4TheoryandGeneral PrinciplesEFFICIENCYThe only reason a practical engineer wants to run an engine at all is to achievea desired output of useful work,whichis,for our present purposes, to drivea ship at a prescribed speed, and/or to provide electricity at a prescribedkilowattage.To determine this power he or she must, therefore, allow not only for thecycle losses mentioned earlier but also for the friction losses in the cylinders,bearingsand gearing (if any)together withthe power consumed by engine-drivenpumps and otherauxiliary machines.He or shemust also allowforsuch things as windage. The reckoning is further complicated by the fact thatthe heat rejected from the cylinder to exhaust is not necessarily totally lost,as practically all modern engines use up to 25 per cent of that heat to drive aturbocharger. Many use much of the remaining high temperature heat to raisesteam,and uselowtemperatureheatforotherpurposes.The detail is beyond the scope of this book but a typical diagram (usu-allyknown as a Sankey diagram), representing the various energy flowsthrough a modern diesel engine, is reproduced in Figure 1.4. The right-handside represents a turbocharged engine, and an indication is given of the kindof interaction between the various heat paths as they leave the cylinders aftercombustion.Note that the heat released from the fuel in the cylinder is augmented bythe heat value of the work done by the turbocharger in compressing the intakeair. This is apart from the turbocharger's function in introducing the extra airneeded to burn an augmented quantity of fuel in a given cylinder, comparedwith what the naturally aspirated system could achieve, as in the left-hand sideofthediagram.It is the objective of the marine engineer to keep the injection settings,the airflow, and coolanttemperatures (not tomention the general mechanicalcondition) at those values, which give the best fuel consumption for the powerdeveloped.Note also that, whereas thefuel consumption is not difficult to measure intonnes per day, kilograms per hour or in other units, there are many difficultiesin measuring work done in propelling a ship.This is because the propeller effi-ciency is influenced by the entry conditions created by the shape of the after-body of the hull, by cavitation and so on and also critically influenced by thepitch setting of a controllable pitch propeller. The resulting speed of the shipis dependent, of course, on hull cleanliness, wind and sea conditions, draughtand so on. Even when driving a generator it is necessary to allow for generatorefficiencyandinstrumentaccuracy.It is normal when definingefficiencytobasethework doneon that trans-mitted to the driven machinery by the crankshaft. In a propulsion system, thiscan be measured by a torsionmeter; in a generator it can be measured elec-trically. Allowing for measurement error, these can be compared with figuresmeasured on a brake in the test shop
Theory and General Principles Efficiency The only reason a practical engineer wants to run an engine at all is to achieve a desired output of useful work, which is, for our present purposes, to drive a ship at a prescribed speed, and/or to provide electricity at a prescribed kilowattage. To determine this power he or she must, therefore, allow not only for the cycle losses mentioned earlier but also for the friction losses in the cylinders, bearings and gearing (if any) together with the power consumed by enginedriven pumps and other auxiliary machines. He or she must also allow for such things as windage. The reckoning is further complicated by the fact that the heat rejected from the cylinder to exhaust is not necessarily totally lost, as practically all modern engines use up to 25 per cent of that heat to drive a turbocharger. Many use much of the remaining high temperature heat to raise steam, and use low temperature heat for other purposes. The detail is beyond the scope of this book but a typical diagram (usually known as a Sankey diagram), representing the various energy flows through a modern diesel engine, is reproduced in Figure 1.4. The right-hand side represents a turbocharged engine, and an indication is given of the kind of interaction between the various heat paths as they leave the cylinders after combustion. Note that the heat released from the fuel in the cylinder is augmented by the heat value of the work done by the turbocharger in compressing the intake air. This is apart from the turbocharger’s function in introducing the extra air needed to burn an augmented quantity of fuel in a given cylinder, compared with what the naturally aspirated system could achieve, as in the left-hand side of the diagram. It is the objective of the marine engineer to keep the injection settings, the air flow, and