The Holy Grail of WWII Aviation Engines - The Rolls Royce Crecy

of all the piston arrow engines throughout world war ii one engine above all others remains shrouded in mystery while books and articles surrounding other power units can be hunted down and scavenged one engine the holy grail of piston arrow engines eludes rediscovery that engine is the rolls-royce cressie this experimental two-stroke sleeve valved v12 monstrosity was so powerful that it would have bridged the technological gap between the piston engine and the jet had there been any delay at all in the jet program as it turned out the jet was adopted extremely quickly and thus the cressie was stillborn however regardless of the fact that it never properly flew it deserves to be remembered as the technological masterpiece it truly was employing various exotic engineering solutions which were seldom used before and never combined into an arrow engine the cressie was the brainchild of famous engine designer harry ricardo who also brought design elements to the cressie that he had been using since he was just a schoolboy like an artist with a unique style the technological quirks ricardo preferred followed him like a signature and by taking a step backward we can see the idea of the crusty forming throughout ricardo's career as an engineer ricardo's first engine the dolphin was a redesign of a pre-existing engine on which ricardo implemented the first design feature he would continue to use throughout his designs at the time designers were troubleshooting the apparent inefficiency introduced whilst throttling an engine via reducing air and fuel together a phenomenon that had been traced back to the reduction of air flowing into the cylinders rather than the reduction in fuel as such engineers of the day were attempting to find ways to throttle petrol engines through fuel adjustments alone one way to partially circumvent this issue was the stratified charged method which ricardo incorporated into his dolphin design in this method by using a bulbous combustion chamber a rich mixture was preserved near the spark plug even when the mixture was made excessively lean beyond the point a normal chamber would no longer combust in this way a petrol engine can be throttled using a wider more versatile range of fuel air ratios another key design feature ricardo became known for prior to world war ii was his love for sleeve valve designs after ricardo finished his education at cambridge he went on to work for a civil engineering firm but he still kept a massive machining garage behind his house so he could continue to work on various engine projects when world war 1 broke out ricardo found himself designing an airship engine for the royal naval air service and additionally he helped design the 150 horsepower engine used in the british mark v tank at this time ricardo also became one of the first engineers to study the phenomenon of knock and ways to circumvent it in discovering knock ricardo was also one of the first to begin developing methods for measuring knock prior to the widespread adoption of the american octane number system ricardo and his colleagues measured the likelihood of fuel detonating in terms of toluene number this knowledge that of the relationship between fuel quality and the ability to delay the onset of the knock phenomenon led ricardo down the path that would eventually lead to his love of sleep valve designs in his own words in those days we had to deal with very inferior fuels with an octane number of less than 60 and i was much impressed by the fact that as compared with our overhead poppet valve engine we could employ just one hole ratio higher compression on the same fuel and gain a very big advantage it was for this reason that ricardo became such a fervent proponent of the sleeve valve designs and as such he was responsible for convincing sir roy fedden of their bristol airplane company to build a complete engine while feden at first had his misgivings he eventually had a change of heart and began investing huge sums of money into developing sleeves able to tolerate the incredible power their later engines would produce for perspective bristol and vickers tried between 50 and 60 compositions of steel and about 1100 heat treatments to get a sleeve that was right for production cost of this research was about 2 million british pounds and incidentally this was about twice what the ministry of aircraft production spent in world war ii on whittle and the power jets in development of the jet engine concerning sleeve valve designs ricardo was later quoted as saying it was perhaps unfortunate that the only firm which was sufficiently interested to take an active part was wedded to air-cooled radial engines that so many years should have elapsed between the completion of the research and its practical development was bitterly disappointing to me all the more so since the advantages of the sleeve valve arrow engine were most apparent in the days when we were using relatively low octane fuels such a statement sheds light on the previous video we did about the napier sabre it would appear that the sleeve valve's biggest advantage was its ability to use inferior fuel and when quality fuel production took off the sleeve valve was already outclassed before the engines that featured this technology entered widespread production nonetheless the choice to use the sleeve valve in the crescent would have other advantages that we'll touch on in a bit it was during this point in history that the promises of the diesel engine also became well known as diesel engines are virtually immune to nock it was briefly thought that a compression engine as they were called may hold the key to the engine of tomorrow as petrol engines at the time again were limited by the low octane number of their fuel an aviation diesel had recently begun achieving higher and higher specific power ratings it made sense at the time to consider the diesel design may soon overtake a petrol engine in terms of viability the first time ricardo incorporated these two ideas that is a diesel using sleeve valves was an experimental engine dubbed the e40 in this early design a cylinder projection of the cylinder head was fitted into the top end of the