Remember the Ducati Supermono? That outstanding racing single of the nineties that was only available as a racing bike? With a capacity of 550 cc it reached a top power of 75 HP at 10000 rpm: not bad for a big single!
Its engine was practically that of an 888 with bigger bore and stroke and minus the top cylinder. It had the same crankshaft as the 888 and BOTH conrods. One went from the crankshaft to the only piston, but what did the other do? It connected the crank to a sort of “balancer” that took the place of the missing cylinder. This balancer had one end pivoted to the crankcase, it had roughly the same weight of a piston and, having the other end connected to the crankshaft via a conrod, it moved nearly as a piston, but without the cylinder around it. In Italy this bike was dubbed “il battacchio”, the bell clapper, because of this feature.
But what is this “clapper” there for? Its reason is to make the single piston engine run like a 90° V twin. The balancer actually helps the piston in its movement, by giving it energy when it needs. Only this way it was possible to move the Supermono’s 100 mm (4 inches!) wide piston back and forth through its 75 mm bore 170 times per second! Incidentally, the balancer also reduces the vibrations of the engine, allowing a lighter frame to be built around it. The Supermono is a single that mimics a 90° V twin.
A 90° V twin may not be the most perfectly balanced engine, nor the most regularly running, not to speak of overall compactness or economy of construction. But it has one interesting feature: each piston effectively helps the other in its movement. To understand how this is achieved, let’s follow the movement of a piston in its cylinder, starting from the bottom dead centre (BTC). At this point the piston is still: it has just came down and had to be stopped dead before it can start its run upwards again. Once the piston reaches the top dead centre (TDC) it must stop again, change direction and start running down the bore once more. When at the “dead centres” the piston is dead still, even at 12000 Rpm. That’s why they are called dead centres. When the piston is at a dead centre, the conrod is aligned with the bore (except in engines that have offset small ends, but this is another matter).
Any object moving at a speed contains a certain amount of kinetic energy. The higher the speed (and the heavier the object) the greater is the energy. When the object is still, it has no kinetic energy. So, to pass from a still state to a state of motion, an object, a piston for example, or a complete motorcycle for that matter, must receive a certain amount of energy, and, in order to stop it, this energy must be taken away from it. That’s what brakes are there for.
The piston is dead still when at the dead centres, it is then quite reasonable to assume that it will reach its maximum speed at the middle of the stroke. It’s not exactly there that this happens, but very near. The rate at which energy is fed (or taken away) depends on the force that is available to the piston to accelerate it through the cylinder bore. The acceleration that a piston must endure is enormous: in just half its stroke it must reach the maximum speed from a standstill, then start decelerating again. A piston that runs through a stroke of 75 mm at 9000 Rpm will reach the speed of 32 m/s (71 mph) in 1/600th of a second! The acceleration is such that the weight of the piston will be multiplied by almost 2000 times! This should clear any doubt as to why it is desirable to have very light yet strong pistons and why it is so difficult to run big pistons at high revs.
It would be a very good thing to have a device that feeds the piston with the energy it needs to be accelerated! A flywheel can do the trick: in fact a flywheel acts as a “reservoir” of kinetic energy, absorbing it when there’s excess and releasing some when there’s need. But on a motorcycle engine a big flywheel can be a bigger problem than the one it wants to solve. First of all, it’s heavy, and this is bad by itself. Secondly, it will absorb energy also when you deliberately want the motor to pick up more of it: for example when you open the throttle. And it will feed energy to the motor when you want it to have less: for example when decelerating. In short, a flywheel will make an engine slow in its response to rev changes, it will add inertia to it, as anyone who rode a big Guzzi knows.
Now, let’s put two conrods on the same crankshaft, side by side on the same pin and with a pistons on each small end. Fit the two pistons into two cylinders placed at 90 degrees one to the other. It must look familiar to any Ducatista and Guzzista and also to some modern Jap bike rider, if they were able to see the engine under all the plastic that conceals it. Let’s follow the movement of one piston, say the one that runs in the vertical cylinder of an engine like the Ducati twin: as I said, the piston reaches its maximum speed at the middle of the stroke, and at this point the related big end is at 90° in respect to the cylinder bore axis, with the conrod at its maximum angle. If the crankshaft is turning from the vertical cylinder towards the horizontal one (as it in fact does in the belt-head Ducati twins), at this very moment the horizontal piston will be at its top dead centre, and dead still, so it will be in strong need of energy to be accelerated down the bore again.
If it’s at the beginning of an expansion stroke, this energy will come from the burning mixture, but if it’s performing an intake stroke it must find its energy somewhere else. But where did we leave the vertical piston? It was exactly half way through its stroke, which also happens to be the point at which it contains its maximum amount of energy! Being the two pistons connected to the same crankshaft via the conrods, it comes as a consequence that the piston at its highest speed will be willing to hand out some of its energy to the piston that must be accelerated. This energy will be taken from the amount which the “fast” piston must release while decelerating towards the top dead centre. Conversely, the accelerating piston will act as a brake for the decelerating piston, in a perfect cooperation that, incidentally, also releases some stress off the conrods. As the crankshaft keeps turning, the vertical piston will reach its BDC and at that point the horizontal piston will have reached its top speed and return the favour.
The result of this subtle cooperation between the pistons of a 90° V twin is a smoother running engine, that vibrates very little, doesn’t need big flywheels to regularise the rotation of its crankshaft nor complicated crankshaft layouts, and revs a lot higher than any other twin piston design. The drawbacks are a more complicated and costly design, and a rather unruly shape that some don’t like, with two pistons poking at such splayed angles. But we don’t mind this, do we?