Engine brake function

General

The engine brake being used is a so-called decompression brake system.

The braking effect is generated in the process according to the following principle:

Shortly after the beginning of the compression stroke, when the piston moves upward in the direction of the top dead center (TDC), one of the two exhaust valves is opened for a short time on the respective cylinder. Therefore exhaust flows out of the exhaust manifold back into the cylinder due to the head pressure. As a result of this, the compression pressure is increased and the upward moving piston located in the compression stroke is braked.

Shortly before the end of the compression stroke, the exhaust valve is opened again for a brief period. Now, part of the compression pressure is removed. As a result the piston is less rapidly accelerated in the direction of the BDC in its next operating cycle.

Operating conditions

The engine brake can be activated under the following conditions:

 Vehicle in overrun mode, i.e. accelerator and clutch pedal not actuated

 Engine rpm > 1000 rpm

 Anti-lock braking system (ABS) not in closed-loop operation

G14.15-3121-79

W14.15-1136-71

The brake cams (6) have two peaks with which the brake rocker arms (3) are actuated and by which the exhaust valves (7) can be opened.

 Peak 1 (17) opens the exhaust valve (7) for a short time shortly before the start of the compression stroke.

 Peak 2 (18) opens the exhaust valve (7) for a short time shortly before the end of the compression stroke.

W14.15-1140-81

G14.15-3122-79

In normal driving mode, if the engine brake is not activated, the engine brake solenoid valve, stage 1 (Y624) and the engine brake solenoid valve, stage 2 (Y625) are not actuated.

This way, no oil pressure can get out of the oil circuit via the oil ducts of the exhaust rocker shaft (10) to the exhaust rocker arms with hydroelement (2). The oil pressure present causes the piston (8) contained therein to be pressed by the spring (9) onto its downstop.

The respective brake rocker arm (3) does shift on the respective brake cams (6) but comes to nothing due to the missing frictional connection to piston (8).

Both exhaust valves (7) are therefore opened or closed as normal according to the cycles of the working cycle.

G14.15-3123-79

In brake stage I, the braking power is provided by cylinders 1 to 3.

The engine management (MCM) control unit (A4) actuates the engine brake solenoid valve, stage 1 (Y624) as soon as the driver activates brake stage 1 via the engine brake switch.

Oil is now pressed out of the oil circuit via the oil duct for cylinders 1 to 3 (14) of the exhaust rocker shaft (10) into the exhaust rocker arms with hydroelement (2) of cylinders 1 to 3.

In the exhaust rocker arms with hydroelement (2), oil first flows through the check valve (16) in flow direction and thus the space under the piston (8) is filled. If there is no longer a gap between the piston (8) and brake rocker arm (3), the check valve (16) is closed by the now accumulating back pressure.

In this way a frictional connection is made between the brake rocker arm (3) and the piston (8). The respective exhaust valve (7) is now opened through the downward movement of the respective brake rocker arm (3).

If brake stage I is deactivated, the solenoid valve, stage 1 (Y624) is no longer actuated. The adjacent oil pressure in the exhaust rocker arms with hydroelement (2) and the exhaust rocker shaft (10) breaks down.

The pistons (8) are pushed back to the downstop by spring force. The missing frictional connection between the brake rocker arm (3) and the exhaust rocker arm with hydroelement (2) means that the downward movement of the brake rocker arm (3) is no longer transferred to the exhaust valve (7).

G14.15-3124-79

In brake stage II the braking power is provided by all cylinders.

As soon as the driver activates brake stage 2 via the engine brake switch, the engine management (MCM) control unit (A4) actuates not only the engine brake solenoid valve, stage 1 (Y624), but also the engine brake solenoid valve, stage 2 (Y625).

Oil is now pressed out of the oil circuit via the oil duct for cylinders 1 to 3 (14) and oil duct for cylinders 4 to 6 (15) of the exhaust rocker shaft (10) in every exhaust rocker arm with hydroelement (2).

In the exhaust rocker arms with hydroelement (2), oil first flows through the check valve (16) in flow direction and thus the space under the piston (8) is filled. If there is no longer a gap between the piston (8) and brake rocker arm (3), the check valve (16) is closed by the now accumulating back pressure.

In this way a frictional connection is made between the brake rocker arm (3) and the piston (8). The respective exhaust valve is now opened through the downward movement of the respective brake rocker arm (3).

If brake stage 2 is deactivated, the engine brake solenoid valve, stage 1 (Y624) and the engine brake solenoid valve, stage 2 (Y625) are no longer actuated. The adjacent oil pressure in the exhaust rocker arms with hydroelement (2) and the exhaust rocker shaft (10) breaks down.

The pistons (8) are pushed back to the downstop by spring force. The missing frictional connection between the brake rocker arm (3) and the exhaust rocker arm with hydroelement (2) means that the downward movement of the brake rocker arm (3) is no longer transferred to the exhaust valve (7).

W14.15-1137-79

In brake stage III the brake power is reached by all cylinders and through an additional increase in the internal pressure of the cylinder.

As soon as the driver activates brake stage III via the engine brake switch, the same processes as for brake stage II are initially triggered. In addition to the now fully switched-in decompression brake also the wastegate valve on the ATL (17) as well as the exhaust gas recirculation positioner (Y621) are actuated.

As a result, a situation is achieved where the internal pressure in the cylinder is increased during the compression stroke, which results in a higher braking torque.

The engine management (MCM) control unit (A4) uses the boost pressure and the exhaust gas turbocharger speed as controlled variables.

Additional functions of the engine brake

Providing support during shift operations

The engine brake is automatically activated for all normal shift operations without the driver having to do anything. Activation of the engine brake significantly lowers engine speed between the individual shift operations, thus achieving quicker synchronization, which, in turn, leads to quicker vehicle acceleration.

The engine management (MCM) control unit (A4) uses the following parameters as a controlled variable:

 Oil temperature

 Differential speed

 Boost pressure

The brake stages I to III are activated based on these.

Supporting stopping the engine

If the ignition is switched off and the injected fuel quantity = 0, the engine brake is automatically activated without any action on the part of the driver. The opening of the exhaust valves strongly reduces the compression so that the engine can "slowly come to a halt" and is not shifted between two ignition TDCs by the compression.

This function is effective only for oil temperatures below approx. 70 °C. In the case of a warm engine it is, therefore, possible that the engine noticeably "surges" when switching off.

Performance feature of a standard and high performance system

The engine brake is offered in two different power categories. The hardware of the two systems is identical.

The differences lie in the software - both systems are fitted with a different code-controlled data record.

Engine brake, standard system

 100 kW at 1,300 rpm

 300 kW at 2,300 rpm

Engine brake, high-performance system

 150 kW at 1300 rpm

 400 kW at 2,300 rpm

The high-performance system functions according to the same mechanical/thermodynamic principle as the standard system. There are, however, parameters in the engine management (MCM) control unit (A4) which ensure that the internal pressure is increased in all cylinders in all engine brake stages over actuation of the wastegate on the turbocharger.

The exhaust gas recirculation positioner (Y621) is also actuated if necessary whereby the fill level of the cylinder is increased again. The result of this is: the piston must do more work to compress the air - the braking torque increases.