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Injection Engine Start-up science (or metaphysics !)

The POH procedure has always worked for me, perfectly.

The tricky bit is the “warm start” and treating it just like the hot start is probably the best advice.

What helps a great deal is a “high speed” Skytec starter.

Administrator
Shoreham EGKA, United Kingdom

High speed starter solved the problems for me too.
I agree warm proves to be harder than hot.

Also agree with pushing forward slowly the mixture when the engine starts. Otherwise, you would loose what was coming to life.

Regardless of the comments above about forgetting what is written in the POH, I respectfully suggest that as the place to start. The same engine in different aircraft will have different fuel system structure. I agree 100% with what Rwy20 wrote:

Ignition will happen anytime the fuel/air mixture in the cylinders is in the combustible range and you add a spark – too little fuel and too much air won’t work, but too much fuel with too little air won’t work either.

It is, however, much easier to gradually add fuel to a pure air environment than dilute pure fuel by adding air. So almost any hot start procedure will result in fuel gradually being added to an engine turning on air only. But that is only a generality and you need to determine the successful procedure for each aircraft that you fly.

There are other factors at play that may need to be addressed before one gets to the point of turning over the engine.

I have a Cessna Hawk XP with a TCM IO-360. This same engine was also installed in early Cirrus and the Cessna 337, plus other aircraft. I suspect the POH and fuel systems for these three aircraft are quite different from each other. Here is where the system structure comes into play. My POH has a fuel system diagram (plus 4 pages describing the system) and if your aircraft has one then I suggest you study it to understand how the system works. When the engine is operating, there is a steady flow of cooling air through the cowl and cooling fuel through the lines. But when a hot engine is turned off, air and fuel stop flowing, and the fuel in the lines can get hot and vaporize, especially if they sit on top of the engine that is no longer being cooled and is giving off heat. I’m not fond of the term “vapour lock” because it can mean different things to different people, but vaporized fuel in the lines will prevent a steady flow of liquid fuel to the injectors whose role is to do the vaporizing. The bottom line is that the fuel line and fuel within it need to be “relatively” cool (i.e. in liquid form) before trying to start the engine (hot or cold). The mixture in the cylinder could be perfect and there is ignition, but without a constant liquid fuel supply to the injectors it will won’t stay running. Maybe fire for a few propeller turns and then quit. My IO-360 fuel system has a fuel reservoir tank between the fuel tanks and the mixture unit (which is the spider sitting on top of the engine feeding the injectors, getting hot after engine shutdown). Between them is an auxiliary fuel pump. Excess fuel that is not delivered to the injectors flows back into the reservoir tank. Part of the hot start procedure is to first cool the fuel system within the engine compartment by running the aux fuel pump about 15 seconds with the throttle half open and the mixture at ICO. 15 seconds seems like a very long time when the pump is running, but this is needed to ensure that cold(er) fuel has circulated through the system. None of it reaches the cylinders due to ICO mixture. Once a steady flow of liquid fuel to the injectors is assured, an attempt to start the engine can be made.

Last Edited by chflyer at 02 Jan 19:35
LSZK, Switzerland

George Braly posted an intersting explanation here

ch.ess wrote:

If you are not quick enough to move the mix forward,

In the first year after the purchase of my plane I had many disappointing experiances concerning a hot start of my Lycoming TIO540. I was already thinking about sale.
Even after just three minutes running the engine from the hangar to the fuel station I could not start the engine again. En route I had to avoid landing for refueling as much as possible, that was really no fun. Various experts gave instructions and tried it partly without success. Finally an old and very experianced FI watched me try to start my hot engine. He realized that I pushed the mixture too slowly after priming for 1,5 sec. That was it! The quick advance of the mixture is very important. I do not have any problems anymore starting my hot engine, many thanks to Max (FI at EDOI)!

Berlin, Germany

Interesting… On a hot start I have always advanced the mixture fast (as soon as the engine starts up).

Then I have to reach for the throttle really fast to catch it and pull it back down, to stop the revs going too high.

It never occurred to me to advance the mixture slowly.

There is another procedure, which I saw about 15 years ago, which works with any injected engine, hot, warm, or cold. It just takes a lot of cranking. Unfortunately I can’t remember the details But it did involve a gradual advance of the lever(s) during the cranking. And I think it is fairly obvious that such a procedure must exist, because a cranked engine is sucking in the air, and the fuel servo is squirting in the right amount of fuel, so eventually it must start up. The procedures we use are designed primarily to not shag the starter motor before the warranty runs out

Administrator
Shoreham EGKA, United Kingdom

How far advanced do you have the throttle? It should be no different to the position for a cold start (let’s say half an inch).

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

On a hot start, all the way forward. Pump on, mixture full for 1 sec then closed. Then start, and when it picks up, mixture to max and reach for the throttle really fast.

On a cold start, about 10mm forward. Mixture on max. Pump on until fuel flow analog gauge just moves (usually takes 5 secs). It doesn’t always move. When it picks up, pull mixture back about 2/3 for peak EGT and adjust throttle for 1200rpm idle, very lean mixture.

Administrator
Shoreham EGKA, United Kingdom

It all depends on what works for you. My SR20 used to be a pig to start when warm, cold starts were easy. However I did what Boscomantico recommends, i.e. learnt why the engine was refusing to fire. In the case of the TCM IO-360 ES, this was due to the warmth of the block / cylinders causing a vapour lock in the injection system. By modifying the starting procedure for warm starts (where a restart is required after approx 5 minutes since shutdown), I no longer face any issues.

Generally it’s mixture full rich, throttle half open, switch my electrical fuel pump to boost and listen for a change in tone as the pump starts pumping fuel rather than vapour through the lines (if no change after 5 – 6 seconds, there was no vapour lock). Now move the throttle back to slightly cracked open – turn the key and she starts.

EDL*, Germany

Peter wrote:

On a cold start, about 10mm forward. Mixture on max. Pump on until fuel flow analog gauge just moves (usually takes 5 secs). It doesn’t always move. When it picks up, pull mixture back about 2/3 for peak EGT and adjust throttle for 1200rpm idle, very lean mixture.

at which point do you crank the starter ?

LFBZ, France
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