13Diesel Power Plants
Diesel plants are more efficient than any other heat engine ofcomparable size. These plants are cheap by way of initial cost, canbe started and stopped quickly and can burn a wide range of fuels.A Diesel plant does not require any warming period; it need not bekept running for a long time before peaking up loads. As a resultthere is no standby losses. Another advantage of such a plant isthat it does not need large amount of water for cooling. A dieselplant can be commissioned in such a much little time comparedwith a hydro, steam or nuclear power station.
In view of these advantages a Diesel station is suitable forlocalities where fuel costs are low, where water supply is limited,where oil is cheaper than coal and where loads are of such magnitudesthat they can be handled by a plant of small capacity.
Another means of generating electricity (i.e. hydro, thermal,nuclear) are rivals to Diesel plants and can be attractive undercertain conditions. Also a gas turbine plant for continuous powergeneration is superior to a diesel plant where fuel is very cheap (asat a refinery or where load factors are very poor).
Not withstanding competition from its rivals a diesel plantprovides the most economical means of generating electricity onsman scale particularly where there is no convenient site for micro-hydroplants, cheap fuels are not available and load factors areconsiderably large.
The important fields of applications of diesel engines are asrail road locomotives, ship propulsion, road building and farmmachinery, electric generators for small supply units for public,industrial and institutional purposes e.g. cinema halls, hospitals,municipalties etc. These are used in freight trucks, and buses.However, since diesel engines can make efficient use of fuels thatare cheaper than gasoline, they are being utilized increasingly inautomobiles.
Diesel electric power plants have been chiefly used as peakload and standby units, for the hydroelectric power plants. These
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are used as emergency standby units which normally remain idleand are run only where there is a failure of the central station andwhere key industrial processes can not be interrupted to avoid financialloss.
132. Diesel Engine : Working Principle and GeneralDescription
An internal combustion engine in which the fuel is ignited byinjecting it into air that has been heated to a high temperature byrapid compression; hence, diesel engines are also called compression-ignition engines. The concept of ignition compression was patentedby Rudolf Diesel in 1892, and first demonstrated in an engine, fiveyears latter. The compression ignition engine is a heat engine (i.e.one that converts heat partially into mechanical work) operating onan approximation to the idealized Diesel cycle in which combustionof the fuel, that is, the heat addition stage, occurs at essentiallyconstant pressure.
Diesel cycle. A repeated succession of operations (or cycle)representing the idealized behaviour of the working fluid in thediesel engine form of heat engine. The diesel cycle is illustrated anddescribed in Fig. (1321.) Following main events are taking place ina cycle.
Fig. ]821. Di('se] cycle.
Suction operation (oa) at constant pressure in which air issucked inside the cylinder from atmosphere at nearly atmosphericpressure.
Adiabatic compression of the working fluid i.e., air (gas) alonga b ; the temperature and pressure are increased.
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Heat addition along beat const?nt pressure; the gas temperatureand volume are increased.
Adiabatic expansion along c d, work is done by the expandinggas, and, the temperature and pressure decrease.
Heat removal (rejection) along d, a at constant volume; thepressure and temperature decrease, and the gas is restored to itsinitial condition at a. Here cycle is completed.
In the description each stage is assumed to have been completedbefore the next stage is initiated. However, in an actual enginethere is a gradual rather than a sharp transition from one stage tonext; hence the sharp points in the figure would actually be roundedoff. In a diesel engine (Fig. 1322), air is down into a cylinder where
Fuel I Both valves ~injector close d
INTAKE STAGE-1 COMPRESSIONSTAGF-2
POWER STAGE-] EXHAUST STAGE-4
Fig. J;1'~'~' "'our stJokc dl(;~'-'I'-'''gllle.
it is compressed adiabatically by the inward motion of the pistonand thereby heated (stage 1). Just prior to maximum compression,fuel is injected and it burns rapidly in the very hot compressed air;heat is thus added to the working fluid at essentially constant pressure(stage 2). The hot combustion gases expand adiabatically and indoing so push back the piston and mechanical work is done stage 3).At stage 4, exhaust valve opens and operation 4 heat rejection andthen exhaust takes place.
