Engine Starting

[uwochris]

Hey guys,

I have some Qs regarding engine start procedures.

For the 172 I used to fly (fuel injected), the basic procedure was:
- master on.
- throttle set.
- fuel pump on.
- mixture rich, then idle (I assume this is to prime??).
- fuel pump off.
- crank, and set mixture rich once it fires.


For the C1 I used the fly (also fuel injected, but Continental engine is used), the procedure was:
- master on.
- mixture rich.
- fuel pump on.
- fuel prime on (separate switch).
- throttle full forward, then set (this is to prime).
- crank and start.

Now, I do not know why the mixture can be full rich in one case, and should not be full rich in the other. Would it even make a difference for the 172 if I left the mixture to rich when I cranked it? I personally do not think so because the checklist calls for you to turn the fuel pump off for the start, but I am obviously missing something here because the engine would likely get flooded if you left it rich. (ie. with the engine not yet running and the fuel pump off, the mixture control should not have any affect whatsoever, or at least this is what I thought).

In the 152 I fly now (carburreted), the mixture is always rich for the start. Again, does anyone know why this is the case, if both the 172 and 152 use Lycomings?

Thanks in advance.

Chris.

 

[U-I pilot]

In carbureted engine, mixture goes rich cause fuel isnt being forced into cylinders. In fuel injected, you were correct as the quick richen of mixture primes the engine. When mixture is rich in this case, injectors are sending more fuel to engine. If you left it rich on the fuel injected engine it would flood the engine.

Someone will surely have a more descriptive explaination than this but its a poor attempt at an answer.

 

[avbug]

Not true, as it all depends on the type of fuel system in use. That the same manufacturer makes the engine is irrelivent. You don't put the same engine parts in a corvette as a geo metro, nor operate the engines the same, though they have the same manufacturer. That lycoming makes two different engines, but the two have different proceedures or operating parameters, should come as no surprise.

Before you delve into the reasons behind the different starting proceedures you need to do a little system self-education...something you should have done before you ever climbed into the airplane...and something you should commit to do before you ever climb into another.

You mentioned priming with the throttle. Do you really believe that? What does a throttle do? Think on that for a while before you answer. Remember the throttle is an air valve. Not a fuel valve. Take it from there.

 

[Stepclimb]

Priming aircraft engines

You mentioned priming with the throttle. Do you really believe that? What does a throttle do? Think on that for a while before you answer. Remember the throttle is an air valve. Not a fuel valve. Take it from there.

It seems AVbug is a little right and a little wrong. 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.
Additionally, most "Fuel injected" engines are actually "Throttle body" engines. The fuel is delivered to the intake manifold just prior to the intake valve as opposed to directly into the cylinder. In some of these engines, the "Fuel servo" as it is called, delivers fuel to the intake manifold when the mixture is rich, and the electric fuel pump is running. The rate of fuel delivery is proportional to throttle position.

http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation.html

This site has some good info on operating Lycoming engines and dispells quite a few "old wives tales"

Best of luck with your quest for knowledge.
-Stepclimb

 

[DenverDude2002]

I just follow what the checklist in the POH says, and for the Piper Archer/Warrior its full rich until the engine starts then lean 1/2 way for taxi.

 

[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. Additionally, if you're going to get a fire, that's the way to do it; your fuel ends up pooling back in the carburetor air box (in an updraft configuration), not in the engine.

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.

The same is true of pressure carburetors.

 

[uwochris]

Thanks for all the responses.

Avbug,
I was always taught that the method to prime the C1 aircraft (with the continental engine) was by turning on the fuel pump and fuel primer switches, and then advancing the throttle and keeping it full open for 3-10 seconds, then back to idle. This always seemed strange to me, as I was used to the 172 method with using the mixture control and fuel pump.

If you can recommend any good reading material in regards to this topic, please share!

Anymore comments are welcome.

 

[FN FAL]

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. Additionally, if you're going to get a fire, that's the way to do it; your fuel ends up pooling back in the carburetor air box (in an updraft configuration), not in the engine.

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.

The same is true of pressure carburetors.

Flame on!

You gotta love those piston engine compartment fires. Klaxons blaring at the airport and people running around like it's December 7th, 1941, looking for for a fire extinguisher.

One time I was riding up to make a skydive and as we are taxing to the end of the runway, I notice flames are licking around the engine compartment of the airport flying club's 152. A couple of the local bone heads are sitting in the plane, fat dumb and happy with the engine running, unaware that all three sides of the fire triangle have met and are happily conducting bidness as usual in the engine compartment. The plane, the plane, the plane is on fire...we don't need no water, let the muthafukk@ burn!

I tell the jump pilot to call them on the radio and tell them they are on fire. They shut down and put it out. It was the same local boneheads that were always trying to buy a BFR from me.

We have a couple of big engined 182's at our drop zone. Four positive primer strokes and all your sky are belong to us.

No throttle pumping...please! Every time you pump the throttle on a plane engine that shouldn't be pumped, god kills a kitten. Please think of the kittens!

 

[52Vincent]

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.

Avbug,
While I completely agree with this, and understand most of the operations of the fuel system, one thing that puzzles me is the priming procedure for most fuel injected engines. The procedure usually involves something like the following (sequence may not be accurate, but it is close):

1/2" throttle, or so
fuel pump on
mixture rich until fuel flow is recognized.
mixture lean
fuel pump off
start engine
enrichen mixture

I have noticed if you do not advance the throttle a bit you will not get any fuel flow. Why does the fuel flow when the throttle is advanced, if it is an air valve?
I looked on the Lycoming site and could not find an explanation.

Thanks in advance

 

[Stepclimb]

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