sleeve and carried piston rings for sealing however in this two-stroke diesel ricardo noted these rings were never really trouble free and in spite of their own separate supply of lubricating oil they were prone to gumming and carbon deposition which rendered them useless to remedy this the e40 was redesigned into the e44 which removed the cylinder head rings altogether and exhausted over the top edge of an open sleeve in this way the cylinder head became a lid containing only the combustion chamber and this idea that is exhausting directly over the top of the sleeve would result in the massive exhaust thrust the cressie would later enjoy but before the cresce's development would properly start ricardo needed to shift from diesel design back to petrol engines that wouldn't occur until war loomed on the horizon and the method of enemy aircraft detection was revolutionized by the invention of radar technology by incorporating radar into the tactical picture fighters in the air war would no longer need to constantly loiter in case enemy aircraft were detected instead they could be scrambled upon enemy detection however an engine able to haul a fighter up to the required altitude in time would be necessary this need gave rise to the proposal of a sprint engine a proposal was first made by henry tiszard at the english subcommittee's december 1935 meeting in that meeting henrik ricardo raised the question of whether a high fuel consumption could in fact be permissible under certain circumstances and if that was the case he believed that the two-stroke petrol appeared to be an attractive design two months after this meeting ricardo officially submitted his report entitled high power two cycle engine for special purpose machines in it it starts the present note relates to the possibility of developing a special purpose aero engine of very high performance for short flights only without the usual regard for fuel or oil consumption but in which high specific output and small frontal area are taken as being the dominating conditions essentially ricardo in this report planned on redesigning his experimental e44 design into a spark ignition and running it as a petrol engine and as we now know this design when scaled up to a full-sized v12 and manufactured by rolls-royce would eventually be named the cressie it's also important to note that while henry tizard had suggested to ricardo that he developed such a spray engine he had not only placed a bet on a radar erected fighter's power plant with ricardo but with hane constant who would design the first british axial flow turbo jet after these meetings with cesar constant would later recall i remember a conversation with mr tizzard the director of the imperial college in which he asked me what was the most worthwhile research to be done in the engine field i told him of my hopes for the turbine and he gave an encouraging reply there is no doubt in my mind that it was tizzard's imagination and enthusiasm that provided the drive during these critical days apparently whittle too had the same experience later saying our best friend was henry tezard it was tizard who made the connections that led to the air ministry funding of power jets in the future i'll probably be covering these early jets but the reason i bring them up is that it's important to put the cressi project into context you see it wasn't as some might think some off-the-wall project made to investigate the feasibility of strange technology rather it was one of several bets made by the british at a time of imminent threat to quickly develop new types of power plants that would be well suited for what would be a reactive type of defense so if the cressi was supposed to be this radically powerful engine capable of scrambling fighters in short order how did it work [Music] in rough terms the cressi engines were liquid cooled upright 90 degree v12 two strokes each cylinder had a single sleeve valve and petrol injection directly into the combustion chamber again ignition was spark driven with charge stratification and the final version of the engine used an exhaust driven turbine to recover some of the power losses incurred by the single staged supercharger however as we'll see even after the turbine there was plenty of exhaust gas to spare additionally the cressie was directly fuel injected used oil cooled pistons was power controlled via fuel metering although some throttling was used because the stratified charge wasn't enough for full power band operation and again it was supercharged by necessity because two-stroke engines won't operate unless the inlet manifold pressure exceeds the exhaust pressure to drive out the exhaust products and introduce fresh air into the cylinder as such the blower was the only means of filling and emptying the cylinders of the cressie to give a quick power overview by the end of the cresce's development henry ricardo had his single cylinder e65 test engine running at a power output equivalent to 5 000 brake horsepower had it been converted to a full scale cressy now that's a lot of power for the time considering the engine only displaced 26 liters in terms of the engine's particulars the cressi featured a 5.1 inch bore a 6.5 inch stroke 1 536 cubic inches or 26 liters a compression ratio of 7 to 1 a 12 inch rotor diameter on the supercharger which was geared at 8.77 and made about 18 pounds of boost using 100 octane fuel and its dry weight was the with the turbine and the contra rotating reduction gears was about 1900 pounds so now that we know the specifics how did this thing actually function well we're going to cover it but first let's do a quick review of some of the differences between a two-stroke and a four-stroke engine remember in the four-cycle engine the exhaust is removed entirely by the piston then recharged with the requisite mixture of fuel and air through the inlet in a two-cycle however the cylinder cannot be emptied because there aren't enough strokes to go around so the residual exhaust must be displaced by the incoming air but if the air supply is throttled the result is that the less exhaust is displaced and in the extreme the cylinder will be just left full of exhaust gas because of this the power of the engine cannot be efficiently