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

[Spindash64]

WWII era engines needed to run very large safety margins by today's standards, since fuel can never be perfectly mixed, and it's entirely possible for a lean spot to ignite prematurely, even if the average is slightly rich

While I know that essentially any application demanding more than 1,000 rated horsepower would make more sense for a Turboprop or turboshaft anyway, I do think it's interesting to ask how much more power could be wrung out of a large V-12 or 18 cyl radial design, if built from the start to take advantage of the following:
-digital computers that can quickly sense when a cylinder is close to knock, and adjust settings accordingly
-improved metallurgy for both base engine components, and for supercharger equipment
-years more experience with the use of Water Injection to prevent knock



I'm aware of Air Racing, and how high power can be boosted in this situation. However, that is a sprint of a few minutes, with the engine being torn apart after for thorough inspection. What I'm asking about is in regards to what engines could give for military rated power: holding a specific amount of power for hours at a time, and their "sprints" still being safe enough to allow the engine to be run at rated power all the way home


Personally, I would imagine that something like a 1700 cu in displacement V-12 could maintain 70" as a "military" rating, and up to 100" with Water Injection, giving the magica 1hp/cu in that the US hyper engine program had been searching for, and jumping to 2,400 - 2,500hp on WEP. I say this because the V-1650-9, on the P-51H, was approved for 90" with Water Injection, and could put out around 2,100hp

 

[ThomasP]

Even without ADI or high supercharging MAP, I would think modern piston engines can have 1 bhp/in3 as a sustained output, at least to the level of what was considered the 'Rated' or 'Normal' power.

As you mention above - modern electronics , modern materials, modern design methodologies and tools - all allow significant increases over what could be done in WWII.

I am sure others can provide better examples of high power engines in the modern auto world, but I would like to use the 1.5L 4-cylinder in my 2002 Prius as an example. The engine is only rated at 70 BHP from a displacement of 91.5 in3 on 87 grade fuel. It uses variable valve timing with intelligence (VVT-i). I have no idea what numbers Toyota used for the designed duty cycle and engine life/MTBO, but it has been used regularly for over 20 years now and has never broken down - not once in over 601,000 miles. I have not even lost a fuel injector, coil pack, spark plug, water pump, etc. The compression is still within 2% in all cylinders. I live in Minnesota so temperatures have varied from +104° to -44°F.

I have not done the regular scheduled maintenance recommended by Toyota (other than oil changes - kind of), and the only thing unusual I have done is use Mobil 1 fully-synthetic oil, and a 50/50 mix for the water/ethylene-glycol long-life antifreeze. Otherwise I have changed the spark plugs 3 times, and replaced the injectors at ~450,000 miles. The oil use is about 1 quart per 2,000 miles, and I top up the oil in between oil changes - I have been averaging ~9,000 miles between oil changes for the last few years. I have also replaced the serpentine belt 3 times, and cleaned the throttle-body and mass air-flow sensor twice. The second time I cleaned the MAF was just a few days ago (I was getting a little bit of roughness/knock until the engine warmed-up) and this is what the bead looked like:

For those of you not familiar with the MAF sensor, the circled bead in the picture above is made of glass and is supposed to be transparent and have a shiny surface. It is a little bit hard to see in the picture but it is obviously not shiny or clear (you would be able to see the wires inside if it was clear). Instead it has a thin dark grey - almost-plastic layer - over the whole surface. A little MAF sensor cleaner and it was all clear and shiny again. I reinstalled it and the roughness/knock went away.

Anyway, aside from the fact that the hybrid system is IMO phenomenally reliable, the gas engine part of the system is also amazingly reliable. I have a mileage and hours running time gauge on my car and it says the gasoline engine has about 14,000 hrs running. The Prius is of course a hybrid, so the gas engine does not do as much work under normal conditions as it would if it was a stand-alone power source.

Now I realize that this is not a high performance aircraft engine, but I think it is a good indicator of what may be realistic for a modern aircraft engine.

Like in WWII, I imagine it would also depend on the situation - ie are we on the run-up to war, already at war, etc - for what would be acceptable for reliability at a given output.

