RCU Forums - View Single Post - Importance of engine break-in ?

05-11-2009 | 02:22 AM

#57

DarZeelon

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From: Rosh-HaAyin, ISRAEL

RE: Importance of engine break-in ?

Quote:

ORIGINAL: Lou Crane

Because of the valve train, 4-cycle engines have practically a built-in RPM limit. Too fast and valves float. Not good in a car, not good in a model engine... So, they do not speed up if the mixture goes leaner.

Lou,

What the needles do in a glow engine, is to adjust the effective ignition timing.

THIS is the reason RPM changes so drastically, when you change the mixture setting...

After all; from very rich to stoichiometric, virtually the same amount of fuel is burnt, which should result in a very similar amount of energy released...

But the power changes considerably because 'timing is everything'.

A richer mixture is cooler and its propensity to be ignited, is lower than that of a leaner mixture.
Also, the flame propagation is much more rapid in a leaner mixture.

Closing the needle in a glow engine, is the same as advancing the ignition timing, in a spark ignition engine...

At a certain point, the ignition will come so early and the flame will propagate so quickly, that very high combustion pressures will occur before the piston reaches TDC.

This will initially slow the piston, causing the RPM to decrease and overheating to set in. But at a certain point, detonation will occur and the piston will be kicked back, causing the engine to stop in a millisecond and the prop to be thrown...

Not only four-stroke engines are subject to this, but also two-strokes, where the 'lighter' form is called a 'lean run'...

The situation with Jorge's TT, however, seems to me to be simple, cowl-induced overheating.

Jorge,

You did not bother to baffle the cowl, to make sure your engine is cooled, did you?

Leaving it to chance and only following the intake/exhaust hole-size ratios, will not cause the cooling air to flow where you want it to...
Only proper baffling will force it to do that.

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