Reciprocating Engine Fuel Metering, Carburetion, and Fuel Injection
Domain 4 covers how reciprocating engines meter fuel and air, and how mixture affects temperature and power.
Carburetion
- A float carburetor uses a venturi, float chamber, main jet, idle circuit, accelerator pump, economizer, and mixture control.
- Impact ice forms in the venturi; carb heat bypasses the filter and heats air, reducing density and enriching the mixture.
- Float carburetors meter by pressure differential; altitude and temperature change the fuel/air ratio.
Fuel injection
- Continuous-flow systems meter fuel at the engine-driven pump/control unit and distribute it to nozzles.
- Air metering is by throttle and intake; fuel pressure and mixture control set flow.
- Boost pumps prevent vapor lock and supply starting fuel.
- EGT peaks near stoichiometric; leaning is done per the POH and often referenced to the richest cylinder.
Worked example
An engine produces 135 hp at 75% power with a brake specific fuel consumption of 0.45 lb/hp·hr. Fuel flow = 135 × 0.45 = 60.75 lb/hr. At 6.0 lb/gal, that is 10.1 gal/hr. The reasoning uses BSFC as fuel mass per horsepower-hour, then converts mass to volume.
Traps
Applying carb heat and not adjusting mixture; leaning at high power; assuming fuel injection eliminates vapor lock; using EGT as a cylinder head temperature gauge; ignoring induction leaks.