GB2114653A - I.C. engine operable on less than all cylinders - Google Patents

Abstract

The combustion characteristics, e.g. valve timing, combustion chamber design and capacity, of groups of engine cylinders differ so that the torque/speed characteristics of the engine when driven by one group of cylinders is different from the torque/speed characteristics of the engine when driven by the other group or groups of cylinders. Control means selectively controls fuel injectors or carburettor throttle valves to direct fuel to all the groups of the cylinders when the engine is operating under high loads but only to one or some of the groups when under low loads, the groups being selected in accordance with engine speed so that the efficiency of the engine under low loads is optimised.

Description

SPECIFICATION Internal combustion engine This invention relates to internal combustion engines.

In order to increase the fuel economy of internal combustion engines having a plurality of cylinders, it is known to provide control means for selectively directing fuel to all or to only some of the cylinders, whereby the engine may be driven by less than the total number of cylinders under predetermined conditions of load and speed. For example, in a 4-cylinder engine, the control system may operate to supply fuel only to two cylinders when the engine is operating under low loads.

According to the present invention, there is provided an internal combustion engine comprising a plurality of cylinders, and control means for selectively directing fuel to all or only some of the cylinders whereby the engine may be driven by less than the total number of cylinders under predetermined conditions of load and speed, characterised in that the cylinders are divided into at least two groups, the torque/speed characteristics of the engine when driven by one group of cylinders is different from the torque/speed characteristics of the engine when driven by the other group or groups of cylinders, and in that the control means selectively directs fuel to one or only some of the groups of cylinders under load conditions in accordance with the engine speed.

By tuning the cylinders in each group to provide different torque/speed characteristics in the engine, and arranging the control means to activate different groups in accordance with the different operating conditions of the engine, the efficiency of the engine when operated by less than the total number of cylinders can be improved with consequent improvements in fuel economy and reductions in emission of undesirable combustion products in the exhaust gases.

Usually, the maximum torque of the engine when driven by one group of cylinders will be generated at a higher engine speed than the speed at which the maximum torque of the engine is generated when driven by the other group or groups of cylinders. This may, for example be achieved by so shaping the cams of cam-operated valves in one group of cylinders, that the valves open for a shorter proportion of the operating cycle of the engine than the proportion of the operating cycle for which the cams of the other group of cylinders open their valves. The differences between the groups of cylinders may however be achieved by other modifications. For example, the cylinders may be provided with different combustion chamber shapes, different valve sizes or different cylinder capacities.The person skiiied in the art of engine design will be familiar with the effect which these and other variations have on the load/speed characteristics of an engine.

The invention is applicable to any type of internal combustion engine, for example engines with 2-stroke or 4-stroke cycles; diesel or petrol engines; and for engines with any type of cylinder configuration e.g. in-line, flat, V-, square, or radial configurations, and with any number of piston greater than 1.

The number of groups into which the cylinders are divided is at least two, but may be more depending upon the number of cylinders and their relative orientations in the engine. For example, in an engine having four, in-line cylinders, division of the cylinders into two groups of two is most convenient. In a V-6 or V-8 engine, division in two groups of 3 or 4respectively, corresponding to the two banks of cylinders may be appropriate. Other combinations are however possible, and the number of cylinders in each group need not be the same. For example in an 8-cylinder engine the groups could be divided into four groups of 2 or into 2 groups of'3 and one group of 2.

A preferred embodiment of the invention will now be described, by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an internal combustion engine embodying the invention; Figure 2 is a graph illustrating the valve timing of the engine of Figure 1; and Figure 3 is a graph illustrating the torque/speed characteristics of the engine of Figure 1.

Referring to the drawings, a 4-stroke petrol engine 1 comprises an engine block having 4 cylinders 2-5 therein, each housing a respective piston drivingly connected to a crankshaft in a conventional manner. Each cylinder is provided with an inlet valve 6 and an exhaust valve 7, which are operated by respective cams 8, 9 on a common camshaft 10 via conventional rocker arms and tappets (not shown). The camshaft 10 is driven at half the speed of the engine crankshaft by means of a timing belt or chain (not shown).

The cams are arranged on the camshaft 10 to open the inlet and outlet valves in accordance with the firing sequence of the cylinders which, in the present case is 2--33--5-4.

Uniike conventional engines, the cams for the inlet and outlet valves are not the same for all the cylinders. The differences in the cams is illustrated with reference to Figure 2.

The solid lines 20, 21 in Figure 2 show the variation in the distance d (in mins) by which the inlet and exhaust valves are open respectively with the phase angle 0 of the crankshaft for a first group of cylinders comprising the first and second cylinders 2, 3 in the firing sequence of the engine.

The broken lines 23, 24 show the same variations for a second group of cylinders comprising the third and fourth cylinders 5, 4 of the firing sequence. It can be seen that the valves for the second group of cylinders 5, 4 are opened for a shorter proportion of the rotation of the crankshaft than the valves of the first group of cylinders 2, 3.

