Patent Grant
7036483
This invention relates to diesel engines and, more particularly, to control of cylinder compression ratio using a dual-lobed intake cam.
It is known in the art to provide means for varying the compression ratio of a diesel engine in order to provide a relatively high compression ratio for cold starting and warm-up, where compression ignition is more difficult, and to provide reduced compression ratios for operating in other modes, particularly at high loads and speeds, to reduce peak combustion pressures and temperatures. Recently the emphasis for such arrangements is primarily to minimize emissions of nitrogen oxides (NOx) by operating at lower compression ratios where this is possible. Many devices have been proposed for compression ratio variation, including variable valve timing mechanisms and engine components such as pistons and cylinder heads with movable combustion chamber walls. In general these devices are relatively complex and add significant cost to the manufacture of an engine.
In spark ignition engines, dual-lobed cams with lobe selection mechanisms are known devices for varying valve timing, duration and lift thus changing valve timing. These devices normally provide for both advancing valve opening and retarding valve closing in order to obtain desirable performance characteristics. It is believed that dual-lobed cams with lobe selection mechanisms have not been utilized on diesel engines because the piston to cylinder head clearance is so small that altering intake and exhaust valve timing may result in contact of the pistons with the valves. A simple and relatively low cost apparatus and method for controlling compression ratio in a diesel engine is desired.
The present invention provides a desired engine combination by the addition of dual-lobed cams with lobe selection mechanism capable of retarding the closure timing of only the intake valves of a diesel engine in order to reduce its compression ratio. A typical diesel engine has cylinders and pistons defining expansible combustion chambers into which air is admitted and compressed during compression strokes of the pistons. Compression increases the air temperature so that injected fuel is self-ignited and burns, creating power to drive a crankshaft. Intake and exhaust valves, actuated by separate crankshaft driven intake and exhaust camshafts, control timed admission of air to and discharge of exhaust products from the combustion chambers.
In accordance with the invention, dual-lobed cams with lobe selection mechanisms are mounted in the valve train and are operable to selectively retard timing of only the intake valves relative to the crankshaft. The purpose of retarding timing of the intake valves is to retard valve closing sufficiently to shorten the effective compression strokes of the pistons and thus reduce the effective compression ratio. This occurs when the intake valves remain open past piston bottom dead center for a desired period into the normal compression stroke phase of engine operation. This reduces compression pressures in the combustion chambers so that combustion temperatures are reduced and exhaust emissions, primarily NOx, may be thus limited under conditions of warmed-up engine operation.
Additional reductions in combustion temperatures can be achieved, in conjunction with use of dual-lobed intake cams in turbocharged or supercharged diesel engines, by increasing the intake boost pressure to maintain constant trapped air mass in the cylinder, even when intake valve closing retard is utilized. This approach allows maintaining lower combustion temperatures, thus inhibiting NOx and soot formation by preventing increases in fuel-air ratio as compression ratio is decreased.
For cold running conditions to avoid excessive hydrocarbon and white smoke emissions from poor ignition and incomplete combustion, a dual-lobed cam can also be used to increase charge temperature by delaying intake valve opening. This increases the pumping losses which are converted into thermal energy thus raising the in-cylinder charge temperature. This increased charge temperature improves ignitability of the charge and completeness of combustion.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.
A diesel engine has a variable compression ratio in accordance with the invention. A diesel engine conventionally includes a plurality of cylinders having therein reciprocable pistons connected with a crankshaft. The ends of the cylinder are closed by a cylinder head so that the cylinders and pistons define expansible combustion chambers.
The cylinder head is provided with intake valves which control the timing and flow of intake air into the cylinders during intake strokes of the pistons. Exhaust valves in the cylinder head control timing and flow of exhaust products from the combustion chambers during exhaust strokes of the pistons. In the engine there may be multiple intake valves and multiple exhaust valves for each cylinder, however, any suitable number of valves provided for operation of the engine may be utilized in accordance with the invention.
The intake and the exhaust valves are actuated by separate intake and exhaust camshafts through rocker arms. The intake and exhaust camshafts exclusively operate their respective intake and exhaust valves, however, both are driven by the crankshaft through a timing chain.
Referring to
Control 38 comprises a conventional microprocessor-based engine or powertrain controller including CPU, ROM, RAM, I/O circuitry including A/D and D/A conversion and serial data bus communications. Control 38 monitors or derives a variety of parameters used in engine and powertrain controls including non exhaustive exemplary parameters such as engine coolant temperature, intake air temperature and mass flow, manifold pressure, exhaust gas constituents, engine speed, crankshaft angles and engine output torque. Control 38 further includes a variety of controlled actuators and control signal therefore such as solenoids and motors including for providing and exhausting pressurized oil to and from the actuation mechanism 37 to effect positional control of pin 35.
Referring now to
For low compression operation in accordance with the first dual-lobed cam 10 in
With continued reference to
For low compression operation in accordance with the second dual-lobed cam 10′ in
In operation, the high compression mode of operation is utilized for cold engine starting and warm-up. This is necessary because the intake air charge must be compressed to a gas temperature high enough to provide reliable and consistent compression ignition of fuel injected into the combustion chambers near their piston top dead center positions. After the engine is warmed up and the cylinder and piston walls are heated, reduction of the compression ratio to a lower range, such as 12/1 to 16/1 depending on the engine configuration, can be utilized to provide effective compression ignition to operate with reduced combustion temperatures in order to control or limit NOx emissions. Thus, during warmed-up conditions, the low compression mode of operation is utilized.
While this will provide reduced combustion temperatures resulting in a reduction of NOx emissions, the effect is limited by fuel heating of the smaller gas charge. With a turbocharged or supercharged engine, the boost level may be increased to provide a trapped mass of the intake gas charge, including air and exhaust gases if needed, that is equivalent to the mass provided without the reduced compression ratio. Burning and expansion of the larger charge with the reduced compression ratio then results in a greater temperature reduction and a resulting greater reduction in NOx emissions.
When the engine is again operated at light loads or during starting and warm-up, the pin 35 is returned to its retracted position, the high compression cam lobe is again effective, and the compression ratio is again increased so that dependable compression ignition of the intake air fuel charge is obtained.
In order to use a dual-lobed intake cam in the manner outlined for reducing the effective compression ratio and resulting compression temperatures of a diesel engine, the cam lobes must not advance the intake valve opening. The variations in valve timing for which dual-lobed cams are utilized in spark ignition engines are not generally usable in diesel engines because the intake valve timing cannot be advanced without the pistons contacting the valves due to the low piston to head clearance.
Thus, the application of dual-lobed cams to a diesel engine is not known to have previously been considered practical. However, the use in the present invention, where only retarding of the intake valves from their nominal timing is utilized, provides a simple and low cost method of controlling combustion temperatures and controlling NOx emissions in warmed-up operation of a diesel engine.
While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.
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| Number | Date | Country | |
|---|---|---|---|
| 20050132982 A1 | Jun 2005 | US |
