PrefaceThe work referred to in the title was carried out

on three Bristol engines, a poppet valve MercuryVI, a sleeve valve Perseus VIII and a sleeve-valvePerseus XII. The paper summarizing these tests isnoted in Reference 1. The purpose of the tests wasto calibrate the engines’ air flow characteristics asa function of operating conditions in anticipationof advanced fuel control system adoption. Theinformation contained in this paper makes possi-ble a direct volumetric efficiency comparison ofsleeve and poppet valve engines at the stage ofdevelopment they had reached in about 1940.

In a previous paper (Ref. 2 ) comparing thecharacteristics of sleeve and poppet valve enginesI was unable to settle definitively the question ofvolumetric efficiencies because I had discoveredonly anecdotal comment on the subject at thattime. Based on flow tests of a Hercules cylinderand sleeve (Ref. 3) I had concluded that there wasprobably not much difference between the two asregards volumetric efficiency. Shilling’s workmakes it possible to resolve this question as wellas some other aspects of sleeve valve engine per-formance characteristics.

I have chosen to use Shilling’s name in the titleof this paper as a way of recognizing an extraor-dinarily competent engineer who seems to beremembered chiefly for solving the Merlinengine’s carburetion problem in the early years ofWW2. This important accomplishment is usuallyreferred to in an inappropriate manner so perhapsthis paper will add a little more perspective to hercareer.

IntroductionThe reader is referred to Reference 2, page 17,

on the AEHS web site for a discussion of thedesign factors that determine the air capacity ofan engine. It will be noted that a number of

dimensionless groups are used to compare per-formance between engines. Besides volumetricefficiency these include the ratio of intake toexhaust pressure and an intake system “Machnumber” that includes piston speed. This numberis referred to as “Z” and is defined in Table 4 ofreference 2. Determining Z requires knowledge ofvalve (or port) areas and flow coefficients for thevalves or ports. This information is not availablefor the Bristol engines we are dealing with here soI have used piston speed as the independent vari-able in my plot of volumetric efficiency versusengine speed.

Shilling’s data is presented as air flow per cycleversus operating conditions that included enginespeed, intake pipe pressure and temperature,exhaust pressure, fuel-air ratio, and cylinder headtemperature. Fortunately, the data taken wasincluded in the report so that it could be analyzedwithout having to extract information from herplots. I used the raw data to put everything interms of volumetric efficiency rather than air flowper cycle.

Comparable tests were carried out by theNACA on two American engines (Refs. 4 and 5) afew years after Shilling’s work and these resultsare used to augment the poppet valve data sincethe Bristol Mercury was a rather old design by1941.

The effect of the sleeve valve on weight perhorsepower and frontal area for a given displace-ment is easily discernible from the engine specifi-cations and has been described and discussed inReference 2. The performance factors that influ-ence these characteristics are:

• Volumetric efficiency versus piston speed• Detonation limits• Friction• The effect of the extra thermal resistance of

the sleeve on volumetric efficiency

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Bristol Engine Tests at the Royal Aircraft Establishment:Work Carried Out by Beatrice Shilling ca. 1941

by Robert J. RaymondOriginal - August 2015; Revised - 14 March 2017

The sleeve valve engines of Bristol had higherdetonation limits than their poppet valve counter-parts and the reasons for this are discussed inReference 2. There have been claims made bothpro and con for the remaining three questions butmost have been anecdotal without hard data toback them up. Shilling’s data gives results for thefirst and fourth questions that appear to be verysound and consistent. The anecdotal claim forequal friction between sleeve and poppet wasshown to be feasible, based on an analysis of thefriction characteristics of the two types, and isgiven in Reference 2.

At the time this data was taken the Bristolsleeve valve engines were in their early adoles-cent years relative to poppet valve engines anddevelopment on them continued until productionended in the 1970s. Therefore the results present-ed here can only be regarded as a snapshot of

where things stood in the 1940s. I still have notseen any Hercules or Centaurus data that wouldallow comparisons to be made of the two valvingsystem types at the climax of large piston enginedevelopment.

Volumetric EfficiencyThe Effect of Exhaust to Intake Pressure Ratio

The set of curves in Figure 1 were done as acheck on the consistency of the data. This set isfor 2,200 rpm and plots at other speeds gave simi-lar results. The volumetric efficiency is normal-ized at a ratio of exhaust to intake pressure of oneand corrected to a manifold temperature of 150°F.The correction factor is based on actual data takenfor each engine and not on some general “rule”,such as a square root factor.

