Speed control driving, function

 The rpm control driving mode function is described by way of example on engine 926.9 (6 cylinder).

G07.15-A029-79

General

If the drive control (CPC) control unit does not demand any idle or working speed control then this means normal driving mode. In driving mode the injection quantity is primarily adjusted according to requests from the driver. This means that the torque specification of the engine is primarily dependent on the position of the accelerator pedal. It is only in certain situations that the engine speed can be adapted independently of the accelerator pedal position or of the specified torque request, for example, for downshifting for rotational speed comparison between the main shaft and the countershaft.

Function

During vehicle operation, the engine management (MR) control unit therefore initially compares the target engine torque incoming from the drive control (CPC) control unit, which is derived, among other things, from the position of the accelerator pedal and provided via drive train CAN, with the current actual engine torque, which represents the engine's actual operating conditions.

If the requested target engine torque is less than the current actual engine torque, the engine management (MR) control unit reduces the injection quantity. If the requested engine specified torque is greater than the current engine actual torque, then it increases the injection quantity.

The engine management (MR) control unit constantly balances the actual engine torque with the target engine torque. While this matching process is continuing, the engine management (FR) control unit adjusts the start of injection in order to balance out the ignition delay which can have a negative effect, particularly at higher engine speeds, and to maintain the pollutant emissions as low as possible.

If the target engine torque is negative, such as in overrun mode, and the engine speed is above 950 rpm, the engine management (MR) control unit interrupts actuation of the single injection pumps for cylinders 1 to 6. In this way the braking effect of the engine is used and the fuel consumption lowered. The engine brake can be activated to reinforce the braking effect.

If the engine speed falls below 950 rpm, the engine management (MR) control unit actuates the single injection pumps for cylinders 1 to 6 again.

There is also a specified engine torque correction taking place in the engine management (MR) control unit, which takes account of the actual mass of fuel. The engine management (MR) control unit uses the fuel temperature to do this.

 The engine management (MR) control unit can limit the target engine torque from the drive control (CPC) control unit on engine protection grounds if the coolant temperature exceeds a certain value or the engine reaches its end or maximum governed rpm, for example. Limitation of the required engine torque always leads to limitation of the injection quantity.