Over run Rotary Engine

Question here for Andre,

Hopefully he sees this and can respond, or anyone that can answer.

My question surrounds rotary engines and the disabling of decel fuel cut on a circuit racing application.

I understand this is done to help cool the engine down on lift throttle.

I've seen circuit race rotaries spitting fire/flames on overrun, I assume they have decel fuel cut turned off as well as heavy 🪨retard on the ignition timing.

I've often heard Andre advocate this to help cool the rotors on overun, my confusion is, with retarded ignition timing and fuel, this would likely increase engine temps pushing the combustion event later in the engine cycle, which would defeat the purpose??

Retarded timing means the energy transfer from the fuel to the engine's combustion surfaces is less, and the burn is cooler, basically more of the energy is going out the exhaust rather than into the engine.

In some cases, as you mention, the fuel is still burning as it passes out the exhaust, which also helps keep the turbo' spooled.

High overlap engines will also suck a lot of charge straight through into the exhaust.

Many race cars also don't have functional oil metering pumps so rely entirely on pre-mix oil in the fuel.

You can happily run timing in the 40s and kissing the 50s on no load decel/'high' vacuum with a rotary. Many people don't because it also tends get rid of most of the 'cool' factor of flames on every shift and decel - similar to 'tuning' for brap in the modern age, sometimes people add inefficiencies just to show off ;)

Let's start by clarifying something. Zero combustion, from disabling injection and moving near ambient temp air through an engine, offers a great deal more cooling than injecting fuel and combusting it at hundreds of degrees.

I have a hunch what Andre meant was if you're going to keep injecting fuel and thus keep combusting on overrun, rather than shutting down injection, then by retarding the ignition advance you move some of that combustion heat from the combustion chamber to the exhaust. You end up with combustion temps lower than if you were running more timing advance, but many times higher than if you weren't injecting fuel.

On a road race car, avoiding thermal shock from sudden severe cooling, and maintaining fuel film for optimal engine torque response when the driver gets back on throttle, are some potential reasons to avoid overrun fuel shut down.

On a rotary specifically, as someone mentioned if the oil metering pump has been removed, then rotor lubrication is coming with the pre mixed fuel, so disabling fuel injection would disable that lubrication.

Mike

Thank you for the insight.

Andre actually advocates just disabling overrun fuel cut by itself vs turning off injection all together to help cool the engine on gear shifts/braking in a racing application.

The retard timing is something I assume tuners are doing on the rotaries in addition to injecting fuel because of the amount of flames I see under braking.

I've first hand experienced coolant temps rising if your ignition retard is too heavy, hence my entire confusion with this post. Why are guys doing this on a rotary road racing application (other than it looks cool) if it's stressing the engine more

They aren't necessarily retarding timing, even carburated (especially bridgeport and PP) cars with timing locked distributors will shoot flames, again, significant overlap and charge pull through.

Thanks for that Michael