The curves for the “ideal cycle” are based on asquare pumping loop with no valve losses or heat

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Fig. 1. Volumetric Efficiency Versus Exhaust to Inlet Pressure Ratio Normalized to Pe / Pi = 1.

transfer to the incoming charge. The equation forthese lines at the two relevant compression ratiosis shown on the plot. The differences betweenengines can be attributed to valve losses, valvetiming, compression ratio, heat transfer, etc.

Table 1 gives the pertinent characteristics of allthe engines.

The Effect of Piston SpeedFigure 2 shows volumetric efficiency versus

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Table 1Engine Characteristics

Fig. 2. Volumetric Efficiency Versus Piston Speed. Pe / Pi = 1; Ti = 150°F

piston speed for five engines. Again, the data hasbeen corrected to an intake pipe temperature of150°F and an exhaust to intake manifold pressureratio of one.

The first question that comes to mind whenlooking at this plot is why was the Perseus XII sixpoints better than the Perseus VIII? Shilling pos-tulated that it had something to do with thelonger induction pipes on the XII but gave nodetails as to their design or relative lengths. Aninquiry to the Rolls-Royce Heritage Trust, BristolBranch, resulted in dimensions of the pipes of thetwo Perseus models and photographs of theengines showing the induction pipe arrangements(Figs 3 and 4). The Perseus VIII induction pipeswere six inches to the center of the manifold beltaround the ports and the XII were 14 inches long.Both had an outer diameter of 3 inches. My esti-mate of the effect of the difference in pipe lengthwould only account for two points out of the sixpoint difference in volumetric efficiency at thehigher piston speeds. It is possible that the differ-ence was more heavily influenced by the transi-tion from circular pipe to the manifold belt. Theearlier arrangement appears to have had a muchmore abrupt elbow and sudden expansion to the

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Fig. 3. Two Bristol Persius VIII Versions.Note the short, angular, abrupt induction pipes on the left, smoother transitions right. (R-RHT, Bristol Branch)

Fig. 4. Persius XII. Note the longer, still smoother intake pipes.(R-RHT, Bristol Branch)

manifold belt. Figure 5 shows an induction pipefrom a late model Centaurus which appears to beeven more aerodynamic than the Perseus XIIpipes.

The information in Figure 2 indicates that, atthe stage of development attained in the early1940s, sleeve valve engines were no better thanpoppet valve in terms of volumetric efficiency asexemplified by the two American engines. Similar

data for two other Wright engines, the R-3350 andR-2600, show higher volumetric efficiencies thanthe Pratt & Whitney R-2800. It is obvious thatBristol was hard at work given the big jump fromthe VIII to the XII and it seems likely that theymay have closed the gap with respect to theWright engines by the end of the era. Could theyhave surpassed them? Perhaps some data as goodas Shilling’s will turn up one day for the Herculesor Centaurus.

I also wondered why the Mercury showed upso poorly in Figure 2. There is nothing in thevalve timing that would make one think it wouldbe worse than the R-2800. I have no informationon the size of the intake valves or details of theintake ports so speculation on this subject is notworthwhile.

The Effect of Cylinder Head TemperatureRicardo’s early work with sleeve valves in liq-

uid cooled engines indicated that the additionalthermal barrier of the sleeve and extra oil film didnot result in higher piston temperatures. Laterwork at Rolls-Royce and Bristol indicated higherpiston temperatures in air cooled sleeve valve air-craft engines by as much as 50°C (see Ref. 2,page10). This should have shown up in the volu-metric efficiency but I was unaware of any harddata on the subject until Shilling’s work.

Figure 6 gives the effect of cylinder head tem-perature on volumetric efficiency for the two

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Fig. 5. Much Improved Centaurus Induction Pipes

Fig. 6. Volumetric Efficiency Versus Cylinder Head Temperature. Pe = Pi; Ti = 150°F

Bristol engines. It is clear that the sleeve valveengine is more sensitive to cylinder head temper-ature rise than the poppet valve engine by about afactor of two. This would account for some of thedifference seen in Figure 2 between the two typesof engines.

Shilling noted the difference in air consump-tion per cycle versus cylinder head temperatureand attributed it to a “characteristic of the sleevevalve design.”

Miscellaneous ResultsIn this section I will discuss other information I

was able to glean from Shilling’s report. Theabsence of any information about the supercharg-ers used on the engines and the pressure riseacross them limits what can be inferred about therelative friction losses and efficiency losses of thetwo engine types. The temperature rise fromengine intake to induction pipe was recorded andI attempted to establish which of the superchargercharacteristics used on these engines best fit the

data. This resulted in the conclusion that therewasn’t much difference between the Mercury andPerseus superchargers used in these tests. A 9.84-inch diameter impeller and 7:1 gear ratio fit thedata best for both engines.

