730 replies to this topic

[Greg Locock]

are you confusing BSFC and BMEP?

[mariner]

No, I was trying to see if there was an equivalent , or a conversion to, BMEP from , essentially BSFC so s to relate gas turbine overal hcycel efficiency to teh piston engine.

Boith do the same thing , they compress outside air from its existing pressure then inject and burn fuel to produce more gas then went in and finally expand the increased gas quantity back to outside air pressure.

Hence my question

Edited by mariner, 04 February 2012 - 16:59.

[Vanishing Point]

No, I was trying to see if there was an equivalent , or a conversion to, BMEP from , essentially BSFC so s to relate gas turbine overal hcycel efficiency to teh piston engine.

Boith do the same thing , they compress outside air from its existing pressure then inject and burn fuel to produce more gas then went in and finally expand the increased gas quantity back to outside air pressure.

Hence my question

You'd need to connect a turbo prop output to a dyno then get a maximum torque figure then find out if it's possible to work out a direct comparison between a piston engine capacity compared to a turbine capacity then use the result to work out a specific torque figure.

[GreenMachine]

You'd need to connect a turbo prop output to a dyno then get a maximum torque figure then find out if it's possible to work out a direct comparison between a piston engine capacity compared to a turbine capacity then use the result to work out a specific torque figure.

Torquemeter?

[mariner]

That is kind of what the big turbo fan the core drives is really a "fan dyno" so it should be possible to calculate the loads from the thrust numbers I would think

[bigleagueslider]

Slightly off topic but with all this discussion of aero piston engines can anybody indicate to me the equivalent bmep for a modern jet engine?

I know BMEP is a piston engine measure but can you convert the measures used for a jet engine to BMEP to see if any piston aero engines match / exceed modern jets?

My understanding is that modern jets are actually very fuel efficient ( especially at high altitude cruise) so I would expect a very high equivalent BMEP if there is such a calculation

mariner,

That's a good question. But the answer is no, there is no direct equivalent to BMEP in turbine engines.

You could make a comparison with BSFC or BTE between a stationary turboshaft engine and a piston engine, since both are based on shaft work. But due to the way aircraft turbofan engines operate, their thermal efficiency and effective work output can vary greatly due to forward speed, air density and temperature.

Like recip piston engines, turbine engine efficiency benefits from economy of scale, but mostly for different reasons. Large displacement recip piston engine efficiency is high due primarily to lower heat rejection. Large turbine engines tend to have better efficiency than smaller turbines due to the better aerodynamic performance possible with the larger compressor and turbine airfoils, and lower stage losses.

Modern large turbofan engines are indeed quite efficient. The most efficient one being Pratt's new PW1000 GTF with an overall pressure ratio close to 50. But even this engine is not as thermally efficient as modern large recip diesel propulsion engines, like those used in massive container ships. Turbine engines still cannot operate at or near stoichiometric combustion conditions due to turbine inlet temperature limitations. If they could, their efficiency would improve significantly.

Regards,
slider

[24gerrard]

Surely thrust is the only basic that can be directly compared.
It should then be straitforward to adjust the figures for altitude and atmospherics.

[Robin Fairservice]

In Sir Stanley Hooker's book "Not Much of an Engineer" Stanley describes visiting Frank Whittle and seeing his gas turbine engine which developed 800 pound thrust. Lord Hines didn't think that was much until Stanley Hooker calculated the thrust of a Merlin engine in a Spitfire, and Hines was convinced that gas turbines were the future. I have lent my copy to a friend and can't quote the numbers.

Edited by Robin Fairservice, 05 February 2012 - 22:30.

[bigleagueslider]

Surely thrust is the only basic that can be directly compared.
It should then be straitforward to adjust the figures for altitude and atmospherics.

If only it were that simple. Purely on the basis of a thrust-per-consumed-fuel-mass rate metric (ie. TSFC), the average helicopter rotor system would win hands down. The helicopter rotor produces huge net thrust for a given installed horsepower or fuel consumption rate. Unfortunately, the thrust vector is not oriented favorably for many aircraft applications.

