR at WOT. Diluting the
charge _WILL_ effectively reduce booth VE and cycle efficiency.
Particularly without an EGR cooler, I doubt that you could ever see a net
gain--I doubt that the detonation suppression effect of the EGR gas could
ever be enough to offset the loss in VE, even if you compensate for the
loss in VE artificially by increasing boost.

  Water injection does
>cause suppression, but kills hp.
>
>Can EGR help under high boost conditions to prevent detonation, and not cause a
>loss of HP, or would it just cause higher EGT, reducing thermal efficiency, in
>the process killing HP.  The proverbial throwing the baby out with the bath
>water.
>
My opinion is that you mechanic has it about half right. Poorly atomized
injected water can scour oil off of parts--the cavitation that happens when
a droplet of water lands on a part, and then boils off of it can easily do
this. With well atomized water injection, this problem is substantially
cured. (Poorly atomized fuel will wash oil off of parts even worse than
water, as the oil is SOLUBLE in the fuel droplets!) In this regard,  one
might observe that a steam cleaner works most effectively when it is set to
spray a MIXTURE of water droplets and steam, not pure steam, out of its
nozzle!

Water that is atomized, but NOT vaporized, before the intake valve closed
does NOT have any significant effect on VE, the volume of the water is
simply too small to have much effect in terms of diluting the intake
charge. On the other hand--crudely injected water--say sprayed on the hot
spot in an intake manifold will have a HUGE effect on VE, Because of the
low molecular weight of water compared to the (average) MW of air, any
water vapor (steam, if you will) formed in the inlet tract before the
intake valve closes will displace a substantial amount of air, thus
reducing power.

On a chemical basis in the chamber, water vapor is a _VERY_ powerful
(perhaps the most powerful one known) detonation supressant, for a number
of reasons. One of the prime ones is that water is a highly polar molecule,
and as such tends to ties up the free radicals which are the immediate
precursors to a detonation event.

In a nutshell--my opinion is that WELL EXECUTED water injection will get
you what you want with no bad mechanical side effects, and that EGR will
not.

Not to mention that properly done water injection can significantly improve
durability by reducing temperatures throughout most of the cycle. There is
a loss in Carnot cycle efficiency due to this effect, but, and again my
opinion, this loss is more than compensated for by a reduction in negative
work during the compression stroke _AND_ a very significant reduction in
heat rejected to the water jackets (due to the lower temps).

Regards, Greg


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