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.
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.
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..