Here's another interesting anomaly to consider when comparing thrust and SFC of aircraft engines: the Pratt J58 engine used on the SR-71. The J58 achieved its best TSFC at speeds close to mach 3. At mach 3, the turbomachinery of the J58 (ie. compressor and turbine stages) were of no use, and they simply got in the way. Above mach 2, all of the engine air compression work was done by the inlet duct.

slider

[24gerrard]

If only it were that simple. Purely on the basis of a thrust-per-consumed-fuel-mass rate metric (ie. TSFC), the average helicopter rotor system would win hands down. The helicopter rotor produces huge net thrust for a given installed horsepower or fuel consumption rate. Unfortunately, the thrust vector is not oriented favorably for many aircraft applications.

Here's another interesting anomaly to consider when comparing thrust and SFC of aircraft engines: the Pratt J58 engine used on the SR-71. The J58 achieved its best TSFC at speeds close to mach 3. At mach 3, the turbomachinery of the J58 (ie. compressor and turbine stages) were of no use, and they simply got in the way. Above mach 2, all of the engine air compression work was done by the inlet duct.

slider

Agreed slider but I think the comparison was requested between the subsonic jet engines of the period and the established capabilities of the piston engines of the period.
The thrust figures for the complete aircraft would be useful for such a comparison.

Helicopters are inefficient simply because they use fuel for thrust and rely only on fuel use for lift. Its a burn, burn burn concept and results in the need for massive fuel support infra-structures much like those for obsolete Mach 3 spy planes.

[bigleagueslider]

Agreed slider but I think the comparison was requested between the subsonic jet engines of the period and the established capabilities of the piston engines of the period.
The thrust figures for the complete aircraft would be useful for such a comparison.

Helicopters are inefficient simply because they use fuel for thrust and rely only on fuel use for lift. Its a burn, burn burn concept and results in the need for massive fuel support infra-structures much like those for obsolete Mach 3 spy planes.

24gerrard,

I re-read mariner's post, it asked about "modern jet engines" versus piston engines, and what metric might be used to compare cycle efficiencies between the two. While my response may have been a bit meandering, the point I attempted to make was that the two types of aircraft propulsion engines can only be compared on a basis of work output versus total energy available in the fuel mass consumed. Since the engines transfer the power they produce through different mechanisms, and operate in widely different conditions, their efficiencies must be considered in the context of the entire aircraft performance.

With turbofan engines, their TSFC is lowest at take-off conditions and falls off quickly at cruise conditions. With recip engines, their BSFC is highest at take-off conditions and lowest at WOT cruise conditions. Aircraft engines tend to spend the majority of their time running at cruise conditions.

Ultimately, it would be a comparison of work in/work out. But with aircraft engine performance, the size, weight and aero efficiency of the airframe itself has a huge impact. And in those terms, an aircraft payload/range metric would be useful. If you look at something like fuel-consumption-per-passenger-seat-mile, a huge turbofan aircraft like the Airbus A380 looks fantastic.

Of course, this whole discussion over relative engine efficiencies ignores some very practical issues like cost or exhaust emissions. But regardless, it has been very interesting and enjoyable.

slider

[gruntguru]

Modern large turbofan engines are indeed quite efficient. The most efficient one being Pratt's new PW1000 GTF with an overall pressure ratio close to 50. But even this engine is not as thermally efficient as modern large recip diesel propulsion engines, like those used in massive container ships. Turbine engines still cannot operate at or near stoichiometric combustion conditions due to turbine inlet temperature limitations. If they could, their efficiency would improve significantly.

The diesels you speak of cannot operate near stoich' either. In both cases efficiency would increase if combustor out temperature could be increased, however rather than running closer to stoich, increasing PR would produce a greater efficiency improvement.

[Bob Riebe]

In all this talk, it is odd the XF-12 , what was probably the peak piston engined aircraft to ever fly has been ignored.

http://en.wikipedia....c_XF-12_Rainbow

Edited by Bob Riebe, 12 February 2012 - 06:01.

