Induction, carburation, fuel metering and mixture control
Induction, carburation, fuel metering and mixture control
This section tests how air enters the engine, how fuel is metered and how mixture is set for power, economy and cooling. Expect carburettor circuits, fuel-injection differences and mixture/EGT reasoning.
Core rules and concepts
- A float carburettor uses a venturi to create a pressure drop that draws fuel through a main jet; the float chamber maintains a stable fuel level. Idle, accelerator-pump and economizer circuits cover off-design operation.
- The mixture control changes fuel flow relative to airflow. Lean mixture reduces fuel consumption but raises combustion temperature; rich mixture aids power and cooling at high load.
- At altitude, lower air density reduces the fuel demand; lean as required, but avoid lean settings at high power. Carburettor ice can form in the venturi and throttle area, so carburettor heat is applied when indicated.
- Fuel injection meters fuel directly to each cylinder; it avoids carburettor icing but needs precise nozzle and linkage adjustment.
- EGT peaks near the stoichiometric region; rich-of-peak is cooler, and lean-of-peak is also cooler but closer to misfire and valve damage.
Worked example
A 200 hp engine has a brake specific fuel consumption of 0.50 lb/hp/h. Fuel flow = 200 × 0.50 = 100 lb/h. At 6.0 lb/gal, that is 100 ÷ 6.0 = 16.7 gal/h. If the gauge shows 12 gal/h at full rich and full power, the mixture or metering is suspect.
Traps
Do not treat carburettor heat as a leaning control-it enriches the mixture; do not lean aggressively at high power; and do not confuse mixture setting with idle speed adjustment.