Smoothest engine?

I’ve managed to find a reference for radial engines; R-7 and R-9 radials, and therefore R-14 and R-18 radials, come out very well, better than the V-12 configurations in fact (ignoring the problems of articulation).

Interestingly the H-24 configuration, such as a Napier Sabre, comes out better than any V-12 too.

The W-12 configuration of the Napier Lion isn’t so good (again ignoring the problems of articulation).

However these are only theoretical measures of ‘smoothness’ for the main moving components and ignore any combustion effects.

And, for wing-mounted engines, the design & construction of the wing itself.

I agree with Anon & Mark XII about rubber mountings. Shack engines are smoother than those in the Lancaster for that reason.

CD is absolutely right on the 3/6/12 cylinder thing. A straight 6 is theoretically perfectly balanced, although thats not the full story, it is the reason why they are so smooth.

One thing I don`t agree with is why anyone would associate what an engine sounds like to how smooth it is mechanically. The two are not linked at all unless an engine is misfiring under high BMEP. The Griffon and Subaru Impreza are two examples of rough-sounding engines which aren`t!

Even the Merlin has periods they don`t want the engine to be run in for extended periods, one is 1200, the warm-up speed! It obviously was not a problem in service because only the very early manuals mention it.

I also agree with Jim Harley, all the piston-engined aircraft I have been in suffer some form of vibration, some of which is the reaction of the propeller to its surroundings.

Pete

…I think there is a bit more to this that more cylinders make for smoother engines…

…Surely it has mainly to do with the firing order and the number of cylinders. Layout must be a factor too…

Absolutely, if we’re talking about fundamental vibration (caused by the greatest moving masses; crankshaft, connecting-rods and pistons) certain engine configurations are better than others; that’s a quantifiable fact.

The problem is that (as has been suggested) we’re really talking about the result of many factors and once you superimpose these other factors (especially the variable combustion factors) the ‘smoothness’ due to the basic engine configuration becomes obscure.

Smoothness

Surely it has mainly to do with the firing order and the number of cylinders.

Performance engines were tuned and timed to run best at high power, which is why most of them can sound pretty rough when on tick-over. It’s at high power where it is most important that an engine is at its smoothest.

Layout must be a factor too. Even opposed-cylinder engines, where you’d think the firing impulses would be cancelled out, the firing stroke is not cancelled out by opposite cylinders firing together as this was, mechanically, impossible to arrange.

Basically, any (piston) engine has a big vibration problem though some layouts did go a long way to solving this, none were able to do so particularly well.

That said, I have heard that the Napier Lion (three banks of four cylinders arranged in broad arrow or W configuration) ran like a sewing machine.

Radials came close and the more banks of cylinders they had the smoother they became. The opposing cylinders did almost cancel each other’s firing impulses out except for the fact that they were on displaced planes and slightly offset. The ingenious Dynafocal mounts that were used to support these engines on their mounts allowed the radial engine to vibrate radially without transferring excessive amounts of resonance to the airframe.

Most piston engines (even modern ones) have an RPM range which has to be avoided and this is usually a function of the propeller and firing order, as well as inherent limitations in the control of the particular vibration mode.

In addition to metallurgical considerations the problems outlined above were another major reason why big piston engines were a heap of trouble in commercial aircraft – and, equally, why the gas turbine was a success.

Anon.

Never flown one or been near one with the engine running, but surely a rotary engine would be extremely smooth on the rest of the aircraft ie lack of vibrations especially once airborne, as the whole thing is turning around with the propellor

Certainly another good point, built in frequencies, harmonics, and resonances.
But I think he meant actual engine set up, if the engine was that smooth on tick over then the cam lift etc wasn’t at its best in the higher driving range rpms.
I suppose car wise it depends on use, ie a Rolls is expected to very smooth most of the time, where as a drag car isn’t.
Could this possibly apply to aircraft, smooth for airliners and rougher at some rpms for high performance fighters?

Remember when I was younger and didn’t mind getting dirty, someone from the garage trade telling me that if an engine is very smooth at tick over than it isn’t performing at its best at higher RPMs, ie there is a compromise.

Critical frequencies.

I remember one of the engine guys at Tickfords telling me there was one engine, a Ford I think, that if held on an engine test bed at a very precise rpm, within he driving range, it would destroy itself.

Mark

I consider the Merlin to be harsh compared to the Allison V-1710…:D

Paradoxically the most common automotive engine, the straight four-cylinder, has inherent vibration problems but you can improve this by joining two to make a V-8 engine. 😉

Really, none of the V8s that I have owned (which is a lot) have been as smooth as a straight four, and certainly nowhere near as smooth as a straight six. But having said that Jag V12 was the smoothest.
I think there is a bit more to this that more cylinders make for smoother engines.
Cam lift, duration, compression, balance of moving parts etc are all very important to smooth running and the overall sound aswel.
Must admit that the Sea Fury’s Centaurus at full chat (understandably rough on tick over) sounds almost as smooth as a jet, in a Nene kind of way, but the same engine type sounds rough as hell on the Beverley, maybe due to compression difference?

Rememeber when I was younger and didn’t mind getting dirty, someone from the garage trade telling me that if an engine is very smooth at tick over than it isn’t performing at its best at higher RPMs, ie there is a compromise.

I used to be able to 50p balanced upright on my 1964 Triumph Speed Twin on tick over. I lost the whole exhust pipe in the way home once due to vibration.

Dave

Im think build quality probably has more to do with it than the layout of the engine. I seem to remember a program showing a Rolls car engine with a 50p piece balanced up right on a running engine.

As with automobiles, the mounting type, solid or rubber, will surely be a factor here.

Mark

Bit out of my league this one but what about the Bristol Mercury.. especially in the Gladiator? Very sweet and smooth sounding, but whats it like from the cockpit?

I’ve been lucky enough to fly a variety of airplanes. The one thing I have noticed, no matter what piston engine I am sitting behind they all shake to varying degrees, especially at idle. The Merlin is the smoothest if properly tuned. The Pratt 1340 in the T-6 shakes the most no matter how well tuned 😛 and in all flight regimes . The 1830s in the B-24 are much smoother than the Wright 1820s in the B-17…but niether is bad or distracting in any way. The shake produced by the 2600s in the B-25 is offset by the noise. In the air, there is no comparison to the Merlin, its like riding behind a sewing machine…again, if properly tuned.

jim harley
http://vimeo.com/15936889

…what was the smoothest piston engine? I know that theoretically the more pistons, the more firing pulses per crankshaft revolution. However, the larger the per cylinder displacement, the more mass moving, the rougher the engine.

Unfortunately it isn’t as simple as that; the number (once you get to three) or size of the pistons have little bearing on the smoothness of an engine. And it depends what you mean by smoothness; presumably lack of vibration rather than just the exhaust sound that it makes.

I cannot remember exactly but I think the smoothest engine is actually a three-cylinder but it isn’t very practical to have a 27 litre Merlin engine with only three cylinders. A straight six-cylinder is typically two three-cylinder units end-to-end and a V-12 is two straight-six units, inclined at 60° to each other, sharing a common crankshaft so all these are very smooth too.

Paradoxically the most common automotive engine, the straight four-cylinder, has inherent vibration problems but you can improve this by joining two to make a V-8 engine. 😉

Not sure about radials, probably pretty good; I’ll have to look it up.

As far as radials go, I was very surprised at how smooth the BMW 801 sounds when compared to the usual bellowing from most radials.

http://www.youtube.com/watch?v=F906prEvkgE

Someone who witnessed the first flight remarked at how unusual the engine sounded when Hinton did a flyby. I’m looking forward to video of the flight if any is made public.