Engine Starting

[FN FAL]

Originally Posted by avbug:
Actually, stepclimb, the accelerator pump in a carburetor is not a primer, and never has been. Use of the accelerator pump in a carburetor to "prime" an engine is poor form and poor airmanship, and shows a misunderstanding of proper engine and engine control useage.



Avbug,
Here is the passage taken directly from Lycoming's site.

"Priming can be best accomplished with an engine priming system, as opposed to use of the throttle. The primer pumps extra fuel directly into the cylinder intake port or induction system. Some float-type and pressure carburetors also provide a supplemental source of PRIMING. Lycoming engines of more than 118 HP have a throttle pump which can be used for priming under moderate ambient temperature conditions while turning the engine with the starter." (emphasis added).
Part of "Airmanship" is to know your systems and understand that there is more than one way to do things. While the accelerator pump on the carb was designed to provide additional fuel for engine accelleration, one "fringe benefit" was the use of said pump to aid in engine start. If it were not meant for that use or its use was prohibited for engine start, surely Lycoming would discourage its use.

Some reasons for its use come to mind.... Fuel pumped directly into the cylinder is liquid vs the atomized vapor that originates in the carb. Second, some engines utilize primer lines on only 1 or 2 of the cylinders-which may not be adequate for engine starting under some conditions.
Lastly, introducing raw fuel into the engine cylinder "washes" away the lubrication provided by the thin film of oil on the cylinder walls and if allowed to sit, dilutes the oil in the sump. (now of course, one could argue the small increase in engine wear, vs ease of starting by use of the primer)
Having written that, I'm hopeful that most pilots of updraft carbureted engines understand the risks vs. rewards of using the accellerator pump and that pumping the throttle should only be done while cranking the starter. Furthermore, they should be aware of the manufacturers procedures for dealing with an induction fire.
Now don't get me wrong...Follow your POH guidance in starting the engine-which most likely advocates using the primer. Just be aware that there are other avenues to use if you understand what is going on beneath the cowl.

Originally Posted by avbug
The throttle is an air valve, not a fuel valve. In engines utilizing a fuel servo (and similiar systems), fuel delivery is not proportional to throttle position, but to airflow.

True, the throttle is an air valve. But in some fuel servo systems, priming is accomplished with fuel pressure from the electric pump, mixture rich with fuel flow proportional to throttle position. Jump in an Arrow, turn on the master and fuel pump switches, put the mixture rich and watch the fuel flow guage as you modulate the throttle position. It is true that after the engine starts, fuel flow is proportional to airflow.
-Stepclimb

So would you guys all agree that the prefered method of starting any aircraft engine, would be to follow the POH?

 

[Snakum]

Every time you pump the throttle on a plane engine that shouldn't be pumped, god kills a kitten.

ROFLMAO!

Thanks ... I needed that.

Minh

 

[DGdaPilot]

Anyone here remember the starting procedure for a Cessna 310R with the 285hp Continentals? I do....it's good food for thought that is in contrast to some of the posted opinions. And it's started that way for a **CENSORED****CENSORED****CENSORED****CENSORED** good reason.........