Warming piston engines

[barbershopquartets]

I recently read an article written by a number of ag pilots who had conducted a few trials on the most effective way to warm a piston engine.

With your standrd warm up procedure it's the usual of low revs until you reach a minimum temp, normally an idling period of 'X' minutes.

However the article claimed that with idle revs, airflow generated by the prop is sufficient to offset heat produced in the engine and that it's difficult to get much above a certain temp.

With trials, they had found that the best technique was to start up and run for 90 seconds, shut down for 90 seconds (allowing heat to dissipate through the engine without cooling airflow) and then starting up again giving an overall higher temp.

In practise it seems to work with my bus. But is it the best technique when it comes to overall engine care?

[Chimbu chuckles]

I think when we 'warm up' a piston engine we are more about warming up the oil than the lump of metal.

I have an IO550b on my Bonanza and I monitor it via an EDM 700 all cylinder monitor. This gives me instant EGT and CHT, accurate to 1 degree, for all the cylinders in real time.

My aircraft lives at a very small airfield and I find that starting the engine, keeping the revs low (around 800-900) and taxiing that way (nice and slow) that by the time I make it to the threshold the engine, and it's oil, is nicely warmed up....and away I go. This only takes a few mintes.

Sitting idling at low revs, or even slighty higher revs, very little air is passing over the engine (even at taxi speed)...the only airflow through the engine cowl will be from airflow around the cowl and over the cowl flaps. This causes a pressure differential between the upper and lower cowl and sucks air out of the cowl through the cowl flaps which is of course replaced by air being sucked into the cowl at the front. Take a look at the first 30 cm of the prop blade...it is round and produces no thrust as such....it is not pushing air into the cowl.

Chuck.

[airag3]

We used to warm the radials for around 20 min'on a cold morning (at around 300rpm) and flats for around 10 min or enough time for a quick cuppa ( never heard of this shut down/start up bizzo, but each to their own).

Interestingly the radial was definately getting cooling flow to all cylinders at idle ,as it also had probes on all cylinders and all would cool right off after around 10 min idling after landing.

[lancaster52]

I was always taught that shock cooling during descent was far more harmful. The important thing is to warm the oil to ensure even distributtion throughout the engine. It is often forgotten that much of the cooling comes from oil and fuel as much as airflow.

[heliduck]

I've seen ag pilots on a cold morning block the airflow through the oil cooler with a piece of rag in order to get the oil temperature to increase. Raises the question of a basic design fault by the manufacturer if people need to do this.
It is very important to have the oil warm but it is even more critical the have the "lump of metal" at operating temperature before subjecting it to the high loads experienced during TO. Engines consist of components made from many different types of metal, & all of these metals expand & contract at different rates when heated or cooled. When these components are seperated by a film of oil the manufacturer has designed them so they have an optimum clearance at operating temperature. If high loads are subjected to these components while they are not at operating temperature(in the green usually) the clearance between them could be greater or smaller than necessary for correct lubrication, leading to premature wear or failure.

[SNS3Guppy]

The engine should be preheated. By preheating the engine, this isn't an issue. A lot of operators don't do this, but if a cold engine is an issue, an engine cover with heat can keep the engine ready to go all the time for very little expense, and the payoff is found in engine life.

The use of any warmup technique in lieu of preheat when it's cold is a poor substitute at best.

That said, regardless fo weather the engine is preheated or weather the ambient temperature is warm, the engine still needs to run for a time before pushing power on it. A preheat reduces that time and allows for better heat distribution early in the process.

More importantly, however, preheating allows the engine oil to circulate more easily and faster, reducing wear on startup, and allowing the engine to heat more uniformly when running, faster.

This is true of any piston engine, opposed or radial.