Engine technology free-for-all

[gruntguru]

Document tabulating WW2 American aero-engine serviceability.. http://www.usaaf.net/digest/t115.htm

MONTHLY MAINTENANCE MAN HOURS PER ENGINE IN USD FROM JULY 1943 TO AUGUST 1945

[J.A.W.]

Bear in mind that V12s in US service were not used for transport/bomber types, with flight engineers to fettle..
& that the majority of US Merlins were used in Mustangs, & were certainly worked pretty hard,
- as the premier air-superiority USAAF machine, inc' escorting B-29s from Iwo Jima..

[trinidefender]

the VDT was intended at speed to reduce the TC exhaust recovery path and open a exhaust jet path ?

Yes, the help use the exhaust thrust. But also eliminate the engine mounted supercharger, relying on turbos only for supercharging, freed up several hundred hp.

the airframe changes (eg needed to the B50) were judged to be excessive

On R-4360 VDT was tried on a B-50. But the flight engineer had to monitor and control it constantly.

and ..... when fuel quality was raised CRs were usually held and mep/boost raised
this should have increased the exhaust thrust even with the usual simple exhaust systems

Higher boost improved performance at low altitude. That is the critical altitude or full throttle height was lower at a higher boost level, above which the extra boost could not be maintained.

Exhaust thrust was far more important at high altitudes, where engine power was reducing and propeller efficiency was lower.

so how much exhaust jet thrust (or equivalent hp) would eg a 2000 hp Mustang or Spitfire have had when high and fast ?

Not sure. There are figures about.

We can say that the Mosquito picked up ~10mph by changing to ejector exhausts.

what did the Merlin 1940 'ejector exhausts do ? (these were then dropped) increase crankshaft power ? increase jet effect by entrainment ?

The aime was to get more exhaust thrust, though the end shape was sometimes modified to reduce exhaust glare for night flying.

The ejector exhausts were refined throughout the war.

Slight correction. Many superchargers of the day were variable speed superchargers. The merlins and griffons (and some American designs), once matured in design, used 2 speed superchargers while the Germans favoured hydraulic couplings for their superchargers.

This allowed them to produce more power lower down with high octane fuel while at the same time at high altitude the superchargers were shifted into a high speed gear where they ran much faster and allowed higher compression ratios of the supercharger. Usually the higher supercharger compression ratios causes detonation/pre-ignition however the raised octane of the fuel allowed the engine to resist the higher operating temps with the increased supercharger compression ratios. In essence the high speed gear/hydraulic drive allowed designers to massively increase the full throttle height of the engine.

[Tommy Cookers]

and ..... when fuel quality was raised CRs were usually held and mep/boost raised this should have increased the exhaust thrust ......

W said ....
Exhaust thrust was far more important at high altitudes, where engine power was reducing and propeller efficiency was lower.
TC says ....
the exhaust thrust was greater at altitude because the ambient pressure was lower
and its efficiency increases with speed (this also tending to increase with altitude)
I think the inevitability of prop efficiency reduction was debunked by this stage (eg the 647 mph turboprop B-47)

[trinidefender]

and ..... when fuel quality was raised CRs were usually held and mep/boost raised this should have increased the exhaust thrust ......

W said ....
Exhaust thrust was far more important at high altitudes, where engine power was reducing and propeller efficiency was lower.
TC says ....
the exhaust thrust was greater at altitude because the ambient pressure was lower
and its efficiency increases with speed (this also tending to increase with altitude)
I think the inevitability of prop efficiency reduction was debunked by this stage (eg the 647 mph turboprop B-47)

TC and others might find this interesting and you guys and girls may have seen these types of graphs before but I'll post them anyway. They are supposed to be the general relationships of the thrust generated on various types of aircraft.





Some relate Mach numbers to thrust while some relate velocity to thrust. All of these graphs pretty much support TC's notion that exhaust thrust was greater at both high altitude and high speed, relating high speed exhaust thrust to a pure jet engine. Note however that most graphs of this type relate the various thrust levels in relation to Mach numbers. As you increase in altitude your Mach speed will be higher for the same true air speed.

[Tommy Cookers]

there is a speed record category for wheel-driven cars
how much exhaust thrust can be obtained ? (eg maybe enhanced by some mixing effect with air)
is (any) exhaust thrust really legal ?

[trinidefender]

there is a speed record category for wheel-driven cars
how much exhaust thrust can be obtained ? (eg maybe enhanced by some mixing effect with air)
is (any) exhaust thrust really legal ?

Are you referring to F1 here or just wheel driven car speed records on the whole?

Well the exhaust, while on throttle, will always be moving faster than the airflow around the car so it must provide some thrust however small.

As per your third question as it relates to F1, I would doubt that the engine designers would really consider this a priority this year with the turbocharger turbine sucking up as much priority as possible. Then again almost all the designs this year used a monkey seat placed to benefit from exhaust flow so there must be some useful energy there in the exhaust flow otherwise they would have tried to locate the monkey seat in free stream airflow.

Thoughts?

[J.A.W.]

Latest Kawasaki, a mechanically blown, 1ltr/inline 4, aero-device encrusted, carbon faceted, street/track bike..
As described by erudite commentator & tuner, Kevin Cameron..
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[gruntguru]

The question of exhaust thrust assisting land speed record cars of the "wheel driven" category would be relevant in all cases but particularly for a gas turbine car. I assume there is a category?

