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Electronic Engine Management Pt.11: Fuel & Air Systems – by Julian Edgar Electronic engine managed cars run fuel and air induction systems that are completely different from those in cars with carburettors. In an engine managed car, fuel is pumped in a circuit from the tank to the fuel injector supply rail and then back to the tank again. This continuous flow of fuel around the system keeps it cool to avoid vaporisation and means that a supply of high-pressure fuel is available whenever it’s needed. Fuel supply components The fuel supply system can be di- vided up into a number of sections, starting with the fuel pump: (1). Fuel Pump: fuel pumps in EFI systems are of the high pres­sure, roller-cell type. Because this type of pump works poorly without a head of fuel, it is located either below the fuel level or is primed by a second, low-pressure pump located in the tank. The pump’s electric motor is cooled by the fuel flowing through it and the pump is protected against outlet line blockage by a pressure release valve. Another valve, which is a non-return type, is located on the outlet side of the pump. Fuel injectors come in a variety of shapes & forms but are all basically electric solenoid valves. 14  Silicon Chip The fuel pressure in Bosch systems is usually held at 250kPa (36psi), although the pump is capable of up to 500kPa (72psi). (2). Fuel Damper: following the fuel pump in some cars is a small inline damper, comprising an internal chamber and a coil-spring diaphragm. As the pump operates, it generates a pulsing effect in the fuel pressure. High volume pulses will deflect the diaphragm in the fuel damper, temporarily increasing the size of the inter­nal chamber and so absorbing the pressure spike. Conversely, momentary drops in pressure cause the diaphragm to move into the chamber, thereby maintaining the output pressure at a constant value. As well as reducing minor pressure fluctuations, the damper can also lower pump noise. (3). Fuel Filter: a large, metal-encased fuel filter follows the fuel damper. This filter may be located close to the fuel tank or can be situated in the engine bay. Particles down to a size of 10 microns (.0001mm) are trapped by the filter to ensure that the fuel injector nozzles aren’t blocked. (4). Fuel Rail and Injectors: fuel flows from the filter to the fuel rail, where the individual injectors take their feed. The fuel rail is of a larger internal diameter than the fuel line from the tank. This is to ensure that as each injector operates, there is still sufficient fuel in reserve to prevent fuel pressure regulator con­trols the return flow to the tank and so maintains the fuel-rail pressure. As Fig.4 shows, this device is divided by a diaphragm into fuel and air chambers, and has a vacuum line to the manifold plenum chamber. Rather than maintain the fuel at a constant pressure above atmospheric pressure, the fuel is kept at a con­stant pressure above that found in the manifold. The injectors are located so that they squirt fuel immediately behind the inlet valves. Note the large volume square cross-section fuel rail above them. pressure variations occurring from injector to injector. The injec­tors can either be held in place by collars and bolts, or by the fuel rail itself. The role of the fuel injectors is critical – they have to be able to respond very quickly (1-1.5ms typical opening time), produce a well-atomised spray, and be durable in the extremes of tempera­ture and vibration under the bonnet. A fuel injector is basically an electrical solenoid valve. It consists of a gauze filter on the inlet, a solenoid winding around the armature, a needle valve and the electrical connec­ tion. The coil resistance varies with different designs, with four ohms resistance being typical. In general, the lower the coil resistance, the faster the response-time of the injector. (5). Cold-Start Injector: some cars, especially those with rela­tively early EFI systems, have an additional injector mounted on the plenum chamber. This injector is activated by the electronic control module during cold starts and enriches the mixture. More recent EFI systems simply run longer injector pulse widths to provide the extra fuel needed. (6). Fuel Pressure Regulator: the Because fuel injectors only have a very small opening lift, they can easily become blocked. To prevent this from happening, the fuel filter blocks particles down to .0001mm diameter. Fig.1: a typical multi-point fuel injection system. August 1994  15 A single cold start injector (see above) is used on some systems to ensure satisfactory running when the engine is cold. It fires into the plenum chamber to enrich the starting mixture. The fuel pressure regulator (right) maintains the fuel at a constant pressure relative to the manifold pressure. The actual fuel pressure constantly varies, however. This means that when an injector opens for a certain length of time, the same amount of fuel will flow irrespective of wheth­er the manifold pressure is high or low. If this weren’t the case, then manifold pressure variations would cause unwanted changes in the fuel injection quantity. Air induction systems Because fuel is added to the airstream just before the engine inlet Fig.2: this diagram shows the in-tank fuel pump & fuel level sender unit used in the Subaru Liberty. 