Tractor is a slow moving machine compared to cars and trucks. The tractor has to be versatile, so that it may give efficiency and economic service in different soils and climate. The speed requirement for different agricultural operations varies from 0.5 km per hour to 25 km per hour. Operations such as ploughing, digging, ridging etc. are heavy jobs, whereas sowing, cultivating etc. are light jobs.
Performance Characteristics of Tractor Engines:
It is noticed that tractor engine produces its transmission power at greatly reduced speeds compared to a car engine. Transmission unit is heavier, larger and rugged in a tractor. A variable speed governor is necessary on a tractor to maintain engine speed when the load varies.
Basic requirements of a tractor engine are different from that of a car engine. It is estimated that a tractor engine requires to be operated at full load approximately 25 to 30 per cent of the operating time. Most of the time, the engine is required to be operated between 50 to 70% of the full load.
An increase in load, applied to a tractor by a hitched implement, demands from the engine a corresponding increased torque. Usually the tractor is subjected to varying load conditions and in order to enable the engine to bear momentary overloads, it is designed to produce its maximum torque at a much lower engine speed than that at which maximum power is developed. Fig. 10.33 shows typical curves drawn between bhp vs speed and torque vs speed of a tractor engine. It may be seen that at the speed corresponding to maximum power at 1900 rpm, the torque is 88% of the maximum torque. The maximum torque is produced at 1400 rpm. The maximum torque is 110% of the torque at maximum power.
The shape of the torque speed curve is dependent upon the shape of volumetric efficiency speed curve In case of a diesel engine; two volumetric efficiencies are to be considered, namely that of a fuel pump and that of the engine pistons and cylinders acting as pumps.
This phenomenon tends to give a flatter torque curve as compared to a spark ignition engine i.e. the power in diesel engine decreases much slower than that of petrol engine. In diesel engine, the torque is considerably high at lower than rated speed. This behavior of diesel engine is called Slogging or Lugging ability. Thus, it is said that diesel engine slogs better than petrol engine.
A tractor has a variable speed governor for maintaining almost a constant speed at different loads. Generally, governor is so adjusted that maximum torque is obtained not at a maximum BHP but at some lower BHP (Fig. 10.33). If there is a difference of 500 rpm between maximum torque and maximum BHP it is said to be a good governor.
Also if the difference is 100 rpm between no load and maximum BHP, it is said to be a good governor. Fig. 10.33 shows that maximum torque is at about 1400 rpm and maximum BHP is at about 1900 rpm, thus there is difference of 500 rpm which is the indication of a good governor setting. The difference between no load and maximum BHP is also about 100 rpm, which is also an indication of a good governor.
Fig. 10.34 shows a relation between specific fuel consumption and brake mean effective pressure. At high loads, the specific fuel consumption curve is almost flat. For economic operations, the load on the tractor should be such that engine may work on the flat portion of curve for longer duration of time. Care has to be taken that fuel consumption curve may not rise steeply as load is reduced.
Fig. 10.35 shows a relationship between drawbar pull and drawbar hp. There is one curve corresponding to each gear of the tractor. The point of maximum drawbar hp of each gear can be joined by an envelope as shown in the Fig. 10.35 by a dotted line.
Gear spacing is the interval between two points of maximum drawbar pull as shown in the Fig. 10.35. Hence greater the number of gears, closer is the gear spacing and better is the working of the tractor. It is always desirable to have large number of equally spaced gears on a tractor engine.
Fig. 10.36 shows relationship between- (i) torque vs rpm, (ii) specific fuel consumption vs rpm, (iii) bhp vs rpm of a typical tractor engine.
The minimum specific fuel consumption occurs at speed of 1650 rpm. This condition corresponds to the maximum efficiency. It is the most economical speed for the engine. The specific fuel consumption per bhp-hr is a measure of overall efficiency of the engine. The best speed of the engine is the speed at which the specific fuel consumption per bhp-hr is minimum.
Tractor engines are designed to produce their maximum efficiency at a certain speed. It is observed that as the speed increases the efficiency increases till the speed for maximum efficiency is reached.
If the speed is further increased, the efficiency goes down. The horse power of the engine also increases with the speed and continues to increase even after the speed for maximum efficiency is passed, but there is more rapid increase in specific fuel consumption beyond this speed.
In short, it can be said that the speed of the engine crankshaft torque and gear spacing are required to be so designed that the torque- speed requirements of the tractor for a variety of farm jobs are satisfied.