 

[Howard Gibson]

WWII era engines needed to run very large safety margins by today's standards, since fuel can never be perfectly mixed, and it's entirely possible for a lean spot to ignite prematurely, even if the average is slightly rich

The top speed of something driven by a propeller is around 528mph. This is a limitation of propellers, not engines. I am curious about ducted fans.

Ten to fifteen years after a war without turbojets and turboprops, fighter aircraft and bombers are pushing that envelope. I think the most interesting thing about all this is defensive armament. Lacking massive speed advantages, fighter aircraft close slowly on the bombers from the rear, giving tail gunners lots of time to sight up on them.

 

[Spindash64]

I'm aware of the limitation of propellers. But that alone wouldn't explain why helicopters use Turbine engines

It's not really important either way, though, because I'm not asking about aircraft performance, just engine performance

 

[pbehn]

I'm aware of the limitation of propellers. But that alone wouldn't explain why helicopters use Turbine engines

It's not really important either way, though, because I'm not asking about aircraft performance, just engine performance

Helicopter turbine engines deliver huge power from a small and light unit.

 

[sotaro]

I think current levels of automotive power output and reliability could be extracted from larger engines. How many pistons and in what shape would be very interesting. In your scenario, would exhaust turbines be allowed? Turbocompounding? If so, the Napier Nomad is a guide to potential power output and that was 68 years ago. If not, 150-200 hp/liter would seem reasonable for expensive aircraft engines if they were unveiled now. Could that be done with 3 liter/cylinder seems a bit doubtful, but perhaps. I would think that X engines would be popular.

 

[wuzak]

The top speed of something driven by a propeller is around 528mph. This is a limitation of propellers, not engines. I am curious about ducted fans.

Ten to fifteen years after a war without turbojets and turboprops, fighter aircraft and bombers are pushing that envelope. I think the most interesting thing about all this is defensive armament. Lacking massive speed advantages, fighter aircraft close slowly on the bombers from the rear, giving tail gunners lots of time to sight up on them.

Tu 95 top speed ~ 575mph.

Current air speed record for piston powered aircraft is 531mph. This was at relatively low altitude, and is an average over a set distance in two directions.

 

[ThomasP]

The 575 mph for the Tu-95 partly due to the turbine engines' exhaust thrust.

 

[wuzak]

The 575 mph for the Tu-95 partly due to the turbine engines' exhaust thrust.

Piston engines have exhaust thrust as well.

The very high boost that Reno racers use may be more beneficial in exhaust thrust than the extra power.

 

[ThomasP]

True, but exhaust thrust from turboprops is usually at least about 4x the best you would get from just piston engine exhaust thrust, and can be as high as 10x(I think), depending on what flight regime the aircraft is designed for.

 

[icepac]

When each engine has near 15,000hp, you will surely get exhaust thrust.

Now Reno racers are still in the stone age and have argued with Mike Nixon many times over how modern engine management would hugely benefit.

 

[ThomasP]

The main area where I have always thought about high power piston engines for aircraft, built to modern standards, is in the area of small(er) countries air forces. I do not know enough about the comparative costs and difficulties of the maintenance, parts, and supporting services/units needed for modern turbine engines vs a modern high power piston to say for sure, but I wonder if it was a missed opportunity for smaller economies. I realize that a modern piston engine would not necessarily be cheap (economy of scale in production pays off for the big boy air forces) but for smaller countries would the ability to maintain a piston engine make a truly significant difference in operational availability? Plus there might be a bonus in the freedom from the larger countries politics and economies. Maybe?

 

[Shortround6]

the comparative costs and difficulties of the maintenance, parts, and supporting services/units needed for modern turbine engines vs a modern high power piston to say for sure,

A lot of that can be answered looking at the commercial aircraft market and prevalence of 500-2000hp turbo props. Granted the turbo props took over before the more modern/sophisticated reciprocating engines showed up. Trying to start up a "new" reciprocating engine using 1990s or newer technology vs existing small turbines would be difficult.
Airlines, even small ones or especially small ones, are very into profit and loss which means purchase price, maintenance costs, durability, flight availability, fuel costs, payload (lighter engine means more cargo/passengers/fuel for same weight) and the turbines have not been standing still.