coolant temperatures (not to mention the general mechanical condition) at those values, which give the best fuel consumption for the power developed. Note also that, whereas the fuel consumption is not difficult to measure in tonnes per day, kilograms per hour or in other units, there are many difficulties in measuring work done in propelling a ship. This is because the propeller efficiency is influenced by the entry conditions created by the shape of the afterbody of the hull, by cavitation and so on and also critically influenced by the pitch setting of a controllable pitch propeller. The resulting speed of the ship is dependent, of course, on hull cleanliness, wind and sea conditions, draught and so on. Even when driving a generator it is necessary to allow for generator efficiency and instrument accuracy. It is normal when defining efficiency to base the work done on that transmitted to the driven machinery by the crankshaft. In a propulsion system, this can be measured by a torsionmeter; in a generator it can be measured electrically. Allowing for measurement error, these can be compared with figures measured on a brake in the test shop
3540%Exhaustfromturbine=5%Chargecooler-Heatfromwalls=10%CoolantPumping=50%=5% Lub oilExhaustRecirctoturbine=5%Radiation=10%Chargeair37%100%FuelTo exhaust15%5%To coolantTo luboil40%8%RadiationTo workEfficiency100%Fuel35% To workNaturallyTurbochargedenginesaspiratedenginesSFIGURE 1.4 Typical Sankey diagrams
Efficiency Naturally aspirated engines 100% Fuel 40% To work 37% 100% Fuel To exhaust Turbocharged engines �5% Radiation �10% Charge air 15% 5% To coolant �5% Lub oil �10% Coolant �50% Exhaust to turbine 35–40% Exhaust from turbine �5% Charge cooler Heat from walls Recirc To lub oil 8% Radiation 35% To work Pumping Figure 1.4 Typical Sankey diagrams
6TheoryandGeneralPrinciplesTHERMAL EFFICIENCYThermal efficiency (Thn) is the overall measure of performance. In absoluteterms it is equal to:Heat converted into useful work(1.1)Total heat suppliedAs long as the units used agree,itdoes not matter whether the heator workis expressed in pounds feet, kilograms metres, BTU, calories, kilowatt hour orjoules.Therecommended units to usenow arethoseof theSI system.Heat converted into work per hour =NkWh=3600NkJwhere Nis the power output in kilowattsHeat supplied = M × Kwhere M is the mass of fuel used per hour in kilograms and K the calorificvalue of the fuel in kilojoules per kilograms3600N(1.2)Thn =MXKIt is now necessary to decide where the work is to be measured. If it is tobe measured in the cylinders, as is usually done in slow-running machinery,by means of an indicator (though electronic techniques now make this poss-ible directly and reliably even in high-speed engines),the work measured (andhence power) is that indicated within the cylinder, and the calculation leads tothe indicated thermal efficiency.If thework is measured at the crankshaft outputflange,it is net of frictionauxiliary drives,etc., and is what would bemeasured by abrake, whencethetermbrakethermal efficiency.(Manufacturers in somecountriesdoincludeasoutputthepowerabsorbed byessential auxiliarydrives,but someconsiderthistogivea misleading impression of thepower available.)Additionally, the fuel is reckoned to have a higher (or gross) and a lower(ornet)calorificvalue(LCV),accordingtowhetheronecalculatestheheatrecoverableif theexhaustproductsarecooledbacktostandardatmosphericconditions,or assessedatthe exhaustoutlet.The essential differenceisthatinthelattercasethewaterproducedincombustionisreleasedassteamandretains itslatent heat of vaporization.Thisis the morerepresentativecaseandmore desirable as water in the exhaust flow is likely to be corrosive. Today thenet calorific value or LCV is more widely usedReturning to our formula (Equation 1.2),if we take the case of an engineproducing a (brake) output of 10 000kW for an hour using 2000kg of fuel perhourhavinganLCVof42000kJ/kg
Theory and General Principles Thermal efficiency Thermal efficiency (Th) is the overall measure of performance. In absolute terms it is equal to: Heat converted into useful work Total heat supplied (1.1) As long as the units used agree, it does not matter whether the heat or work is expressed in pounds feet, kilograms metres, BTU, calories, kilowatt hour or joules. The recommended units to use now are those of the SI system. Heat converted into work per hour kW h kJ N 3600 N where N is the power output in kilowatts Heat supplied M K where M is the mass of fuel used per hour in kilograms and K the calorific value of the fuel in kilojoules per kilograms Thη 3600 N M K (1.2) It is now necessary to decide where the work is to be measured. If it is to be measured in the cylinders, as is usually done in slow-running machinery, by