controlled by air throttling but only by adjusting the quantity of fuel delivered essentially power control is more similar to a diesel or a jet engine than a typical four stroke also in a four-cycle engine air displacement is limited by the volume the piston is capable of displacing whereas in a two-cycle engine there's a period in which both inlet and exhaust valves are open at the same time creating a period in which air can escape because of this at any given boost pressure the same weight of air will pass into and through the cylinder per unit of time whether the engine speed is 300 or 3000 revolutions per minute in the 300 rpm case maybe 10 cylinder volumes of air move through the per cycle whereas in the 3000 rpm case perhaps only one regardless it's important to remember that without positive blower pressure a two-stroke will not scavenge exhaust gases nor will it create any power again remember that exhaust pressure is one of the biggest advantages of the two-stroke because it has doubled the number of power strokes as a four-stroke so while a merlin was expected to provide about 150 pounds of exhaust thrust a cressie delivered 300. this will be important to remember when we compare the cressie to the griffin and the merlin later on and while we're on the topic of exhaust let's talk about the ridiculousness that was the krusty's exhaust note you see because the cressie exhaust ports opened almost instantly unlike a poppet valve design and it opens 91 degrees after the top dead center when the pressure in the cylinder is still high you're hearing 36 000 pulses per minute at 3 000 revolutions per minute that's twice as many as a four-stroke v12 this combination of rapid opening high pressure exhaust and the high frequency of the pulses produced an exhaust sound like no other which incidentally is the most memorable feature of the cressi apparently when the engine first ran the air raid sirens went off in nottingham 15 miles away now we don't actually have any recordings of the cresty unfortunately and while it wasn't a diesel the closest engines we have recordings of are high displacement two-strokes such as the detroit diesel while the cressie would have no doubt been louder than the detroit diesel the exhaust note would have been somewhere in the ballpark so without further ado for your viewing pleasure behold a large screaming two-stroke so so now that we've refamiliarized ourselves with two stroke operations and maybe pulled the cotton out of our ears how did the power cycle of the cressie with its fancy sleeve valve actually work well fortunately the folks at rolls-royce historical society have provided us with this excellent graphic that splits the two-stroke power cycle of the cressie into six steps in the first image the piston is at top dead center while the sleeve has begun a downward stroke 15 degrees ahead of the crank as combustion occurs we move on to step two in the second image we can see the top of the sleeve is now cutting across the cylinder ports at the same time the piston skirts for cooling are open to blast air from the intake valve in phase 3 the inlet opens 52 degrees prior to the bottom dead center as the top of the piston cuts across the sleeve boards at this time both the piston and the sleeve are traveling downwards and scavenging has begun at this time the exhaust ports remain three quarters open and the sleeve port is still drawing in air which is cooling the rings in the top of the piston and moving upwards in a column to clean out the bulb of exhaust residuals all the while the exhaust port continues to fully scavenge the remaining exhaust in the fourth image the piston is at bottom dead center and fuel injection begins at this point the extreme end of the spray catches air as it enters and eddies around the sleeve port bars breaking up the fuel particles as the exhaust closes at 59 degrees after bottom dead center the injection step ends with an intentional dribble of fuel to ensure the stratified charge of the bulb remains rich now the top end of the ascending sleeve closes the exhaust ports and the cylinder fills with air until the incoming blower pressure is attained inside in the final step before combustion the top end of the piston cuts across the sleeve port and the sleeve and the piston are moving upwards compression finally commences with the richest charge retained in the bulb and that's it while the sleeve makes it seem slightly more complicated the engine's total cycle is actually more simple than a comparable four stroke but that being said if it was so simple and so powerful and so amazing as we were making it out to be why didn't it succeed that's a great question in the end the deck was stacked against the cressie due to a lack of priority status not enough qualified engineering talent typical teething problems in the rapid acceptance of the gas turbine the cressie program was cancelled in 1946 interestingly david birch one of the authors of my primary research source claims that if only mrs whittle and von ohane had been born 10 years later what a glorious swan song there might have been for the piston aero engine however while we can't appreciate any actual service from the cressy many engineers of the day were keenly interested in performing hypothetical tests and collecting data that i think is extremely interesting to start we know that in fact the cressi was actually intended to be mounted originally in the hawker henley and one was actually delivered on march 28 1943 to be converted to a cressi power unit but unfortunately due to delays in the program the henley sat without an engine for the remainder of the war interestingly this was not the first aircraft intended to be mounted with a cressi as the spitfire 2 was fitted with a cressy mock-up in the summer of 1941. however tragically this exercise proved futile as well as production of the spitfire 3 never occurred that being said though the performance of the spitfire with the cressie engine was compared with the merlin 61 in a report prepared by wattol chellier who was hucknile's chief performance engineer in the report we can see three estimates of performance two by the royal aircraft establishment featuring both single and two-stage supercharged