Tractor Design & Testing 3(2+1)

Lesson 5. Design requirements for tractor engine components and systems.

2. Stroke-Bore Ratio:

Major principles set by a French Engineer for an IC engine that combustion should have smallest possible surface to volume ratio, expansion process should be rapid and compression at the start of expansion should be as high as possible must be strictly followed. Stroke-bore ratio in engines is a design consideration because higher compression ratios are generally permissible with small bores and higher thermal efficiencies may be attained with the higher compression ratios. However, assuming equal compression ratio and displacement per stroke, a cylinder with a large stroke-bore ratio will have a higher surface to volume ratio. Obviously, a decrease in the length of the stroke necessitates an increase in the bore in order to maintain the same displacement. This larger surface ratio permits more heat transfer to the combustion chamber walls, resulting in a decreasing efficiency. The short stroke engine having less piston travel has lesser friction. The breathing capacity or volumetric efficiency of high speed engines can be improved by decreasing the stroke bore ratio, which permits the use of larger or multiple valves. This ratio for tractors tested at Nebraska in 1982, the average stroke to bore ratio in tractor engines was decreased to 1.01 from 1.29 in the year 1920. Average stroke-bore ratio of the tractor engine in India was about 1.33 in 2001 (Table 2). There is the tendency to further reduce this stroke-bore ratio hence recent engines have a smaller stroke-bore ratio which allows higher engine speeds, resulting in more compact size and reduced vibration. One model Swaraj 744 and three models of Escorts range of tractor viz. Farmtrac 45, 50 & 55 have stroke-­bore ratio equal to one (Table 2). Even in one model of Escort, this ratio is less than one i.e. 0.95. Technology of turbocharging the engine has also been introduced recently in the Indian tractors.

3. Specific Fuel Consumption (SFC):

Fuel efficiency is becoming an increasingly important factor in farm economics. Obviously, tractors with the lowest SFC are the most efficient. There is the continuous effort by all the manufacturers to reduce the SFC for their newly introduced models. Among the six major players of tractor in India, the Mahindra 575DI has the lowest SFC at max. power i.e. 233 g/kWh. It is clear from the table 2 that wt/power ratio of the tractors in India is being reduced, which results in the tractors travelling faster, since improvements in traction have not kept pace with the increase in power. Efforts are needed to improve the traction of the tractor. Indian manufacturers have vastly improved the weight to power ratio of their prime mover by either the slimming process on castings derived through CAD/CAM designs or by change of designs to accommodate higher break mean effective pressures reaching peak values close to 23 bars.

4. Compression Ratio:

The thermal efficiency of an engine does not approach the theoretical efficiency of an Otto cycle engine as is given by
                                                                                  ... (5.1)

Where

r = Compression ratio

n = C_p/C_v (Ratio of specific heat of air at constant pressure and constant volume)

Hence, it is apparent that the efficiency of an ideal otto cycle increases as the compression ratio increases.
There are many reasons why the actual efficiency is substantially below the theoretical efficiency. Some of the discrepancies can be accounted for by the following factors

1. The compression and expansion of gases is not adiabatic

2. There are friction losses in the engine

3. Work is required to draw in and exhaust the gases

4. For a free breathing engine (not supercharged) the volumetric efficiency will be less than 100%

5. Complete combustion does not occur

Large changes in the compression ratio of diesel engines are not practical. Most diesel engines have compression ratios that range from 16:1 to 22:1

