8 Main Elements of Diesel Electric Power Plants

A diesel electric power plant comprises of the engine proper and the auxiliary equipment as follows: 1. Diesel Engine 2. Engine Air Intake 3. Engine Fuel System 4. Engine Exhaust System 5. Engine Cooling System 6. Engine Lubricating Oil System 7. Engine Starting 8. Diesel Engine Generator.

Element # 1. Diesel Engine:

This is the main component of the power plant which develops power. The diesel engines em­ployed for diesel electric power plants may be four or two stroke engines. In a 4-stroke engine the complete cycle of operations is performed in four strokes namely suction, com­pression, expansion (working or power) and exhaust strokes and two revolutions of the engine.

In the case of a two- stroke diesel engine all the four operations are completed within two strokes of the piston or during one revolution of the crank shaft—the power and exhaust operations are completed during the travel of the piston in backward direction (i.e., toward crank end of the cylinder) while intake and compression operations are completed during the forward travel of the piston.

A two-stroke engine is rugged and compact in construction, simple in mechani­cal design, easily reversible, cheaper in cost and develops more power for the same speed and piston displacement because of elimination of two idle strokes. Flywheel re­quired is also smaller one because of development of more uniform torque.

However, a four-stroke engine has lower specific fuel consumption and more effective lubrication, more flexibility, less noisy exhaust, simple and better cooling, better scavenging and higher efficiency than a two-stroke engine.

The engine may be horizontal or vertical type. Hori­zontal engines are used for comparatively smaller output while vertical engines with multi-cylinder construction are employed for larger outputs. The arrangement of cyl­inders is important as it affects the foundations, building space and maintenance problems.

Vertical in-line arrange­ment of cylinders is the most commonly used. To make the engine more compact, the cylinders may be arranged in V-shape (used for medium and large aircrafts). De­pending upon the requirement of power the number of cylinders is decided. Six to eight cylinder engines are commonly used in power stations.

The ignition may be through compression ignition or spark ignition. In the compression-ignition engine, the combustion starts by spontaneous ignition of fuel and air because of high temperature developed by the compres­sion of the air while in the spark-ignition engine, air-fuel mixture is compressed in the cylinder and at the end of compression, and the mixture is ignited with the help of a spark. The diesel engines are compression-ignition type.

The diesel engines are available in low (lesser than 350 rpm), medium (lying between 350 and 1,200 rpm) and high speed (exceeding 1,200 rpm) ranges. It has been found that a higher rotational speed means a greater de­gree of compactness resulting in reduction of costs of both engines and generators and simultaneously in build­ing requirements. Speeds commonly used are in the range from 500 to 1,000 rpm.

The rating of an engine which should be specified correctly is the net output in kW developed continuously at the crankshaft coupling by the engine in good operat­ing condition, at a height of less than 450 m, at a tem­perature of 32°C and a barometric pressure of 717.5 mm Hg. Diesel engines are available in sizes from 75 kW to 3,750 kW.

Ambient conditions differing from the above have a marked effect on the output power. The power output is reduced by the high temperature firstly due to reason of diminished cooling effect of air and secondly by the ex­pansion of air due to higher temperatures and its conse­quent thinning resulting in less fuel energy being con­verted into mechanical energy during each working cycle in a cylinder of a diesel engine.

At high altitudes the rarefied air produces an even greater reduction of output in the manner just described and in addition, its cooling power is reduced further by rarefication over and above the reduction produced by higher temperature.

The selection of engine size depends on the plant capacity. The overload capacity of the plant is very low; its operation at loads less than full load is uneconomical and its use is limited to emergency and special services, the determina­tion of plant capacity is very important.

The size and number of engines depends upon plant capacity, purpose for which plant is being set up and the load characteristics. For small scale industries only one or two engines are sufficient. The total space available in the installation is also an important consideration in de­termination of number and size of units.

Diesel engines, large capacity industrial engines, have efficiencies in the range of 35-42%.

Element # 2. Engine Air Intake including Air Filters, Ducts and Supercharger (Integral) with the Engine:

Air in­take system is provided to supply necessary air to engine for fuel combustion. Air requirements of large diesel elec­tric power plants are considerable (about 4-8 m³ per kWh). The air filters are provided to remove dust and other sus­pended impurities from the air to be supplied to the engine.

The supercharger is usually employed to increase the pres­sure of intake air above atmospheric one in order to develop an increased power output. The air is drawn from outside the engine room and delivered to the inlet manifold through ducts or pipes and filters. In cold season, the intake air is heated by the heat from exhaust gases, before injection.

The filters are to be cleaned periodically. Superchargers are usually driven by the engines. Sometimes, a silencer is required in between the engine and the intake since the noise may be transmitted back to the outside air via the air intake system. There should be minimum pressure loss in the air intake system, otherwise specific fuel con­sumption will increase and the engine capacity will be reduced.

Element # 3. Engine Fuel System including Fuel Storage Tanks, Fuel Transfer Pumps, Strainers, Heaters and Connect­ing Pipe Work:

Fuel transfer pumps are required to trans­fer fuel from delivery point to storage tanks and from stor­age tanks to daily consumption tanks and then to engine. Strainers are provided to remove suspended impurities and thus ensure clean fuel supply to the engine. Heaters are required to heat the oil specially during winter season.