Following four strokes are taking place in one cycle.
1. Intake or Suction. The piston moving downward (i.e., outof the cylinder) draws air into the cylinder by way of the openintake valve. The exhaust valve is closed (operation oa).
2. Compression. The intake valve is closed and piston movingupward (i.e., into the cylinder) compresses the air. The pressure isincreased to about 35 to 40 atm. (35 to 4 MPa), and the airtemperature rises to 450 to 500C.
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3. Power. Just before the point of maximum compression, withboth valves closed, a spray of very small droplets of fuel is injectedinto the top of the cylinder. At the existing high temperature of theair the fuel burns rapidly and produces extremely hot compressedgases. The gases expand and push back the piston ; this is thepower stroke in which mechanical work is done. Not all of this workis available, however since part is utilized in the other strokes,especially in the compression stroke.
4. Exhaust. The piston moving upward pushes the some whatcooled gases out through the open exhaust valve.
The network in a Diesel cycle in the difference between theworkdone by the working fluid in stages 2 and 3 and the work doneon the fluid in stage 1. The thermal efficiency (i.e. the fraction ofthe heat supplied in stage 2 that is converted into net mechanicalwork) is increased by increasing the temperature at c and bydecreasing that at d. An equivalent statement is that an increase inthe compression ratio (volume at a divided by volume at .b) anddecrease in the cut ofTratio (volume at c divided by the volume at b)increase the thermal efficiency. The minimum value of the cut offratio is unity.
Four Stroke and Two-Stroke Engines
Diesel engines like spark ignition engines can operate on fourstroke or two stroke cycle (A stroke is an in or an out motion of thepiston). In the four-stroke cycle there are two in and two out motion(i.e., two revolutions of the crankshaft) per cycle. However, only oneof these four strokes is a power stroke ; hence there is only onepower stroke for two rotation of the crankshaft. In the two strokeengine, on the other hand, there, are one in and one out operation(i.e., one rotation of the crank shaft) per cycle. Consequently thereis one power stroke in each rotation of the crankshaft.
The two-stroke diesel engine are designed without valves andwith only two ports in the cylinder wall; the ports are opened andclosed when they are uncovered and covered, respectively, by themoving piston.
The advantage of a two-stroke cycle in providing a power strokefor each revolution of the engine crankshaft, rather than onepower stroke in two revolutions in a four stroke cycle, is outweighted in a spark ignition(gasoline) engine by the associatedpower losses. In two stroke diesel engines, however, especiallythose operating at low and medium speeds these losses aregreatly decreased. There is no loss of fuel through the exhaust
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port because the fuel is not added until both ports are closed.Consequently, because of its design simplicity and increased powerfor a given engine speed, the two-stroke diesel engine is quite common,whereas the corresponding spark ignition engine has found onlylimited use.
Thus advantages of two stroke cycle over four stroke cycle are:more power output, less frictional loss per horse power, compactand simple mechanical design, no trouble from valves, lighter fly-wheel due to improved turning moment. But a two stroke engineoverheats on heavy loads and under light loads the running is erratic.Moreover, there is always a certain loss of fuel which escapes throughthe exhaust port before the compression. A four stroke engine iseconomical on lubricating oil and fuel consumption. Moreover, theengine cooling is simple and better as more time is available for theremoval of heat. Also, the combustion gases can be completely clearedfrom the cylinder. The arrangement of cylinders is also importantsince it effects the foundations, building space and maintenanceproblems. Vertical in line arrangement is most commonly used. Tomake the engine more compact, the cylinders may be arranged inV-shape. Two stroke radial diesel engines require minimum spaceand foundations.
Engines in the speed range of 200-1000 r.p.ro. are more common.Each cylinder is designed for around 75 kW and multi-cylinder engineshaving upto 16 cylinders; arranged vertically, are used for higheroutputs.
Diesel Fuels. A diesel engine can use a wide variety offuels, ranging from natural gas to fairly heavy petroleum distillateoils which are cheaper than gasoline. High-speed diesel enginesuse lighter fuels than do those operating at lower speeds. Theheavier fuels require larger times to be injected and to vaporizeprior to combustion and hence are more s