Improvements to compression or spark ignition four-strok internal combustion engines having a variable compression ratio enabling high supercharging pressure levels.
This invention relates to the concept of an engine having a variable compression ratio, which consists in varying the volume of the combustion chamber in function of the intake air density and temperature, the engine speed and the engine temperature, in order to produce a hypersupercharging effect on the engine by means of a single or double supercharging pressure with intercooling.
A compression ignition engine having a variable compression ratio is disclosed in WO 89/03476, which represents as an example a preferred embodiment of the invention, relating to a compression ignition two-stroke engine provided with an axial assembly of two adjacent cylinders and two crankshafts arranged centrally in the crankcase, provided with a kinematic chain wherein the two crankshafts are coupled at the same rotational speed, the engine being a parallel flow scavenging engine with intake and exhaust located at the opposite ends of the larger of the two cylinders. The engine is further provided with a coupling means to synchronize cyclically the position of the precombustion chamber piston at the end of the compression phase, in order to vary the compression ratio in function of the different engine speeds. However, said means do not enable the two-stroke engine to operate under normal conditions, since at each exhaust stroke, the exhaust gases in the smaller of the two cylinders are partially scavenged from the cylinder. This causes a loss of efficiency, since the intake of fresh air in not a parallel flow. According to another embodiment of the invention, a four-stroke engine may also be provided with a kinematic chain wherein the smaller crankshaft is coupled at half rotational speed to the larger crankshaft; this however invalidates the operation of the four-stroke engine due to a desynchronization of the piston of the smaller cylinder with respect to the engine phases, for example, the expulsion of exhaust gases from the smaller cylinder to the larger cylinder during the intake phase, and of exhaust gases drawn in from the larger cylinder by the smaller cylinder during the exhaust phase.
This invention describes a new combination of a four-stroke engine having a variable volume combustion chamber. The engine has a kinematic chain wherein the two crankshafts are coupled at the same rotational speed via a variably timed transmission. The angular displacement between the two crankshafts, effected between the start of travel and the end of travel of the variably timed transmission, is provided by an appropriate ratio between the displacements of the paired cylinders and between the volume of said two cylinders and the clearance space, so that the compression ratio of the engine may be varied without any desynchronization of the piston of the smaller cylinder with respect to the engine phases. A means of varying the relative angular position of two coupled shafts, in particular a crankshaft and a camshaft mounted in a machine casing, is also disclosed in DE-A-3 616 234. The main component of said means is the camshaft onto which all the mechanical parts are mounted in a rotational motion, with one of said parts capable of effecting combined translational and rotational motions in order to define the angular displacements between the camshaft and the crankshaft. The combined translational and rotational motions of said part are effected by a double-action piston and hydraulic cylinder assembly, separate from the camshaft. The axial resistance force generated by the stroke of the piston at each angular displacement of said means is proportioned by the resistance torque of the camshaft (resistance due to cam friction on the valve stems), and generates axial loads on the camshaft, the latter being axially locked by a thrust bearing. The camshaft only generates low resistance torques of approx. 1 to 3% of the torque of the crankshaft, so that the axial loads on the camshaft are low. Mechanically speaking, this enables the use of a low load-carrying thrust bearing, so that a small-size thrust bearing may be mounted onto the camshaft. As far as the removal of the angular displacement variation means from the engine unit is concerned, the latter requires removal of many mechanical parts which form part of said means and are supported by the camshaft, the latter being also part of said means.
This invention also describes a new variably timed transmission assembly suitable for regulating a four-stroke engine having a variable volume combustion chamber. This new variably timed transmission assembly is coupled to the two crankshafts of the engine and offers the advantage of varying the angular displacement between said crankshafts, without any axial load on the latter, whatever the torque load on the variably timed transmission. According to this new arrangement, the variably timed transmission may be removed from the engine unit and uncoupled from the two crankshafts as an interchangeable mechanical assembly. Means are also provided an said variably timed transmission to enable an accurate angular adjustment between the coupling of the two crankshafts and said variably timed transmission.
