Opposed pistons

There is a debate as to whether 8 or 4 cylinders are best, but it’s not what you’ve got, it’s how you use it. Apparently. If you look back 100 years you will see all sorts of ideas for novel engines, it sometimes seems a little odd that we have become so used to only using the 4 stroke piston engine.
A four-stroke engine only actually produces power on one stroke out of four, which is only a quarter of the time. Two-stroke engines produce power every down stroke, that’s twice as often, so for the same engine size it should produce about twice the power. Which is nice.
In reality there are some problems with 2 strokes; getting fresh mixture in and the exhaust out at the same time, near bottom dead centre, whist still keeping the ports closed enough to allow a reasonable amount of the stroke to produce power. This is why they need some form of forced induction to push new mixture in and the exhaust out – on bikes this tends to be by feeding the mixture through the crank case and using the underside of the piston as a compressor on the down stroke. But then you need to mix oil with the fuel to keep the crank happy, which reduces power and produces rubbish emissions. The bigger engines, used in military vehicles and such, use a supercharger to do the work instead thus solving the oil problem at the expense of weight.

Genius of engineering.

But how about if we pack more pistons into the same space; the Rolls Royce K60 Range multi-fuel engines put two pistons in the same cylinder, one at the top one at the bottom and a crank shaft at each end. It’s an idea that has been around for some time, one of the greatest exponents of it was the German Junkers company in the Second World War, the experimental Jumo223 engine having 4 banks of 6 cylinders and thus 48 pistons, the larger Jumo224 was 73 litres and produced 4500bhp at 3000rpm but weighed the same as a small lorry.
The opposed piston design only works with the two-stroke system of piston covered inlet and exhaust ports in the cylinder walls, in fact in the Rolls Royce K60 opposed piston engine, one piston uncovered the exhaust ports and the other one did the intake. The two cranks were geared together with an offset of 20 degrees in order to get the exhaust and intake timing right. The top crank thus produced only a third of the total power and drove the supercharger, alternator, water pump and other gubbins so there was only a small proportion of power left going to the bottom crank, so the connecting gears could be quite small and light. Very clever.

A slice of lovelyness

Although there have been attempts at opposed piston spark ignition engines in the past, the problem of getting the spark plug into the cylinder wall without compromising the design of the combustion chamber (formed by the two piston crowns) and cooling system proved to much of a buggerance. So most Opposed piston engines are compression ignition, that’s diesel to you.
The remarkable thing about the K60 engine is the types of fuel it uses, basically almost anything that catches fire, diesel, kerosene, JP4 jet fuel and petrol with and octane rating of less than 80. Remember the octane rating is the tendency of a fuel to go bang when you squash it (ie knock), so a low octane fuel will burn in a diesel engine but a high octane one won’t, or at least not very well. Of course this does mean that the power per litre has to be compromised in fine British tradition.
The K60 was a relatively small engine, at 6.57 litres and produced 240bhp at a crank speed of 2400rpm, but because of the way the two cranks were geared together the output flywheel would be doing 3750rpm so that it could directly replace the older B80 range of straight 8 cylinder petrol engines in things like the 432 armoured personnel carrier. It was also used in the Swedish S tank along with a 300bhp turbine engine, as tanks go its well worth looking into, really funky.
Amusingly the K60 was marketed as being light, as it only weighed 757kg! but compared to the Leyland L60 engine, which was a flat version of the same thing and bigger at 19 litres, which was well over a ton the K60 was a feather weight.
The secret is in its long studs. Although that sounds like the title of a spam email, it is the same principal that was used decades later on the Rover K series engine. The crank main bearing caps are held on with studs that goes right through the engine and out the top, in the Rover case they held the cylinder head on, in the K60 case they hold the top crank bearing caps on too and tie the whole engine together very well. Making a very reliable engine.
In order to get fresh mixture into a cylinder that was still venting the exhaust gasses, a Roots supercharger was fitted. Later versions also had a turbo, you still need the supercharger because otherwise, because the turbo makes no boost at idle, you will never get the engine started. But once on song, the turbo added more boost and improved power and efficiency in the traditional turbo diesel way. Supercharged and turbocharged, now that does sound like a good thing.
Now, me being me, I have wondered if I could make up a home brewed modern equivalent, there is after all a lot going for opposed piston engines, for a start you lose the weight of a cylinder head. Maybe using parts from two BMW 6 cylinder diesels welded together. The standard 4 stroke can be tuned to about 300bhp, so 2-stroking it could make maybe 500, then having the two nailed together we might be in the 900bhp range. The engine might weigh less than 400kg too which, compared to my old Jaguar V12 at 350kg, isn’t too bad. Hmm, a 12 piston opposed diesel Jaguar with 900bhp and really good fuel economy – tempting. It is of course a really stupid idea and should only inhabit the area near a bar. Mind you, I bet there is someone out there who could do it..


