What if “military class” reciprocating aircraft engines and aircraft were still being designed and built?

[Howard Gibson]

But then the eventual top developments of the Merlin were twice the power and the comparably sized Roll Royce Crecy tests pointed to eventually getting twice that again. Not to mention we can look to more power recovery from thermal excess and electronic engine management in addition. From 27 litres it is not unreasonable to think of having 6,000bhp to hand by now. As to what one might do with it is another matter. The Rolls Royce Crecy and Napier Nomad are, perhaps, the best OTL pointers (in different directions) to what might have been.

Merlins put out 2000HP on 150 octane fuel, and 2600HP on 150 octane and water injection. If I could increase my Honda's compression ratio, I could take advantage of 94 octane gas. There are absolute physical limits to what you can do with a piston engine, especially if you can't use tetraethyl lead.

 

[ThomasP]

I have mentioned my 2002 Prius in other threads. I do not know what Compression Ratio it usually runs at with typical E10 (87 PN) fuel, but I have run it on E85 (100-105 PN) a couple of times and It ran just fine. MPG was the same as far as I could tell (by the estimator and by the pump), power was the same or better as far as I could tell, and it ran quieter (with E10 87 PN there is almost always a tiny bit of knock) - though there was some rpm 'hunting' on start-up.

The 1.5L 4-cylinder in my Prius has variable valve timing and can run upto 13:1 CR (in theory).

 

[Shortround6]

A few side notes.

1. Mercedes Benz fielded a Grand Prix engine in 1939 that ran on some rather exotic fuel.
It was a 3 liter engine (2962cc) and made 480hp at 7500rpm. The BMEP was 305psi and intake pressure was 2.41 Atm. also used two stage supercharger.

It is not like engineers of the time didn't know how do things. There was a racing Austin 7 750cc engine1936 that made 116hp at 7600rpm with double overhead cams and blower that gave 2.5Atm using a totally different exotic blend of fuel.

Granted a modern engine would be lighter and made of better materials.

However there are only two ways to get more power out of an engine.
1. Get more air and fuel into the engine for each revolution. (many, many ways to do this)
2. Increase the rpm.

everything boils down to these two things.

The practical problems show up with "power for how long" and "power to weight" of the engine.

Both the British and Germans knew how to get 156-162hp per liter in the late 30s. What they could not do was keep the engine running for very long (more than a few hours) or at light weight. The little Austin engine weighed 260lbs and it's block was made of RR50 Alloy. The Mercedes 3 liter V-12 went 603 lbs.

The V-12 aircraft engines were usually rated 100 hours or more during a test that kept them at or near 90% power for over 80% of the time, and around 10% of the time at their max rated power. One reason that RR was confident in the Merlin and did not jump to the Griffon very quickly was that in the testing for the "Speed Spitfire" they had not only hit around 1800hp with the test engine but they had run it at 1600hp or above for over 10 hours (in 1938?) which showed them there was room for increased power with little modification needed. By the end of the war large aircraft engines had a service life of 400-500 hours or more even allowing for the 1945 peak power levels (7 1/2 hours)

Modern over the road cars spend a tiny fraction of their time, even under test, anywhere near max power.
Materials have gotten a lot better,

The "2009 Honda Fit has a 1.5l engine that puts out 117HP at 6600rpm" is about twice the size of that old Austin 7 race engine. The Austin was also using a roots supercharger which is not a very efficient supercharger. The Mercedes was using two roots superchargers.

We could do better with modern technology but lets not forget that the engine designers of the time were trying to build engines that were high powered and reliable.

Also do not ever forget that at about 21-22,000ft the Honda Fit engine would be making about 58hp due the the thin air.
So now you need to add a supercharger and provide cooling in the thin air.

 

[Howard Gibson]

A few side notes.
...

Also do not ever forget that at about 21-22,000ft the Honda Fit engine would be making about 58hp due the the thin air.
So now you need to add a supercharger and provide cooling in the thin air.

Well, there goes that plan. Thanks for ruining my weekend.

 

[Howard Gibson]

I have mentioned my 2002 Prius in other threads. I do not know what Compression Ratio it usually runs at with typical E10 (87 PN) fuel, but I have run it on E85 (100-105 PN) a couple of times and It ran just fine. MPG was the same as far as I could tell (by the estimator and by the pump), power was the same or better as far as I could tell, and it ran quieter (with E10 87 PN there is almost always a tiny bit of knock) - though there was some rpm 'hunting' on start-up.

