Dumb Questions. Engine RPM.

[GregP]

OK, the collective military engine industry of Lycoming, Pratt Whitney, and Curtiss-Wright in the USA couldn't do it, but you can?

Lycoming needs you in their engineering department.

 

[PWR4360-59B]

Yeah either them or some other engine manufacture does. But do you think that will happen? Naaa
Oh and the reason those out fits you mention couldn't do it is they are all stuck in the box.

 

[GregP]

I might point out the guys stuck in the box won the war. Also, the best pistons produce about 1.1 HP per pound or so, perhaps slightly more, but not enough to matter here. If you kept up with that best, your 14,500 Hp engine would weigh close to 13,200 pounds.

The turborpop in the Tu-95 Bear that produces 14,500 HP weighs close to 6,400 pounds or slightly less, including the gear case, and so is less than half as heavy as your projected piston engine alone. By the time you add the gearcase to fly your engine, you'll be well over 15,000 pounds for 14,500 HP installed weight, and you still haven't added a radiator (if liquid cooled), the coolant, the collant pump, or the accessories to attach the plumbing to the engine. Of course, that assumes you need a gearcase and a contraprop due to the immense torque produced by a single propeller.

I'd say with the reliaibiliy of turboprops as a known factor compared with pistons plus the weight disadvantage, your projected 14,500 HP piston engine would have few takers.

What do you say? Hopefully in more than two sentences ... can you explain your claim that you could do it? Technically. How would you make a piston engine light enough to fly and competitive in weight, givien that no piston engine of the aviation variety to date can do it or even come CLOSE to it?

 

[Shortround6]

Once again, the big difference between race car/boat engines and race airplane engines is that if you blowup the car/boat engine you can walk/swim/hitch a ride to the pits and get the vehicle towed. Blowing up an airplane engine doesn't leave the driver/pilot with those options.

There is also the question of duration. IF making power at those dragster levels was so easy why aren't the Speed record breakers at Bonneville using those power levels? Maybe making power for 6-8 seconds and replacing the crankshaft after less than a minute of run time (at power) doesn't work so well even for a measured mile speed run?

 

[PWR4360-59B]

To make the power levels you mention, they would have had to get out of the recip engine box they made for themselves back then.
Won the war? By shear numbers. And a steady manufacturing environment in the states. Unlike the countrys that lost, that had thier manufacturing base and raw materials supplys constantly destroyed.
Explain my claim? You mean give away certain idea's? I will say every, piston aircraft engine ever made is lacking. And I will not go into details, and give the ideas away. So this majic 14,500 hp number used, how does that relate to some of the jet engines today? I thought they were much higher than that. And what are the weights of those jet engines? A wild guess is about 15,000 pounds for a 747 size engine at what 60,000 lbs thrust? And how much HP is that?
I thought you had to be going 375 mph or so at sea level, so not sure how the jet is rated. Well quick numbers here if 60k was the hp figure at sea level and at speed, then the estimated fuel consumption is 61 gallons per minute. The nice thing is when you arrive at what ever HP number with the recip you are going to shave off the fuel consumption numbers. I remember some years back they were taking about making pusher props on some jet liners, so a prop is not out of the question. No matter what a jet is a constantly flushing toilet when it comes to fuel economy.

 

[Shortround6]

Just a few real world numbers. A 1988 Rolls-Royce RB211-524G had a take-off thrust of 58,000lbs, it's dry weight was 9,499lbs. At cruise (35,000ft and mach 0.85) it gives 11,813lbs of thrust at a specific fuel consumption of 0.570 lb/h/lb.

on the other end of the spectrum we have the R-R Viper 680 used in the MB-339 trainer. Take-off thrust 4360lbs for a dry weight of 836lbs, specific fuel consumption of 0.97lb/h/lb at take off.

When comparing fuel consumption figures for the piston engine please remember that thrust is what counts and when converting engine HP to thrust the propeller is only 80 something % efficient.

