This invention relates to a variable compression device for a two cycle diesel engine and more particularly to an improved valve arrangement for controlling a pair of auxiliary or sub-exhaust ports for varying the compression ratio of an engine.
It has been proposed to provide an arrangement for varying the compression ratio in an engine and particularly a two cycle engine so as to improve its performance through out its load and speed ranges. This expedient is particularly useful in conjunction with diesel engines as it permits the use of high compression ratios for starting, low and mid speed operation without providing a high compression ratio at high speed that can give rise to thermal and emission problems. One way in which the compression ratio can be varied, is by providing a pair of auxiliary or sub-exhaust ports that open into the cylinder bore at a higher point than the main exhaust port. By opening and closing the sub-exhaust ports, the timing of the beginning of the exhaust cycle can be changed and the compression ratio effectively altered. However, there are certain problems with the prior art constructions that can be best understood by reference to FIGS. 1 and 2 of the drawings.
FIGS. 1 and 2 are respectively cross sectional views taken through a plane extending perpendicular to the cylinder bore axis and parallel to the cylinder bore axis. As may be seen, a cylinder 51 is formed with a cylinder bore 52 in which a piston 53 reciprocates. A main exhaust port 54 extends through a side of the cylinder 51 and permits exhaust gases to flow, through a main exhaust passage 55 that extends through the cylinder 51 and which communicates with an exhaust manifold (not shown).
At circumferentially spaced locations above the main exhaust port 54, there are provided a pair of sub or auxiliary exhaust ports 56 that communicate with auxiliary exhaust passages 57 formed in the cylinder 51 and which intersect the main exhaust passage somewhere downstream of the main exhaust port 54.
An exhaust control valve assembly, indicated generally by the reference numeral 58 is supported within a transversely extending bore 59 formed in the cylinder 51 for controlling the opening and closing of the auxiliary exhaust ports 56. As may be seen, when the auxiliary exhaust ports 56 are closed, the piston 53 will sweep past them but no exhaust gases can be discharged from the exhaust passage 55 until the main exhaust port 54 is opened. This has the effect of providing a high effective compression ratio. To lower the compression ratio, the exhaust control valve 58 is rotated from its closed position to an opened position so that the auxiliary exhaust ports 56 will open earlier in the downward movement of the piston 53 and reduce the effective pressure and, accordingly, the compression ratio.
As may be seen, when the exhaust control valve 58 is provided for controlling both of the auxiliary exhaust ports 57, as is desirable to maintain a simple construction, the valving surfaces 61 of the valve 58 must be spaced at a location from the cylinder bore 52 to provide a thin body of cylinder block material 62 between the bore 59 and the cylinder bore 52 for strength purposes. As a result, there are dead spaces 63 formed between the ports 56 and the valve surfaces 59 which in effect lower the compression ratio and hence do not make the engine as efficient as is desirable. In addition, because of the intervening body of cylinder block material 62, the exhaust control valve 58 is not effectively cooled and thermal problems can occur.
It is, therefore, a principal object to this invention to provide an improved variable compression device for an internal combustion engine which permits a high compression ratio and yet a simple and compact construction.
It is a further object to this invention to provide a simplified and compact variable compression ratio mechanism for an engine that will not adversely effect the compression ratio or the cooling ability for the exhaust control valve.
It is a further object to this invention to provide an improved variable compression device for an internal combustion engine wherein the exhaust control valve that controls the exhaust port timing will be adequately cooled.