1. Field of the Invention
The present invention relates to an overhead-valve type forced air-cooled engine having an intake valve and an exhaust valve provided in a cylinder head thereof.
2. Background of the Prior Art
In such a forced air-cooled engine, a cylinder and/or a cylinder head are adapted to be cooled by the cooling air flow generated by a cooling fan. However, there exist portions locally kept at a high temperature in the cylinder and/or the cylinder head.
In this basic engine, a push rod chamber is provided at one side of the cylinder chamber, hence a partition wall between the cylinder and the push rod chamber is exposed to a high temperature from a combustion chamber at one side thereof and is isolated from the cooling effect of cooling air flow by the push rod chamber at its other side. It is, thereof, apt to reach such a high temperature that thermal distortion is caused in the cylinder due to uneven temperature distribution in the cylinder wall.
In such high-powered engines, this is a serious matter because a large amount of heat is generated in the engine. Such thermal distortion is apt to cause poor contact between a piston ring and an inner surface of a cylinder, which results in a decrease of engine power and engine durability due to uneven abrasion at the inner surface of the cylinder and, in a worst case, may cause piston squeezing.
And, since a peripheral wall of a divided combustion chamber provided in a cylinder head is apt to be heated locally with the combustion heat generated therein in a divided chamber type forced air-cooled engine, a thermal distortion is caused in the cylinder head and the cylinder head cracks in a worst case.
An example of such a conventional engine, in which the decreases of engine power and engine durability by such thermal distortion of the cylinder are substantially prevented by obtaining an even temperature distribution in the cylinder wall, an oil-cooled engine is disclosed in British Laid Open Patent Publications Nos. 2,127,487 (refer to FIG. 11 hereof) and 2,000,223 (refer to FIG. 12 hereof).
In FIGS. 11 and 12, a cylinder jacket 100 for cooling a whole cylinder 24 is spirally formed around a cylinder wall, and a cylinder head 3 is provided with a head jacket 101. The cylinder jacket 100 is in communication with an oil pan 103 below the crankcase through the head jacket 101 and an oil cooler 102, and the inlet port 104 thereof is in communication with a delivery port 107 of an oil pump 106 in a forced lubrication system 105.
As for the cooling of the cylinder head, there is provided a cylinder jacket through the whole cylinder head as shown in FIG. 11.
However, the above-mentioned prior art has the following disadvantages because the whole cylinder block and/or the whole cylinder head has to be cooled:
(1) the engine becomes larger in size and heavier in weight, because a large oil pump 106 and a large oil pan 103 are required for supplying a large amount of lubricating oil to the cylinder jacket 100 and/or the head jacket;
(2) oil cooler 102 becomes larger in size because of the large cooling capacity required thereof for cooling lubricating oil which also serves to absorb a considerable amount of the heat transferred from the whole cylinder block and/or the head jacket.