CNG Fuel Injection, Air/Fuel Metering, and Engine Controls
How CNG is metered
CNG engines use either port fuel injection (gas injected near the intake valve, common in bi-fuel and converted engines) or direct injection (high-pressure gas injected into the cylinder, common in dedicated OEM engines). Port injection meters gas by injector pulse width; direct injection adds a high-pressure gas pump and injectors that must handle cylinder pressure.
Air/fuel control fundamentals
Stoichiometric air/fuel ratio for methane is about 17.2:1 by mass - leaner than gasoline (~14.7:1). Candidates must know that the ECM calculates fuel mass from measured air mass (MAF) or speed-density (MAP + RPM + temperature) and commands injector duration to hit target lambda. Closed-loop control uses an oxygen sensor (wideband or switching) to trim fuel. CNG's high octane allows more spark advance and higher compression, which is why dedicated CNG engines often run higher compression ratios.
Sensors and actuators
Know MAF, MAP, IAT, ECT, throttle position, oxygen sensor, and the gas pressure/temperature sensor. The pressure sensor is critical: gas density changes with pressure and temperature, so the ECM must correct injector duration for actual gas conditions.
Traps
- Using gasoline stoichiometry for CNG calculations is a common error - remember ~17.2:1.
- A lean code on CNG may be a low gas supply pressure, not a bad O2 sensor.
- Bi-fuel vehicles have separate fuel maps; a complaint may be specific to one fuel mode.
Rule of thumb
Fuel mass = air mass ÷ target AFR. If air measurement is correct and the engine is lean, suspect fuel delivery (pressure, injector, regulator) before the sensor.