If recips were made nowadays

[PWR4360-59B]

I'm not talking about the small opposed ones for general aviation. Just the big stuff.
I'm thinking power densities of the nowadays motorcylce engines and the what ifs if aircraft engines of old even came close.

 

[michaelmaltby]

The simplicity of turbines tops the impressive advances in 4-stoke motorcycle engines .... these engines aren't upwardly scaleable beyond a certain point ... and to be 'large' they would be complicated. Big recips in big aircraft were not all that the movies would make them out to be ....... complicated and very labour intensive.

MM

 

[gumbyk]

Motorcycle engines get their power densities from high-revving. They get the high revs because they are small.
As things like pistons get bigger, they have more inertia, so everything needs to be stronger, more so than simply scaling the small engine up.

Flame front speed also places limitations on the effective size of cylinders/RPM of the engine.

 

[Shortround6]

What are the power densities in terms of weight?

A 100lb engine that makes 100hp even if no bigger than 750cc is not the basis for scaling up to airplane size.

A motorcycle engine with a 50mm stroke turning 9000 rpm has pistons moving at the same speed (feet per minute) as a Merlin or Allison doing 3000rpm.

 

[davebender]

How about a Wankel engine? Could that be scaled up to produce 3,000+ reliable hp?

 

[Shortround6]

Can you get the seals to last 3,000-5,000 hours??

TO compete in the modern world any 2,000+ HP engine is going to have to go at least 3,000hrs between overhauls and preferably closer to 5,000 hours.

 

[tyrodtom]

The largest Wankel built was a 2 rotor of 1100hp, by Ingersol-Rand, in the late 70's.
Wankels have the same problems of any other IC engine, their size is limited by combustion speed in the combustion chamber, you can only go so big, bigger rotors mean slower rpm.
Wankels have never been noted for their fuel efficentcy.

 

[davebender]

Neither have jet aircraft. Especially those in service prior to the 1970s.

 

[michaelmaltby]

Turbines will burn a wide large of non-octane fuels ... not so Wankels

 

[davebender]

AAI RQ-7 Shadow - Wikipedia, the free encyclopedia

Scale the engine up to 3,800 hp.

 

[tyrodtom]

AAI RQ-7 Shadow - Wikipedia, the free encyclopedia
View attachment 210890

Scale the engine up to 3,800 hp.

IT's already been pointed out, but you apparently missed, internal combustion engine can't be just simply scaled up. The combustion process speed, or flame front in the combustion chamber can only go so fast. The same applies to wankels.
The 1100 hp wankel I mentioned was a 2 rotor, about 3 feet in diameter, developed it's hp at 1200 rpm. If you went bigger, the rpm would have to be less. The only way to more practical hp with a wankel is more rotors, but then you run into limits with the center shaft.

I had a Mazda RX3 in the early 80s, a lot of power in a small car, but in no way economical on gas.

How did this thread go from reciprocating engines to rotaries?

 

[tyrodtom]

Rotec still makes small radials for lightplanes.
Compare their 220 ci, 150 hp 9 cylinder radial to a late 30's R-5 Kinner 540 ci, 160 hp 5 cylinder radial. 275 versus 330 lbs.
The Rotec is a lighter, and smoother running engine no doubt, but not exactly a big leap in performance for a engine designed 70 years later.

So maybe those state of the art big radials of the 40's aren't so bad after all, once they had the bugs worked out..

 

[michaelmaltby]

"... I had a Mazda RX3 in the early 80s, a lot of power in a small car, but in no way economical on gas."

And I an RX2 in the mid-70's. Seals went twice in 3 years.

TURBINES ARE "OPEN" COMBUSTION ... that's why they are scaleable like no-other.

 

[Shortround6]

For a "NEW" engine to make it in the over 500hp category (and if we are talking horsepower we are talking propellers, jets are different) it has to be aimed at the commercial market with eventual sales of hundreds if not thousands of engines.

Commercial operators are interested in profitability, second only to not crashing.

This means things like life cycle costs, first cost of engine, frequency of overhauls, cost of overhauls, length out of service for overhaul, fuel costs, routine maintenance costs and so on.

