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Priming

SigTauPilot

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I am up for my multi-engine checkride in the Beechcraft Duchess, and i am curious as to why the priming system only primes the 1,2, and 4 cylinder. Why not the 3rd? I would appreciate all the help, Thanks!
 

bigD

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I was told that each cylinder has only one port, and the EGT is what uses the port on the #3 cylinder.
 

tarp

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Ditto.

Lycoming O-360's are drilled with one port in each Cyl. Head. Three are used for priming. The warmest one Cyl # 3 is usually reserved for a CHT probe if installed on your airplane.

I think EGT probes are usually found in the Exhaust stacks. Sorry.
 

bigD

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Eek - DUH that's what I meant. I really should proofread before hitting the "Submit Reply" button!
 

SigTauPilot

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"Tarp", thanks for the clarification, much appreciated. Now i must find if it has a CHT probe and what that does.
 

FlyinBrian

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A CHT probe is just a probe that measures the cylinder head temperature. I'm pretty sure that everyone here is correct about the Duchess having one on number three. If you're unsure just ask a mechanic to show you. They know that pilots are dumb, so you won't be changing his opinion of you. You can really learn a lot about systems by hanging out at a maintanence hangar.
 

Data

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Ask any mechanic if it is always the #3 cylinder and you might be surprised. It depends on who wired it so to speak. The CHT can be put in any port and most of the time you'll find that it's not the same one (#3) on both engines. Another reason you don't want all 4 cylinders being primed is if you flood it.
 

Hovernut

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CHT???

Unless y'all's Lycoming engines are WAY different from our Seminoles', I think you may be mis-informed about the placement of the CHT.

I've wondered about this for some time, too, and upon consulting our mx staff, they said that in fact all 4 can be primed, and in some instances in northern climates, all 4 are! ...for those cold starts...

Our Seminoles have the Lycoming O-360-F1A6 engines and the manifold has two ports at the cyl head (one upper and one lower). The lower ports are where the prime lines are plumbed, and the upper is plumbed for the MANIFOLD PRESSURE GAGE in one cyl, while the others are plugged. Typically, the manifold press. gage is located @ the cyl that is the furthest from the carburetor....that'll be the one that experiences the greatest amount of headloss (pressure drop) due to the length of manifold between it and the carb.

I may have to clear something else up, too...the prime and manifold pressure taps into the intake manifold, not the cylinder head itself. They tap into the intake manifold just upstream of the intake valve. A lot of folks think the prime goes directly
into the cylinder itself.

The CHT probe is threaded into the meat of the cylinder head itself, typically on the bottom of the cylinder head. There is a well drilled out in the head so the probe's thermocouple gets in contact with the metal that is very close to the combustion chamber. I believe ours are mounted on one of the aft cylinders maybe because they run hotter being behind the warm air flow from the front cyls, and possibly a reduced air flow. I'm not sure...

One more thing. Our local feds at a recent safety seminar said they much prefer starting engines with prime instead of the accelerator pump. I started doing this on the old seminoles with pump primers and found most of the primers to be dry due to inactivity! The new seminoles use the electric fuel pump and solenoid valves to prime. Engine starts have been much nicer lately!

I hope I cleared some things up....now another pet peeve...the governor...uh, maybe next time... 1:1 gov-prop
 
Last edited:

avbug

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Hovernut,

Most horizontally reciprocating aircraft engines accept prime directly in the cylinder, rather than the manifold. Many series of engines are able to be carbureted or fuel injected and provision is available to place the line directly into the cylinder head. In the case of injected engines, the injectors are used to "prime" the engine, and use the same tapped holes for injectors that carbureted versions of the same engine use for priming ports.

Manifold pressure isn't lost between the carburetor and the cylinder on a normally aspirated engine, unless there is a leak in the induction system. The cylinder isn't pressurized by the carb or intake; it's sucking air. If anything, manifold pressure will be lowest when operated at the cylinder. There is no "head" pressure to be lost. Manifold pressure is affected by throttle plate position. Closing the throttle is akin to placing one's hand over the hose on a vacum cleaner; pressure in the hose drops. Same for the engine. Measuring the pressure in the hose will be the same at the vacum end of the hose as the end where your hand is unless there is an internal leak in the hose.

In most cases, manifold pressure is taken directly from the manifold itself at a comon juncture before branching to the various cylinder or delivery tubes.

"Priming" isn't done with an accelerator pump. This is a fallacy and an improper technique made popular by instructors who don't know any better. This does nothing to prime the engine, especially in an updraft carburetor setup. Use of the throttle in this manner demonstrates a lack of understanding of the purpose of the accelerator pump, and it's use. More than that, often I've seen pilots pumping away madly on the throttle because that's what they were taught...on aircraft that don't even have accelerator pumps.

On the subject of prime, one should always be careful to ensure that a plunger type primer is secured before takeoff; backing out in flight can cause an engine failure in some cases, as can application of the loaded primer.
 

skeezer

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I have a priming question.

I used to fly a C182Rg with the 235HP carbureted Lycoming. Anyway, in the before starting checklist in the POH it said to pump the throttle to prime the engine.... wtf?

The aircraft had a pump primer so that is what I always used to start the engine. I never saw the point in pumping the throttle to prime the engine because it was an updraft carb. The engine always started fine the way I did it.

Was Cessna just looney with their priming procedure in the POH, or was I doing a bad thing?

Thanks for the help.

Skeezer
 

tarp

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Hover,

I stand by my answer for older O-360 engines found on the Cherokee 180's, Archers, Seminoles (at least up to 1979 which is the manual I own), etc. I have no idea what extra "piping" may be on the 2000 model Seminoles.

Skeezer,

The reference to "pumping" the throttle forward twice is to activate what many call the "accelerator pump" inside a carbureted engine. The accelerator pump is just a simple valve that allows a little bit of gasoline into the throttle body (venturi) of the carburetor without the aid of the vacuum pressure from the engine.

A lot of folks advocate using this as a "primer", however it can be dangerous in the event of a backfire. Since you have now provided a ready fuel source so high in the system, you can literally set your air filter on fire or worse if the engine backfires.

Generally, two pumps of the throttle on a warm engine without any other "prime" will be plenty of fuel to get the engine to turn over. The danger comes in when people keep pumping and pumping loading up the carb with fuel.

Follow the manufacturers suggested starting procedures but always use the "cold", "warm" and "hot" engine sections of these starting procedures as they are written.
 

DC9stick

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When I flew the Twin Beech, The real Twin Beech P&W 985 tailwheel, When the weather got chilly we pumped the throttle as fast as we could while cranking the engine, a large puddle of fuel would form under the engine and sooner or sometimes later the engine would backfire setting the puddle on FIRE! By the time you got out of the seat crawled out the window ran down the wing opened the cargo door grabbed the extinguisher and put out the fire the engine had been preheated from the bottom and would then start. On really cold days you repeated the preheating process until succesful or the battery was dead.

I have witnessed numerous pilots cranking and pumping the throttles on FUEL INJECTED engines, this does nothing except cause the butterfly valve in the injection unit to flap open and shut and is seldom sucessful.
 
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