Engine technology free-for-all

[J.A.W.]

Supercharger efficiency, or allowing the engine to "breathe as deeply" in the air at rarefied altitude was key..

The American method was via massively complex turbo set-ups, which did best in bombers running steady state
in the careful hands of dedicated flight engineers..

The fairly large DB 605.. in the fairly small/light Bf 109.. had less work to do..

The Merlin 2-stage 2-speed units worked most efficaciously in the slick aero-airframes of Mosquito & Mustang,
both of which were significantly faster than the Spitfire running similar Merlins at equivalent power settings ..

Stanley Hooker & co at R-R had got them running pretty hard..

[trinidefender]

Those charts just tell me that raising the boost pressure of the supercharger resulted in a lower full throttle height (or critical altitude, whichever you prefer) for the RR Merlin 66 at 20,000 ft it shows that at full throttle the wastegate would be fully closed closed and you would get 18 lbs of boost. It isn't saying that the two different pressures give the same power, it is saying that at that altitude this supercharger will give that boost pressure. If I remember correctly the 5 minute combat limit for the merlin 66 was something like 18 lbs of boost pressure although I cannot remember on which fuel that was.

It is a hp chart, so where the lines meet is the same hp.

Note that the lines that rise from left to right are constant boost lines. That is, on the Merlin chart +25psi boost is maintained from 0ft to ~4,000ft in MS (medium supercharge) gear. In contrast, +18psi boost is held from 0ft to 9,500ft.

Boost is controlled by the throttle plate. As the altitude increases the throttle is gradually opened until it is fully open.

Naturally more boost can be produced than the engine could cope with if the throttle was opened earlier. This is how the boost was increased when better fuels were available.

The lines that fall from left to right are the full throttle lines. The throttle is wide open, and boost (and power) falls away with altitude. At 20,000ft the Merlin 66 is giving +18psi boost, whether it was set to do +25psi or +18psi.

The limit, whether +18psi or +25psi, is 5 minutes.

As far as I'm aware they never ran (authorised at least, who knows what the mechanics did in the field) boost pressures up to 28 lbs in the RR Merlin 66.

I think maybe very late war they were authorised.

Although the you should have used the Merlin 63 as a better example as its super charger was redesigned and re-heard to work at higher altitudes. The MK 66 was purposefully designed to produce more power at lower altitudes.

The Merlin 63 had the same supercharger as the 66, just different gears.

The higher FTH sacrificed power at lower altitudes.

The Mk 63 merlin was fitted in the Spitfire F MK IX while the MK 66 Merlin was fitted to the spitfire LF MK IX, the lower altitude derivative of the MK IX Spitfire.

The 63 was the earlier engine. I believe the 66 had some strengthening compared to the 63, and I'm not sure if the 63 was allowed to use +25psi.

The HF.IX used the Merlin 70.

You also have to remember that the DB601 was 34 litres and the DB605 was an even bigger 35.7 litres. The merlin on the other hand was a piffling 27 litres.

From my knowledge it is more this large capacity that allowed higher full throttle heights than the high compression/low boost mantra. I'm not saying that they don't help but it always seemed that the large capacity had the most impact.

The extra capacity certainly helped in the power stakes, but I still think the pressure ratio of the supercharger was important for high altitude performance.


If you use the Merlin as an example, if you raised the CR and lowered the boost the FTH would be considerably higher.

Yes I know how to read that graph.

I really shouldn't have but I used the term wastegate. Yes it is the throttle valve that controlled the boost pressure. However the merlin and I think the griffon too, were equipped with automatic boost controllers that would limit boost to whatever the limit was set at on the ground regardless of how much you opened the throttle (until of course you hit full throttle height). This automatic boost control mechanism altered the throttle plate to control boost pressure so it took a large workload off of the pilot.

Here is a nice link that describes the mechanism in some detail, http://www.enginehistory.org/Piston/Rol ... nABC.shtml

P.s. Go up and read my edited post and the bit about the Jumo 213 engine. Love that thing for high altitdue

[J.A.W.]

As regards the Jumo 213, are you referring to actual service proven performance, or only mooted/projected?
Some of the late-war 'desperation' stuff may have been a bit 'pie-in-the-sky' for what was an ex-bomber mill..

