At 06:40 AM 4/1/2004 -0600, you wrote:

>Greetings,

>

>I haven't had a chance to do anything else with my O2 testing, but I've

>had an odd thought.

>

>The full story, is that I originally had erratic O2 readings with the

>rev-2 configuration.  Primarily, this was staying on the mid, to low end

>of the scale, but would then just go away completely, as if it was way too

>lean.

         I don't know if this will help, but I thought I would mention it

anyway.

         When the engine is running at high manifold pressure, fuel will

coat the manifold walls (after the fuel injectors.) When you shut the

throttle, the manifold pressure drops and the fuel coating the manifold

evaporates, leading to a very rich mixture. Depending on the length and

roughness of the manifold runners, this rich condition can last quite

awhile with a shut throttle. The flow is very low so the fuel can take

quite some time to clear out.

         Would this explain the behavior of your engine?

 

Good point!  Maybe another reason to keep the injectors close to the rotors.

 

Interesting post from the Mistral Engine (formerly Mechanair) on the ‘other’ list:

 

> François Badoux wrote:

>

> I have noticed on the ACRE site, as well as on the pictures of several

> engine installations, that there was a tendency to locate the

> injectors and the associated fuel rail on the spark plug side of the

> engine, away from the turbo and/or the muffler, for fire prevention. 

> Well, we at MISTRAL Engines know a little bit about that because we

> did exactly the same ... and I feel I should comment.  Initially, we

> located the injectors on the turbo side... where Mazda had put them,

> as close to the intake ports as possible (see first picture of engine

> on bench).  It was before my time but I was told that insufficient

> heat shielding "cooked" the seals, resulting in two consecutive engine

> fires... thankfully with no serious consequences!  It prompted my

> predecessors to relocate the injectors to the side of the engine

> opposite from the turbo (see second picture) as often recommended.  It

> solved the problem... no more fire.

>

> Unfortunately, during more recent testing of ours to improve the

> engine's BSFC, it was quickly discovered that this arrangement, while

> safe, is very unfavorable to achieve a proper mixture in the

> combustion chambers.  The injectors are just way too far from the

> chambers.  Fuel is being sprayed onto the intake tubes.  Back pressure

> waves from the closing of the intake ports compound the problem of the fuel liquefying onto the tubes.  Part of the fuel evaporates again, of course, but it is enriched in the lighter fraction of HC.  To make it short, by placing the injectors 20-24'' from the intake ports, you end up feeding the engine a cocktail of vaporized mixture enriched in hexane (C6H14 ), and liquid fuel enriched in its heavier fractions.  You end up having two systems in one:

 

1) a rotary engine working on a mixture enriched in light HC which is more detonation prone, and

 

2) an oil stove which burns liquid heavy

> fuel in the exhaust (or in the chimney when running it on a bench),

> resulting in a red orange exhaust system.  Of course, I exaggerate a

> bit to make my point, it's in fact only a question of a few percent

> shift, but this injection configuration does seem to result in a kind

> of "pre-distillation" of the fuel and a very unoptimal mixture.  This

> is especially true with mogas which has a very wide distribution of

> hydrocarbons.  By using 100LL avgas which has a much higher

> concentration of heptane, we very noticeably reduced the problem.

>

> In addition, you run the risk of having icing occur in the intake

> tubes, especially on a normally aspirated engine.  We never had icing

> on a turbo-supercharged engine because of the higher intake air

> temperature, except during very prolonged idling.

>

> After all, if Mazda installed its injectors very close to the intake

> ports, and operating in a "semi-direct" injection mode, it might be

> for a good reason.

>

> So we moved back the injectors to the "muffler" side of the engine,

> taking extra precaution to heat shield the fuel system and to route

> any possible leak away from the hot muffler or turbo.  It made a world

> of a difference.  BSFC dropped down very sensibly, the engine worked

> much better, especially in transients, the EGT dropped more than 100C

> and the exhaust system went from bright orange to dark cherry red...

> and the insulation in the chimney stopped burning (sorry, I don't have

> more scientific data to submit on exhaust temperatures past our EGT

> probes)!

 

> Sure, many of you are flying happily with the injectors away from the

> turbo... and so will we initially, as our airworthy new intake

> manifolds will not be delivered before next month.  Nevertheless, I

> believed I should share with this great group of people some knowledge

> of ours in return for all we have learned reading this newsletter. 

> Have any of you made similar observations?

 

> Francois Badoux