fatherlarry
04-22-2002, 06:16 PM
Improving The Engine's Mechanical Efficiency
A significant part of the power developed by the expansion of the gas in the cylinders is used for overcoming friction (between piston and cylinder and in the bearing of the connecting rod and crankshaft) and for driving the water- circulation pump, oil pump, dynamo, camshaft and valves. Hence only a certain proportion of the theoretical power output is available as effective output. This proportion is termed the “mechanical efficiency” of the engine. Depending on the type and design of the engine and on its state of maintenance, the mechanical efficiency is usually between 0.75 and 0.85.
More than half the loss of power is due to friction of the pistons and bearings. The piston friction depends on the pressure developed in the cylinder and on the piston speed, which is determined by the stroke and the speed of rotation. Generally speaking, the rotational speed should be as high as possible. Therefore the only possible means of reducing the friction is to shorten the piston stroke. The friction developed at the piston rings depends on the number of rings per piston. To minimize the loss of gas, it is necessary always to have two compression rings; in addition, each piston has an oil-scraper ring.
Friction in the crankshaft bearings can be reduced by the use of lighter connecting rods. This also reduces the lubricant requirement of the bearings, so that the oil-pump power input is lessened. A crankshaft rotating at high speed causes frictional losses due to turbulence and foaming of oil in the sump. For this reason high-speed engines have dry-sump lubrication. In this system, oil entering the crankcase is immediately extracted by suction and is returned through a filter and a cooler to the oil tank. A second pump delivers the oil from the tank to the bearings.
A water-cooled engine is usually equipped with a fan. The fan is necessary only when the cooling water temperature is high. For a substantial proportion of the engine’s running time the fan is absorbing power without performing any useful function. For this reason fans have been developed that are switched on and off automatically, controlled by the temperature of the cooling water or air.
A significant part of the power developed by the expansion of the gas in the cylinders is used for overcoming friction (between piston and cylinder and in the bearing of the connecting rod and crankshaft) and for driving the water- circulation pump, oil pump, dynamo, camshaft and valves. Hence only a certain proportion of the theoretical power output is available as effective output. This proportion is termed the “mechanical efficiency” of the engine. Depending on the type and design of the engine and on its state of maintenance, the mechanical efficiency is usually between 0.75 and 0.85.
More than half the loss of power is due to friction of the pistons and bearings. The piston friction depends on the pressure developed in the cylinder and on the piston speed, which is determined by the stroke and the speed of rotation. Generally speaking, the rotational speed should be as high as possible. Therefore the only possible means of reducing the friction is to shorten the piston stroke. The friction developed at the piston rings depends on the number of rings per piston. To minimize the loss of gas, it is necessary always to have two compression rings; in addition, each piston has an oil-scraper ring.
Friction in the crankshaft bearings can be reduced by the use of lighter connecting rods. This also reduces the lubricant requirement of the bearings, so that the oil-pump power input is lessened. A crankshaft rotating at high speed causes frictional losses due to turbulence and foaming of oil in the sump. For this reason high-speed engines have dry-sump lubrication. In this system, oil entering the crankcase is immediately extracted by suction and is returned through a filter and a cooler to the oil tank. A second pump delivers the oil from the tank to the bearings.
A water-cooled engine is usually equipped with a fan. The fan is necessary only when the cooling water temperature is high. For a substantial proportion of the engine’s running time the fan is absorbing power without performing any useful function. For this reason fans have been developed that are switched on and off automatically, controlled by the temperature of the cooling water or air.