The Germans were intending to rev them out to 3,700rpm, which rather raised British eyebrows piston speed-wise..

[trinidefender]

As regards the Jumo 213, are you referring to actual service proven performance, or only mooted/projected?
Some of the late-war 'desperation' stuff may have been a bit 'pie-in-the-sky' for what was an ex-bomber mill..

The Germans were intending to rev them out to 3,700rpm, which rather raised British eyebrows piston speed-wise..

I'm not talking about numbers that were shown on actual aircraft. More numbers that were bench tested by Junkers themselves when designing the engine. Usually I prefer to use real world examples but in this case with the limited resources Germany had it just wouldn't be fair to use the numbers put out by factory production engines.

Saying it is an "ex bomber mill" is quite harsh. It was totally resigned. Stronger crank with most of the internals beefed up to take the extra power. Pressurised cooling system was added. Fully housed supercharger with 2 speeds and 2 stages was added. A charge air cooler was used on some models. It was updated to run much faster, something like 3,250 rpm by wars end.the exterior and cooling passages of the block were redesigned to decrease engine frontal area and to take advantage of the increase in cooling. Intake manifold after the supercharger was changed to allow a freer flow and not to mention many other changes. I would call it a new engine.

[J.A.W.]

Harsh, but accurate, since all the German 'hyper' types such as the Jumo 222 had failed to come good..
& the production facilities set up to mass produce Jumo V12 bomber engines were urgently needed for fighters..
( a similar remark re 'bomber' engines could be applied to most radial aero-mills likewise)..

As posted earlier, here is Roy Fedden's post-war appraisal for 'Flight', inc' notes on 213 development..
Basic architecture - bore & stroke unchanging - largely determines a developmental refinement, a do-over..

http://www.flightglobal.com/pdfarchive/ ... 02394.html

[Tommy Cookers]

TC this may be of some interest to you. It is a Wikipedia link I know but is cited all the same, if only the citation was a book I could get my hands on now. http://en.m.wikipedia.org/wiki/Rolls-Royce_Merlin

Specifically look at this, "During tests, 70 pounds-force (310 N; 32 kgf) thrust at 300 mph (480 km/h), or roughly 70 horsepower (52 kW) was obtained which increased the level maximum speed of the Spitfire by 10 mph (16 km/h) to 360 mph (580 km/h)." It doesn't say which version of ejector exhausts were fitted or which mark merlin was used however it may still be relevant.

thanks for this reference
this would be what I call the 1940 ejector exhaust
to me this EE gave more thrust than earlier night-suitable exhausts, but not more thrust than was available from day flying exhausts
subsequently Spits etc went to individual stub (ie day type) exhausts which must surely have given at least as much thrust as EEs
I now assume this was because in 1940 Hurri and Defiant (and Spit even) night flying was important in GCI of night attacks
but by 1941 Ai took over, and the Spit in particular was not required to have exhausts well suited to night combat

this EE's thrust power of 70 hp .....
implies that eg the Mk 19 Spitfire had an exhaust thrust power ('jet power') of about 200 hp
at its much greater maximum tas/altitude, and having/using about 2.5x the power in the EE referred above
(the pressure drop to ambient is greater, thrust is greater relative to exhaust energy at this greater tas, and exhaust energy is 2.5x)
at 49000' the crankshaft power and the propellor thrust will be quite low
so the ratio of exhaust 'jet thrust' to prop thrust is higher in the Mk 19 than the 16:84 jet thrust:fan thrust ratio in the A380 ?

also thanks for links, to Wuzak and J.A.W
good supercharger efficiency was no problem at low and medium altitudes as pressure ratio was small or quite small
so the USAAC/AAF saw the turbo as no more burdensome than other ways of getting the 2nd stage needed for altitude
partscountwise the turbo was more-or-less an exhaust system option to the standard (mechanically-supercharged) engines

the turbo was more efficient at high altitude/high speed (eg P-38 and P-47) because the mean exhaust pressure is raised
as eg in turbo road car engines, delta P is close to zero, NACA showed this improves efficiency because .....
raising mean exhaust pressure (density) better conserves pressure pulses ie reduces blowdown pressure loss (upstream of turbine)
so the cost in crankshaft power of driving the supercharger is less (with turbocharging compared to mechanical supercharging)
but the turbo aero-engines exhaust thrust is no less than with mechanical' supercharging
and can be greater if it has a variable exit nozzle (though this was in principle and even in actuality available to the MS engines)
IMO

