Piston Engines - Fuel Injection — Page 127, Lesson 163

We’re starting a new topic now: fuel injection for piston engines, and specifically the diesel engine’s fuel-supply system. Let me walk you through this. First, the fuel-supply system in terms of storage is similar to that of a conventional aircraft. In a light aircraft, the wing tanks—which are rigid—store the bulk of the fuel. That fuel is then sent, using the effects of gravity and ram air, to a common strainer, then a selector-valve, and then via water-traps, fuel heaters, and filters, to an engine-mounted delivery system. So the path is: wing tanks, gravity and ram air push the fuel, common strainer, selector-valve, water-traps, fuel heaters, filters, and finally the engine-mounted delivery system. Now, delivery to the cylinders can be done in many ways, but the preferred system is called ‘common rail’. In a common rail system, a high-pressure supply—1800 bar, which is 26,000 psi—is maintained locally, right next to the cylinders. That’s the key idea: you keep a reservoir of very high-pressure fuel close to where it’s needed. Then we have the FADEC, which stands for Full Authority Digital Engine Control. It’s an electronic, computer-controlled, dual-channel unit. The FADEC opens electronically operated ‘nozzles’ at the appropriate time and duration, according to demand. So the FADEC decides when to inject and for how long. Let me show you the general arrangement of an indirect fuel injection system. Now, in a common rail system, the old-style distributor injection pump is replaced by a single extremely high-pressure pump—2000 bar, or 29,000 psi—that feeds a single storage manifold known as the Common Rail. So the common rail is that storage manifold. It distributes the high-pressure fuel to computer-controlled injector valves. Each injector valve is activated by either a solenoid, or, more recently, by piezoelectric actuators. A solenoid is an electromagnetic device; a piezoelectric actuator uses a crystal that changes shape when voltage is applied. Here’s the common rail system diagram. In modern aircraft such as the DA40, both the timing and the fuel quantity per injection are under the control of the FADEC. The FADEC receives data from various sources, such as air temperature, air density, and throttle position. So it takes those inputs and decides the injection timing and quantity. The combination of the high-tech injector-valves and computer control leads to greater fuel efficiency and more effective power management. That’s the core of the common rail diesel fuel injection system.