Reciprocating Engines can be up to 15 percent of the Powerplant test, and its calculation questions are among the most predictable. This guide covers the definitions and arithmetic from chapter 1 of FAA-H-8083-32B.
Valve timing
- The intake valve closes about 50 to 75 degrees after bottom dead center so the momentum of the incoming gases charges the cylinder more fully.
- The exhaust valve opens before bottom dead center so residual pressure forces the gases out quickly and carries waste heat away.
- Valve overlap is intake valve lead combined with exhaust valve lag. It helps cool the cylinder internally, increases the charge inducted and aids scavenging.
Pre-ignition and detonation
Pre-ignition is combustion that begins before the timed spark, typically from an incandescent deposit or hot spot. Detonation is spontaneous combustion of the unburned charge after the spark has ignited it.
Worked example: compression ratio
Compression ratio compares the cylinder volume with the piston at the bottom of its stroke to the volume with the piston at the top. A cylinder holding 144 cubic inches at the bottom and 18 cubic inches at the top has a compression ratio of 144 / 18 = 8, or 8:1.
Worked example: indicated horsepower
Indicated horsepower = PLANK / 33,000, where P is indicated mean effective pressure, L is stroke in feet, A is piston area, N is the number of power strokes per minute in each cylinder and K is the number of cylinders. For a six-cylinder engine with 120 psi, a stroke of 0.5 foot, a 20 square inch piston and 1,200 power strokes per minute: (120 x 0.5 x 20 x 1,200 x 6) / 33,000 = 8,640,000 / 33,000, about 262 hp.
Worked example: mechanical efficiency
Mechanical efficiency is brake horsepower divided by indicated horsepower. An engine that develops 200 indicated horsepower and delivers 170 brake horsepower is 170 / 200 = 0.85, or 85 percent efficient. The 30 hp difference is friction horsepower.
Worked example: specific fuel consumption
For a reciprocating engine, brake specific fuel consumption is fuel flow in pounds per hour divided by brake horsepower. An engine that develops 150 brake horsepower and burns 60 pounds of fuel per hour consumes 60 / 150 = 0.4 pound per bhp per hour.
Volumetric efficiency
Part-throttle operation, long small-diameter intake pipes and sharp bends in the induction system all reduce the charge drawn in and lower the volumetric efficiency of a naturally aspirated engine.
Radial engine facts
- On a single-row radial the odd-numbered cylinders fire in order, then the even-numbered: 1-3-5-7-9-2-4-6-8 for nine cylinders.
- Each row has one master rod and the rest are articulated rods, so an 18-cylinder, double-row radial has two master rods and 16 articulated rods.
- To clear a hydraulic lock in a lower cylinder, remove a spark plug and pull the propeller through in the direction of rotation.
Practice FAA Aviation Mechanic Powerplant questions with explanations and timed mock tests.