In internal combustion engines, it is desirable to vary compression ratio during engine operation. Compression ratio strongly affects in-cylinder processes and provides an exceptional degree of control over engine performance.
Conventional engines, however, have fixed compression ratios. Their performance is a compromise between conflicting requirements.
The provision of variable compression ratio in compression ignition engines improves exhaust emission characteristics, overall fuel efficiency, cold startability and multi-fuel capability. It provides control over peak cylinder pressures and therefore permits considerable increase of specific power output through supercharging without sacrificing engine durability.
The present invention is applicable to a category of reciprocating VCR engines in which compression ratio is varied by altering the phase relation between two pistons operating in separate cylinders interconnected through a transfer port.
In VCR engines of this category, compression ratio is defined as maximum combined cylinder volume divided by minimum combined cylinder volume. The combined cylinder volume is the sum of individual cylinder volumes at any instant and the transfer port volume.
Compression ratio is maximum when both pistons move in phase, reaching their respective top dead center positions simultaneously. Any phase shift from that maximum compression ratio phase relation alters compression ratio by changing both the maximum and the minimum combined cylinder volume. Said phase shift is measured in terms of crank angle relative to a phase relation which corresponds to the two pistons reaching their respective top dead center positions simultaneously. The value of the phase shift angle can be arbitrarily assigned a positive or a negative sign to indicate whether the movement of one or the other piston is advanced or delayed relative to the combined cylinder volume changes. Within the phase shift angle range of practical interest in VCR engines, the greater the absolute value of the phase shift angle, the lower the compression ratio.
In practical application of this method of varying the compression ratio, the phase shift angle range need not include the zero value. In other words, the highest compression ratio utilized in an engine may correspond to a phase shift angle other than zero. On the other hand, certain applications may require that the phase shift angle range include zero as well as positive and negative values.
For the purpose of understanding the present invention, it will be assumed that the phase shift angle range is restricted to zero and those positive values which correspond to the compression ratio range of practical interest in VCR, CI, internal combustion engines. Furthermore, in each pair of pistons operating in interconnected cylinders, the piston whose movement is progressively advanced relative to combined cylinder volume changes as the phase shift angle is increased from zero will be termed the leading piston. Accordingly, the other piston of same pair will be referred to as the trailing piston since its movement is progressively delayed relative to combined cylinder volume changes as the phase shift angle is increased from zero. Engine cylinders will be referred to as leading or trailing, depending on whether leading or trailing pistons operate therein. Similarly, engine crankshafts will be termed leading or trailing based on whether the leading or trailing pistons are linked thereto.
The following prior art discloses mechanisms which vary compression ratio by altering the phase relation between two pistons operating in interconnected cylinders. These two pistons are generally linked to separate crankshafts and their phase relation is altered by varying the phase relation between those crankshafts. A multitude of such piston pairs may, of course, be incorporated in an engine.
U.S. Pat. No. 1,457,322 discloses a two crankshaft engine which employs a VCR mechanism comprised of helical gears, some of which are axially movable. Specifically, this mechanism includes two pairs of helical gears which couple the two crankshafts to an axially movable phase shaft. Each pair of those gears consists of a helical gear mounted on a crankshaft and, engaged therewith, a helical gear mounted on the axially movable phase shaft. The crankshafts ar situated side by side and the phase shaft transversely thereto. Helix angles and directions of helices of those gears are arranged to alter the phase relation between the two crankshafts in response to axial displacement of the phase shaft. The principal advantage of this VCR mechanism is mechanical simplicity. However, the location of the phase shaft in an extension of the crankcase results in a significant increase of engine length. In addition, the operation of helical gears on nonparallel shafts considerably reduces their load carrying capacity and/or useful life.
U.S.S.R. Pat. No. 300643 discloses a VCR mechanism which employs helical splines to vary the phase relation between two crankshafts of an opposed piston engine. The mechanism is incorporated in a transverse shaft geared to both crankshafts and consists of two separate helical spline couplings which couple two segments of the shaft to an axially movable member located coaxially between those segments. The helix angles and directions of helices of the splines are arranged to vary the phase relation between the two segments of the transverse shaft and, consequently, between the two crankshafts, in response to axial displacement of the movable member. The principal disadvantage of this VCR mechanism is the mechanical complexity of the whole crankshaft phasing system. Due to the sliding fit requirement and the resultant presence of backlash between mating surfaces, the durability of spline couplings which are subject to heavy alternating loads is also compromised. U.S. Pat. No. 3,961,607 discloses a two crankshaft VCR engine incorporating a planetary gear set in the crankshaft phasing system. The phase relation between crankshafts is varied by rotating the planetary gear carrier around its axis. This VCR mechanism is mechanically complex.
In 1984, an article authored by C. M. Bartolini, V. Naso and this inventor was published in a Polish journal, "Archiwum Termodynamiki", Vol. 5, No. 2. It disclosed a VCR mechanism employing two pairs of helical gears which couple the two crankshafts of the engine to an axially movable phase shaft. Each pair consists of a helical gear mounted on a crankshaft and, engaged therewith, a helical gear mounted on the movable phase shaft. The crankshafts are situated side by side and the phase shaft parallel thereto. The two crankshaft mounted gears are located at the opposite ends of the engine. Helix angles and directions of helices of the VCR mechanism gears are arranged to alter the phase relation between the two crankshafts in response to axial displacement of the phase shaft. In order to accommodate changes in the relative axial position of those gears associated with compression ratio variation, the face width of the crankshaft mounted gears is greater than the face width of the phase shaft mounted gears. This particular parallel configuration of the crankshaft and the phase shaft is favorable from the standpoint of gear durability and load carrying capacity but results in a considerable increase of engine length.
The method of varying compression ratio by altering the phase relation between two pistons operating in separate cylinders interconnected through a transfer port imposes a complex and unique combustion chamber configuration. Each combustion chamber in a VCR engine of this category consists of two interconnected chambers formed in the respective cylinders within the confines of the cylinder head, cylinder liner and the top surface of the piston at any instant. The variation of the phase relation between pistons operating in each pair of those interconnected cylinders affects the phasing of individual cylinder volume changes relative to combined cylinder volume changes. It has a significant effect on the rate and timing of mass transfer between the two cylinders throughout the whole operating cycle of the engine.
A monography entitled "A New Type of Internal Combustion Engine" authored by V. M. Kushul and published in the U.S.S.R. in 1965, as well as U.S.S.R. Pat. Nos. 956827 and 1002627 disclose combustion systems characterized by combustion chambers situated in paired engine cylinders. However, these combustion systems relate to spark ignition engines, while the present invention is applicable to CI power plants. Furthermore, the phase relation between pistons operating in each respective cylinder pair is invariable. The said combustion systems therefore differ from the present invention in that their compression ratio is fixed.
A publication entitled "A Variable Volumetric Ratio, Self Ignition, Internal Combustion Engine" by Gilbert Avermaete of Luxemburg discloses a variable compression ratio CI engine having combustion chambers consisting of paired engine cylinders of unequal diameter. The compression ratio is varied by altering the phase relation between pistons operating in each respective cylinder pair. The combustion system is of the precombustion chamber category, the fuel being injected by a side-mounted pintle-type fuel injector into the smaller cylinder of each pair. This combustion system suffers from the drawbacks of precombustion chamber CI engines namely, excessive fuel consumption and extreme thermal loading of combustion chamber walls.