Exhaust Port relocation ideas.

[NoDOHC]

Gorilla's post is too long to requote, so please bear with the new reply.

Let me make sure that I understand what is going on here:

The exhaust reversion is occurring under high vacuum conditions for the same reason that is occurs on a piston engine, exhaust pressure exceeds intake pressure and overlap exists. Tuned runners don't help this appreciably, as they are tuned for the speed of sound at WOT and the speed of sound is significantly slower at lower manifold pressures. The air also lacks enough velocity to actually dynamically charge the cylinder.

What I hear you saying is that the intake port is basically opening too late, causing the vacuum that is generated in the chamber to pull air back out of the exhaust. Closing the exhaust port later will exaggerate this. I see how this design could cause additional exhaust reversion at high vacuums (ie: cruising speed).

I still think that the reversion would be far less drastic than with a bridgeport. At 32 ATDC, A rotary engine has displaced about 8% of the total volume of the engine. This means that the effective intake displacement is actually only 92% of 0.65L or 0.6 L. This is the effect that the bridge port removes. I don't see how this is changed appreciably by relocating the exhaust port, as the engine already has this effect and already has a small amount of overlap (16 degrees) in the least shielded position. The only overlap added by this modification will be while the rotor face is very close to the housing (should be mostly ignorable).

I am curious how the small amount of exhaust reversion (should never be greater than 20%) can cause the engine to misfire. This may be obvious, but I am not getting it. All my knowledge of camshaft design is from the piston engine world and I admit that I may have drawn the parallel of the two types of engine too far.

The understanding that I have of the above explanation is that the exhaust gas continues to build up in the chamber and eventually the engine misfires. I see how this could occur at very high vacuums (> 60 kPA), but under normal operating conditions, the engine should force enough exhaust gas out each time to keep net air flow through the engine. The exhaust dilution should reach a still-functional maximum.

Kenichi Yamamoto mentions the reversion misfire effects in his paper on the rotary engine. He only speaks of it as a serious problem under deceleration conditions.

Here is some data from page 52 of his paper:


From what I see, it improves low and high end power, while make little effect on midrange. The misfiring data given is at 0 degrees BTDC, While a good idle timing will be ATDC.

I can say that on my '88 4 port engine (Naturally aspirated) I did have misfire issues if I leaned the fuel mix up too much or if I advanced the timing too far (running a lot of intake vacuum) however, the fueling (and hence the VE) matched what I expected from a piston engine. In fact, the rotary VE seemed to be higher at high vacuums than the piston engine. This would seem to contradict any theory that I can form about the exhaust reversion causing the misfiring, as any reverted exhaust is directly subtracted from the VE of the engine. If the timing was retarded enough or the fuel mix was rich, the car idles (I should say idled, as it barely runs right now with my fuel system problems) smooth as butter.