When comparing piston aircraft engine power to some race car engines, the race car engines may be using alcohol for fuel. It has a lot of advantages for race cars but one big disadvantage for airplanes. It's BTU content per gallon is about 1/2 (depending on type) that of gasoline meaning you have to carry twice as much.

I will say that GregP is correct on some points (some others I don't know so will refrain from comment) gearboxes grow in weight the more power they have to transmit. Bigger reciprocating engines need bigger starters. There are definite limits on how big you can make a cylinder (bore and stroke) and run it at certain rpms unless your thinking out side the box can change the speed of the flame front moving across the cylinder, dual ignition helped here but again there are limits. Internal friction ( and bearing loads/stress) goes up with the square of the speed so high RPM is great for power output per cu in but lousy for fuel economy and engine weight per hp.

 

[GregP]

Hi Shortround,

Unless I completely misremember, Thrust = (HP * 375) / moh or (HP * 325) / knots. This is a very good first-order approximation. So, in your first case aboove (Mach 0.85 @ 35,000 feet), I believe the conversion is as follows:

1) Assume for teh sake of argumenta standard day. At 35,000 feet, the temperature is -65.4°F, speed of sound is 663.3 mph. So Mach 0.85 = 0.85 * 663.3 = 564 mph.

2) Using the mph part of the equation above, with 11,813 pounds of thrust and 564 mph, the HP = 17,767.

Let's say you are speaking of a Boeing 747-8. The engines are GE nx-2B67 units and make 66,500 lbs of thrust for takeoff. They have 105 inch diameter fans and an overall pressure ratio of 43 with a bypass ratio of 8.6 : 1. The dry weight is 12,400 pounds each. The HP at takeoff, assuming liftoff at 150 mph, is 27,000 HP each.

The Boeing 747-8 has a maximum takeoff weight of 975,000 pounds and the total for the four engines is 49,600 pounds, or 5% of the maximum takeoff weight.

So, let's say this proposed piston engine weighs in at 12500 pounds, and about 14,500 pounds with a gearcase and make 14,500 hp. If the engines made up 5% of the maximum takeoff weight, enguys piston transport would weigh in at 1,160,00 pounds and would have a power loading of 80 pounds per HP. For a rough comparison, a Cessna 172 weighs in at 2,450 pounds and has 160 HP for a power to weight ratio of 15 pounds per HP.

We can estimate the performance of the transport as not very good if a Cessna 172 has a power to weight ratio more than 5 times as good as the proposed transport. That's why turbines are so darned good! Thye make a lot of HP for the weight. Pistons just don;t, by comparison.

Sorry enguy, it just ins't practical, even thinking out of the box, and most engineers, this one included, don't think it is possible.

You might be right ... I don't think so, and I'll let it go at that.

Good luck in your engineering endeavors and I'm done with this exercise, at least as far as arguing it goes. If we see a big piston aircraft powerplant come into use, I'll get very interested again.

 

[PWR4360-59B]

So the RR at cruise is using about 17 or so gallons a minute?
I do have a question. Is the fan used on the jet more effcient than a prop? How fast is that large 747 fan spinning?
One other interesting advantage of a recip is, it can be used in air with volcanic ash. Since it is more posssible to use an air filter for it, on a jet its impossible.
Overhaul cost of a recip would be less, fuel consumption would be less, and it can burn the same fuel the jet does if need be.

 

[GregP]

What did you mean by "the RR cruises at 17 gallons per minute?" I never mentioned a Rolls Royce or fuel consumption. I talked about horsepower and weight. If you want to get into fuel consumption, let's talk about real numbers. Let's use the GE nx turbofan, such as used on the new Boeing 787 Dreamliner.

In cruising flight, the 787's total fuel flow is 11,402 pounds per hour, or 1,689 gallon per hour. The 787 weighs 450,000 pounds at mid-weight cruise.

A Cessna 172 burns 9.5 gallons per hour at 75% power (75% of 160 HP , ro 120 HP) and 120 knots at 2500 pounds weight.

If we scale the Cessna up to weight 450,000 pounds we must multiply by 180. Multiplying 180 by 9.5 means we would scale the engine fuel burn up to 1,710 gallons per hour.

