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2 Answers

What`s in the intake manifold?

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Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot

 

My textbook says that the MAP gauge measures the absoulte pressure of the fuel/air mixture whitin the intake manifold. But the diagram I have shows that there is only air in the intake manifold, not fuel/air mixture.

Im inclined to think that it`s only air pressure at the intake manifod, otherwise if we have a mechanical MAP gauge it will mean that a mix of fuel and air are actually going into the instrument case.

Is it possible to have an aircraft with constant speed prop and a carburetor as well?

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2 Answers



  1. John D. Collins on Dec 30, 2012

    To answer your last question first, yes it is possible for an aircraft with a constant speed prop to also have a carburetor as well. A few prominent examples include the C182 and the early versions of the Bonanza. With a carburetor, air and fuel are mixed in the carburetor and distributed to the cylinders thru the intake manifold. With a fuel injection system, air is delivered to the cylinders thru the intake manifold and fuel is injected into the cylinders just above where the intake valve sits. When the intake valve is open, the air from the intake manifold (runner) and the injected fuel into the intake chamber are sucked into the cylinder as the piston moves down during the intake stroke. Since the intake valve is only open during the intake stroke and the injectors continue to spew fuel above the valve in the intake chamber, a portion of the time, fuel will be drawn into the intake manifold by the low pressure of the air passing by in the intake manifold. Depending on the geometry of the intake manifold, some fuel is bound to mixed with the air and be delivered to other than the intended cylinder. The TCM IO470/IO520/IO550 family of engines are notorious for this mal-distribution and cause uneven fuel distribution to the cylinders. This uneven fuel distribution doesn’t cause rough running when the engine is operated rich of peak EGT (rich mixtures), but does cause rough running when the engine is operated lean of peak EGT (lean mixtures). A company called Gamijectors provides tuned injectors that permit smooth operation of these engines when operated lean of peak and this permits the pilot the option of saving a significant amount of fuel. I operate my IO520 in my Bonanza fuel injected engine with Gamijectors routinely at 12.5 gallons per hour using lean of peak operation at 65% power verses using a richer mixture that is rich of peak and uses over 14 gallons per hour for the same power.

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  2. Robert Gruber on Jun 11, 2013

    Good question regarding the MAP gauge.No you will not have a mixture of fuel and air inside the instrument case since the intake manifold is under negative pressure due to the suction created by the pistons during the intake stroke. So in a worst case scenario ,say you decide to smash the instrument with a hammer in flight ,the engine would now run lean due to additional air being sucked in from the cabin. Remember that a MAP is calibrated to read inches of mercury which tells you how much negative(suction) pressure is being produced in the intake. It is NOT calibrated to read PSI (pounds per square inch) which would be used to show positive pressure. A tire is under positive pressure so the air would leak out if you puncture it. The intake manifold is under negative pressure so if you puncture it air would be sucked into it.

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