The 1.5L 4-cylinder in my Prius has variable valve timing and can run upto 13:1 CR (in theory).

The energy value of gasoline is not affected by the octane number. If your engine runs fine on 87 octane, it will run just as fine on 100 octane, i.e., no better, no worse. High octane numbers allow you to build engines with higher compression ratios, and it lets you run at higher supercharger boosts. For the latter, it helps if you have a supercharger.

 

[ThomasP]

Sorry, I was not clear. E85 has a lower energy content than E10, but the combination of Toyota's VVTi technology and knock sensors allows the engine to effectively use higher compression with the 100-105 PN fuel - which compensates for the lower BTU content of the E85. In effect the Toyota engine runs as if it has variable compression, although I do not have specific data to tell how much it can do so.

 

[Snautzer01]

Volkswagen w16 ? Already 1000+ hp.

 

[pbehn]

Any high power engine would probably drive some sort of fan, which was what Whittle originally started his research into. In the early days of this type of fluid dynamics he calculated that the "engine" wasnt needed, and sustainable power (surplus thrust) could be produced just by using a turbine in the exhaust to drive a compressor, many others disagreed probably because they hadnt thought about it.

 

[Alan Stevens]

Tu 95 top speed ~ 575mph.

Swept wings to 35 degrees, huge turboprop engines at 15,000 shp each, contra-rotating propellors to reduce their size to where the tips won't run supersonic.

Rather unusual aircraft

 

[MIflyer]

In the 1970's, with the supply of R-1340 engines that had not been rebuilt multiple times drying up, a company in California was taking the cylinders from R-2800 engines and modifying them to fit on R-1340 cases. The R-2800 was called the Double Wasp because in many respects it was just that, two R-1340's stock back to back. That is one reason the R-2800 was such a tremendous success from the very start with few teething problems; it was based on over a decade of R-1340 experience. But unlike the R-1340, the R-2800 development continued, adding superior materials and design features as compared to the earlier engines. In other words, we could go a lot further with other aircraft piston engine designs, even if we only brought them up to the 1945 level of the technology.

 

[marathag]

besides engine life, turbines are lighter in weight for same shaft HP and have far less vibration making that power

 

[Shortround6]

In the 1970's, with the supply of R-1340 engines that had not been rebuilt multiple times drying up, a company in California was taking the cylinders from R-2800 engines and modifying them to fit on R-1340 cases. The R-2800 was called the Double Wasp because in many respects it was just that, two R-1340's stock back to back. That is one reason the R-2800 was such a tremendous success from the very start with few teething problems; it was based on over a decade of R-1340 experience. But unlike the R-1340, the R-2800 development continued, adding superior materials and design features as compared to the earlier engines. In other words, we could go a lot further with other aircraft piston engine designs, even if we only brought them up to the 1945 level of the technology.

Well, P & W and Wright and Bristol all put a lot of effort into large radial engines in the post war era. At least for the commercial market and for some military aircraft. It took a long time to get turbines to give long range (fuel economy) and at times they were pushing a little harder than the technology allowed.
I believe it was Hives who said something about designing the simplicity out of the jet engine.
P&W with the 28 cylinder R-4360 and Wright with the turbo compound were trying their best

For modern piston engines there are a few things that are stumbling blocks. There are some things that are easy to do.
Better materials can greatly reduce the weight for one.
But if you use gasoline you have to deal with the speed of the flame in the cylinder, Using more advance helps or going to triple ignition but large cylinders have certain restrictions just due to the flame burn. Doesn't matter it you can build a modern R-2800 that will turn at 5600rpm without coming apart (and please note that the stress on the rotating parts will be 4 time higher, not double) unless you can get the fuel to burn in that large cylinder in 1/2 the time. If you can't do that then you don't even have to work on the valves and inlets/exhaust ports to increase the flow of the gases.
Then you have to figure out how to cool the thing. Better machining and metallurgy can increase the fin area, but by how much, or change to liquid cooling?

There are certain economies of scale. Unfortunately cooling is not one of them. Displacement goes up with the square of the bore. Cooling surface doesn't.

Trying to translate over power levels from small cylinder, high rpm engines to large cylinder aircraft engines doesn't work so well. Also consider that a 1968 Chevy Z-28 engine at 6,000rpm had the same piston speed as a Merlin or Allison did at 3,000rpm. Many modern engines went back to the under-square bore to stroke ratio for emissions reasons.
Under square engines have a little easier time with high rpm (less piston mass reversing direction).

