Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. The exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake and exhaust valves may also be opened during positive power at various times to recirculate gases for improved emissions.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle.
In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams can make it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide variable valve actuation and incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid trigger valve and/or a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston, and correspondingly, the engine valve.
Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system. Lost motion systems of the prior art have accordingly not been variable such that they may assume more than one length during a single cam lobe motion, or even during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
The lost motion system and method of the present invention may be particularly useful in engines requiring variable valve actuation for positive power, engine braking valve events (such as, for example, compression release and bleeder braking), and exhaust gas recirculation valve events. Each of the foregoing events not only make the engine operate, but also relieve the high pressures and temperatures that occur in the engine. If left uncontrolled, which may occur with the failure of a lost motion system, positive power, engine braking, and/or exhaust/intake gas recirculation could result in pressure or temperature damage to an engine. Moreover, if the exhaust valve should fail to open during positive power, the “exhaust” stroke will actually be an engine compression stroke starting with an abnormally high cylinder pressure. The peak cylinder pressures produced could be substantially higher than the structural limits of the engine. Therefore, it may be beneficial to have a lost motion system which is capable of providing exhaust valve events should the lost motion system fail.
The present invention provides such a system. The present invention is a valve actuation system capable of automatically and virtually immediately providing at least a portion of a desired engine valve event should the lost motion system fail.
One advantage of an embodiment of the present invention is that it may provide a system and method for actuating at least one engine valve should a primary variable valve actuation system fail to operate.
Another advantage of an embodiment of the present invention is that it may provide a system and method for actuating at least one engine valve should a primary variable valve actuation system be turned off.
Another advantage of an embodiment of the present invention is that it may provide a system and method for automatically actuating at least one engine valve using an auxiliary system should a primary variable valve actuation system fail to operate.
Another advantage of an embodiment of the present invention is that it may provide a system and method for virtually immediately actuating at least one engine valve using an auxiliary system should a primary variable valve actuation system fail to operate.
Additional advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.