The Effect of Cowling on the Local Cooling Effectiveness of Engine Air-Cooled Cylinders

In this work, the effect of providing air-cooled cylinders with cowlings on the local cooling effectiveness was investigated. Cast-iron specimens representing air-cooled cylinders were manufactured with rectangular and trapezoidal fins. Fifteen cowlings were manufactured to provide for various cowling parameters namely: cowling diameter to fin diameter ratio, cowling tail length to fin diameter ratio, cowling tail width to fin diameter ratio and cowling tail angle. A specially designed test rig was manufactured for this purpose. It mainly consists of an air blower, air heating box, orifice plate flowmeter and a test-section. The test variables included the air flow velocity, air temperature and heat flux. Local heat-transfer data were gathered using micro-thermo-couples embedded in the front, side and rear of the specimens representing air-cooled cylinder. Particular attention was given to the rear part of the finned cylinder (opposite to the direction of air flow). That part was always the hottest part and thus can promote cylinder scuffing and large thermal stresses due to the large circumferential temperature variation. In this respect, the optimum cowling parameters were put forward that provide for optimum local cooling effectiveness. Also, correlation of data was attempted to enable a prediction to be made.