The engine brake function is described using engine 926.9 (6 cylinders) as an example.
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Shown on engine 926.9 |
A3 | Drive control (CPC) control unit | Y602 | Cylinder 3 single injection pump | |
A6 | Engine management (MR) control unit | Y603 | Cylinder 4 single injection pump | |
B15 | Crankshaft position sensor | Y604 | Cylinder 5 single injection pump | |
B16 | Camshaft position sensor | Y605 | Cylinder 6 single injection pump | |
CAN 4 | Drive train CAN | Y617 | Pneumatic constant throttle solenoid valve | |
Y49 | Hydraulic constant throttle solenoid valve (only for 6 cylinder engines) | A | Input signals | |
Y600 | Cylinder 1 single injection pump | B | Output signals | |
Y601 | Cylinder 2 single injection pump |
Function requirements
Vehicle in deceleration mode (driving and clutch pedal are not actuated)
Engine speed above 900 rpm
To prevent the engine from being switched off inadvertently, the constant throttle and the engine brake flap (8) are automatically switched off at an engine speed of less than 900 rpm.
The anti-lock braking system (ABS) is not in closed-loop operation.
Engine oil temperature above a defined threshold (only for 6 cylinder engines)
General
The engine brake used is a constant throttle in the cylinder heads and an engine brake flap (8) downstream of the exhaust gas turbocharger.
Function
The constant throttles are throttle valves (2) that are integrated in the cylinder heads. When open, the throttle valves (2) create an additional connection between the combustion chamber and the exhaust port and thereby increase the engine braking power. When the engine brake is switched on, the throttle valves (2) are opened and the engine brake flap (8) closed all at the same time.
6 cylinder engine
Through the actuation of the hydraulic constant throttle solenoid valve, the throttle valves (2) in the cylinder head are opened hydraulically. The hydraulic constant throttle solenoid valve opens a feed hole to the control line of the constant throttle pistons (1) which then open the throttle valves (2) using the oil pressure of the engine.
Through the actuation of the pneumatic constant throttle solenoid valve, the engine brake flap (8) is closed by means of a pneumatic engine brake cylinder (7).
4 cylinder engine
Through the actuation of the pneumatic constant throttle solenoid valve, the throttle valves (2) in the cylinder head are pneumatically opened and the engine brake flap (8) closed via the pneumatic engine brake cylinder (7) all at the same time.
The request to switch on the engine brake is sent by the drive control (CPC) control unit to the engine management (MR) control unit via the drive train CAN.
The engine management (MR) control unit takes over the actuation of the hydraulic constant throttle solenoid valve and the drive control (CPC) control unit the actuation of the pneumatic constant throttle solenoid valve.
Illustration of the principle of the constant throttle, shown on cylinder 1 |
1 | Constant throttle piston |
2 | Throttle valve |
3 | Outlet valve |
4 | Inlet valves |
5 | Injection nozzle |
6 | Piston |
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On the compression stroke (2nd cycle), only a small amount of air escapes through the throttle valves (2) into the exhaust port during the quick movement of the piston (6) from the bottom to the top reversal point. As a result, the desired compression work is achieved.
During the brief standstill of the piston (6) at top dead center, the compressed air escapes, for the most part, through the throttle valve (2) into the exhaust port.
Thus, the pressure on the piston (6) is significantly lower in the 3rd cycle. The usable difference between compression and expansion work is therefore considerably larger and, as a result, so is the engine braking power. When the constant throttle is activated, the actuation of the single injection pumps for cylinders 1 to 6 is interrupted and the fuel is delivered directly to the fuel return.
Illustration of the principle of the engine brake flap (8) |
7 | Engine brake cylinder |
8 | Engine brake valve |
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When the engine brake is actuated, the actuation of the single injection pumps for cylinders 1 to 6 is interrupted, the engine brake cylinder (7) is pressurized with compressed air and the engine brake flap (8) is closed. The engine braking effect is increased as a result.
Providing support during shift operations
The engine brake is automatically activated for all normal upshifting operations without the driver having to do anything. Activation of the engine brake reduces the engine speed between the individual shift operations more quickly. This results in faster synchronization, in turn leading to faster vehicle acceleration.
The engine management (MR) control unit uses the following parameters as controlled variables:
Boost pressure
Differential speed
Supporting stopping the engine
If the ignition is switched off and the injected fuel quantity = 0, the engine brake is automatically activated. The compression is strongly reduced through opening the throttle valve (2) so the engine can "slowly come to a halt" and not be shifted between two ignition TDCs by the compression. This function is effective at oil temperatures below about 80 °C. In the case of a warm engine it is, therefore, possible that the engine noticeably "surges" when switching off.