The present invention relates to internal combustion engines of reciprocating piston type and is particularly, though not exclusively concerned with engines of the general type disclosed in EP-A-0591153.
This prior document discloses an engine in which the or each piston is caused to move over at least a portion of the cycle at a rate which is such that the graph of its displacement against time differs from the sinusoidal shape which is inherently produced in conventional engines in which each piston is connected to a respective crank on a crankshaft by a respective connecting rod. In such a conventional engine attempts are made to match the combustion of the fuel/air mixture to the motion of the piston but the philosophy underlying the construction of the prior document is that the combustion is permitted to proceed in the optimum manner and the piston is caused to move in a manner which "follows" the combustion and is related to the nature and progress of the combustion process.
More specifically, the prior document discloses an engine in which the piston is caused to decelerate and thus to move more slowly than in a conventional engine at or around the point in the cycle at which ignition of the fuel/air mixture occurs and then to speed up again prior to reaching the top dead centre position (TDC). This is based on the recognition that in a conventional engine the piston is moving at substantially its maximum speed at the point at which ignition occurs and the compression ratio is altering at substantially its maximum rate and thus impedes the rate of propagation of the flame front through the fuel/air mixture and thus impairs the nature and completeness of the combustion process. However, slowing the piston down at around the ignition point means that the rate of increase in the pressure of the fuel/air mixture at the time propagation of the flame front commences is substantially less than is usual which results in the flame front propagating through the fuel/air mixture very much more rapidly than as usual.
The prior document also discloses that the piston is caused to reach its maximum acceleration and maximum speed at something between 0 and 40.degree. after TDC, instead of 90.degree. after TDC as in a conventional engine and thereafter to move more slowly than in a conventional engine in the latter portion of its working stroke prior to reaching the bottom dead centre position (BDC). This results in a decreased temperature of the exhaust gases and thus in reduced emissions of NOx and reduced erosion of the exhaust ports and valves.
Extensive tests have been conducted on engines constructed in accordance with EP-A-0591153and these have shown that the engine does indeed have a substantially increased efficiency by comparison with conventional engines and also dramatically reduced emissions of unburnt hydrocarbons CO and NOx. Indeed, these tests have shown that the combustion process in the engines in accordance with the prior document proceeds in a manner which is fundamentally different to that in conventional engines, as evidenced by the fact that, for instance, the rate of pressure rise in the cylinder during combustion is about 6.5 bar per degree of rotation of the output shaft, as compared with about 2.5 bar in a conventional engine and that the combustion is complete within about 22.degree. rotation of the output shaft after TDC, as compared to about 60.degree. in a conventional engine.
However, the engine disclosed in the prior document incorporates profiled cams cooperating with the pistons and not a conventional crankshaft and whilst such cams are wholly functional and technically satisfactory it would be preferable for the engine to incorporate a crankshaft of generally conventional type because mass manufacturing facilities for crankshafts are already available and the technology for manufacturing crankshaft type engines is more familiar and tried and tested than that for cam type engines.