Fuel Injection Systems

Before gasoline can burn in a piston engine it needs to be vaporized and mixed with oxygen in the right quantities. This process is done by either a carburetor or by an high pressure injection system. For this process to be as perfect as it can be the system needs to take into account, power setting, mixture control and such.

Piston powered aircraft engines can use either a carburetor or a modern injection system, our previous page talked about the basic float carburetor, here we will dive into the fuel injection system used on high performance aircraft engines, both gas and diesels; although these system are not alike.

These injection system do not suffer from ice formation on the throttle valve and have a number of other advantages warranting their higher cost of installation.

Operating Principles

With an injection system fuel is pressurized and introduced just ahead of the inlet ports, directly in to the combustion chamber (GDI, gasoline direct injection) or at the super/turbocharger impeller. Normal carburetion uses a pressure differential to vaporize the fuel before it enters the cylinders.

There are two types of fuel injection systems in use, these are continuous flow and direct injection. We will describe both of them here.

Continuous Flow

This system provides a continuous flow of pressurized fuel at each inlet port of the cylinders and basically it uses the following parts:

• Injection pump, this is a positive displacement vane type pump driven by the engine. Feeds excess fuel, which is returned to the tank in use, to maintain a positive pressure under all circumstances. Fuel is delivered in liquid form, vapor is separated and has a bypass check valve so that an electric pump can deliver fuel for starting the engine.
• Fuel metering unit, Filters the fuel and sets the mixture ratio. The cockpit mixture knob is connected to this unit and leans the fuel. The cockpit throttle is also connected and regulates the butterfly controlling the air flow to the manifold.
• Manifold valve, distributes the fuel to all cylinders and you will see it on top of the engine with stainless steel tubing running to the inlet ports of the cylinders. It resembles a spider with its legs over the engine.
• Injector nozzles, these inject pressurized metered fuel of the exact right quantity and ratio with air. The design is such that it favors improved fuel atomization for good combustion and power developed by the engine.

Super- and turbocharged engines can also use a fuel injection system but they will need modifications to adjust the fuel flow with rapid throttle openings combined with air pressure sensors in the manifold. Introducing more air should mean more fuel (and power) if the engine is to run smoothly without faltering when opening the throttle.

Direct Injection

With this system fuel is pressurized and injected directly into the combustion chamber bypassing the inlet valve, much like a diesel engine but with the two spark plugs. It is used on higher powered engines mainly.

This system uses almost the same parts as described above with the continuous flow fuel system with the only difference the point where the fuel is being injected.

Fuel injection Dis / Advantages

As with any system there are always two sides to the picture. Fuel injection can provide each cylinder with the correct fuel air mixture for the operating conditions at that particular time where carburetors supply the same fuel/air mixture to all cylinders at once. This could mean that one cylinder could run cool where others run possibly hot. Which is why you need CHT sensors on all cylinders.

Summarizing the advantages:

• No more refrigeration ice
• Higher engine efficiency, lower consumption
• Improved fuel air mixture / ratio
• Uniform delivery to all cylinders
• Smoother running engines
• Easier starting

Some disadvantages are:

• Possible vapor lock in the steel fuel lines above the engine, making it difficult to start a hot engine
• Fuel must be cleaner as the part are like fine clockwork
• The need for fuel return lines to the tank in use, or a separate header tank
• Fuel injection systems are a bit more expensive compared to carburetor engines

To conclude: fuel injection systems maybe more expensive initially but they will save fuel and prolong engine life thus saving on maintenance cost and increase engine reliability and safety for those involved in the flight. As such the advantages are something to think about!