 

[MIflyer]

For modern piston engines there are a few things that are stumbling blocks.

One of the biggest and potentially the easiest are the ignition systems. Magnetos are fine for lawnmowers, but we should not have to suffer with the things on airplanes, given that they have managed to combine all of the faults of poor reliability, high cost, excessive weight, awkward installations, absurdly short overhaul times, and lackluster performance in one handy unit. I doubt very much that there are any pilots who have NOT had to deal with a mag problem. We need two of the damn things because they are so unreliable, but since we have two not enough people get killed by them to force a better system design. The lawyers have made sure we have to suffer through with that 1930's hardware; non aircraft applications, such as industrial engines, got solid state replacements for mags 50 years ago. Finally, recently, a few companies have come put with modern replacements that can provid better reliability as well as radical new ideas such as advancing the spark to improve power, but even they require one of the old Depression Era units as a backup.

 

[Shortround6]

One of the biggest and potentially the easiest are the ignition systems. Magnetos are fine for lawnmowers, but we should not have to suffer with the things on airplanes, given that they have managed to combine all of the faults of poor reliability, high cost, excessive weight, awkward installations, absurdly short overhaul times, and lackluster performance in one handy unit. I doubt very much that there are any pilots who have NOT had to deal with a mag problem. We need two of the damn things because they are so unreliable, but since we have two not enough people get killed by them to force a better system design. The lawyers have made sure we have to suffer through with that 1930's hardware; non aircraft applications, such as industrial engines, got solid state replacements for mags 50 years ago. Finally, recently, a few companies have come put with modern replacements that can provid better reliability as well as radical new ideas such as advancing the spark to improve power, but even they require one of the old Depression Era units as a backup.

What you say is true.
However the Mag has the "advantage" that it is a separate system, unrelated to the Generator/alternator and/or battery. Granted generator/alternators and batteries have also gotten a lot better.
They were fooling around with variable ignition timing in WW II, The Germans more than the Allies. But P&W was starting to put variable ignition timing on R-1830s by the end of the war. Perhaps some late R-2800s or R-4360s? I don't know about some others?
I can't remember if it was the Soviets or the Germans who limited the size of their V-12 cylinders because they thought they would need to go to triple ignition to use larger diameter cylinders.
A lot of the problems with ignition systems in the race cars in the 40-50-60-70s that forced some people to go to magnetos were solved by modern solid state ignition.
But it is not the ignition systems that really limit power. Unless they are not working right. People had been able to design high performance ignition systems that allowed for engine performance even at the expense of reliability. One really bad example that I know of was a few of our really old fire trucks that had dual ignition for "reliability" but in order to get that "reliability" they used two dual point 12 cylinder distributers and four coils. Plenty of stuff to go wrong but the engine would still run on one distributor

Some of the tricks that modern ignition can do is monitor the engine and adjust the timing for ideal advance for best power with different fuel or atmospheric conditions and/or prevent knock pre-ignition. It may not make anymore power but it allows you to run closer to the edge with less danger of catastrophic failure.

 

[yulzari]

One thing to bear in mind is that, at the end of the day, you are merely turning a shaft. Today with more speed and torque but still turning a shaft. The same as the axle of an ox cart. Unless you are using the infernal combustion engine just as a gas producer, a hideously complex jet combustion chamber. However, commercial air traffic is currently stuck at transonic speed at best so there might be room for a compromise, using it to drive a by pass fan. Propellor technology has moved far on today with materials and design methods and is probably a more productive area for modernisation than the power unit turning their shaft. Even in the late 1940s we saw double Bristol Centaurus engines jointly driving the airscrew unit of the Bristol Brabazon and Saro Princess, so nigh on eighty years ago they could make use of over 6,000bhp in normal service without supersonic speed propellor speeds.

On a different tack, I spoke years ago with an ex Rolls Royce engineer who was given the exercise of seeing what power could be found from using Merlin exhaust gases in an afterburner. His conclusion was that an unladen (ie less war load and armour with minimum fuel) early Spitfire with a late Merlin engine could manage to take off and fly without the propellor. I am not qualified to comment technically on the validity of the tale but it does show why engineers rejoiced at the simplicity of early turbojet engines and their controls. Why do it the easy way when you can make it very, very hard for yourself?

 

[MIflyer]

On a different tack, I spoke years ago with an ex Rolls Royce engineer who was given the exercise of seeing what power could be found from using Merlin exhaust gases in an afterburner.

Here is a shocking chart.