So the Boeing 787 at 450,000 pounds and Mach 0.85 is slightly more efficient than a 450,000 equivalent Cessna 172 at 120 knots. Call it a wash.

What is your objection? Specifically, to the Boeing? It seems to be doing a pretty good job in relation to one of our most economical airplanes, a Cessna 172.

 

[PWR4360-59B]

Shortround6 had the numbers. 1710 gph? Hmmm still thinking about them Allisons .
So what bsfc for the 172? Use the bsfc of the Napier Nomad, and remember that engine is old stuff. I'm not thinking just SI engines.

 

[GregP]

Hi engguy. Good one about 1710 and Allisons.

C'mon, I have been doing the digging and calculating in our bantering. You do the BSFC for the two planes and tell me. I already told you the gallons per hour. Just for the record, these are US gallons, not Imperial gallons. For jet fuel, use 6.75 pounds per gallon on a standard day ... not that we ever experience a standard day. I know I haven't, particularly when flying. If I have passengers, it is always warmer than standard. When I don't have time to fly, it can sometimes get colder than standard. But almost never standard.

 

[Shortround6]

Shortround6 had the numbers. 1710 gph? Hmmm still thinking about them Allisons .
So what bsfc for the 172? Use the bsfc of the Napier Nomad, and remember that engine is old stuff. I'm not thinking just SI engines.

Any savings in fuel from the Nomad would be eaten up in maintenance costs

Just to show why the Nomad didn't go anywhere. BSFC of 0.33 for the Nomad, 2,392shp and 145lbs thrust max continuous (altitude not given) for a weight of 3,580lbs (dry). R-R Tyne turbo prop. BSFC 0.41, 2,270shp + 150lbs thrust at 25,000ft/425mph max continuous for a weight of 1935lbs without propeller. Later versions of the Tyne got below 0.40 SFC.

 

[PWR4360-59B]

I'm bad. I was using 6.5 lbs per gallon. I thought gasoline and kerosene were close to that. Serious Shortround6? I didn't think any turbine ever got that good of bsfc. GregP I'm sorry I'm kind of a lazy bum sometimes. I thank you for your hard work in tracking down that good info. And you to Shortround6 thanks for the good info.

 

[GregP]

You're very welcome engguy. Technical is fun, but not after too many beers, too late in the evening ... sometimes I get lazy too. See three posts above ...

 

[vanir]

I've no idea of its voracity but I watched an entertaining documentary on Whittle including his original notes and recordings, in which he essentially describes his development of the jet idea as originating in the simple design of a high altitude piston engine. This was at the time and in the absence of alternatives to piston engines, the limitations of the piston engine had been reached and it's all about altitude. So he started off by designing a complicated blower and intake design where the thrust prop was also encased in a tube, and helped engine air intake, to raise the service ceiling of his proposed engine.

Eventually as he refined the design successively, the blower got bigger and morphed with the prop, which he drove with the jet exhaust unit, so the piston engine just became an oversized and largely superfluous starter motor. This was the original Whittle jet engine design, part of the show was about it.

So it appears looking at it from this perspective that jet turbine propulsion was rather a linear evolution of piston aero engines due to altitude and thus performance limitations. What I mean to say is if you come up with an innovative new piston aero engine design you'd be reinventing the wheel and designing a rudimentary jet like Whittle wound up doing, just by accident, because that was the way forward. Coincidental, independent jet technology evolution concurrantly is the supportive argument and it's uber convincing on that basis alone.

 

[Siegfried]

The combustion process in a SI or spark ignition engine is known as 'deflageration' it is not an explosion but a controlled burn along a flame front at at sonic speed emanating from the sparking plug. Simultaneous combustion at multiple points throughout the cylinder is known as detonation, is damaging to the engine and occurs at supersonic speed at multiple points in the mixture propagated by infrared radiation. Hot spots or end gas species can also cause pre-ignition and detonation.

