System Operation

Fuel injection in the Saab Trionic is sequential, which means that fuel injection follows the engine's firing order.

The fuel injection time is calculated by the control module chiefly on the basis of the following information:
- pressure in the intake manifold
- intake air temperature
- engine speed
- oxygen content of the exhaust gases
- any knocking
- throttle position

Pre-injection
When the ignition switch is turned to the Drive position, the main and fuel pump relays operate. At the same time, all the injectors open to inject a small amount fuel onto the intake valves. The open time is determined by the engine coolant temperature. The aim is to achieve a shorter starting time.





When the ignition switch is turned ON, both the main and fuel pump relays operate for a few seconds. After fuel pressure has built up, fuel injection dependant on engine coolant temperature takes place with the four fuel injectors simultaneously. This means that short starting times are achieved.

If the engine is not started, this function is blocked for 15 minutes. If the engine is started and is switched OFF a short time afterwards, another pre-ignition is obtained after waiting for at least 45 seconds with the ignition turned OFF.





Calculation of Fuel Injection Time
To determine how much fuel has to be injected into each intake manifold, the ECM calculates the air mass drawn into each cylinder.

The calculation is made as follows: (each cylinder in the engine has a capacity of 0.5 |):
0.5 | air with a certain density has a particular weight. The density is calculated using the pressure and temperature in the intake manifold.

The ECM keeps the injection valve open until the correct amount of fuel has been injected in relation to the measured air mass.

The fuel injection time is corrected by means of the oxygen sensor so that lambda = 1.0 is obtained. If the throttle is wide open, the lambda control is disconnected and the fuel/air mixture is made richer at full load for maximum performance. When the throttle is opened, a richer mixture is produced during acceleration and a leaner mixture is produced during deceleration. When starting from cold and during engine warm up before lambda control has been activated, a richer mixture is produced depending on engine coolant temperature.

If the engine has warmed up and battery voltage is normal, the fuel injection times vary between approx. 2.5 ms during idling and approx. 18 ms at wide open throttle.





Fuel Shut Off
With the throttle completely closed and an engine speed above 1800 rpm, in 3rd, 4th and 5th gear, fuel shut off takes place after a certain delay (about 1 second).

On cars with automatic transmission, fuel shut off takes place in all gears.

The injectors are re-activated when the engine speed drops below 1400 rpm.

Fuel Consumption
The lead from the ECM to injector 3 is also connected to the main instrument. The main instrument calculated the fuel consumption from the length of injector pulses. Fuel consumption is used to obtain an accurate picture of the average fuel consumption in the Saab Information Display (SID).