[wuzak]

The question of exhaust thrust assisting land speed record cars of the "wheel driven" category would be relevant in all cases but particularly for a gas turbine car. I assume there is a category?

Current wheel drive LSR is held by a gas turbine.

There is a sub category for piston engined vehicles, Diesels, steam powered, electric powered.

[wuzak]

W said ....
Exhaust thrust was far more important at high altitudes, where engine power was reducing and propeller efficiency was lower.
TC says ....
the exhaust thrust was greater at altitude because the ambient pressure was lower
and its efficiency increases with speed (this also tending to increase with altitude)
I think the inevitability of prop efficiency reduction was debunked by this stage (eg the 647 mph turboprop B-47)

Not sure on that one Tommy.

B-47 with turbojets did jet over 600mph.

Just found this:
http://www.joebaugher.com/usaf_bombers/b47_10.html

Performance is listed as 597mph @ 13,500ft. Note that it had 2 turbo props and 2 jets.

[wuzak]

Slight correction. Many superchargers of the day were variable speed superchargers. The merlins and griffons (and some American designs), once matured in design, used 2 speed superchargers while the Germans favoured hydraulic couplings for their superchargers.

Daimler Benz engines had the hydraulic coupling on its engines. Junkers and BMW engines had multi-speed gear boxes. Two speed in the BMW 801 and Jumo 211, up to 3 speeds in the Jumo 213.

Most American designs used distinct gear ratios too.

The two stage V-1710 used fluid couplings, and there was a fluid coupling drive for the R-2800 that powered the F8F-2 (post war). Also an experimental V-1650 (Merlin) with a fluid coupling supercharger drive built by Packard.

This allowed them to produce more power lower down with high octane fuel while at the same time at high altitude the superchargers were shifted into a high speed gear where they ran much faster and allowed higher compression ratios of the supercharger. Usually the higher supercharger compression ratios causes detonation/pre-ignition however the raised octane of the fuel allowed the engine to resist the higher operating temps with the increased supercharger compression ratios. In essence the high speed gear/hydraulic drive allowed designers to massively increase the full throttle height of the engine.

http://www.spitfireperformance.com/merlin66hpchart.jpg
http://www.spitfireperformance.com/griffonhp_b.jpg

You can see from these charts that the Merlin 66 had the same power at around 20,000ft whether its max boost was set to +28psi or +18psi.
For the Griffon it is around 26,000ft for +25psi or +18psi.

You are correct that the high gear can give more boost (and power) than maintaining low gear.

But you also notice that the Full Throttle height is reduced with increased boost.

Daimler Benz's high CR/low boost formula meant that the supercharger wasn't required to provide high pressure ratios, so its full throttle or critical altitude was higher.

[trinidefender]

Slight correction. Many superchargers of the day were variable speed superchargers. The merlins and griffons (and some American designs), once matured in design, used 2 speed superchargers while the Germans favoured hydraulic couplings for their superchargers.

Daimler Benz engines had the hydraulic coupling on its engines. Junkers and BMW engines had multi-speed gear boxes. Two speed in the BMW 801 and Jumo 211, up to 3 speeds in the Jumo 213.

Most American designs used distinct gear ratios too.

The two stage V-1710 used fluid couplings, and there was a fluid coupling drive for the R-2800 that powered the F8F-2 (post war). Also an experimental V-1650 (Merlin) with a fluid coupling supercharger drive built by Packard.

This allowed them to produce more power lower down with high octane fuel while at the same time at high altitude the superchargers were shifted into a high speed gear where they ran much faster and allowed higher compression ratios of the supercharger. Usually the higher supercharger compression ratios causes detonation/pre-ignition however the raised octane of the fuel allowed the engine to resist the higher operating temps with the increased supercharger compression ratios. In essence the high speed gear/hydraulic drive allowed designers to massively increase the full throttle height of the engine.

http://www.spitfireperformance.com/merlin66hpchart.jpg
http://www.spitfireperformance.com/griffonhp_b.jpg

You can see from these charts that the Merlin 66 had the same power at around 20,000ft whether its max boost was set to +28psi or +18psi.
For the Griffon it is around 26,000ft for +25psi or +18psi.

You are correct that the high gear can give more boost (and power) than maintaining low gear.

But you also notice that the Full Throttle height is reduced with increased boost.

Daimler Benz's high CR/low boost formula meant that the supercharger wasn't required to provide high pressure ratios, so its full throttle or critical altitude was higher.

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.

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.

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 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. 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.

Hence compare the full throttle height of the RR Merlin to the RR Griffon and notice that they use very similar compression ratios throughout their lives.

Also something to note was that BMW traded altitude performance that they enjoyed with their larger engine for simply more weight that they had to out around.

Now let's look at probably one of my favourite WWII aero engines. The Junker Jumo 213E specifically. 6.5:1 compression ratio. It combined large capacity with high boost pressures of a two stage, two speed supercharger into probably the best high altitude WWII fighter engine, especially considering that it ran on 87 octane fuel or 100 octane fuel if ever it was available.

I'm part Brit and even I'll admit that the best high altitude WWII aero engine was, in my view the Jumo 213.

[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.

[wuzak]

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.