I made some assumptions about the indicatedefficiency relative to the fuel-air cycle efficiencyand calculated the imep for a number of cases forall three engines. The only conclusion I was ableto reach was that the difference in brake efficiencywas more than can be attributed to the differencein compression ratio. When looking at the differ-ence between imep and bmep which ran about 20psi, there was always about a 5psi differencebetween the Mercury and Perseus in favor of thesleeve. Shilling attributed the higher brake effi-ciency of the sleeve engines to “superchargers,compression ratio and indicated efficiency.” Iwonder at the absence of friction in this list. Wasit an oversight or did she believe the claims orhave access to motoring data?

Table 1 lists the spark advance used in the

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Fig. 7. Temperature Rise, Carburetor to Inlet Pipe Versus Exhaust to Intake Pressure Ratio.Normalized to Pe / Pi = 1; N = 2,200 RPM

Bristol tests and the difference clearly supportsthe higher flame speed of the sleeve valve enginesand consequent higher detonation limits.

Another aspect of the sleeve valve performancewhich was noticeably different from poppet valvewas the temperature rise from carburetor tointake pipe as a function of exhaust to intakepressure ratio. This is shown in Figure 7, wherethe temperature rise is normalized to a pressureratio of one.

Despite the very small overlap relative to thepoppet valve engines, the sleeve valve enginesexhibit a relatively high temperature rise whenthe pressure ratio is greater than one. With noleakage from exhaust to intake, one would expectthe temperature rise to be a function of the over-lap and clearance volume and the poppet valveengines in this plot seem to bear this out. Both ofthese values are on the low side for the sleevevalve engines as seen in Table 1. The only expla-nation I can come up with for this phenomenon isleakage from the exhaust back into the inlet beltand pipe. This could be circumferential betweenthe sleeve and cylinder, from the volume con-tained in the port which is common to intake andexhaust, and past the piston rings in the cylinderhead which are closer together than the height ofthe ports, allowing a momentary short-circuit.This is a rather academic question since very littleoperational time is spent with exhaust to intakepressures greater than one. Some fresh chargecould leak into the exhaust when the situation isreversed but if it were significant it would showup as increased fuel consumption, but the datagiven here does not indicate this is significant.

SummaryThe results given here indicate that ca. 1941 the

volumetric efficiency of the Bristol Perseus sleevevalve engine was better than the poppet valveBristol Mercury engine and comparable to thePratt & Whitney R-2800. It was not as high as thatof the Wright R-1820 and other Wright engines ofthat period. The later version of the two Perseusengines tested showed a marked improvement involumetric efficiency which apparently was due

to changes made to the intake system. Continuedefforts in that area could have closed the gapbetween sleeve and poppet valve with the BristolHercules and Centaurus engines, unfortunatelythat data has not, to my knowledge, surfaced.

The results also show that the extra thermalbarrier of the sleeve and oil film did act to reducethe volumetric efficiency of the sleeve valveengine as compared to the poppet valve. It is notlikely that this characteristic would have beenovercome in later model Bristol engines.

AcknowledgementsI am indebted to David Robinson for bringing

the Shilling paper to my attention and providingme with a copy. David was also helpful with hiscomments on a draft of this paper and informa-tion regarding Mercury and Perseus superchargerdimensions and gear ratios.

Patrick Hassell of the Rolls-Royce HeritageTrust, Bristol Branch provided intake pipe dimen-sions and the photos shown here as Figures 3 and4. I am very grateful for his help.

Kim McCutcheon provided information onBristol superchargers for the period of interesthere.

References1. Shilling, B. “Measurements of the Air Consumption of a

Poppet and Sleeve Valve Air Cooled Radial Aircraft Engines withVarying Exhaust Back Pressure and Intake Temperature, ReportNo. E 3800” Royal Aircraft

Establishment, Farnborough, United Kingdom, February 1941.2. Raymond, R. “Comparison of Sleeve Valve and Poppet Valve

Aircraft Engines” AEHS http://www.enginehistory.org/mem-bers/articles/Sleeve.pdf, April, 2005.

3. Thomson, Mark R. “Flow Tests of a Bristol Sleeve ValveEngine Cylinder” M.I.T. Archives, Massachusetts Institute ofTechnology, Cambridge, Mass., 1951.

4. Shames. S. and Howes, W., “Determination of Air-Consumption Parameters for Two Radial Aircraft Engines” NACAMemorandum Report

MR No. E5H21, August 21, 1945.5. Humble, L.V., et. al., “Effect of Exhaust Pressure on the

Performance of an 18 Cylinder Air-Cooled Radial Engine with aValve Overlap of 62°” NACA Technical Note TN No. 1232,January 2, 1947.

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