is however most accurate for the SAAB 900. Fuel Fuel injector valves The fuel injector valves are of a solenoid type with needle and seat. They are opened by a current flowing through the injector's coil and are closed by a strong spring when the current is switched off. To ensure as optimal combustion as possible and with that lower exhaust emission the injectors are equipped with four holes, which gives a good distribution of the fuel. The squirts of fuel are very exact positioned (two jets on the backside on each inlet valve). This put very high demands on the fixation of the injectors. To secure this fixation the injectors are fixed in pairs by a special retainer between cylinders 1 – 2 and 3 – 4. The injectors are electrically supplied from the main relay, while the ECU grounds the injectors. Fuel injection Pre-injection When the ignition is switched on, the main relay and fuel pump relay are activated during a few seconds. As soon as the ECU gets the cranking signal (from the crankshaft sensor) it initiates a coolant temperature dependent fuel injection with all four injectors simultaneously which ensures a fast engine start. If the engine is started and shortly after is switched off a new pre-injection is initiated after the ignition has been switched off for 45 seconds. Calculating of injection time To decide how much fuel needs to be injected into each intake runner the ECU calculates the air mass that had been drawn into the cylinder. The calculation makes use of the cylinder volume (the B204 engine has a displacement of 0.5 litres per cylinder). That cylinder volume holds equal amount of air which has a density and thus a certain mass. The air density is calculated using the absolute pressure and temperature in the intake manifold. The air mass for combustion has now been calculated and that value is divided by 14.7 (stoichiometric relation for gasoline mass to air mass) to determine the required fuel mass for each combustion to inject. Since the flow capacity of the injector and the density of the fuel (pre programmed values) are known, the ECU can calculate the duration of the injection. Using the oxygen sensor 1 the injection duration is corrected so stoichiometric combustion is obtained. When hard acceleration occurs, the lambda correction is masked and Wide Open Throttle (WOT) enrichment occurs for maximum performance.