Torque control: General
Engine torque control is used to ensure the correct engine torque is supplied by the engine. ECM controls engine torque by regulating the air mass per combustion (cylinders' capacity) and also by regulating the ignition timing.
The maximum engine torque allowed for the engine is stored as a table in the control module memory. The table specifies the highest torque the engine is allowed to generate at various engine speeds.
Engine torque is primarily calculated using air mass per combustion and engine speed. But, ignition timing and current lambda value also affect the torque calculation. Retarded ignition timing generates lower engine torque. This is the case in knock control. As regards the lambda value, there is an optimal lambda value for each operating point. If the engine runs richer or leaner than the optimal value, engine torque reduces.
Note that a lambda value lower than the optimal value (richer mixture) produces lower torque.
The calculation also takes the engine's own torque consumption, i.e. internal friction, into consideration. The following values are used for this calculation:
- Air mass
- Engine speed
- Coolant temperature
- Idling torque
Idle speed control is used to regulate the engine torque so that the balance between the torque developed by the engine and the torque required to keep the engine and its auxiliary equipment running is maintained.
The nominal idling speed of a warm engine is 650 rpm. The engine speed is higher just after starting, especially if the engine is cold. Nominal idling speed also depends on coolant temperature.
Idle speed control is active when the accelerator pedal is released and the vehicle speed is 0.
Several different functions, whether they be external or internal control module functions, can request engine torque. This request can be either positive or negative (limitation). ECM combines these sometimes opposing requirements.
The following functions can request positive engine torque. The highest value is selected and sent on to "limitations."
- Pedal request (from driver)
- Cruise control
- TCS/ESP
- Idle speed control
- Compensation for A/C, generator
- Active surge dampening
But it is not always that the requested engine torque can be attained. This can be due to the maximum permissible engine torque for the engine speed in question has already been attained or that TCS/ESP has requested a reduction in torque. The following functions can limit engine torque:
- Maximum permissible engine torque
- TCM
- Manual gearbox
- Engine protection function
- Brake
- Safety-critical fault in throttle control.
- Misfiring
- Emission limitation
- Active surge dampening
- Maximum permissible vehicle speed
Once the engine has been started, all torque requests are combined into a joint driving torque request. When starting the engine, there is a starting torque request that is selected by upon start of the engine. The result - a starting or driving torque request - is sent on to be converted to an air mass request.
Two potentiometers are integrated in the accelerator pedal bracket. Potentiometer 1 provides ECM with information on the driver's torque request in the form of a voltage signal 0-5V.
Potentiometer 2 is a safety potentiometer that provides ECM with information on the pedal position in the form of a voltage signal 0-2.5V. This voltage is used to ensure that information from potentiometer 1 is reliable.
If the information from the potentiometers does not agree, one of them must be malfunctioning. The engine throttle control will then go into limp-home mode.
Torque can be requested from this function in order to keep the specified engine torque independent from the prevailing load from the A/C compressor and generator. Consequently, this is a positive request.
The maximum engine torque allowed for the engine is stored as a table in the control module memory. The table specifies the highest torque the engine is allowed to generate at various engine speeds.
The highest value is chosen from among the functions that can limit torque, e.g. accelerator pedal 400 Nm. This value is sent on to the function for limitations, at which time the lowest value is selected. It is usually "MAXIMUM PERMISSIBLE ENGINE TORQUE " that is the limiting function. This value may change as it is dependent on engine variant. Let's say the value is 350 Nm. The driver requests 400 Nm via the accelerator pedal, but the system only allows 350 Nm for this engine variant. The value 350 Nm is the one chosen by the system.
If the engine begins to knock due to poor fuel, "KNOCK CONTROL " limits torque to 300 Nm, for example, to prevent engine damage. In this case "KNOCK CONTROL " is lower than "maximum permissible torque", thus "knock control is selected.
The system's torque request is sent on to the sub-functions:
- Throttle adjustment
- Ignition
- Turbo adjustment
- CVCP control