Engine shutdown and engine shake

I have over 300 hours in my RV-7A (XP IO-360 FI) and need some input.

During engine shutdown the engine shakes a bit more then I see other aircraft engines shake. If I start up the engine taxi a short distance to the fuel pumps and shut down the engine shake is minimal. But when the engine is hot after a flight, there is much more engine shake at shutdown.

I have tried to minimize the engine shake with different throttle settings, control sequences with the mixture at ICO but I haven't found a good combination that improves the shake that much.

What is your engine shutdown procedure that I might try to improve this situation?

Thanks,

Paul,
You can try killing the ignition after mixture pull to minimize additional firing during shutdown. But there is a difference between a shake during shutdown, and getting a some firing during shutdown. I would not be worried about the shake. But the fire of the fuel while its trying to quit can be annoying.

I have a purge valve. My engine does that occasionally, but pulling the purge valve shutting of the fuel supply stops it promptly. Per Don Rivera, the engine should be shut down with the purge valve and not simply pulling the mixture to ico.

YMMV, especially since purge valves aren't common on bendix fuel controllers. I have an airflow performance fuel controller.

Kahuna said:

Paul,
You can try killing the ignition after mixture pull to minimize additional firing during shutdown. But there is a difference between a shake during shutdown, and getting a some firing during shutdown. I would not be worried about the shake. But the fire of the fuel while its trying to quit can be annoying.

Click to expand...

It is in the last revolutions of the prop that that "strong" shake occurs. In the pilots seat it appears more violent then what I see from watching other aircraft shut down.

I was taught this shutdown method by a more experienced friend. I wonder if it would help.

RPM to 1200. Slowly lean out mixture until you peak EGTs and keep going. When the engine gets rough, then quickly mixture to ICO.

Leaning prior to ICO helps purge lead and maybe get rid of excess fuel that leads to shaking??? i'll have to experiment myself.

The Mooney used to do that...

It was much better when the mixture was pulled at about 1500 rpm.

In the Grumman world, this is known as the "wet dog shake". Unfortunately it's also believed to be the cause of cracks in the tail structure of some high-cycle airplanes (not just Grummans).

The shake is more pronounced on a warm engine, or so it seems. Logic would suggest compression on a warm engine is greater than that of a cold engine, so the last few swings of the prop through a compression stroke result in larger amplitude of movement of the engine and airframe.

Depending on your engine/prop combo, you may find the amplitude of the shake is reduced by pulling the throttle back to the lowest idle setting before killing the ignition. An airplane I flew years ago liked it best when the throttle was retarded to idle, then, as the engine was slowing down, mags off and mixture to ICO at the same time. Every airplane has its own personality...

The new airplane we just started flying is the first I've had with a purge valve, and I have to agree that so far, anecdotally, killing it with the purge valve makes the wet dog shake less severe. And yes, shutting down from about 1500 RPM always seems to be smoother, with any of our planes.

Paul

Lycomings recommendation is to bring RPM to 1100-1200, wait for temps to stabilize, then go ICO (no delay). I follow this and have never had shaking. With this procedure, the mis-firing that happens as you go to cut-off (inevitably a stroke or two in transition will have just enough fuel to light and it doesn't happen consistently with all four cylinders) happen with more centrifugal force/momentum and it doesn't result in shaking.