applications and the rolls-royce figures in this instance the spitfire used for comparison like i said was using a merlin 61 operating at 15 pounds of boost as the engineers discovered the most notable trade-off of using the cressi was a slight reduction in climb rate for a huge boost in top speed with the two-stage supercharger we can see that the rae expected a 58 mile an hour increase in top speed over the merlin however in that configuration we can see that the spitfire would lose a considerable amount of its climb rate in this instance i think a more solid trade-off would be the single stage cressie which offered an impressive top speed of 352 across the ground and 431 at 19 000 feet while still making a respectable 3600 feet per minute in the climb at the end of this report shelier states it will be seen that the estimated performance is very high both speed and ceiling the high speed is due to the large ejector thrust power which is much greater than in a four stroke engine and amounts to over 25 percent of the engine power at high speeds and altitudes so in this report we can see this data sort of backing up the later claims that the two-stroke should have been the bridge between the jet and the four-stroke piston aero engine as the exhaust jet thrust was a massive advantage there was also an interesting comparison made between a griffon-powered spitfire and the cressi however it's important to note that farnborough based its estimates on a two-stage supercharged cresce which was a hypothetical configuration that was never achieved in reality as rolls-royce instead opted to use the single speed supercharger in a power recovery turbine instead in any case the resulting comparison is interesting to say the least what we see in this chart is that their performance is nearly identical however it's critical to remember that the griffin was 37 liters while the cressie only displaced 26. what's also interesting is to see again how much exhaust thrust is contributing to top speed in the griffin we're only seeing a boost of about 44 miles per hour due to exhaust thrust while the crest seas exhaust is increasing the top speed by 88. later as ricardo kept making improvements to his experimental e65 engine upon which the full-sized crescey was based farnborough made an updated estimation of performance as ricardo was getting 270 pounds of brake mean effective pressure while running at 3500 rpm with a shorter stroke and real quick if you don't know what bmep is it's the pressure in pounds per square inch which if exerted on the piston for the full stroke would produce the power measured by the dyno with zero frictional losses it doesn't matter if that doesn't make any sense just know that he was making big power and he was also forcing enough air into the cylinder to fill it 2.89 times per cycle so he was using a big blower in any case the farnborough boys extrapolated this new data out to the full 12-cylinder engine and the results were insane estimating a maximum output of 3500 brake horsepower on 26 liters which would propel a spitfire to they estimated about 535 miles per hour at 32 000 feet again these numbers were thought to be achievable primarily due to the jet thrust of the cressie however it was later determined that the crescey was probably too powerful for the spitfire and interest moved toward mating this beast instead with a mustang after the allison was swapped with merlin a study was conducted concerning the performance of a mustang fitted with a cressy engine this study actually planned on mounting the engine mid fuselage and entertained three possible exhaust configurations in the first the exhaust provided ejector thrust in the normal manner just through exhaust stacks in the second configuration an exhaust driven turbine was fed back into the crankshaft to relieve the energy that drove the propeller and the supercharger in a typical turbo compound configuration but in the third proposal was when things got interesting in this configuration an exhaust drive turbine called a thrust augmenter was used along with an outer annulus that was fed with the air from the radiator and oil cooler the resultant mixture was exhausted into a jet pipe and a combustion was assisted by the ignition of any unburnt fuel within the air perched through the exhaust ports during the scavenging cycle essentially it was a bypass jet or the start of the very first motor jet which is awesome and also a topic for another video just know that essentially at this point we're so close to just being a pure jet engine that the reciprocating bit is maybe superfluous but it is pretty awesome and pretty crazy when we look back at it so in closing while it is perhaps a disappointment that the kressy's amazing performance was never fully realized it isn't surprising like many of the ingenious wonderful piston engine innovations of world war ii the death knell of the cressi came alongside the first pure jet engines however it should be remembered that in the words of henry tizard himself the research had revealed that the two-stroke sleeve valve cylinder was capable of a maximum power output of nearly double that of a contemporary four-cycle engine nearly 200 horsepower per liter thusly it seems probable that had the turbine not materialized the two-stroke would have proved the next step in the development of the aero engine as always guys thank you so much for tuning in to flight dojo if you made it this far we hope that you enjoy our content enough to hopefully subscribe also you asked and we answered the flight dojo channel now has its own official patreon admittedly it isn't too fancy at this point mainly it's just a digital tip jar but if you enjoy the research going on here at flat dojo and want to be involved in deciding which content we cover next you can hop on over to the patreon at patreon.comflightdojo and send us a few bucks that being said thanks again for watching and we will see you guys next time

Channel: Flight Dojo

Views: 548,091

Keywords: aviation history, wwii, rolls royce crecy, wwii history, engineering history, engine history, rolls royce, ww2 aviation, ww2, warbirds, petrol engine, two stroke

Id: oOFn0eTgdE8

Length: 24min 2sec (1442 seconds)

Published: Thu Jul 14 2022