1. Field of the Invention
The present invention relates to compression release retarding or braking systems for internal combustion engines. More particularly, the present invention relates to those retarding systems in which an engine valve is opened at a particular time in the engine cycle to release the compressive energy within the engine cylinder associated with the valve and retard or brake engine operation.
2. Description of the Prior Art
In known engine brakes or retarders of the compression release type, compression in an engine cylinder is released by opening the cylinder exhaust valve(s) when the piston within the cylinder is at or near the top dead center (TDC) of a compression stroke. The engine then acts essentially as an air compressor due to the release of compression, the energy expended by the engine on the compression stroke being lost. This loss of energy converts the engine from a power source into a "brake" having the retarding power approaching the power generating capacity of the engine.
Ideally, a compression release brake should release all of the compressed air in a cylinder at the end of its compressive stroke, thereby dissipating the maximum amount of compressive energy. An ideal brake would also allow the exhaust valve to close almost immediately after compression release so that air is not re-ingested through the open exhaust valve. In known engine brakes such as disclosed in U.S. Pat. No. 4,706,624 of which I am a co-inventor, the release of compressive energy from a cylinder has been accomplished by hydraulically actuating a slave piston within the brake to force an exhaust valve open. The pressurized hydraulic fluid used to actuate the slave piston is generated by a "master" piston and can be delivered either directly or indirectly to the slave piston.
Directly displacing the slave piston of one cylinder with a master piston has several disadvantages. In such a direct displacement system, the master and slave pistons are connected by a hydraulic circuit so that any displacement of fluid by the master piston displaces the slave piston. The slave piston associated with one engine cylinder is advantageously displaced by a master piston, the lifting cam and pushrod associated with the intake or exhaust valve of a certain other cylinder. The timing and motion of the directly displaced slave piston, however, depart substantially from the ideal. Displacement of the master piston by the comparatively slow rise and long dwell of an exhaust or intake cam dictates that the slave piston start to open the exhaust valve well before TDC of the compressive stroke in order that maximum displacement of the slave piston occur close to TDC. On the other hand, if a mechanical fuel injection system is used on the engine, it is preferable that the master piston be displaced by the fuel injector cam and pushrod of the cylinder with which the slave piston is associated because the lifting motion builds relatively quickly to a maximum near TDC, the approximate time at which the cylinder should be decompressd. The fuel injector cam, however, and mechanical fuel injectors are not found on all engines.
Indirectly displacing the slave piston by a master piston overcomes some of the above disadvantages associated with direct displacement. In an indirect system, the master piston supplies the high pressure hydraulic fluid to an accumulator and then triggers release of the accumulated hydraulic fluid to the slave piston at the appropriate time. Such a braking system is the type disclosed in my U.S. Pat. No. 4,706.624 referenced above. In the patent, an accumulator or plenum containing hydraulic fluid, typically lubrication oil, exerts pressure on one side of a "free" piston. The other side of the free piston is connected via passageways to both the master and slave pistons. The passageway between the free and the slave piston is normally closed by a trigger valve so that there is no direct connection between the master and slave pistons. The initial travel of the master piston forces fluid against the free piston. When the force exerted by the master piston on the one side of the free piston exceeds the opposing force exerted by the plenum fluid on the other side plus a small spring force, the free piston is displaced towards the plenum causing the plenum pressure to rise. At this point, the trigger valve prevents any pressure from reaching the slave piston. After travelling a predetermined distance, the master piston opens the trigger valve and allows a volume of fluid displaced by the motion of the free piston to be discharged to the slave piston. Discharge of sufficient high pressure fluid displaces the slave piston against the exhaust valve and opens the valve.
However, low plenum pressure will not cause a discharge of fluid sufficient to displace the slave piston. The discharged hydraulic fluid is then channeled into the plenum to increase the plenum pressure. Over several engine cycles, the pressure within the plenum increases until a sufficient operative level is reached.
Using the accumulated high pressure fluid and the master piston as a trigger allows an exhaust valve to be opened and closed almost instantaneously at any time. The rapid opening of the exhaust valve approaches the ideal compression release engine brake as discussed above.
In known indirect slave piston displacement systems, a one-piece master piston is used to open the trigger valve. The trigger valve must be opened at the exact time near TDC when the slave piston is to be displaced. The length of the master piston as well as the lash or clearance with the pushrod displacing the master piston must be adjusted and maintained so that the trigger point occurs at the proper time. Due to the inherent tolerances of engine components, it is impossible to determine in advance at what point the pushrod will have moved the master piston far enough for the trigger valve to be allowed to open. As a result the length of the master piston can only be determined and adjusted after the engine brake has been installed on the engine and the pushrod clearance or lash is known. Adjustment of the master piston length is thus necessary.
To ensure that the exhaust valves are opened a correct distance, the lash between the slave piston and the valve crosshead must also be adjusted at installation. Component wear and changes in settings further require that the master and slave piston be adjusted at regular intervals to maintain correct triggering point and lash.
Use of a free piston in conjunctin with a master piston also means that the pressure release is determined solely by the fluid displaced by the master piston travel prior to the trigger valve opening. If insufficient fluid is displaced by the master piston due to wear or misadjustment, the slave piston stoke decreases and braking action diminishes.
Accordingly, a general object of the present invention is to provide a compression release retarding system or engine brake which constitutes an improvement over the prior art.
Another object of the present invention is to provide a compression release retarding system which can be installed on an engine without the need for post-installation adjustment of the master piston.
A more specific object of the present invention is to provide a compression release retarding system in which a slave piston establishes and maintains zero lash clearance with the valve actuating mechanism in the braking mode.
Another object is to provide a compression release retarding system in which the total distance the slave piston can travel per engine cycle is regulated.
Still another object is to provide a compression release retarding system in which the slave piston is prevented from keeping the valves open after the engine cylinder has been decompressed.
It is still another object of this invention to provide a compression release retarding system in which the volume of hydraulic fluid released for the purpose of opening the exhaust valves need not be equal to the volume of fluid accumulated by the master piston travel.