cold starting

[addicted to air]

why does it help when starting engine to push the throttle in one or two times before or during cranking

 

[Skip Miller]

why does it help when starting engine to push the throttle in one or two times before or during cranking

The accelerator pump adds more fuel with each throttle stroke.

Do this while cranking. With the typical GA updraft carb, more of that fuel will get into the cylinders than if you pump without cranking, when fuel is likely to drip out of the carb.

Others will say that the need to pump the throttle means you are not priming properly. Well, maybe so but most primers feed one or two cylinders at the most. Again, the gurus will say this is sufficient but I have thought, and experienced, that you will have more luck with all cylinders breathing a richer mixture for starting.

 

[oldShar]

Better yet:
Properly & completely pre-heat
keep the oil loose and let it do it's compression job (as well as providing for slickeriness)
and the engine should start without the need for calesthenics

 

[tyndall]

most primers feed one or two cylinders at the most.

Say what, now?

 

[Skip Miller]

most primers feed one or two cylinders at the most.

Say what, now?

Sorry, but it is true for the typical Cessna and Piper carbureted engine airplanes. Remember the priming system is installed by the airframe manufacturer, not by the engine manufacturer. They cheap out on this!

If you don't believe it, just decowl your engine and trace the primer tubing.

-Skip

 

[Dan Thomas]

Sorry, but it is true for the typical Cessna and Piper carbureted engine airplanes. Remember the priming system is installed by the airframe manufacturer, not by the engine manufacturer. They cheap out on this!

If you don't believe it, just decowl your engine and trace the primer tubing.

-Skip

Cessna primes three out of four cylinders on their carbed Lycs. Continental either primes all of them separately, or injects it into the manifold above the carb so they all get it. Nobody's cheaping out on anything. The biggest problem with priming is the lack of understanding of what's happening when one does it and how much is too much or too little and when to prime before cranking. The guy or gal that understands can get a cold engine fired up with no trouble. The pilot that doesn't understand blames the manufacturer. He might have a mechanic that doesn't maintain his primer nozzles. They get coked up with flashed-off fuel when the engine heats up so they don't atomize the fuel properly, or are clogged altogether.

 

[Skip Miller]

Cessna primes three out of four cylinders on their carbed Lycs. Continental either primes all of them separately, or injects it into the manifold above the carb so they all get it. Nobody's cheaping out on anything. The biggest problem with priming is the lack of understanding of what's happening when one does it and how much is too much or too little and when to prime before cranking. The guy or gal that understands can get a cold engine fired up with no trouble. The pilot that doesn't understand blames the manufacturer.

Thanks, Dan. I've seen a single cylinder prime setup on a C-150 and was told "they're all like that." Sorry to be misleading.

-Skip

 

[paflyer]

Cessna primes three out of four cylinders on their carbed Lycs. Continental either primes all of them separately, or injects it into the manifold above the carb so they all get it. Nobody's cheaping out on anything. The biggest problem with priming is the lack of understanding of what's happening when one does it and how much is too much or too little and when to prime before cranking. The guy or gal that understands can get a cold engine fired up with no trouble. The pilot that doesn't understand blames the manufacturer. He might have a mechanic that doesn't maintain his primer nozzles. They get coked up with flashed-off fuel when the engine heats up so they don't atomize the fuel properly, or are clogged altogether.

Do tell.

I've read that the 'throttle pump can' lead to engine fires.

 

[Dan Thomas]

Do tell.

I've read that the 'throttle pump can' lead to engine fires.

Improper priming of several sorts can lead to fires. In cold weather that fuel is reluctant to vaporize and it just lands on the induction system walls and flows down and out the carb and airbox. Overpriming can cause a rich mixture that is very slow to burn, and it's still burning in the cylinder when the intake next opens. That's a long time, and if it happens you get a backfire through the carb that can ignite that fuel in the airbox and cowling.

Priming with the accelerator pump can do it even quicker. The fuel is already low down in the system, and the accelerator pump nozzle squirts the fuel so it mostly strikes the throttle plate as it opens and closes, breaking up the stream and throwing it against the carb wall, where the air can't pick it up and carry it to the cylinders. It flows down and makes a mess or sometimes a fire. You want to have the engine cranking when you do this, to get that air drawing the fuel in. Pumping the throttle while not cranking does more harm than good.

When using the primer, crank immedialely after priming. Don't wait. Get that fog of fuel into the cylinder before it settles onto the induction system walls.

 

[Capt. Geoffrey Thorpe]

You need a combustible mixture in the vicinity of the spark plug for it to ignite and start the engine. Enough fuel, but not too much.

When the engine is cold, you have a number of things working against you. First, the manifold is dry - much of the fuel that gets to the cylinders actually flows along the surface of the manifold and gets mixed with the air at the intake valve. Second, the manifold and port are cold so you get much less evaporation from the fuel from the carburetor or the fuel film on the manifold walls. Third, the cranking speed is slow so the air flow rate is low so you get less mechanical mixing of the fuel and air.

Bottom line is that you need to add extra fuel to the system so that enough of it makes it to the vicinity of the spark plug in vapor form - and the colder it is, the more fuel you need to add to compensate for the fuel that just doesn't want to get involved.

There are a few ways to add the extra fuel. The primer puts fuel somewhere into the manifold - the exact location will vary. Pumping the throttle activates the accelerator pump to add fuel at the carburetor - this pump is installed because of the lag between opening the throttle (more air) and the extra needed fuel getting to the intake valve (because it is moving in liquid form along the manifold), but it also works for adding fuel during cranking. Other methods, such as a choke are less common on aircraft operations - one exception would be the Rotax which uses a bypass in the carb to add fuel instead of an actual choke plate.

Typically, one would use primer and/or pumping the throttle when starting an aircraft engine. Priming or throttle pumping while cranking can help the fuel mix with the air as it flows by. Priming and letting it sit can give time for some of the fuel to evaporate, but in some applications it just runs down the intake manifold and into the air cleaner.

In a modern car, the extra fuel necessary is added automatically by the injection system in response to environmental parameters such as the air inlet and cooling water temperatures.