5. Design of Combustion Chamber:

The shape and size of the combustion chamber has a decided effect upon the proper mixing of the fuel and air and also upon the fuel detonation. In general the combustion chamber is designed to create turbulence of the mixture for better combustion and greater flame propagation velocity, thereby decreasing the combustion time and improving antiknock characteristics. A short and compact combustion chamber generally requires lesser time for combustion and improves the antiknock characteristics of fuel
There are many variations in the design of combustion chamber of diesel engines fitted in the tractors. Some variations are due to the different methods of starting the combustion chamber process in the cylinder. Because the fuel to air ratio is very low as the fuel is first injected into the cylinder, ignition will not begin until the ratio is in the combustible range. Thus if the fuel were injected directly into the combustion chamber, there would be an unnecessarily long delay before ignition begins, resulting in a very high initial pressure.
Four types of combustion chambers are used in diesel engines namely direct injection (DI), pre, swirl or turbulence and auxiliary combustion chambers. Most common method of diesel injection is the direct-injection (Dl) in which combustion chamber generally employs a concave piston head. M&M ltd. has to introduce in tractor engines to increase the thermal efficiency, results to lower the fuel consumption claimed this method of injection first. In DI engines there is also tine atomisation of the fuel in the combustion chamber. Two manufacturers HMT and Eicher have claimed the turbulence or swirl combustion chamber in their tractor engines. Due to turbulence or swirl action in these type of combustion chamber there is a greater utilisation of air therefore higher mean effective pressure is attained at the clean exhaust from the engine. These engines are also called indirect injection (IDI) engines. IDI engines are also quieter than the DI engines and can be run on fuel with a lower cetane number. In the swirl combustion chamber, burning of fuel-air mixture is caused by swirl action thereby improving the mixing and subsequent combustion in the main combustion chamber. The swirl chamber contains from 50 to 90% of the compressed volume when the piston is on top dead center (TDC). Eicher ltd. gave the term Ricardo type engines to these IDI engines, since Ricardo first used that design. With the better swirl control in these engines, noise level was also reduced in greater extent. Some of the models of different tractor with their specifications shown in Table 5.1.

6. Fuels:

Almost all of the fuels commonly used in farm tractors are products of crude petroleum. Crude petroleum is made of combined carbon and hydrogen in approximately the proportion of 86% carbon and 14% hydrogen. The atoms of carbon and hydrogen may be combines in many ways to from many different hydrocarbon compounds from crude oil. The two grades of diesel fuels commonly used in US are Grade No. 1-D and 2-D. Grade no. 1-D diesel fuel is volatile fuel is applicable for use in engines used for relatively wide variations in loads and speeds. Grade No. 2-D type fuels are lower volatile used for the engine used relatively at high loads and uniform speeds or in engines not requiring fuels having the higher volatility. Fuel quality is specified because engine design and operation affect the type of fuel best suited for an engine.

I. Cetane number:

The ignition quality of diesel fuel is determined by its cetane number/index. In general, high cetane fuels permit an engine to be started at lower air temperatures, provide faster engine warm-up without misfiring or producing white smoke, reduce the formation of varnish and carbon deposits and eliminate diesel knock. Generally diesel fuels marketed in USA have a range from 33 to 64 cetane numbers but for tractor diesel fuels having 38 to 52 cetane numbers are generally used.

Proper viscosity or resistance to internal flow of the diesel fuel is required.  The injector pumps perform best when the fuel has proper viscosity. If the viscosity is too low, more frequent maintenance and repair of injection system may result. If the viscosity is too high, excessively high pressures may result in the injection system. The desired kinematic viscosity for diesel fuel is 1.5 (Min) to 4.0 (Max) centistokes at 40^o C temperature.

A diesel fuel often has a tendency to form carbon deposits in the engine. Carbon residue tests are very effective in predicting carbon formation of base fuels, but they may be in error when the fuels contain additives such as ignition improvers like hexyl nitrates. Carbon residue in diesel fuel generally ranges from 0.15 to 0.35%.

II. Flash point:

The flash point is not directly related to engine performance. It is however of importance in connection with legal requirements and safety precautions involved in fuel handling and storage and is normally specified to meet insurance and fire regulations.

Diesel engine injectors are precision made and therefore are quite sensitive to any abrasive material in the fuel. Since the ash content is directly related to wear of the injection system, it must be kept low. Diesel fuel has a maximum allowable ash content of 0.01%.The additives in diesel fuel are for engine protection and to improve engine performance. Antioxidants, metal deactivators and corrosion inhibitors are used in diesel fuels. 

III. Biodiesel:

As fossil fuels become scarce, other fuels like bio-diesel will have to be used. There are many factors that must be considered in the selection of an alternative fuel. Some of these factors are cost per unit of work done, availability, compatibility with the engine, safety, storage, management and convenience.Biodiesel is a clean, oxygenated fuel made from renewable agricultural resources such as Jatropha, soybeans, rapeseeds etc.. It is simple to use, biodegradable, and free of sulfur. Using biodiesel fuel reduces particulate emissions, as well as decreases dependence on crude oil. It can be used as blend with diesel as B2 fuel (2 percent biofuel and 98 percent diesel). John Deere has approved B5 fuel for tractor engines and many other manufacturers in the industry are following the lead. The technical problems often reported include excess carbon deposits on pistons and exhaust valves, fuel filter cogging and engine oil dilution with the use of plant oils for IC engines. These problems must be solved before plant oils can become a viable alternative fuel.

