Diesel Locomotives
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| Figure 3: A diesel-mechanical locomotive is the simplest type of diesel locomotive. It has a direct mechanical link between the diesel engine and the wheels instead of electric transmission. The diesel engine is usually in the 350-500 hp range and the transmission is similar to that of an automobile with a four speed gearbox. Other parts are similar to the diesel-electric locomotive but there are some variations and often the wheels are coupled. Diagram: Author. Fluid Coupling In a diesel-mechanical transmission, the main driveshaft is coupled to the engine by a fluid coupling. This is a hydraulic clutch, consisting of a case filled with oil, a rotating disc with curved blades driven by the engine and another connected to the road wheels. As the engine turns the fan, the oil is driven by one disc towards the other. This turns under the force of the oil and thus turns the driveshaft. Of course, the startup is gradual until the fan speed is almost matched by the blades. The whole system acts like an automatic clutch to allow a graduated start for the locomotive. Gearbox This does the same job as that on an automobile. It varies the gear ratio between the engine and the road wheels so that the appropriate level of power can be applied to the wheels. Gear change is manual. There is no need fora separate clutch because the functions of a clutch are already provided in the fluid coupling. Final Drive The diesel-mechanical locomotive uses a final drive similar to that of a steam engine. The wheels are coupled to each other to provide more adhesion. The output from the speed gearbox is coupled to a final drive and reversing gearbox which is provided with a transverse driveshaft and balance weights. This is connected to the driving wheels by connecting rods. Hydraulic Transmission Hydraulic transmission works on the same principal as the fluid coupling but it allows a wider range of "slip" between the engine and wheels. It is known as a "torque converter. When the train speed has increased sufficiently to match the engine speed, the fluid is drained out of the torque converter so that the engine is virtually coupled directly to the locomotive wheels. It is virtually direct because the coupling is usually a fluid coupling, to give some "slip. Higher speed locomotives use two or three torque converters in a sequence similar to gear changing in a mechanical transmission and some have used a combination of torque converters and gears. Some designs of diesel-hydraulic locomotives had two diesel engines and two transmission systems, one for each bogie. The design was poplar in Germany (the V series of locomotives, for example) in the sand was imported into parts of the UK in the s. However, it did notwork well in heavy or express locomotive designs and has largely been replaced by diesel-electric transmission. Wheel Slip Wheels slip is the bane of the driver trying to get a train away smoothly. The tenuous contact between steel wheel and steel rail is one of the weakest parts of the railway system. Traditionally, the only cure has been a combination of the skill of the driver and the selective use of sand to improve the adhesion. Today, modern electronic control has produced a very effective answer to this age old problem. The system is called creep control. Extensive research into wheel slip showed that, even after a wheelset starts to slip, there is still a considerable amount of useable adhesion available for traction. The adhesion is available up to a peak, when it will rapidly fall away to an uncontrolled spin. Monitoring the early stages of slip can be used to adjust the power being applied to the wheels so that the adhesion is kept within the limits of the "creep" towards the peak level before the uncontrolled spin sets in. The slip is measured by detecting the locomotive speed by Doppler radar (instead of the usual method using the rotating wheels) and comparing it to the motor current to see if the wheel rotation matches the ground speed. If there is a disparity between the two, the motor current is adjusted to keep the slip within the "creep" range and keep the tractive effort at the maximum level possible under the creep conditions. Diesel Multiple Units (DMUs) The diesel engines used in DMUs work on exactly the same principles as those used in locomotives, except that the transmission is normally mechanical with some form of gear change system. DMU engines are smaller and several are used on a train, depending on the configuration. The diesel engine is often mounted under the car floor and on its side because of the restricted space available. Vibration being transmitted into the passenger saloon has always been a problem but some of the newer designs are very good in this respect. There are some diesel-electric DMUs around and these normally have a separate engine compartment containing the engine and the generator or alternator. Diesel Engine Background The diesel engine was first patented by Dr Rudolf Diesel (1858-1913) in Germany in 1892 and he actually got a successful engine working by. By 1913, when he died, his engine was in use on locomotives and he had setup a facility with Sulzer in Switzerland to manufacture them. His death was mysterious in that he simply disappeared from a ship taking him to London. The diesel engine is a compression-ignition engine, as opposed to the petrol (or gasoline) engine, which is a spark-ignition engine. The spark ignition engine uses an electrical spark from a "spark plug" to ignite the fuel in the engine's cylinders, whereas the fuel in the diesel engine's cylinders is ignited by the heat caused by air being suddenly compressed in the cylinder. At this stage, the air gets compressed into an area 1/25th of its original volume. This would be expressed as a compression ratio of 25 to 1. A compression ratio of 16 to 1 will give an air pressure of 500 lbs/in² (35.5 bar) and will increase the air temperature to over F (427°C). The advantage of the diesel engine over the petrol engine is that it has a higher thermal capacity (it gets more workout of the fuel, the fuel is cheaper because it is less refined than petrol and it can do heavy work under extended periods of overload. It can however, in a high speed form, be sensitive to maintenance and noisy, which is why it is still not popular for passenger automobiles. Diesel Engine Types There are two types of diesel engine, the two-stroke engine and the four-stroke engine. As the names suggest, they differ in the number of movements of the piston required to complete each cycle of operation. The simplest is the two-stroke engine. It has no valves. The exhaust