[Wuzak]

In all this talk, it is odd the XF-12 , what was probably the peak piston engined aircraft to ever fly has been ignored.

http://en.wikipedia....c_XF-12_Rainbow

Big call, but an interesting aircraft nonetheless.

The Rainbow carried a number of cameras, as required by the reconnaissance role, and a film development suite as well.

An airliner version was proposed, but the cost would have been several times that of its competitors - especially some of the war surplus aircraft available after the war. The cost would have been cheaper had the Air Force chosen it instead of converting B-50s and B-36s.

[Magoo]

Proposed use: "Photo reconnaissance," i.e., you have to claim it does something or Congress might cut off your money.

[Kelpiecross]

In all this talk, it is odd the XF-12 , what was probably the peak piston engined aircraft to ever fly has been ignored.

http://en.wikipedia....c_XF-12_Rainbow

Unfortunately it is also pretty ugly - it just doesn't look "right".

[Wuzak]

Proposed use: "Photo reconnaissance," i.e., you have to claim it does something or Congress might cut off your money.

Proposed for the same requirement as the Hughes XF-11.

The XF-11 and XF-12 received development contracts and funding for the construction of a pair of prototypes each.

Congress' funding went to Boeing (for the RB-50) and Convair (RB-36) instead.

Edited by Wuzak, 12 February 2012 - 22:29.

[Wuzak]

Unfortunately it is also pretty ugly - it just doesn't look "right".

I think it looks good - apart from the nose.

The nose shape was desigend for aerodynamic cleanliness, but at some stage during the design period (mockup review perhaps) it was figured that the screen would not allow for good vision in difficult landing conditions. So the nose was redesigned so that the windows retracted, leaving a flat window for the pilot to see through for landing.



You can see the bulkhead behind the nose. The top two quarter windows rotated down.

Edited by Wuzak, 12 February 2012 - 22:33.

[bigleagueslider]

The diesels you speak of cannot operate near stoich' either. In both cases efficiency would increase if combustor out temperature could be increased, however rather than running closer to stoich, increasing PR would produce a greater efficiency improvement.

gruntguru,

I agree. Turbines and recips are different animals. The recip engine has higher peak cycle pressures and temperatures than the turbine, but this is only possible in the recip engine due to the fact that the combustion process is intermittent.

The turbine engine guys are now pushing overall PR's higher mainly due to the fact that they've reached the limits of what can be done with TIT increases. 1st stage nozzles on modern turbine engines run at EGT's that could quickly melt them if it weren't for the sophisticated system of film cooling they have.

Obtaining higher PR's in turbine engines is not so easy either. Higher PR usually means more stages, and to get good performance from lots of stages you typically need several shafts running at different speeds. While this is possible with large engines, like the 3 shaft Rolls Trent, it is not so easy with smaller turbine engines.

slider

[Vanishing Point]

In all this talk, it is odd the XF-12 , what was probably the peak piston engined aircraft to ever fly has been ignored.

http://en.wikipedia....c_XF-12_Rainbow

What if the Germans had been clever enough to build something like that instead of the FW Condor.

[Wuzak]

What if the Germans had been clever enough to build something like that instead of the FW Condor.

Yep, the Wetsren Allies would have been in big trouble if teh Germans had managed to build aircraft with mid '40s technology in the mid '30s....

[dovatf]

Here is a photo of the DB 604 (X24 similar to the RR Vulture) I took at the Mercedes Museum in Stuttgart in 1983. Unfortunately this and other aircraft engines of WW II are no longer displayed in the new Mercedes Museum.



AFAIK its development was stopped around 1942.

Edited by dovatf, 13 February 2012 - 09:26.

[Wuzak]

Here is a photo of the DB 604 (X24 similar to the RR Vulture) I took at the Mercedes Museum in Stuttgart in 1983. Unfortunately this and other aircraft engines of WW II are no longer displayed in the new Mercedes Museum.



AFAIK its development was stopped around 1942.

