Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, and may also be used to produce auxiliary valve events, such as engine braking, and exhaust gas recirculation (EGR). During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be opened during positive power at various times to recirculate gases for improved emissions.
Engine valve actuation also may be used to produce engine braking and exhaust gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine 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.
Engine valve(s) may be actuated to produce compression-release braking and/or bleeder braking. The operation of a compression-release type engine brake, or retarder, is well known. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392 (Nov. 1965), which is incorporated herein by reference.
The operation of a bleeder type engine brake has also long been known. During engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke. An example of a system and method utilizing a bleeder type engine brake is provided by the disclosure of Assignee's U.S. Pat. No. 6,594,996 (Jul. 22, 2003), a copy of which is incorporated herein by reference.
The basic principles of exhaust gas recirculation (EGR) are also well known. After a properly operating engine has performed work on the combination of fuel and inlet air in its combustion chamber, the engine exhausts the remaining gas from the engine cylinder. An EGR system allows a portion of these exhaust gases to flow back into the engine cylinder. This recirculation of gases into the engine cylinder may be used during positive power operation, and/or during engine braking cycles to provide significant benefits. As used herein, EGR may include brake gas recirculation (BGR), which is the recirculation of gases during engine braking cycles.
During positive power operation, an EGR system is primarily used to improve engine emissions. During engine positive power, one or more intake valves may be opened to admit fuel and air from the atmosphere, which contains the oxygen required to burn the fuel in the cylinder. The air, however, also contains a large quantity of nitrogen. The high temperature found within the engine cylinder causes the nitrogen to react with any unused oxygen and form nitrogen oxides (NOx). Nitrogen oxides are one of the main pollutants emitted by diesel engines. The recirculated gases provided by an EGR system have already been used by the engine and contain only a small amount of oxygen. By mixing these gases with fresh air, the amount of oxygen entering the engine may be reduced and fewer nitrogen oxides may be formed. In addition, the recirculated gases may have the effect of lowering the combustion temperature in the engine cylinder below the point at which nitrogen combines with oxygen to form NOx. As a result, EGR systems may work to reduce the amount of NOx produced and to improve engine emissions. Current environmental standards for diesel engines, as well as proposed regulations, in the United States and other countries indicate that the need for improved emissions will only become more important in the future.
An EGR system may also be used to optimize retarding power during engine braking operation. As discussed above, during engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. By controlling the pressure and temperature in the engine using EGR, the level of braking may be optimized at various operating conditions.
In many internal combustion engines, the engine intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle drivability may be obtained if the intake and exhaust valve timing and lift can be varied. The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift to optimize them for various engine operating conditions.
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.
Proper control of the engine valve lift and actuation timing when utilizing a lost motion system may improve engine performance and reliability during engine braking, positive power, and/or EGR/BGR operation. For example, during engine braking, the main exhaust event may experience an added valve lift because lash in the system may be taken up. This added valve lift may create an increased overlap between the main exhaust event and the main intake event, and cause excess exhaust gases to flow back into the cylinder and into the intake manifold. This result may lead to braking and EGR performance issues, such as higher injector tip temperature and lower engine retarding power. In addition, the added valve lift may cause reliability issues, including increased potential of valve-to-piston contact. Accordingly, by reducing the added valve lift during engine braking, braking performance and engine reliability may be improved.
Proper control of the engine valve lift and timing may also lead to improvements during positive power operation. For example, main intake event timing may be modified such that the intake valve closes earlier than a standard main intake valve event. This process is known as a Miller Cycle. Controlling the main intake event valve timing may lead to improved fuel economy and emissions.
Cost, packaging, and size are factors that may often determine the desirableness of an engine brake. Additional systems that may be added to existing engines are often cost-prohibitive and may have additional space requirements due to their bulky size. Pre-existing engine brake systems may avoid high cost or additional packaging, but the size of these systems and the number of additional components may often result in lower reliability and difficulties with size. It is thus often desirable to provide an integral engine braking system that may be low cost, provide high performance and reliability, and yet not provide space or packaging challenges.
Embodiments of the systems and methods of the present invention may be particularly useful in engines requiring valve actuation for positive power, engine braking valve events and/or EGR/BGR valve events. Some, but not necessarily all, embodiments of the present invention may provide a system and method for selectively actuating engine valves utilizing a lost motion system. Some, but not necessarily all, embodiments of the present invention may provide improved engine performance and efficiency during positive power, engine braking, and/or EGR/BGR operation. Additional advantages of embodiments 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.
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