[trinidefender]

TC this may be of some interest to you. It is a Wikipedia link I know but is cited all the same, if only the citation was a book I could get my hands on now. http://en.m.wikipedia.org/wiki/Rolls-Royce_Merlin

Specifically look at this, "During tests, 70 pounds-force (310 N; 32 kgf) thrust at 300 mph (480 km/h), or roughly 70 horsepower (52 kW) was obtained which increased the level maximum speed of the Spitfire by 10 mph (16 km/h) to 360 mph (580 km/h)." It doesn't say which version of ejector exhausts were fitted or which mark merlin was used however it may still be relevant.

thanks for this reference
this would be what I call the 1940 ejector exhaust
to me this EE gave more thrust than earlier night-suitable exhausts, but not more thrust than was available from day flying exhausts
subsequently Spits etc went to individual stub (ie day type) exhausts which must surely have given at least as much thrust as EEs
I now assume this was because in 1940 Hurri and Defiant (and Spit even) night flying was important in GCI of night attacks
but by 1941 Ai took over, and the Spit in particular was not required to have exhausts well suited to night combat

this EE's thrust power of 70 hp .....
implies that eg the Mk 19 Spitfire had an exhaust thrust power ('jet power') of about 200 hp
at its much greater maximum tas/altitude, and having/using about 2.5x the power in the EE referred above
(the pressure drop to ambient is greater, thrust is greater relative to exhaust energy at this greater tas, and exhaust energy is 2.5x)
at 49000' the crankshaft power and the propellor thrust will be quite low
so the ratio of exhaust 'jet thrust' to prop thrust is higher in the Mk 19 than the 16:84 jet thrust:fan thrust ratio in the A380 ?

also thanks for links, to Wuzak and J.A.W
good supercharger efficiency was no problem at low and medium altitudes as pressure ratio was small or quite small
so the USAAC/AAF saw the turbo as no more burdensome than other ways of getting the 2nd stage needed for altitude
partscountwise the turbo was more-or-less an exhaust system option to the standard (mechanically-supercharged) engines

the turbo was more efficient at high altitude/high speed (eg P-38 and P-47) because the mean exhaust pressure is raised
as eg in turbo road car engines, delta P is close to zero, NACA showed this improves efficiency because .....
raising mean exhaust pressure (density) better conserves pressure pulses ie reduces blowdown pressure loss (upstream of turbine)
so the cost in crankshaft power of driving the supercharger is less (with turbocharging compared to mechanical supercharging)
but the turbo aero-engines exhaust thrust is no less than with mechanical' supercharging
and can be greater if it has a variable exit nozzle (though this was in principle and even in actuality available to the MS engines)
IMO

About the extractor exhausts. As far as I am aware in the later war years the extractor exhausts weren't used to the same effect. I don't mean to say that they weren't used but that they weren't relied upon as much as the original models. The extractor exhausts were great at high speed and high altitude but for slow speed maximum climb performance they were detrimental compared to the power loss that they provided. The most distinctive EE's were the fan/fishtail ones. Later on in the war Rolls Royce and Supermarine went back to more tubular shaped exhausts that were more focused on pure crankshaft power.

TC all that about the turbocharger being more efficient is great however you are forgetting one thing. This:


There was something like, if I remember correctly, over 11 feet worth of turbocharger piping. This meant the setup was massive, heavy, very vulnerable to enemy fire and was hell to maintain with the piping often springing leaks, especially common on the the P-38, due to the expansion and contraction due to heating.

You say that the turbocharger exhaust thrust can be equal to the mechanical supercharger however none of the WWII designs that were turbocharged seemed to use the exhausts to provide thrust. I do have a question though, the turbocharger turbine does absorb a lot of energy from the exhaust and the exhaust flow does come out the turbine at a lower temperature so how would a turbocharged unit still have nearly as much exhaust thrust?