The supercharging principle of piston engines consists in increasing the air masses without increasing the displacement. For constant compression ratio engines, this results in an increase of the combustion pressure and a higher specific output (with reference to the cylinder volume in liters). However, when the supercharging pressure is increased, the mechanical and thermal stresses increase on the engine members. This main drawback is due to the fact that the compression ratio generated by the combustion chamber and the piston stroke cannot be altered and adapted to pressure and temperature variations of the intake air and to variations in engine speeds and temperatures.
Consequently, engine manufacturers respect certain construction rules by defining, on the one hand, a limit to the amplitude of intake pressure variations and, on the other hand, by achieving an average compression ratio between the atmospheric intake pressure and the supercharging pressure. Since the fact of defining an average compression ratio is a comprise which conciliates at best the different engine loads and speeds, the pressures and temperatures of the atmospheric intake are too low and the supercharging pressures and temperatures are too high.
According to the invention, this new engine has two crankshafts, one with a long-stroke crank and the other with a short-stroke crank. Both crankshafts are coupled at the same rotational speed via a gear train and a variably timed transmission, the coupling gear of which is part of the gear train and may be shifted at an angle with respect to the short-stroke crankshaft, in order to provide an infinite number of lead angles between the two crankshafts without interrupting the coupling of the latter.
According to the invention, the variably timed transmission is designed so that it may be removed from the engine independently of the short-stroke crankshaft, which offers the advantage of easy and fast replacements of defective parts or permits a standard exchange of the variably timed transmission. The cylinders, having different displacements, are respectively arranged above one of the two crankshafts. The crank of the short-stroke crankshaft coacts with the connecting rod of the piston in the smaller cylinder and the crank of the long-stroke crankshaft coacts with the connecting rod of the piston in the larger cylinder. The two cylinders are connected one by one, from one row to the other, through a recess in the cylinder head, so as to form a pair of cylinders in communication with each other, permitting gases to flow from one cylinder to the other, irrespective of the position of the piston in each of the cylinders.
According to the invention, in the case of an engine provided with compression ignition means, the engine comprises at least one fuel injector in the clearance space, the fuel being injected in mesh, at half speed, with the long-stroke crankshaft.
According to the invention, in the case of an engine provided with spark ignition means, the engine comprises at least one spark plug in the clearance space, the ignition being achieved through means known in the art, in synchronism, at half speed, with the long-stroke crankshaft.
According to the invention, engine timing is achieved through at least one camshaft in mesh, at half speed, with the long-stroke crankshaft, so as to connect periodically the pair of cylinders with the intake and exhaust pipes via the intake and exhaust valves, at definite moments of the four-stroke cycle. The expansion phase is effected simultaneously on each piston of the paired cylinders, so as to cause the two crankshafts to cooperate to the motive force The long-stroke crankshaft is connected to the outer transmission line of the engine, so that the vaiably timed transmission only conveys the engine torque of the short-stroke crankshaft to the long-stroke crankshaft; the motive force on the variably timed transmission is thus dependent on the smaller cylinder of the paired cylinders.
The different lead angles effected by the variably timed transmission between the two crankshafts, alter at the end of the compression phase ( top dead center of the piston in the smaller cylinder) an additional volume in the smaller cylinder. This additional volume is defined with the clearance space, so as to alter the compression ratio of the engine towards a maximum at the start-of-travel of the vaiably timed transmission and towards a minimum at the end-of-travel of the variably timed transmission.
According to the invention, a hydraulic force amplifier having a controlled thrustor acting on the variably timed transmission, alters the additional volume of the smaller cylinder in proportion to the supercharging pressure, so as to maintain the engine under optimal running conditions with a minimum of pollution.
Also according to the invention, a programme pre-established on a prototype engine permits the elimination of excessive pressure and temperature stresses. Each running condition of the engine is stored in a point-progression scale, so as to encompass all the engine output capabilities. Each point is a combination of values measured by four sensors : the intake air pressure, the intake air temperature, the engine speed and the engine temperature. Each combination is recorded simultaneously with the pisition of the thrustor actuating the variably timed transmission.This programme permits the automatic control of the standard type engine in the same way as that of the prototype engine. The fuel quality specifications should also be identical, so that the same running conditions are reproduced exactly on the standard type engine, by means of a high frequency monitorng of the values measured by the four sensors.
The invention will be more fully understood from the following description, taken together with the accompanying drawings, as an example only and a non restrictive embodiment of the invention.