The fuel oil is supplied at the plant site by rail or road. The fuel oil is stored in bulk storage tanks which are usually situated outdoor for safety. The fuel oil is transferred from these tanks to the small storage tanks called day tanks daily or at suitable intervals by means of transfer pumps through filters. From the day tank the fuel oil is passed through the strainers and fired into the engine through fuel filter and fuel injection pump.

The capacity of the daily consumption tank should be atleast the 8 hour requirement of the plant. Nowadays all diesel power plants employ mechanical injection system in which fuel at a pressure of about 100 kg/cm² is delivered mechanically, through the atomizer nozzle, into the compressed air.

Element # 4. Engine Exhaust System including Silencers and Connecting Ducts:

This system is provided to discharge the engine exhaust to the atmosphere outside the building. The exhaust manifold connects the engine cylinder exhaust to the exhaust pipe provided with a muffler to reduce pressure in exhaust line and eliminate most of the noise which may result if the waste gases were discharged directly into the atmos­phere.

As the temperature of the exhaust gases is sufficiently high, therefore, the heat of these gases is utilised in heating oil or air supplied to the engine. In case of diesel electric generating units installed in steam power plants exhaust system also includes water heaters and steam boilers in order to utilise the heat of waste gases.

Element # 5. Engine Cooling System including Cooling Pumps, Cooling Towers or Spray Ponds, Water Treatment or Filtration Plant and Connecting Pipe:

It is known that the heat generated by the burning of fuel in the engine cyl­inder is partially converted into useful work. The remainder is wasted as heat in the outgoing exhaust gases and in heat­ing the engine, and if not removed may disintegrate the lu­bricating oil film on the cylinder walls and damage the cyl­inder liners, heads, walls, piston and rings. Small engines may be air-cooled but large engines mostly make use of forced water cooling.

In the forced water cooling cold water is sent through the cylinder jacket with the help of a pump. The hot water is cooled in spray pond and is re-circulated. The water used for cooling engine cylinder is softened by water treatment or water filtration plant in order to avoid formation of scales etc., in it. Hot well is provided in the surge tank to make a provision for expansion of jacket water.

It is also necessary to keep the exit temperature of the cool­ing water around 70°C. In case it is too low, the lubricating oil film will not spread properly and will result in wear of cylinder and piston and if it is too high, the lubricating oil will burn. This requires a control of the flow of cooling water according to the load on the engine.

Element # 6. Engine Lubricating Oil System including Lubricating Oil Pumps, Oil Tanks, Filters, Coolers, Purifiers and Connecting Pipe Work:

This system is of great importance and is provided for lubricating the moving parts, removing the heat from the cylinders and the bearings, helping the piston rings to seal the gases in the cylinders and for carrying away the solid dirt particles from the rubbing parts. The life of the engine and its efficiency depend largely on its lubrication system.

The parts of the engine requiring lubrication are piston and cylinders, gears, crankshaft and connecting rod, bearings etc. Piston and cylinder require special lubricating oil. The forced feed lubrication is mostly employed. In this system of lubrication the lubrication oil is drawn from the sump by means of a pump and is passed through a strainer and then through a filter in order to remove impurities.

Usually the lubricating oil is hot when drawn from a sump, if it is not, it should be heated, first before passing through the filters, in order to decrease its viscosity and make the filtration easy. The oil is then cooled through a heat exchanger and then sent into the engine. The lubricating oil costs of a diesel power plant are about 30% of the fuel costs.

Element # 7. Engine Starting including Battery, Starter, Com­pressed Air Supply etc.:

This system is provided to rotate the engine initially, while starting, until the firing starts and the unit runs under its own power. Small sets are usually started manually by handles but for sets of large capacity, say above 75 kW, the compressed air system is mostly employed for starting diesel engines. Battery driven motors can also be used for starting the diesel engine sets.

In compressed air system of starting, the compressed air, from a tank, at high pressure (say 20 atmospheric pressure) is admitted to a few of the engine cylinders making them to act as reciprocating air motors to turn the engine shaft. The fuel is admitted to the remaining cylinders which causes the engine to start under its own power. A motor driven com­pressor is employed for supply of air to the compressed air tank.

Element # 8. Diesel Engine Generator (Alternator):

The al­ternators used in diesel electric power plants are of rotat­ing field, salient pole construction, speed ranging from 214 to 1,000 rpm (poles 28 to 6) and capacities ranging from 25 to 5,000 kVA at 0.8 pf lagging. Their output voltages are 440 V in case of small machines and as high as 2,200 V in case of large machines. Voltage regulation is about 30%.

They are directly coupled to the diesel engines. They are provided with automatic voltage regulators to allow close voltage regulation and satisfactory parallel opera­tion. The excitation is usually provided at 115 or 230 V from a dc exciter of rating about 2-4% of the alternator rating, usually coupled to the engine shaft either directly or through a belt.

In addition to above, the diesel power plants are pro­vided with different instruments such as thermometers or pyrometers (for measuring temperatures of lubricating oil, exhaust, jacket water, fuel oil, injection air etc.), pres­sure gauges (for measuring pressures of inter-stage and final stage of starting air system, lubricating oil system before and after cooling, cooling water, jacket water and fuel oil before and after straining), ammeters (for meas­uring excitation and output currents), voltmeters (for meas­uring excitation and output voltage), watt meters (for measuring power outputs), synchronizing equipment, vis­ible and audible alarms for giving indications of high jacket water temperature, high lubricating oil tempera­ture, low lubricating oil pressure, low jacket water pressure, incorrect starting, blower motor overcurrent etc.