Then there are things like payload. Look at the DC-3 powered by turbines. The lighter weight of the turbines allowed for a 40 in fuselage stretch and and an increase of about a ton of useful load. Carrying 3/4 to 1 ton more per cargo flight ( or 4 more passengers per flight) for the same gross weight ( raising gross weight requires approval from the FAA and a whole batch of new tests/documentation.) Many flights are not to max range and the turbine powered aircraft don't need full tanks to make the trip. I believe there used to be ( and may still be) a weight break in the Part 23 (if I have that right) regulations that aircraft under 12,500lbs only need one pilot so many 12-20 passenger commuter planes with turbines need only the pilot. Using Piston engines and keeping under 12,500 pounds might have meant 2-3 less seats. 2-3 less fares per flight ( how many flights per day?) is a lot less total revenue.

Even back in the 60s things did not look good for big Piston engines. An account of the difference of the Convair 580 "This aircraft proved extremely beneficial for North Central as actual cost per aircraft mile dropped by 31 cents compared to the 340/440. The 580 was so efficient for North Central's system that 24 of the turboprops remained in service when NCA purchased Southern Airways in 1979 to form Republic Airlines. When Northwest bought Republic in 1986, 13 of the 580s were still in service and used through the late '80s." 35 planes had been converted from 1966 to 1969.

Jet fuel is available at every major and most minor airports. High octane/performace number fuel is no more and even large quantities of 100LL (several thousand gallons per fill-up) may be hard to come by. And what is the price per gallon compared to the jet fuel? Commercial operator cares about how much it costs per hour, not the grams of fuel per hour. 15% more fuel per hour is OK if it costs 15% percent less per gallon.

 

[gumbyk]

Commercial operators are interested in profitability, second only to not crashing.

The two are kind of interlinked - no-one is going to fly with someone who has unreliable aircraft, but, yes I agree with your sentiment there.

This means things like life cycle costs, first cost of engine, frequency of overhauls, cost of overhauls, length out of service for overhaul, fuel costs, routine maintenance costs and so on.

In a number of ways, the maintenance costs are less for piston engines. When I worked for a helicopter company, we used to budget around $135 per hour for lifed items and overhaul on the 250-C20 (420 shp)(that was cheap, because we did all the work in-house) in 2008. The turbine had a heavy inspection and lifed components at 1750 hours. The compressor was 3500 hour TBO
Now, with inflation, I see that an IO-720 (400 shp) base exchange price is around $66,000, with a 1800 hour TBO. This equates to just under $37 per hour.

Although the TBO is greater for the turbine engine, the lifed components effectively nullify this.

I completely agree with what you say regarding the weight advantages. This is why you don't see many piston engines in helicopters. The weight is worth the cost penalty.

What I see as the end for big piston engines is that turbines were more reliable (we now see failures measured in the per hundred-thousand flight hours, I don't think we would have ever seen that reliability with pistons), and that they simply weighed less.

 

[fastmongrel]

A modern piston engine is going to have to be as quiet and smooth as a Turbo. Not an easy job with all those pistons bouncing up and down and going suck squeeze bang blow. You could probably get a piston as quiet but thats going to kill a lot of horsepower and you will never get it as smooth.

 

[michaelmaltby]

On a related note .... while I detest the noise and exhaust from my 2-cycle chain saw and Ski Doo engines ... I appreciate the fantastic power-to-wight ratios that 2 cycles offer. A new breed of 2 cycles that uses ceramic composite materials instead of aluminum therefore permitting much higher temperatures and therefore cleaner combustion .... a new breed of such engines could be a simple power plant for unmanned drone aircraft.

 

[swampyankee]

Wander over to oldengine.org enginehistory.org (Aero Engine Historical Society) for some anecdotal history of big recip operations. In general, the big radials were much less reliable than turbines, and much more maintenance intensive.

 

[Shortround6]

It is said that those engines went through 3 stages, in the beginning power was low and the engines were troublesome, after a few years they gained power and became reliable. After a few more years they gained more power and went back to being troublesome.

 

[Trilisser]

Can you get the seals to last 3,000-5,000 hours??

TO compete in the modern world any 2,000+ HP engine is going to have to go at least 3,000hrs between overhauls and preferably closer to 5,000 hours.

AFAIK the Bristol Hercules and Centaurus had a TBO of at least 3000 hrs. Finnish AF Twin Wasps in the DC-3 had a TBO of 1800 hrs, though 3000 hrs was possible, if the "upper end" (cylinders and valve gear) was overhauled at 1800 hrs.

Of course, for a large piston engine to be truly viable today diesel operation would be an absolute, preferably a turbocompound diesel not unlike the Napier Nomad.