Aircraft Engine Performance Analysis

1 Aircraft Engine Historical testing and Performance modeling to quantifyengine and supercharger air flow characteristics in supportof Rolls- royce merlin development began in the late status of this work was summarized in an internalRolls- royce Report in March, 1941 and made public by theRolls- royce Heritage Trust in paper introduces a generalized method of predictingand comparing Aircraft Engine Performance under flightconditions. Information in the Rolls- royce Report is ana-lyzed in this generalized manner, allowing comparison ofMerlin and Allison V-1710 Performance , which helps vali-date the from the Rolls- royce Report is reconciled with otheravailable data to conclude:V-1710 volumetric efficiency was somewhat higherthan the merlin 's and is readily explained by differencesin valve timing, intake passage design, and compressionratio;An error exists in the method for determining Merlinfriction and pumping characteristics described byStanley Hooker in his autobiography;Friction and pumping characteristics of the Merlinand V-1710 are similar.

2 Supercharger Performance of the ca 1941 merlin XXis similar to that of the ca 1943 uninterested in the engineering may proceeddirectly to Summary and the end of summer, 1940, the Battle of Britain was over,the victors having flown Spitfires and Hurricanes poweredby the Rolls- royce merlin Engine . The vast majority ofthese engines were equipped with single stage and singlespeed superchargers, which would soon be replaced bymore advanced marks including two speed, two stagesuperchargers with aftercooling. These developmentsallowed the Aircraft they powered to maintain a crucialadvantage over the German Aircraft they were fightingthroughout World War II despite the fact that their oppo-nent s engines were significantly larger in testing at Rolls- royce in support of these developmentsbegan in the late 1930s and involved establishing the airflow characteristics of the Engine and supercharger.

3 The sta-tus of this work as of March, 1941 is summarized in aninternal Rolls- royce report titled The Performance of aSupercharged Aero Engine by Stanley Hooker, Harry Reedand Alan Yarker [1] and was made available to the publicin 1997 by the Rolls- royce Heritage Trust. This reportmakes no mention of it, but the design of a two stage super-charger had begun a year before the report was written andit is obvious that the work described was at least partiallyin support of two stage supercharger development. Theauthors state that their motivation was to better character-ize the Performance of their Engine at altitude so as to mini-mize arguments between Engine and airframe builder as towhy the Performance of new or modified Aircraft did notmeet expectations.

4 This is a valid reason for the work butwould seem to be somewhat secondary given the militarysituation in 1941 when the outcome of the conflict was stilluncertain and superior Performance at high altitude was alife or death issue. I would guess that their primary goalwas to get more power at altitude and settling argumentswith Hawker and Supermarine was rather object of this paper is to analyze the information inthe Rolls- royce report and present it in a more generalizedmanner. This will allow the comparison of merlin perform-ance with that of the Allison V-1710, which while dimen-sionally similar to the merlin had significantly differentintake manifold and cylinder head intake passage second document, Sir Stanley Hooker s autobiographyNot Much of an Engineer[2], contains an appendix that out-lines the 1941 report and adds the results of some addi-tional Analysis carried out in an attempt to infer theMerlin s friction and pumping characteristics; informationthat also allows comparison with the V-1710.

5 Hooker s dis-cussion also makes clear that the goal was superior per-formance at altitude stating these gains came at a time inthe war when the odd extra thousand feet and extra speedmeant the difference between death to the enemy fighter ordeath to the Spitfire . Beyond comparing some of the per-formance characteristics of the merlin and V-1710, my motivation is to provide data and validation for a techniqueI am developing for predicting Aircraft Engine performanceunder flight conditions. The data in the Rolls- royce reporton breathing and supercharger Performance is very valu-able in this respect. Indicated horsepower (the power deliv-ered to the piston), besides determining how much powergets to the propeller, influences bearing loads, thermal load-ing of the piston and cylinder head and detonation limits ofthe Engine .

6 The ability to estimate indicated horsepower is,therefore, important for all aspects of Engine Analysis andthe two documents analyzed here contribute significantly tothis speed of soundCP specific heat at constant pressureeV volumetric efficiencyF fuel/air ratiok ratio of specific heatsMi mass of fresh charge ingested per cyclemep mean effective pressureN Engine speedpi intake manifold pressureAircraft Engine Performance Analysisat Rolls- royce ca. 1940by Robert J. RaymondMarch, 2011 Aircraft Engine Historical exhaust manifold pressurepa atmospheric pressurePR pressure ratio across the superchargerQc heating value of fuelR Universal gas constantr compression ratios piston speedTi intake manifold temperatureTa ambient temperature Tc temperature rise across the superchargerU impellor tip speedV Engine ( or cylinder ) displacement charge flow, air plus fuel fuel flow air flow choking mass flow c Adiabatic efficiency gB Gearbox efficiency ( for merlin XX)

7 I Indicated Engine efficiencyIntroductionPredicting the output of a piston Engine rests on thermo-dynamic and fluid mechanic principles combined withexperimental data taken in as general a manner as can set a limit to the indicated efficiency(the efficiency with which the heat released by the fuel isconverted to work on the piston) based on the compressionratio and the fuel/air ratio; but how close can a real engineapproach that limit? The same reasoning applies to thesupercharger; none are 100% efficient. When it comes tofrictional and pumping losses, one is even more dependenton experimental data. In the subject report the Rolls-Royceengineers were not attempting, at least at the time thereport was written, to determine indicated, friction orpumping horsepower.

8 In general, to get the brake horse-power (BHP, the power to the propeller) one must deter-mine the indicated horsepower (IHP) and subtract the com-pressor (supercharger) (CHP), friction (FHP) and pumping(PHP, getting charge in and out of the cylinder) powers, asfollows:We can re-arrange this equation to illustrate how the Rollsengineers attacked this problem, Aircraft Engine Historical defined a shaft horsepower as follows:Since the charge flow is equal to the air flow plus fuelflow the following relationship is easily derived:Rolls- royce engineers then carried out a series of tests atvarious speeds and manifold pressures to vary the chargeflow. They measured the brake horsepower and calculatedthe supercharger power at each point.

9 The sum of these twodivided by the charge flow is the left side of Equation (3),above. The results were then plotted against charge flowrate and are shown as Figure 13 in their report for fiveengine speeds. I have plotted the same results from theirTable I in Chart 1 for one Engine speed. This indicates thatthe technique works well since the data is from two differ-ent supercharger gear ratios. A look at the right side ofEquation (3) will indicate what is going on. I have substi-tuted the first law expression for indicated efficiency for theindicated horsepower in the first term that eliminates itsdependence on the charge flow rate so as long as thefuel/air ratio stays constant, and Table I [1] indicates that itdid, and if the spark advance was reasonably optimal thenthis term would not vary as the charge flow was second term would become larger as the charge flow isreduced since the friction horsepower would remain con-stant at a constant rpm and the pumping power wouldincrease slightly as the manifold pressure was reduced.

10 Thisis why the curve shown in Chart 1 drops off sharply as thepower is reduced. With this technique established whatremained was to develop a method for predicting thecharge flow under all conditions of Engine speed, intakeand exhaust manifold pressures, and ambient this was accomplished will be described in the follow-ing section. The Rolls- royce technique for predicting airflow will be examined and generalized to a volumetric effi-ciency so that it can be compared on a one-to-one basis withthe V-1710 and I will attempt to explain the differences onthe basis of design differences between the two friction and pumping loss characteristics of the V-1710will be presented and discussed with reference to anattempt at the same for the merlin as described by Hooker[2].