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Basic Engine Performance

Characteristics

The basic characteristics describing the performance of an engine are the engine torque, power, fuel economy, and efficiency.

Some of the heat energy evolved in the process of burning of fuel in the cylinders of an engine is converted into mechanical energy. The gas pressure force acting upon the piston is transmitted through the connecting rod to the crankthrow and thus applies torque to the crankshaft.

Torque is the product of the force rotating the crank and the crank radius. Torque is expressed in units of newton meters (N m). While an engine is developing torque, it performs work. The amount of work done in unit time is termed power.

Engine power is measured in kilowatts (kW).

A distinction is made between the indicated power and the effective power of an engine. The indicated power is the power developed by combustion gases in the cylinders of the running engine. The effective, or brake, power is the power that is delivered at the crankshaft. The effective power of an engine is 10 to 12% less than its indicated power, because some power is lost in overcoming friction forces in the engine mechanisms and in driving auxiliary equipment (pumps, fan, generator, etc.). The engine power consumed in overcoming these losses is known as friction power. The engine power increases with the gas pressure force in the cylinder, crankshaft rotation frequency, and engine displacement. The effective power of an engine is given by

where Pe is the mean effective gas pressure in the cylinder, MPa (Pe = 0.5 to 0.8 MPa for four-stroke diesel engines); V is the engine displacement, l; n is the crankshaft rotation frequency, s ; and t is the engine cycle factor (τ = 2 for four-stroke engines and τ = 1 for two-stroke engines).

The engine cycle factor is the number showing how many revolutions of the crankshaft it takes to complete the cycle of operations of the engine.

It is seen from the above formula that with τ and Pe remaining the same, the engine power depends on the engine displacement and the crankshaft rotation frequency. Increasing the crankshaft rotation frequency without changing the engine displacement will enhance the power to weight ratio of the tractor or automobile the engine is used on. The running speed of such a tractor or automobile in all gears will increase proportionately with the rotation frequency of the engine crankshaft. It is precisely through augmenting the crankshaft rotation frequencies that the running speeds of farm tractors has recently risen to 2.5-4.16 m/s (9-15 km/h).

Increasing the displacement of an engine increases its overall dimensions. The higher the class of the tractor as regards the drawbar pull, the greater the displacement of the tractor engine, hence its power output.

The mechanical efficiency of an engine is the ratio of its effective (brake) power to indicated power. It depends mainly on the quality of machining and lubrication of the engine components and workmanship in assembling the engine. The numerical value of this characteristic ranges between 0.8 and 0.9.



The thermal efficiency of an engine is the ratio of the amount of heat actually converted by the engine to mechanical energy to the amount of heat contained in the fuel used by it. The thermal efficiency of automobile and tractor engines is in the range from 0.26 to 0.37. In carburetor engines, it tends toward the lower limit and in diesel engines, the upper limit of the range. An engine in good working order converts to effective power around 30% of the thermal energy it receives from its fuel. The rest of the thermal energy is consumed in overcoming mechanical losses (about 10%) and heating the cooling liquid (about 35%) and the engine (about 10%) and is carried away together with the hot exhaust gases (about 15%).

The fuel economy of an engine is characterized by its specific fuel consumption which is determined by dividing the hourly fuel consumption of the engine by its effective power. In diesel engines, the specific fuel consumption does not exceed 72 mkg/J. An engine with worn, improperly adjusted, or poorly lubricated rubbing parts will have its power output and fuel economy reduced.

 


Date: 2015-12-24; view: 916


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