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Performance of Automobile

The pressure developed by the burning of fuel in the engine cylinder is transmitted to the crankshaft by the piston and connecting rod and a turning force or effort known as torque is produced. The crankshaft is coupled to the driving road wheels through clutch, gear box, propeller shaft, differential and axle shafts in an automobile. Thus, torque produced by the engine is transmitted through the drive line to the road wheels to propel the vehicle. The torque depends upon the pressure exerted on the piston and the length of crank arm and it is measured in newton-metre (N-m). The actual power delivered by the engine is known as brake power (B.P.) and is measured in watts (W) or kilowatts (kW). The power actually developed by the engine cylinder is known as indicated power (I.P.). The mechanical efficiency is the ratio of brake power (B.P.) to the indicated power (I.P.).

The torque increases with the increase in engine speed up to a certain point after which it starts to fall down even though the engine speed continues to increase. The number of revolutions per minute (r.p.m.) at which the torque begins to decrease, depends upon the engine design. At higher speeds, engine vacuum falls down and less fuel enters the cylinder resulting in lesser force available at the piston and thus there is a fall in torque. The power and torque curve of a particular petrol engine with respect to engine speed is shown in figure below:

The torque available at the contact between driving wheels and road is known as tractive effort. The gearbox and final drive at differential act as leverage to multiply torque which is inversely proportional to the speed. If the gear speed is lowered, the torque shall be increased in the same ratio and vice-versa. Thus, the torque at the driving wheels,

It may be noted that the ratio between the engine r.p.m. and the vehicle speed depends upon the overall gear ratio. A vehicle having four speed gearbox shall have four different speeds and the ratio between the engine r.p.m. and the vehicle speed shall be different. Thus, R.P.M. of the driving wheel,

From the above, we see that engine torque can be increased by reduction in gearing. The torque transmitted by the engine through gearbox and propeller shaft to the final drive is increased in every gear speed except in top (direct) and overdrive. The torque transmitted by propeller shaft is further increased by means of gear reduction of final drive (drive pinion and ring gear at differential). The torque of final drive, provided a differential is fitted, is always equally divided between each axle shaft irrespective of speed of road wheel, although this does not apply to limited-slip type differential.


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