About Ralph Hosier

I love exploring everything the world has to offer, the fabulous beauty and intricacies of nature, the stunning majesty and grandeur of the universe, and the fascinating range of chocolates available from the local sweety shop. I have led a charmed life, sure there has been extremes, but the highs far outweigh the lows. I get paid for arsing about in very fast cars, I get to write about them and amazingly get paid for this too. My days are usually filled by making prototype and concept cars for car companies, a dream job. I have lived many of my dreams, worked all over the world, raced cars built by my own hand (and hardly ever crashed really badly), seen things and done stuff. But nothing compares to the love of Diana and my son Peter, beyond my greatest hopes. I am a chartered engineer, a member of the Institute of the Motor Industry (IMI), and of the Institute of Engineering and Technology (IET) and I am a member of the Guild of Motoring Writers. A pleasing fact is that there are now more letters after my name than there are in it ;) R.Hosier B.Eng(Hons) C.Eng MIET MIMI MGoMW
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8 Responses to Opposed pistons

  1. Pique Talk says:

    How have you written an article on opposed piston engines and not mentioned the awesome Napier Deltic….!?
    Nevertheless, a very enjoyable read, thanks.

    • ralphhosier says:

      Ah! Of course this would be incomplete without mentioning the magnificent Napier Deltic which took this idea, which is not a new one, a step further by having three cylinders in a triangle and a crank at each corner. These powered all sorts of big things like train locomotives and motor torpedo boats. The noise from these is bowel quakingly loud, in an entirely inspiring way.
      Then there is the Comer flat six, which was similar to the K Range but with the cylinders lying horizontally under the floor, remarkably though it used only one crank nailed on above the middle of the cylinders, which was connected to the pistons via levers, splendidly mad. If you bump started them in reverse, the engine could run backwards, badly, with exhaust billowing out the air filter!

  2. Andrew Michael says:

    “Later versions also had a turbo”
    Do you have any actual evidence of this? I’ve never seen a turbocharged one and I own three K60s!

    • Ralph Hosier says:

      Yes, they were a bit rare. I used to work at Rolls Royce in Crewe, part of which was Vickers Specialist Engines. One of the engineers that developed the turbo version indulged my interest and explained how they had developed the engine. I think it was used in some versions of the Swedish S tank.

      • Andrew Michael says:

        That’s VERY interesting – do you happen to have any photots or other information? I know that at one stage MOD were looking at re-engining the FV432s with a K60TI before they eventually decided to use a plain old Cummins instead.

      • Ralph Hosier says:

        The only document I have these days is a Rolls Royce brochure from ’64 which mentions it as a future development at 300bhp. I don’t know if it was ever used in production but it was certainly developed to a saleable condition in the late sixties.

  3. luis lucas says:

    I’m guessing the turbocharged version was meant to compensate for the loss of atmospheric pressure at high altitude.

    Still, another topic of interest would be the the so called the turbo compound engine in which the turbocharger not only provides extra air, it also adds power directly to the crank.

    And the way things are going, I wouldn’t be too surprised to see regular alternators and electric starters replaced by a single electric engine, together with a sterling engine in the exhaust/cooling system to provide extra electrical power and thus better fuel economy.

  4. Tracy Lemme says:

    What about Fairbanks Morse? These engines have been around for 75 years or more and I believe that the US Navy still buys theme.

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