a) Field of the Invention
The present invention relates generally to engine retarders and, more particularly, to a system and method for supercharging or precharging an engine brake mounted to a diesel engine.
b) Description of Related Art
The engine of a vehicle is often used as an auxiliary brake to retard vehicle speed. This particularly applies to heavy vehicles, such as trucks and buses, whose developments over recent years have resulted in engines of much greater power with unchanged cylinder volume. As a result, the average speed at which such vehicles are driven uphill has increased considerably, creating a need for greater braking power when driving downhill. In an exhaust brake, some form of throttle valve is incorporated in the exhaust system to achieve improved retarding power. This power, however, is relatively low and often less than half the driving power of the engine. In an engine brake, on the other hand, the exhaust valves are hydraulically opened near top dead center of the compression, thus absorbing power.
Furthermore, the resistance of such heavy vehicles to driving has decreased over recent years, meaning that the wheel or service brakes of the vehicles are subjected to greater loads. When driving in hilly terrain, the wheel brakes should be used as little as possible, primarily for safety reasons. The average speed of the vehicle in hilly terrain is therefore greatly influenced by the available engine retarding power, which increases the requirement for a more effective retarding that will also be capable of reducing wear and tear on the wheel brakes and thereby improve running economy.
When free-running, a four-stroke diesel engine, i.e. when the wheels of the vehicle drive the engine, a certain braking effect occurs as a result of the internal resistance in the engine, i.e., due to friction. This braking effect is relatively small, however, and has been further reduced in modern engines.
A well known method of improving the engine retarding power is to mount a throttle device, for instance a butterfly valve, in the exhaust system. When the valve is closed, an overpressure is generated in the exhaust system which causes the work to increase during the exhaust stroke, with a commensurate increase in retarding power. In this manner, retarding power can be increased by placing the combustion chamber in the cylinder in communication with the exhaust system during the latter part of the exhaust stroke and during a smaller or greater part of the intake stroke.
Alternatively, with an engine brake, the cylinder is placed into communication with the exhaust system at the end of the compression stroke and somewhat during the expansion stroke. This can be achieved, either by opening the conventional exhaust valve or with the aid of a separate valve. As a result, air compressed in the combustion chamber during the compression stroke will flow partially into the exhaust system, meaning that a large part of the compression work carried out during the compression stroke is not recovered during the expansion stroke, therewith increasing the retarding power. One known arrangement for carrying out this method utilizes the conventional exhaust valve. The exhaust valve operating cam is provided with an additional cam lobe which is operative to achieve the additional opening of the exhaust valve. The extent to which the exhaust valve is lifted by this additional lobe is relatively slight, and when the engine is used as a power source the valve clearance is sufficiently large to render the additional lobe inoperative. When braking vehicle speed with the aid of the engine, a hydraulic valve-clearance adjuster is brought into operation, such as to reduce the valve clearance, therewith bringing the additional lobe into operation The extent to which the exhaust valve is lifted during the conventional valve-opening sequence will at the same time be correspondingly greater, however, and this must be taken into account so that problems will not occur by impact of the exhaust valve against the piston.
It has been known that increasing boost pressure of the intake manifold increases retarding power with an engine compression release brake. Engaging a mechanical supercharger when retarding is a costly and complicated means to achieve increased boost. Likewise, adopting a supercharger with a variable area turbine is another costly and complicated means to achieve more boost.
When an exhaust brake is used in conjunction with an engine brake, some of the high pressure in the exhaust manifold may be tansferred under appropriate conditions to the cylinder to provide boost pressure. However, this type of combination brake creates increased exhaust manifold pressure by restricting the airflow through the engine, resulting in increased temperature in the exhaust valve, piston and nozzle components. The increase in critical component temperature and the corresponding increase in boost pressure must be limited to avoid injector nozzle needle seat and exhaust valve seat softening and wear.
The need therefore exists for an improved system and method to provide engine braking without suffering from the foregoing drawbacks.