Hydraulically-actuated fail-safe stroke-limiting piston

Details Hydraulically-actuated fail-safe stroke-limiting piston Hydraulically-actuated fail-safe stroke-limiting piston

1999-08-19

2001-08-14

Solis, Erick (Department: 3747)

Internal-combustion engines

Engine speed regulator

Responsive to deceleration mode

C123S090120, C123S090460

Reexamination Certificate

active

06273057

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to compression release brakes for internal combustion engines. In particular, the present invention is directed to a high-pressure, decompression braking system with high-speed actuation and means that allows independent adjustment of the slave piston lash and the maximum slave piston stroke.
BACKGROUND OF THE INVENTION
Compression release-type engine brakes are well-known in the art. Engine brakes or retarders are designed to temporarily convert an internal combustion engine of either the spark ignition or compression ignition type into an air compressor. The fundamental braking power is achieved by preventing fuel injection during the compression stroke of a piston, compressing the captured air mass, and releasing the compressed air at or near a top-dead-center position of a piston into an exhaust manifold. The energy expended in compression release braking systems is controlled, for the most part, by the volume of gas compressed, the timing of the release of the gas into the exhaust manifold and the amount of gas released. A compression release brake decreases the kinetic energy of an engine by opposing the upward motion of the engine's pistons on the compression stroke. As a piston travels upward on its compression upstroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. When the piston nears the top of its stroke, an exhaust valve is opened to “release” the compressed gases. The pressure having been released from the cylinder, the piston cannot recapture the energy stored in the compressed gases on the subsequent expansion downstroke. In so doing, the engine develops retarding power to help slow down the vehicle. This provides the operator with increased control over the vehicle.
A properly designed and adjusted compression release-type engine retarder can develop retarding power that is a substantial portion of the power developed by the engine on positive power. Compression release-type retarders of this type supplement the braking capacity of the primary vehicle wheel braking system. In so doing, these retarders may substantially extend the life of the primary wheel braking system of the vehicle.
The basic design of a compression release type engine retarding system is disclosed in U.S. Pat. No. 3,220,392 to Cummins, which is incorporated herein by reference. The compression release-type engine brake disclosed in the Cummins patent employs a hydraulic control system to operate the exhaust valves to effect the compression release event. The hydraulic control system engages the engine's existing valve actuation system, namely, the rocker arms of the engine.
When the engine is operating under positive power, the hydraulic control system of the compression release retarder is disengaged from the valve control system, so that no compression release event occurs. When compression release retarding is desired, the engine is deprived of fuel and the hydraulic control system of the compression release brake engages the valve activation system of the engine. The valve activation system drives the compression release brake to produce compression release events at the appropriate times.
Typically, it is desirable to use the compression release-type engine retarder to open an engine exhaust valve as late in the engine cycle as possible. In this way, the engine develops greater compression, allowing more energy to be dissipated through the compression release retarder. Delaying the opening of the exhaust valve in the compression release event, however, may substantially increase the loading on critical engine components. The force required to open the exhaust valve during the compression release event is transmitted back through the hydraulic system.
In a compression release engine retarder it is desirable to provide accurate timing of exhaust valve opening. To this end, it is advantageous in these systems to apply sharp hydraulic pulses to the slave pistons so that they open the exhaust valves rapidly. In order to both stop the slave pistons' motion and prevent excessive opening of the associated exhaust valves, stroke-limiting mechanisms have been employed to reduce the hydraulic fluid pressure when either the hydraulic fluid pressure reaches the predetermined maximum or the slave pistons have reached the end of their desired stroke. The term stroke-limiting generally refers to modification of the forward motion of the slave piston in order to limit the total travel of the slave piston or to reduce the length of the slave motion event. The disadvantages of excessive slave piston travel include excessive exhaust valve travel and possible contact of exhaust valves with the engine piston, increased overall braking apparatus and engine height, and overtravel of the slave piston return spring.
In the present invention, Applicants have designed an innovative slave piston for a common rail, variable valve actuation system. The present invention has been designed to overcome limitations in stroke-limiting and lash adjustment design found in the prior art.
There are several categories of stroke-limiting designs for common rail variable valve actuation systems. One design relies on precise trigger valve timing to avoid valve-to-piston contact. This design may be unacceptable with respect to potential failure modes. Other designs that use hard stops with oil squeeze films may have difficulty meeting long range durability requirements, considering start-up conditions when there could be insufficient oil in the squeeze film. Still other designs that limit the stroke only by bleeding high-pressure oil behind the slave piston have excessive oil utilization and require an unacceptable increase in the capacity of the high-pressure pump. Other designs that employ a separate stroke-limiting piston in addition to the slave piston are excessively complex. Additionally, designs having an occluding orifice on the slave piston side of the stroke-limiting piston are not fail-safe with respect to degradation of the flow metering edges. Designs that cut off the flow only to the slave piston, such as U.S. Pat. No. 5,531,192 assigned to Caterpillar, are not suitable for decompression braking because valve-to-piston contact could result from entrained air or check valve failure. The present design avoids the risk of valve-to-piston contact due to entrained air or failure of a check valve.
The electronically-controlled, common rail, decompression braking system of the present invention provides variable timing of the opening of the engine exhaust valve to optimize retarding power. The present invention comprises a high-pressure common rail, high-speed electronic trigger valve, means of stroke limiting, and slave piston positioned over the engine exhaust valve or cross head. Opening the trigger valve routes high-pressure hydraulic fluid to a plenum above the slave piston, which displaces the slave piston and opens the exhaust valve. The displacement of the engine exhaust valve must be limited to avoid valve-to-piston contact. Closing the trigger valve connects the slave piston plenum to drain pressure, which causes the slave piston and the exhaust valve to close.
The present invention is directed to a stroke-limiting slave piston design in which the travel of the slave piston is limited by dropping the pressure above the slave piston to drain combined with the force of the exhaust valve and slave piston springs. Dropping the slave piston pressure to drain while the trigger valve is open is accomplished by closing the port between the slave piston plenum and the trigger valve while opening a port between the slave piston plenum and drain. The port flow areas are defined by the mating of grooves, circumferentially-arranged holes, or slots in the slave piston and slave piston cylinder. The flow area versus piston displacement characteristics are built-in and fail-safe. They are defined to provide acceptable valve overshoot, pressure spikes, and utilization of high-p

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