Piston Engines - Fuel Injection — Page 127, Lesson 160

Let's begin with indirect fuel injection. This is the system you'll find on many aircraft piston engines, and the key thing to understand right away is that it's a low-pressure, continuous-flow type. That means fuel isn't squirted in bursts timed to the engine cycle — it's sprayed continuously into the induction pipe, as close to the inlet valve as possible. That continuous spray is what gives the method its name. Now, why do manufacturers choose this? The advantages claimed are fourfold. First, low operating pressure — the system doesn't need high fuel pressures. Second, good fuel distribution — because the fuel is sprayed continuously, it mixes evenly with the incoming air. Third, freedom from icing problems — this is a big one; carburettor icing is a real hazard, and injecting fuel downstream avoids the cooling that causes ice to form. And fourth, the ability to use a pump which does not have to be timed to the operating cycle — the pump just delivers a steady flow; it doesn't need to synchronise with the pistons or valves. Now, some fuel injection systems work on a similar principle to the carburettor, but they inject fuel under pressure into the intake. The indirect system we're looking at does exactly that. Let me walk you through how the mixture is controlled, because this is where the system's logic lives. The air throttle metering valve varies the pressure of the fuel according to engine speed. So as you open the throttle and the engine speeds up, that valve adjusts the fuel pressure to match. Then, mixture strength — the ratio of fuel to air — is varied by a manually operated mixture control valve. That valve adjusts the fuel pressure for altitude or operating conditions as necessary. So you, the pilot, lean the mixture as you climb, and that manual valve is what does the adjusting. Here's a nice practical benefit: because of the way the injector operates, no special idling arrangements are required, and a separate priming system for engine starting is unnecessary. The continuous flow handles both of those jobs. Now, the main components. There are four of them. A fuel pump, which delivers the fuel. A fuel/air control unit, which is the brain that meters fuel against airflow. A fuel manifold (distribution) valve, which takes the fuel and distributes it evenly to each cylinder. And discharge nozzles for each cylinder — one nozzle per cylinder, spraying the fuel into the induction pipe. In addition to those four, there's a normal throttle valve that controls airflow to the engine, and a fuel pressure gauge fitted so you can make mixture adjustments — you read the pressure and adjust the mixture control valve accordingly. Let me show you the general arrangement so you can see how these parts sit together. That's the layout. The system itself is illustrated in Figure 10.2, which shows the full indirect fuel injection system with all the components in place. So, to tie it together: the fuel pump delivers fuel, the fuel/air control unit meters it, the manifold valve distributes it, and the nozzles spray it continuously into the induction pipe near the inlet valve. The throttle valve controls airflow, the air throttle metering valve adjusts fuel pressure with engine speed, and the mixture control valve lets you lean or enrich for altitude. Low pressure, continuous flow, no icing, no priming, no special idle — that's the indirect system in a nutshell.