from the combustion and the air for the new stroke is drawn in through openings in the cylinder wall as the piston reaches the bottom of the downstroke. Compression and combustion occurs on the upstroke. As one might guess, there are twice as many revolutions for the two-stroke engine as for equivalent power in a four-stroke engine. The four-stroke engine works as follows Downstroke 1 - air intake, upstroke 1 - compression, downstroke 2 - power, upstroke 2 - exhaust. Valves are required for air intake and exhaust, usually two for each. In this respect it is more similar to the modern petrol engine than the 2- stroke design. In the UK, both types of diesel engine were used but the stroke became the standard. The UK Class 55 "Deltic" (not now in regular mainline service) unusually had a two-stroke engine. In the US, the General Electric (GE) built locomotives have stroke engines whereas General Motors (GM) always used stroke engines until the introduction of their SD90MAC 6000 hp "H series" engine, which is a stroke design. The reason for using one type or the other is really a question of preference. However, it can be said that the stroke design is simpler than the stroke but the stroke engine is more fuel efficient. Size Does Count Basically, the more power you need, the bigger the engine has to be. Early diesel engines were less than 100 horsepower (hp) but today the US is building 6000 hp locomotives. Fora UK locomotive of 3,300 hp (Class 58), each cylinder will produce about 200 hp, and a modern engine can double this if the engine is turbocharged. The maximum rotational speed of the engine when producing full power will be about 1000 rpm (revolutions per minute) and the engine will idle at about 400 rpm. These relatively low speeds mean that the engine design is heavy, as opposed to a high speed, lightweight engine. However, the UK HST (High Speed Train, developed in the s) engine has a speed of 1,500 rpm and this is regarded as high speed in the railway diesel engine category. The slow, heavy engine used in railway locomotives will give low maintenance requirements and an extended life. There is a limit to the size of the engine which can be accommodated within the railway loading gauge, so the power of a single locomotive is limited. Where additional power is required, it has become usual to add locomotives. In the US, where freight trains run into tens of thousands of tons weight, four locomotives at the head of a train are common and several additional ones in the middle or at the end are not unusual. To V or not to V Diesel engines can be designed with the cylinders "inline, "double banked" or in a "V. The double banked engine has two rows of cylinders inline. Most diesel locomotives now have V form engines. This means that the cylinders are split into two sets, with half forming one side of the V. AV engine has 4 cylinders set at an angle forming one side of the V with the other set of four forming the other side. The crankshaft, providing the drive, is at the base of the V. The V was a popular design used in the UK. In the US, V is usual for freight locomotives and there are some designs with V engines. Engines used for DMU (diesel multiple unit) trains in the UK are often mounted under the floor of the passenger cars. This restricts the design to inline engines, which have to be mounted on their side to fit in the restricted space. An unusual engine design was the UK 3,300 hp Class 55 locomotive, which had the cylinders arranged in three sets of opposed Vs in a triangle, in the form of an upturned delta, hence the name "Deltic". Tractive Effort, Pull and Power Before going too much further, we need to understand the definitions of tractive effort, drawbar pull and power. The definition of tractive effort (TE) is simply the force exerted at the wheel rim of the locomotive and is usually expressed in pounds (lbs) or kilo Newtons (kN). By the time the tractive effort is transmitted to the coupling between the locomotive and the train, the drawbar pull, as it is called will have reduced because of the friction of the mechanical parts of the drive and some wind resistance. Power is expressed as horsepower (hp) or kilo Watts (kW) and is actually a rate of doing work. A unit of horsepower is defined as the work involved by a horse lifting 33,000 lbs one foot in one minute. In the metric system it is calculated as the power (Watts) needed when one Newton of force is moved one metre in one second. The formula is P = (F*d)/t where Pis power, F is forced is distance and t is time. One horsepower equals 746 Watts. The relationship between power and drawbar pull is that a low speed and a high drawbar pull can produce the same power as high speed and low drawbar pull. If you need to increase higher tractive effort and high speed, you need to increase the power. To get the variations needed by a locomotive to operate on the railway, you need to have a suitable means of transmission between the diesel engine and the wheels. One thing worth remembering is that the power produced by the diesel engine is not all available for traction. Ina hp diesel electric locomotive, some 450 hp is lost to on-board equipment like blowers, radiator fans, air compressors and "hotel power" for the train. Starting A diesel engine is started (like an automobile) by turning over the crankshaft until the cylinders "fire" or begin combustion. The starting can be done electrically or pneumatically. Pneumatic starting was used for some engines. Compressed air was pumped into the cylinders of the engine until it gained sufficient speed to allow ignition, then fuel was applied to fire the engine. The compressed air was supplied by a small auxiliary engine or by high pressure air cylinders carried by the locomotive. Electric starting is now standard. It works the same way as for an automobile, with batteries providing the power to turn a starter motor which turns over the main engine. In older locomotives fitted with DC generators instead of AC alternators, the generator was used as a starter motor by applying battery power to it. Governor Once a diesel engine is running, the engine speed is monitored and controlled through a governor. The governor ensures that the engine speed stays high enough to idle at the right speed and that the engine speed will not rise too high when full power is demanded. The governor is a simple mechanical device which first appeared on steam engines. It operates on a diesel engine as shown in Figure 4. Modern diesel engines use an electronic governor system that replicates the requirements of the mechanical system. Download 0.84 Mb. Share with your friends:
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