Cylinder Priming Question

P

Pilot997

New Member
Hey guys, I'm waiting on a CFI intial ride at the moment and was trying to look more in depth into my checkride aircraft's (C172-RG, Carb, not fuel injection) systems. One question I have that no one is able to provide me with an accurate answer, is the reason for priming only 3 of the 4 cylinders.

Does anyone know why the primer is for 3-cylinders only? And, if there is a specific reason, which cylinder is not primed? I've heard a few different explanations on the subject, such as:

1) The cylinder that is not primed is the location of where the measurement for the CHT is taken.

2) It's just a manufacturer design, and its cheaper to do it like that.

3) If the engine is flooded, one that one cylinder is able to allow the engine to start.

Thank you for reading and any help you can offer is greatly appreciated.
 
This question often comes up, and for airplanes with fixed pitch props, I don't think anyone has come up with a definitive answer.

If you have a constant speed propeller and the MAP gauge that goes with it, then it is easier to answer. The primer tubes go into the cylinder head just out side of the intake valve. This very same port is also used to connect the MAP gauge to the engine, so there is only room for 3 primer tubes on a 4 cylinder engine.

It has nothing to do with the CHT probe. The intake side of the head is the coolest part, most of the heat is dumped into the cylinder head by the hot exhaust gas exiting the exhaust port at a very high velocity. So measuring the temperature here would not do us much good. Most CHT probes are either a special washer that goes under one of the spark plugs, or they thread into another hole located under the bottom spark plug, which is between the intake and exhaust ports, providing more of an average temperature.
 
Ditto. I have heard that the 4th cylinder is left unprimed to leave space for a MAP sensor for aircraft using a constant speed prop. I am guessing its just a standard setup when the engine is built so the MAP sensor can be easily added (or not) if required. Obviously the engines we are talking about start fine with just three cylinders primed so it makes sense that they would just leave the 4th one without priming even with no MAP.

1) Not for CHT as ananoman explained

2) It probably is very slightly cheaper, but I doubt that is why it is done.

3) No idea
 
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