Yes, development was stopped in 1942 - but I have no idea why.

It was 46l vs the 44l of the DB603 V12, but comfortably more powerful. It was about 100kg heavier, though.

AEHS Picture of DB604

AEHS Daimler-Benz Engine Ratings

The DB604 is 5th down the list, and is rated at 2660PS for takeoff, and 2410PS at 20,600ft for, I guess, war emergency.

[dovatf]

I read they found vibrations and bearings problems that would have required a lot of development work to be sorted out, so the RLM preferred that they concentrate on developing the DB 605 and larger DB 603 - and ordered the termination of the program. Only 5 were built.

Thanks for the links!

Edited by dovatf, 13 February 2012 - 13:12.

[Vanishing Point]

Yep, the Wetsren Allies would have been in big trouble if teh Germans had managed to build aircraft with mid '40s technology in the mid '30s....

I was thinking more along the lines of a totally redesigned Condor 2 around early 1940's would probably have been soon enough to have done the job .It would have meant having access to an engine supplier who was ahead,in realising that it's the big radial idea that seems to provide the best compromise for heavy aircraft that need to operate at sustained high speeds,though.

[gruntguru]

AEHS Daimler-Benz Engine Ratings

The DB604 is 5th down the list, and is rated at 2660PS for takeoff, and 2410PS at 20,600ft for, I guess, war emergency.

The bore, stroke and displacement specifications don't add up.
140mm x 135mm = 49.88L
135mm x 140mm = 48.09L

[Wuzak]

The bore, stroke and displacement specifications don't add up.
140mm x 135mm = 49.88L
135mm x 140mm = 48.09L

The original bore and stroke were 135mm x 135mm.

[Wuzak]

I was thinking more along the lines of a totally redesigned Condor 2 around early 1940's would probably have been soon enough to have done the job .It would have meant having access to an engine supplier who was ahead,in realising that it's the big radial idea that seems to provide the best compromise for heavy aircraft that need to operate at sustained high speeds,though.

BMW was developing the 801 at that time, and was in production in mid 1940.

The Condor was an airliner adapted for military use. It was, perhaps, the best airliner of the immediate pre-war era. It certainly could fly the atlantic non-stop (which caught the attention of the US).

It's role was maritime patrol, and thus its endurance was far more important than speed.

The Heinkel He177 was the strategic bomber design, but that had many problems, a lot of them to do with the powerplant. There weren't any radial engine alternatives in the same power class (2500hp), however.

BMW did design the 802 (18 cylinder version of the 801) and the 803 (liquid cooled 28 cylinder radial), but these came to nothing.

[Vanishing Point]

BMW was developing the 801 at that time, and was in production in mid 1940.

The Condor was an airliner adapted for military use. It was, perhaps, the best airliner of the immediate pre-war era. It certainly could fly the atlantic non-stop (which caught the attention of the US).

It's role was maritime patrol, and thus its endurance was far more important than speed.

The Heinkel He177 was the strategic bomber design, but that had many problems, a lot of them to do with the powerplant. There weren't any radial engine alternatives in the same power class (2500hp), however.

BMW did design the 802 (18 cylinder version of the 801) and the 803 (liquid cooled 28 cylinder radial), but these came to nothing.

But if it could have been given sufficient performance to outrun Liberators or the catapult launched hurricanes used on merchant ships to defend against the Condor attacks which were causing havoc in the Atlantic the results would have been even more severe for the convoys caught by them.

http://www.Asisbiz.c...200-Condor.html

[bigleagueslider]

The Heinkel He177 was the strategic bomber design, but that had many problems, a lot of them to do with the powerplant. There weren't any radial engine alternatives in the same power class (2500hp), however.

Of course there was also the FW 190, which used both the Jumo 213 liquid-cooled V12 and the BMW 801 air-cooled radial

[Wuzak]

Of course there was also the FW 190, which used both the Jumo 213 liquid-cooled V12 and the BMW 801 air-cooled radial

Neither of which were 2500hp engines.