As cylinders get bigger the propagation speed becomes an issue and the flame front can become unstable. This limits the maxim cylinder size. The biggest cylinders were the Daimler Benz DB603 at about 3.7L each and the Mikulin AM-38 at about 3.9L each, the R-3350 worked out only at 3.2L a cylinder. One solution is to simply increase the number of cylinders, other partial solutions are to use multiple spark plugs and to create rich stratified charges around the spark plugs. (Used in the giant Curtis Wright Wankel engines). In a normal reciprocating piston a large number of cylinders, multiple spark plugs, vibration along long shafts becomes difficult or exhausting to design, build and maintain.

Diesels are not limited by maxim cylinder size, hence the Napier Nomad had only 12 cylinders. The CI (Compression Ignition) Diesel is somewhat heavier (50% again) than the SI due to the higher stresses of the high compression ratio and the peaky pressure pulse of the injection and burn. Diesels are however made to be turbo-supercharged, they love it, the exhaust is much cooler due to the greater efficiency and dilution by air of the exhaust (less stress on the turbine and exhaust manifolds) while there is no danger of pre-ignition. The stunning performance of modern automotive high speed diesels and light aviation engines shows what can be done. The hyperbaric diesel of a Leopard 2A6 can produce 1500hp and nearly 2800 if used in amphibious vehicles.

If it were not for the invention of the jet engine long range cruise airliners would be using diesels. The injection of an oil also ensures cylinders is less worn than the injection of a solvent (petrol).

During the second world war the Germans tried to replace spark plugs with injection of a special fuel that would ignite under compression, these were known as 'ring cycle' engines and are today known as HCCI "homogeneous charge compression ignition engines" they are very efficient, perhaps 50% and may one day power our cars. Although these are CI engines Unlike diesels the engines fuel is mixed throughout the cylinder prior to injection and unlike petrol engines they can run lean: ignition is achieved via careful mixture of exhaust gas recirculation or through the use of a special injected fluid, injected at a rate sufficient to maintain idle speed. I suspect these engines would not be limited by cylinder size.

There are some CIOS files here of the German WW2 work:
T.O.M. Microfilm Reel 248
http://www.fischer-tropsch.org/Tom Reels/Linked/TOM 248/TOM-248-0228-0243 FD2866-46-Lt35.pdf

I'm not a combustion engineer, I am an electrical engineer so take what I say as being somewhat simplified.

 

[vanir]

Interesting post Siegfried, thanks man. Hey I read a German wartime doc (got it saved) all about synthetic fuel development which of course talked about compression rates and the pinging issue with large bore cylinders. Most of the DB-605 development in lieu (ie. continued development after production) was to do with combustion chambering and trying to control that flame front. The document specifically discusses it, going so far as to say the German philosophy was that with carefully designed chambers lower grade fuels did not compromise on engine output. Another thing it discussed was the definite preference for lower rather than higher octane in common use particularly with synthetics because the chemical cocktail of high octane synthetic was too volatile and corrossive (which at the end of the war was comparable to 150 grade despite being listed as 100 octane, I spoke with a rebuilt Fw190A operator who swears he has documentation for this).

The document mentions specific chemicals in high octane synthetic that actually ruin the engine in prolonged use, so the German preference and aim in all aero engine development is formally stated as always in 87 octane or "normal premium grade" for its longevity and improved running condition. Use of 100 octane synthetic was pretty much on a need to do basis when development was lagging, say for the 601N because the 601E wasn't ready yet, or for the 801D because the bugs just couldn't be ironed out running 87 octane (never did solve hot running and short military operation, west front tactics were to tag team interceptions, got a great account of one of these over france).

Anyways Germany liked big swept capacities for easy good throttle altitudes, and carefully designed, extensively developed chamber design for the synthetic fuel grade that was nicest to the engine. This wound up mainstream tradition, notably the jumo 213 broke into the Merlin direction for smaller bores/higher rpm but typically german, was built so bulletproof it's as big as a db603.

So the main thing that document talks about is flame fronts and combustion chambers, central focus of german engineers on aero engines at the time.

