Fuel Injection Systems

6.fuel-injection-systems. Fuel Injection Systems

A fuel injection system delivers fuel directly into each cylinder's intake port (or, in some designs, into the intake manifold near each cylinder) rather than mixing it with air in a carburetor upstream of the induction system. Most reciprocating aircraft engines used in general aviation training fleets that are fuel injected use a continuous-flow, port injection system (such as the Bendix RSA or Continental TCM systems). These systems are mechanically driven and meter fuel continuously while the engine is running — unlike automotive systems, they are not timed or pulsed to individual intake strokes.

Major Components

• Engine-driven fuel pump — Provides fuel under pressure to the fuel/air control unit. A separate electric auxiliary (boost) pump is provided for engine start, takeoff, landing, high-altitude operation, and as a backup if the engine-driven pump fails.
• Fuel/air control unit (fuel servo or throttle/mixture control body) — Located at the induction air inlet. It meters fuel in proportion to airflow through the throttle body and to the position of the mixture control.
• Fuel manifold valve (flow divider/spider) — Mounted on top of the engine. It receives metered fuel from the control unit and distributes it equally to each cylinder through individual fuel lines.
• Fuel discharge nozzles — One per cylinder, threaded into the cylinder head near the intake port. Each nozzle sprays a continuous stream of atomized fuel into the intake air just before it enters the combustion chamber.
• Fuel pressure (or fuel flow) gauge — Displays manifold pressure at the flow divider, which is calibrated to indicate fuel flow in gallons per hour (gph) or pounds per hour (pph).

Advantages of Fuel Injection

• More uniform fuel distribution to all cylinders, producing smoother operation and more even cylinder head temperatures.
• Improved specific fuel consumption when leaned properly.
• Better engine response to throttle changes.
• Easier cold-weather starting because fuel is delivered directly to the cylinder.
• No carburetor icing in the venturi sense — there is no venturi where fuel vaporizes and cools the air below freezing. Induction system icing from impact ice on the air filter is still possible.

Disadvantages of Fuel Injection

• Vapor lock susceptibility, particularly after shutdown on a hot day. Fuel lines retain heat from the engine, vaporizing fuel in the lines and making restart difficult (hot start procedure required).
• Difficulty restarting a hot engine — Standard procedure typically involves the mixture at idle cutoff, throttle cracked, auxiliary fuel pump on briefly to purge vapor, then a normal start with mixture advanced as the engine fires.
• Greater susceptibility to flooding if priming or hot-start procedures are mishandled.
• More expensive components and tighter tolerances than a carburetor.

Operating Principles

The pilot controls fuel flow through two cockpit levers: the throttle, which sets airflow through the throttle body, and the mixture control, which sets the fuel-to-air ratio at the fuel servo. Air entering the throttle body creates a pressure signal proportional to airflow; the fuel servo uses this signal, together with regulated fuel pressure from the engine-driven pump, to meter fuel to the flow divider. From the flow divider, equal fuel flow goes to each cylinder's discharge nozzle.

Because the system is continuous flow, there is no accelerator pump as on a carburetor; instead, smooth throttle movement is important to avoid lean stumble. Leaning in cruise is accomplished by referencing the fuel flow gauge or, more precisely, an exhaust gas temperature (EGT) gauge — leaning to peak EGT or a specified number of degrees rich of peak (ROP) or lean of peak (LOP) per the POH.

Priming and Starting (Typical Cold Start)

1. Mixture — RICH.
2. Throttle — open approximately 1/4 inch.
3. Auxiliary fuel pump — ON until fuel flow indication stabilizes (commonly 3-5 seconds).
4. Auxiliary fuel pump — OFF.
5. Mixture — IDLE CUTOFF.
6. Starter — ENGAGE; as the engine fires, advance mixture to RICH.

Hot Start (Typical)

1. Mixture — IDLE CUTOFF.
2. Throttle — cracked open.
3. Auxiliary fuel pump — ON briefly to purge vapor, then OFF.
4. Starter — ENGAGE; advance mixture to RICH as the engine fires.

Always follow the specific procedures in the POH/AFM for the airplane being flown; injection system designs vary, and Continental and Lycoming engines have noticeably different hot-start techniques.

Induction Icing Considerations

Fuel-injected engines are not subject to refrigeration (venturi) icing, so there is no carburetor heat control. However, an alternate air source is provided. If the induction air filter becomes blocked by impact ice or other debris, alternate air either opens automatically (spring-loaded door) or is selected manually by the pilot, drawing unfiltered air from inside the engine compartment.