16  Silicon Chip valves in the majority of EFI cars, the air induc­tion system can be designed almost solely for greatest airflow. In carburettor designs, the inlet manifold has to be kept short and sometimes heated to prevent fuel droplets from Fig.3: the fuel injectors are positioned so that they spray fuel into the intake port immediately behind the intake valves. This photo shows a modified Subaru Liberty Turbo air intake system. At the bottom left is a new fabricated intake duct to the air-filter box. The airflow meter is located just behind this box & the airflow then passes through a rightangled duct to the shiny section of pipe, which was made to replace a silencing resonator volume. The right-angled rubber bend then takes the induction air to the turbo inlet. These modifications reduced the pre-turbo intake pressure drop by 40%, with a commensurate in­crease in performance. forming on its walls. In addition, the throat size needs to be restricted so that intake gas velocities remain high Fig.4: a fuel pressure regulator consists of a fuel chamber & an air chamber with a vacuum line to the intake manifold. The manifold pressure controls a diaphragm that separates the two cham­bers (Holden VL Commodore). in all driving conditions. In EFI cars by contrast, resonant tuning of long intake runners is employed and the ducts can be sized to provide the lowest pressure drop at full load. (1). Intake Silencers: the combustion air is generally drawn from outside the engine bay to avoid the induction of hot air. In some cars, it then passes into a silencing volume, often comprising a plastic box located under the mudguard. A duct then carries the air to the air-filter box. (2). Air Filter: the air filters used in modern cars are general­ly flat corrugated paper element types. They are located in plastic boxes positioned to one side of the engine bay. The volume of the air-filter box is sometimes part of the intake resonant tuning which is employed to gain better cylinder fill­ing. In addition, the air-filter box also generally acts as an additional intake silencer. This intake air silencer volume is from a Subaru Liberty & is located inside the mudguard. (3). Airflow Meter: in cars not employing a MAP sensor, the air­flow meter follows the air filter. The airflow meter can be of the vane, vortex or hot-wire type. August 1994  17 Right: This view shows a typical 4-cylinder intake manifold. The plenum chamber is at the top, while below it are the individual cylinder runners. The throttle body is a simple butterfly valve which controls the airflow into the engine. Fig.5: the fuel pressure is typically regulated so that it is 250kPa greater than the manifold pressure. This ensures that the fuel injectors deliver a constant amount of fuel for a given pulse length, regardless of manifold pressure variations. This photo of a modified Mazda rotary engine clearly shows the throttle body, with the plenum chamber located behind it & the intake port runners in the foreground. Note that this racing engine does not use an air bypass valve & so the throttle blade is set so that it is slightly ajar at idle. 18  Silicon Chip (4). Throttle Body: the throttle body is the main butterfly controlling the airflow into the engine. The vast majority of throttle butterflies are controlled by a cable linkage to the accelerator pedal, although some exotic cars now run ‘drive-by-wire’ arrangements. A bypass passage is often built into the throttle body, with an adjustment screw to vary idle speed. (5). Auxiliary Air Valve: when the engine is cold it needs more fuel and air to idle satisfactorily than it does when warm. The extra fuel is supplied by either a wider injector pulse width or by a cold-start injector. The extra air is provided by the aux­iliary air control valve (sometimes called the bypass air control valve). Its function is to allow intake air to bypass the throt­tle body butterfly. These valves can be either controlled mechani­ cally by engine coolant temperature or can be electrically pulsed or otherwise controlled by the ECM. This valve can also be used – in conjunction with ignition timing control – to give a constant idle speed, irrespective of engine loads like the air-conditioner. In some cars, additional air-bypass valves perform the idle-speed control function, with the auxiliary air valve used only during warm-up. (6). Plenum Chamber and Inlet Runners: following the throttle body, the air enters a plenum (or surge) chamber, before flowing to the inlet valves through long runners. The res- The powerful Nissan Skyline GT-R twin turbo sports car uses a huge log-type intake manifold & six throttle butterflies. Fig.6: a typical air intake system for a fuel injected car. Fig.7: typical air intake flow diagram for a fuel-injected, turbocharged & intercooled engine (Subaru Liberty). onant intake tuning is related to the length and diameter of the individual cylinder runners and the volume of the plenum chamber. Tests have indicated that an increase in maximum engine torque of as much as 25% can be gained by appropriate tuning of this system. Some engines, notably those with four valves per cylinder, use two intake runners for each cylinder. The two sets of intake ducts are of different lengths and are activated by ECM-controlled butterfly valves located within the induction system. Sophisti­cated twin Helmholtz resonance intake systems SC are used on some cars. August 1994  19