 

[vanir]

You know dave, one of the big differences in the whole rpm and horsepower equation between aero and auto engines is altitude. Really altitude is part of the running of the engine in a plane, it's not in a car just something you have to compensate for with tuning. It's a whole different world, effectively rpm = altitude. And you think of engines as a physics equation, so much work for so much fuel divided by efficiency.

Now one of the big things here is the relationship between torque and horsepower. Torque is the power and horsepower the angular momentum, and altitude is rpm so aero engines run around using mostly torque, peak torque in the neutral model 4 stroke is 3000rpm so aero engines generally aim for a peak horsepower figure close to this engine speed. Then you run at maximum efficiency under most normal operating conditions. Maximum output will be a matter of how strong the motor is, and how much breathing the layout can feed, but it's counterproductive for it to be very far from normal military. Engines built for high rpm get loose around bearings at cruise, etc.

Aero engine power is expressed in horsepower, like truck engines because the idea is to demonstrate a figure of resistance this engine will overcome. Brake horsepower is measured by placing resistance on the crankshaft. But the motive power the aero engine is using is torque. Automobiles run on horsepower, continually varying engine speed from 800-5500rpm and more. That would kill a plane.

 

[Siegfried]

SNIP Most of the DB-605 development in lieu (ie. continued development after production) was to do with combustion clambering and trying to control that flame front. The document specifically discusses it, going so far as to say the German philosophy was that with carefully designed chambers lower grade fuels did not compromise on engine output. SNIP

So the main thing that document talks about is flame fronts and combustion chambers, central focus of German engineers on aero engines at the time.

I suspect that had SI piston engines had to develop, due to some technical issue with turbines, some problems relating to maximum cylinder size might have been solved. Perhaps we would have seen cylinder volume expanded such that we would see 60L or 72L V12's (V-3840, V-4608) or 90L (R-5760) 18 cylinder radials I am not a big fan of large numbers of cylinders as in the R-4360 as being affordable. The giant wankel was another alternative.

The approach taken by Allison with its V-3420 (two Allisons bolted back to back with a common crankcase but seperate crankshafts geared together) strikes me as realistic. Fairey did this with its Fairey Prince (H-16) engine which consisted of two flat opposed 8 cylinders engines which were arranged vertically in two separate blocks, driving contra-rotating propellers via separate shafts and gears. Each bank of cylinders could be shut down in flight to drive only one propeller. That kind of arrangement gets two big V12's to a total of about 5000hp. The problems of the Germans had with the DB606 seem to have been unusual and related to the installation as much as the engine.

Interesting also is the BMW 802, often described as a 18 cylinder version of the 14 cylinder BMW 801 it had little in common apart from the bore. The stroke was much longer so that the BMW 802 would have been described as a R-3600 (56.25L engine). It had the inlet and exhaust valves arranged front to back to ensure good cooling flow to the rear cylinders but most interestingly it was to use variable exhaust valve timing to allow the engine to be manifold tuned at certain RPMs for exhaust gas scavenging.

I don't think any engine with more than 18 cylinders made commercial sense. A flight in a super-constellation complicated turbo compounded R-3350 from Australia to the UK used to cost over 1/2 of a years salary and I'm sure that a transatlantic flight was prohibitive as well. Large numbers of cylinders add a lot of cost.

 

[vanir]

But historically every "back to back bolted" engine type like the DB-610 etc. failed miserably in the servicability stakes which is unsurprising to me. And the terrific issues with swept capacities Daimler and BMW were using as it was were related directly and proportionately to their pinnacle research on combustion chamber design, to run on synthetic fuels with the comparative quality they had. All terrific and scientific accomplishments of the highest order of the day.
I think so much was at stake, so much backing available, such a quintissential marketplace, that aero engineering surrounding the war years functioned at its very apex possible. I think if you could take what you know now back to there in a time machine, not a thing would be different. Those materials, those machining tools, and almost every new design completely original and many groundbreaking. People were independently inventing the same things from completely different paths in different countries, that's how racey the arms race was, blistering pace.