Transcript Slide 1
Engine Brake VEB+ Engine Brake VEB+ VEB+ is the name of the Volvo engine braking system and is a further development of the VEB engine brake. Engine brake VEB+ consists of two interacting braking systems: the exhaust regulator or EPG (Exhaust Pressure Governor) and the compression brake or VCB (Volvo Compression Brake). The exhaust regulator builds up an exhaust back pressure that increases the effect of the compression brake. The braking effect of the compression brake is achieved by valve opening. The exhaust valve opens during the inlet stroke and also just before TDC during the compression stroke, so the braking force of the compression stroke is harnessed. Mechanically this is achieved by a special camshaft equipped with an extra cam with two cam lobes, a brake rocker arm and a control valve. The control valve which regulates the oil pressure to the rocker arm shaft is situated on the cylinder head between cylinders 3 and 4. The inlet is connected to the lubricating oil channel in the engine block and the outlet is connected to the rocker arm shaft. If the engine is equipped with VEB+ this is shown on the front engine identification label on the left side of the valve cover. Compared to the previous engine brake the mechanical stresses are now divided between two rocker arms instead of one which makes it possible to increase the braking force without reaching maximum mechanical stresses. With VEB+ the gas flow increases and the thermal stresses on the fuel injection nozzles decrease since the increased flow cools the nozzles. Camshaft and exhaust rocker arms The camshaft (1) on an engine with VEB+ has four cams for each cylinder: inlet cam, injection cam, exhaust cam and brake cam. The brake cam has two cam lobes, a charging cam lobe (3) and a decompression cam lobe (4), which opens the exhaust valves and eliminates the power of the combustion stroke. The braking force of the compression stroke can thus be used to the maximum. Each exhaust rocker arm (5) has two pistons, the pump piston (7) and the power piston (14), one check valve (6), a piston (10) and a spring (13). The power piston is located straight above the exhaust valve calliper and when the piston is affected the calliper is pressed down and opens the exhaust valves. The pump piston is located directly below the brake rocker arm and the pump piston can be operated by the brake rocker arm (9). The exhaust rocker arm (5) and the brake rocker arm (9) are designed so that the brake rocker arm is positioned over the exhaust rocker arm and when oil pressure reaches oil system pressure the clearance between the rocker arms decreases and the brake rocker can press down the pump piston. The pump and the power piston are connected with an oil channel and when the pump piston is pressed down the oil beneath the piston is pressed forward to the power piston. At the same time the check valve closes and the high pressure results in depression of the power piston and the exhaust valves opens. Exhaust Pressure Governor, EPG The exhaust pressure regulator (called EPG in the front cover engine identification label on the rocker cover) is placed in direct connection with the turbine housing of the turbocharger, and it has two tasks: 1. When idling, the engine is kept warm by creating back pressure in the exhaust manifold. 2. When the accelerator pedal is released it acts as an exhaust brake. The exhaust pressure regulator consists of a shutter housing, a shutter and a pneumatic cylinder. The illustration above shows schematically the path of the exhaust gases when the shutter is closed. The illustration also shows the butterfly shutter which too is closed. Compressed air is taken from the vehicle’s pneumatic system and is controlled by an air valve called the AVU valve (Air Valve Unit) which is situated on the engine’s right side just below the AC pump. The valve is of PWM (Pulse Width Modulated) type and consists basically of a solenoid valve, an air valve and a printed circuit board. It regulates the pressure continuously within the 0.5-7.5 bar range. The valve has a built-in reduction valve which releases different pressures for the various amounts of braking power. Regulation is achieved by a PWM signal from the engine electronic control unit via the connection. To meet the varying pneumatic needs of the exhaust brake the engine electronic control unit sends a PWM signal to the AVU valve where the signal pulse width (as a percentage) determines the output air pressure to the pressure cylinder. The AVU valve does not receive electrical current during normal driving, the shutter is completely open and the exhaust gases are free to pass. The AVU valve has three exits and in addition to the EPG also regulates compressed air to the actuator of the turbocharger and to the butterfly air cylinder.