A modern 500-2000hp turbine has over ten times (very roughly) the overhaul life of a late war piston engine. Has a lot less maintenance issues and so on. A modern reciprocating engine would be a number of times better than WW II engine, but is it enough better to compete with a modern turbine?

A P & W PT6A-68C as used in a Super Tucano

gives 1600hp for just under 600lbs. And that is 1600hp max continuous, not max for 5 minutes or max war emergency only.
Super Tucano has a max speed of 370mph.

That would be the market for a modern high power reciprocating engine. That and 12-20 seat passenger aircraft.

 

[z42]

Also, Jet A-1 is cheaper, safer and better available than AVGAS.

I think more likely than modern piston engines clawing back market share from turbines, is turbines expanding downward into smaller sizes previously still dominated by piston engines. See e.g. TurbAero | Turboprop Engines | Light Aircraft and UAV | General Aviation and About Us | TURBOTECH | Innovative turbines, Turboprops, Turbogenerators & Range-Extenders, both of which use recuperation to compensate for the otherwise inherently low efficiency of small turbines.

 

[tonycat77]

Russian tank engines develop around 1200hp-1500 using a crankshaft derived from t-34's (and ww1 bmws!) that originally had 500hp.
Tank engines are also highly detuned due to the much larger stress and torque needed.
Assuming a 3x factor, we would get early turbojet class power out of a modern merlin or allison, but much higher weights.

 

[Shortround6]

The other thing to take into account is the engine life.
tank engines are not rated at thousands of hours. Most turbine engines are and modern turbines are modular. You can replace one section of the engine and not replace/overhaul the whole thing. Now tank turbines need better filters (or at least larger) because they need more air.
In the 1960s and 70s turbines on AG aircraft were being rated in the 3-5,000hr area, They may have cost more than 450-600hp radial but they lasted several times longer so the cost averaged out.
Manufacturing has gotten better but aircraft engines were sort of like mass produced race car engines. And that is what you need in for a modern piston aircraft engine.
A higher standard of manufacture and/or inspection.

 

[yulzari]

The other thing to take into account is the engine life.
tank engines are not rated at thousands of hours. Most turbine engines are and modern turbines are modular. You can replace one section of the engine and not replace/overhaul the whole thing. Now tank turbines need better filters (or at least larger) because they need more air.
In the 1960s and 70s turbines on AG aircraft were being rated in the 3-5,000hr area, They may have cost more than 450-600hp radial but they lasted several times longer so the cost averaged out.
Manufacturing has gotten better but aircraft engines were sort of like mass produced race car engines. And that is what you need in for a modern piston aircraft engine.
A higher standard of manufacture and/or inspection.

Yes. The Pratt & Whitney turboshaft that Lotus used in their Lotus 56 1968-71 Formula One/Indy car was rated in a rebuild life in hundreds, if not thousands, of hours. The contemporary Ford DFV ICE was rated in single hours.

 

[Howard Gibson]

My 2009 Honda Fit has a 1.5l engine that puts out 117HP at 6600rpm. This works out to a BMEP of 1100N/m^2. A Rolls Royce Merlin II has 27l displacement. It put out 1030HP at 3000rpm for a BMEP of 1140N/m^2. Compared to WWII engines, my Honda pollutes less, and I hope it is more efficient. 150 octane gas is no longer available.

There are absolute physical limits to what you can do with a piston engine turning a propeller.

 

[yulzari]

My 2009 Honda Fit has a 1.5l engine that puts out 117HP at 6600rpm. This works out to a BMEP of 1100N/m^2. A Rolls Royce Merlin II has 27l displacement. It put out 1030HP at 3000rpm for a BMEP of 1140N/m^2. Compared to WWII engines, my Honda pollutes less, and I hope it is more efficient. 150 octane gas is no longer available.

There are absolute physical limits to what you can do with a piston engine turning a propeller.

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.

 

[tonycat77]

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.

Don't forget variable valve timing!