means of an indicator (though electronic techniques now make this possible directly and reliably even in high-speed engines), the work measured (and hence power) is that indicated within the cylinder, and the calculation leads to the indicated thermal efficiency. If the work is measured at the crankshaft output flange, it is net of friction, auxiliary drives, etc., and is what would be measured by a brake, whence the term brake thermal efficiency. (Manufacturers in some countries do include as output the power absorbed by essential auxiliary drives, but some consider this to give a misleading impression of the power available.) Additionally, the fuel is reckoned to have a higher (or gross) and a lower (or net) calorific value (LCV), according to whether one calculates the heat recoverable if the exhaust products are cooled back to standard atmospheric conditions, or assessed at the exhaust outlet. The essential difference is that in the latter case the water produced in combustion is released as steam and retains its latent heat of vaporization. This is the more representative case—and more desirable as water in the exhaust flow is likely to be corrosive. Today the net calorific value or LCV is more widely used. Returning to our formula (Equation 1.2), if we take the case of an engine producing a (brake) output of 10 000kW for an hour using 2000 kg of fuel per hour having an LCV of 42000kJ/kg
Working cycles73600×10000X100%(Brake) Thn =2000×42000= 42.9% (based on LCV)MECHANICALEFFICIENCYoutput atcrankshaft(1.3)Mechanical efficiencyoutput at cylindersbhpkW (brake)(1.4)ihpkW (indicated)The reasons for the difference are listed earlier.The brake power is nor-mally measured with a high accuracy (98 per cent or so) by coupling theengine to a dynamometer at the builder's works. If it is measured in the shipby torsionmeter,it is difficultto matchthis accuracy and,if thetorsionmetercannot be installed between the output flange and the thrust block or the gear-box input, additional losses have to be reckoned due to thefriction entailed bythesecomponents.The indicatedpower canonlybemeasuredfromdiagramswherethesearefeasible, and they are also subject to significant measurement errors.Fortunatelyfor our attempts to reckon the mechanical efficiency,test bedexperience shows that the'friction'torque (i.e. in fact, allthe losses reckoned toinfluence the difference between indicated and braketorques) is not very greatlyaffected by the engine's torque output, nor by the speed. This means that thefriction power loss is roughly proportional to speed, and fairly constant at fixedspeed over the output range. Mechanical efficiency therefore falls more andmore rapidly as brake output falls. It is one of the reasons why it is undesirabletolet an engine run for prolonged periods at less than about 30 per cent torqueWORKINGCYCLESA diesel engine may be designed to work on the two-stroke orfour-stroke cycle-both of these are explained below.They should not be confused with the terms'single-acting or 'double-acting',which relate to whether the working fluid (thecombustion gases) acts on one or both sides of the piston.(Note,incidentally,thatthe opposed piston two-stroke engine in service today is single-acting.)TheFour-StrokeCycleFigure 1.5 shows diagrammatically the sequence of events throughout the typicalfour-stroke cycle of two revolutions. It is usual to draw such diagrams startingat TDC (firing),but the explanation will start at TDC (scavenge).TDC is some-times referred toas innerdead centre (IDC)
(Brake) Th % (based on LCV) η 3600 10 000 2000 42 000 100 42 9. % Mechanical efficiency Mechanical efficiency output at crankshaft output at cylind ers (1.3) bhp ihp kW (brake) kW (indicated) (1.4) The reasons for the difference are listed earlier. The brake power is normally measured with a high accuracy (98 per cent or so) by coupling the engine to a dynamometer at the builder’s works. If it is measured in the ship by torsionmeter, it is difficult to match this accuracy and, if the torsionmeter cannot be installed between the output flange and the thrust block or the gearbox input, additional losses have to be reckoned due to the friction entailed by these components. The indicated power can only be measured from diagrams where these are feasible, and they are also subject to significant measurement errors. Fortunately for our attempts to reckon the mechanical efficiency, test bed experience shows that the ‘friction’ torque (i.e. in fact, all the losses reckoned to influence the difference between indicated and brake torques) is not very greatly affected by the engine’s torque output, nor by the speed. This means that the friction power loss is roughly proportional to speed, and fairly constant at fixed speed over the output