Air is fed to the cylinders 2-5 via an inlet manifold 1 , and fuel is fed thereto via four fuel injectors 12-1 5, each associated with inlets into respective ones of the cylinders 2-5. The fuel injectors 1 2-1 6 are operated electronically by means of a control unit 18, the general construction of which will be known to persons skilled in the art.

The control unit 18 receives input signals from the engine indicating the phase angle of the crankshaft, engine speed and engine load and actuates the fuel injectors in accordance with a predetermined programme so that an appropriate volume of fuel is delivered to the cylinders at the correct moment in the firing sequence of the engine.

The control unit 1 8 is conditioned to operate in three modes in accordance with the prevailing engine speed and load conditions. In the first mode, the fuel injectors from all four cylinders are actuated in each cycle of the engine. In the second mode only the fuel injectors 12 and 13 for the first group of cylinders 2, 3 are actuated so that only the first group of cylinders is supplied with fuel. In the third mode, only the fuel injectors 14, 1 5 for the second group of cylinder 5, 4 are actuated so that only the second group of cylinders is supplied with fuel.

The control unit 1 8 operates in the first mode when the engine is under high loads, regardless of engine speed. At low engine loads, the control unit 1 8 operates in the second mode at lower engine speeds (e.g. below 50% of the maximum rated speed) and in the third mode at higher engine speeds (e.g. above 50% of the maximum rated speed).

The performance of the engine in these three modes of operation is illustrated in Figure 3 which is a graph showing the engine torque Q on the ordinate as a percentage of the maximum torque, and the engine speed V on the abscissa as a percentage of its maximum rated speed.

The solid line 30 illustrates the torque 1 speed characteristics of the engine when operating in the first mode, in which all four cylinders are supplied with fuel. This line exhibits the usual maximum at about 50% of the maximum rated engine speed.

The dotted line 32 illustrates the torque/speed characteristics of the engine when operating in the second mode, in which only the first two cylinders 2, 4 in the engine firing sequence are supplied with fuel. Since only two of the cylinders are operating, the torque output of the engine is reduced by about 50%. However, since the cams of the first group of cylinders 2, 3 hold the inlet and exhaust valves open for a relatively long proportion of the rotation of the crankshaft, the maximum torque output of the engine when operating in this mode occurs at relatively low speeds, for example at about 25% of the maximum rated engine speed.

The dashed line 33 illustrates the torque/speed characteristics of the engine when operating in the third mode. The torque output is again reduced to about 50% of that of the first mode of operation.

However, since the cams of the second group of cylinders 5, 4 hold the inlet and exhaust valves open for a relatively short proportion of the rotation of the crankshaft, the maximum torque output of the engine occurs at relatively high speeds, for example at about 75% of the maximum rated engine speed.

In use therefore, when operating under light loads, the amount of fuel consumed by the engine will be reduced. Additionally, the efficiency of the engine, represented by the envelope of the two torque/speed curves 32 and 33 will remain at a relatively constant level since the overall torque/speed curve at low loads exhibits two maxima.

The chain-dotted line 35 illustrates the torque/speed characteristics of the engine incorporating a conventional camshaft in which the cams for the inlet and outlet valves of each cylinder are of similar shapes, and controlled by a conventional control unit which cuts off the supply of fuel to two cylinders at low engine loads. This line exhibits a single maximum at an engine speed corresponding to 50% of the maximum rated speed. As a result, the engine operates less efficiently than that of the present invention at low and high speeds.

Although the invention has been described in detail with reference to an electronicallycontrolled fuel injection engine, similar results can be obtained using a multi-barrel carburettor in which each barrel of the carburettor supplies fuel/air mixture to respective groups of cylinders, the modes of operation being chosen by selectively closing barrels of the carburettor in accordance with changes in engine load and speed. Such a construction is especially suitable for an engine having cylinders arranged in two inclined or opposed barrels.

Claims (5)

1. An internal combustion engine comprising a plurality of cylinders and control means for selectively directing fuel to all or only some of the cylinders whereby the engine may be driven by less than the total number of cylinders under predetermined conditions of load and speed, characterised in that the cylinders are divided into at least two groups, the torque/speed characteristics of the engine when driven by one group of cylinders is different from the torque/speed characteristics of the engine when driven by the other group or groups of cylinders, and in that the control means selectively directs fuel to one or only some of the groups of cylinders under low load conditions in accordance with the engine speed.

2. An internal combustion engine according to Claim 1 wherein the maximum torque of the engine when driven by one group of cylinders is generated at a higher engine speed than the speed at which the maximum torque of the engine is generated when driven by the other group or groups of cylinders.

3. An internal combustion engine according to any one of Claims 1 or 2 wherein the cylinders are divided into two groups.

4. An internal combustion engine according to any one of Claims 1 to 3 wherein each cylinder is associated with valves for permitting flow of gas into and out of the cylinder, and a cam for operating the valve, the cams of one group of cylinders being arranged to open the valves for a smaller proportion of the operating cycle of the engine than the proportion of the operating cycle for which the cams of the other groups of cylinders open their valves.

5. An internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.