IV. Fuel Injection Systems:

Generally three types of fuel injection systems used in tractors are: individual or in-line type, distributor type and unit injector type (Fig. 5.1). In function the distributor system and inline or individual pump systems are the same. The difference is that the distributor system replaces the individual pumps with a single pump plus a distributor rotor.  Because of compact nature of the distributor type pump, its principle of operation is more difficult. The distributor pump performs the different functions like metering the quantity of fuel determined by the governor, delivering the fuel at high pressure in the range from 10000 to 20000 kPa to the injectors and timing of the fuel injection that is the function of engine speed. Unit injector combines the function of high pressure pump and injector into one unit. One advantage of such a system is that there is no high pressure line to complicate the timing and metering of the fuel. The unit injector is commonly driven by a separate cam shaft.  TAFE and New Holland have introduced rotary type FI pump in the fuel injection system of their tractors.

 

Fig. 5.1: Fuel injection system
(Source: CIGR Handbook of Agricultural Engineering Volume III)

7. Air Cleaning System:

Dust entering a tractor engine is often the principal cause of wear. The location of the air inlet affects the quantity of dust to be removed by the air cleaner. The dust concentration was highest near the engine and lowest in the region directly above the engine. For this reason, tractor designers usually select an air inlet above or near the top of the engine housing. Dry type and wet type or oil bath type air cleaners are generally used for the tractors (Fig.5.2 and 5.3). Efficiency of dry type air filters exceeds that of the oil bath type. Because a tractor operates most of the time at less than full load, the relative cleaning efficiency at one half load is significant. At less than rated airflow the efficiency of the oil bath cleaner tends to decrease, whereas that of the dry type filter remains high. Almost by all the manufacturers 3-stage air cleaning first by pre-cleaner followed by oil bowl and paper element has been introduced for tractor engines.

Fig. 5.2: Oil bath type air cleaner

(Source: http://vintagetractorengineer.com/2009/01/oil-bath-air-cleaners-for-tractors/)

Fig. 5.3: Oil bath type air cleaner assembly
(Source: http://www.metlonics.com/images/TRACTOR/L7.jpg)

8. Cooling System:

The proper design and maintenance of a cooling system (Fig. 5.4) are extremely important because the amount of heat to be dissipated is high. The heat rejected to the cooling water by a diesel engine at 2000 rpm is approximately 0.58kW for each kW of output. Because a tractor usually operates at a high load factor, a relatively large cooling capacity must be supplied compared to that for an automobile. Water flow for the cooling system ranges from 0.7 to 1.4 l/kW of engine output and pumps are usually selected to deliver 25 to 90 ml/s per kW. Fan power is normally 5 % of gross engine power, with a range of 2.5 to 10%. Radiator frontal areas of 19 to 29 sq.cm/kW engine power output. Temperature drop through the radiator can be assumed to be 5.5 to 8.5 Degree C. 

Fig. 5.4: Cooling system
(Source: http://www.crankshaftcoalition.com/wiki/File:Engine-cooling-system.jpg)
(http://web.utk.edu/~tprather/FoothillsTractorClub/TechTips/cooling.gif)

9. Governing System:

The engine control device is called a governor. The governor can be mechanical or an electronic device. In general, governors for IC engines are of the centrifugal-force, spring loaded type.

A good governor for agricultural tractor engines is taken to be one giving performance such that the difference between no load speed and maximum load speed is less than 100 rpm.  The maximum load and no, load are at maximum governor settings on both sides. As with the increase of load the crankshaft torque rises to a peak and that maximum torque is developed at a speed well below governed speed and therefore well below the speed at which maximum power is obtained. It is an important phenomenon. When the engine is operating at the full throttle, its speed and power are controlled solely by load and some reserve is provided by governor in order to cope with sudden increase of load. If at any particular speed of engine set by the operator, the increase or decrease in load will decrease or increase the speed is regulated by the governor without changing the gear. But a tractor fitted with a suitable diesel engine having more than 500 rpm in reserve that is the difference in maximum power speed and maximum torque speed, will possess good slogging ability.