range. Mechanical efficiency therefore falls more and more rapidly as brake output falls. It is one of the reasons why it is undesirable to let an engine run for prolonged periods at less than about 30 per cent torque. Working cycles A diesel engine may be designed to work on the two-stroke or four-stroke cycle— both of these are explained below. They should not be confused with the terms ‘single-acting’ or ‘double-acting’, which relate to whether the working fluid (the combustion gases) acts on one or both sides of the piston. (Note, incidentally, that the opposed piston two-stroke engine in service today is single-acting.) The Four-Stroke Cycle Figure 1.5 shows diagrammatically the sequence of events throughout the typical four-stroke cycle of two revolutions. It is usual to draw such diagrams starting at TDC (firing), but the explanation will start at TDC (scavenge). TDC is sometimes referred to as inner dead centre (IDC). Working cycles
TheoryandGeneral Principles8Proceeding clockwise around the diagram,both inlet (or suction)andexhaust valves are initially open. (All modern four-stroke engines have poppetvalves.)Ifthe engine is naturally aspirated, or is a small high-speed typewith acentripetal turbocharger,the period of valve overlap (i.e.when both valves areopen)willbeshort,andtheexhaustvalvewillclosesome10°ATDC.Propulsion engines and the vast majority of auxiliary generator engines run-ning at speeds below 1000rev/min will almost certainly be turbocharged and willbe designed to allow a generous throughflow of scavenge air at this point in order tocontrol theturbine bladetemperature (seealso Chapter7).In this case the exhaustvalve will remain open until exhaust valve closure (EVC) at 50-60°ATDC. As thepiston descends to outer or bottomdead centre (BDC)on the suction stroke,it willinhale a fresh charge of air. To maximize this, balancing the reduced opening as thevalve seats against the slight ram or inertia effect of the incoming charge, the inlet(suction valve)will normally be held open until about 25-35°afterBDC (ABDC)(145-155°BTDC). This event is called inlet valve closure (IVC)The charge is then compressed by the rising piston until it has attained a tem-perature of some550°C.At about10-20°BTDC(beforetopdead centre)(fir-ing). depending on the type and speed of the engine, the injector admits finelyatomized fuel which ignites within 2-7° (depending on type again) and the fuelburns overaperiod of 30-50°whilethepistonbegins todescend on theexpansion stroke, the piston movement usuallyhelping to induce air movement to assistcombustion.TDC (firing)TDCFuel valveclosed (fulload)Injection commences(scavenging)=10-20°BTDCTDC (firing)ExhaustInlet valvevalve closesopen=7080°=5060°TDCInletvalvecloses145-155°BTDCExhaustvalveopens=120150ATDCBDCFIGURE1.5Four-strokecycle
Theory and General Principles Proceeding clockwise around the diagram, both inlet (or suction) and exhaust valves are initially open. (All modern four-stroke engines have poppet valves.) If the engine is naturally aspirated, or is a small high-speed type with a centripetal turbocharger, the period of valve overlap (i.e. when both valves are open) will be short, and the exhaust valve will close some 10° ATDC. Propulsion engines and the vast majority of auxiliary generator engines running at speeds below 1000rev/min will almost certainly be turbocharged and will be designed to allow a generous throughflow of scavenge air at this point in order to control the turbine blade temperature (see also Chapter 7). In this case the exhaust valve will remain open until exhaust valve closure (EVC) at 50–60° ATDC. As the piston descends to outer or bottom dead centre (BDC) on the suction stroke, it will inhale a fresh charge of air. To maximize this, balancing the reduced opening as the valve seats against the slight ram or inertia effect of the incoming charge, the inlet (suction valve) will normally be held open until about 25–35° after BDC (ABDC) (145–155° BTDC). This event is called inlet valve closure (IVC). The charge is then compressed by the rising piston until it has attained a temperature of some 550°C. At about 10–20° BTDC (before top dead centre) (firing), depending on the type and speed of the engine, the injector admits finely atomized fuel which ignites within 2–7° (depending on type again) and the fuel burns over a period of 30–50° while the piston begins to descend on the expansion stroke, the piston movement usually helping to induce air movement to assist combustion. Fuel valve closed (full load) Injection commences �10–20°BTDC Inlet valve closes �145–155° BTDC Exhaust valve closes �50–60° TDC Exhaust valve opens �120–150 ATDC TDC (firing) TDC (scavenging) BDC Inlet valve open � 70–80° TDC (firing) Figure 1.5 Four-stroke cycle