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Why do engines shudder on shutdown?

It has to be caused by uneven fuel distribution when the fuel is shut off, because the mechanical balance of the engine is obviously just the same.

Are cylinders robbing fuel from one another (inside the inlet manifold) or is the fuel distributor failing to maintain a properly distributed fuel flow to all cylinders? Or is it just that as the mixture becomes very lean, the combustion is extremely sensitive to the exact ratio?

Administrator
Shoreham EGKA, United Kingdom

More likely to be a mechanical resonance IMHO. The engine is a heavy lump mounted on relatively flexible rubber mounts, which will result in a low resonant frequency – much lower than normal engine rpm. However, as the engine stops, it winds down through the speed range from idle to 0 rpm, passing through the resonant frequency at some point. You can see a similar effect in washing machines and older car engines.

Top Farm, Cambridgeshire, United Kingdom

One simple way to find out – shut down using the magneto switches. If it is still shaking, it has nothing to do with uneven combustion…

Biggin Hill

The low torsional natural frequency of the engine on its mounts is excited. The forcing function is the substantial inertia of the rotating propeller being reacted by engine compression as it rotates to a standstill.

Last Edited by Silvaire at 06 May 13:10

I observe this mainly with Rotax engines …

EDxx, Germany

A PT6 does it as well although not as much as a piston.

EGTK Oxford
I have never understood the wisdom of cutting off the fuel for stopping the engine except for an emergency. There are numerous engines with automatic altitude mixture, no manual mixture there. You simply switch off the mags, no kick back due to overly lean fuel before the engine dies. Vic
vic
EDME

The reason is probably self-ignition. The prop is directly connected and can do a lot of harm. If fuel remains in the engine it could theoretically self ignite.

Silvaire is correct.

In simple terms, the isolators have an elastic stiffness – K – and the engine has a mass – M .

The system will resonate at a frequency sqrt(K/M).
This is designed to be below the shaft frequency in normal operation, but you will pass through that frequency on shut down and start-up.

The same effect can be seen on cars and washing machines.

KHWD- Hayward California; EGTN Enstone Oxfordshire, United States

Also the engine mount itself probably isn’t particularly stiff in torsion (low K)

Last Edited by Silvaire at 06 May 17:31
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