P.s. As far as I'm aware the A380's trent 900 option has a bypass ratio of about 8.5-8.7:1, not 5.25:1 (84:16) as you said.

[wuzak]

There were a couple of prototypes that tried to use the exhaust from the turbo to provide thrust.

The XP-67



The XF-12

[J.A.W.]

The US Navy preferred mechanical supercharging for their fighters,
& although radial mills meant more convoluted/less effective exhaust thrust arrangements, it was beneficial..

In this Corsair article 190lbs/20mph is claimed from exhaust thrust utilization..
http://legendsintheirowntime.com/F4U/F4 ... 08_DA.html

[J.A.W.]

This rare P-51H clearly shows the attention to detail N.A.A. gave to its slick aero, inc' the ejector stubs..
..which are both finely faired & big bore - to enable the Merlin-9 to run its rated 90"/ADI for 2,200+hp at high speed..
http://www.youtube.com/watch?v=HfPOVWOIL8M

Certainly, R-R reckoned that for high speed fighter performance.. turbochargers were not as effective..

[gruntguru]

You say that the turbocharger exhaust thrust can be equal to the mechanical supercharger however none of the WWII designs that were turbocharged seemed to use the exhausts to provide thrust. I do have a question though, the turbocharger turbine does absorb a lot of energy from the exhaust and the exhaust flow does come out the turbine at a lower temperature so how would a turbocharged unit still have nearly as much exhaust thrust?

It won't. The mass flow will be the same but the velocity will be lower unless the designer was to allow an extra-ordinarily high back pressure (which would of course reduce the bhp). Fighters with the turbocharger behind the pilot would be even worse on exhaust energy due to heat loss in the plumbing.

[Tommy Cookers]

IMO the turbo potentially has competitive exhaust thrust ......
because the mean exhaust pressure downstream of the turbine is not lower than in the mechanically supercharged engine
because the mean exhaust pressure above the turbine will be higher or much higher than in the MS engine

and because P&W thought so, they did the VDT

MS engines in WW2 had fixed geometry exhaust exits
but ideally there should be variability of exhaust outlet area of maybe 3x to match eg both a low altitude sprint and a high altitude cruise
to an extent the turbo will naturally regulate the exhaust pressure

note to self -
in principle an aircraft turbocharger turbine could be operated as pure blowdown, these never were
but those few more recent light aircraft (true) turbocharged engines presumably were blowdown operated (for various reasons)


J.A.W, thanks for the useful links, eg to Corsair exhaust thrust engine power context
now I'm wondering about the 3-1 pipes re intervals and ('tuned') length effects (eg any benefits to crankshaft power)
(having suspicions regarding the Turbocompound exhaust length effects)

@ trini
the 84% fan hrust for the Trent (A380) engine was stated by RR, bypass ratio will not directly represent the fan thrust %

[gruntguru]

But velocity will be lower because volume flow is lower because specific volume is lower because temperature is lower.

P&W knew there would be "some" thrust. If it was only half I wouldn't categorise it as "competitive" (the term used by trini)

[wuzak]

I really shouldn't have but I used the term wastegate. Yes it is the throttle valve that controlled the boost pressure. However the merlin and I think the griffon too, were equipped with automatic boost controllers that would limit boost to whatever the limit was set at on the ground regardless of how much you opened the throttle (until of course you hit full throttle height). This automatic boost control mechanism altered the throttle plate to control boost pressure so it took a large workload off of the pilot.

Cirrect. It is the Automatic Boost Control taht is adjusted to allow for increased boost with higher grade fuels.

[Tommy Cookers]

But velocity will be lower because volume flow is lower because specific volume is lower because temperature is lower.
P&W knew there would be "some" thrust. If it was only half I wouldn't categorise it as "competitive" (the term used by trini)

why would the temperature be lower ?
the mean exhaust pressure above the turbine is far higher than the the mean exhaust pressure in the MS engine

and why should velocity be the dominant factor ?
for a given energy the thrust will be greater with lower velocity and higher pressure ( via momentum/Froude ? efficiency)

unrelated .....
the Corliss throttle (re Merlin era) was chosen via needing less force from the operator and 'had a smaller pressure drop'
I have in the past asked 'experts' whether or not the necessary property of a throttle is to drop pressure