Now, have a look at the above shown engine from the IAV company! So what?, you might say and ask yourself what's so special about a four-cylinder in-line engine. It would be great if the position of the pistons would cause you to be a little uneasy with your judgement, after all, in which in-line 4-cylinder would you find two directly adjacent pistons at the same height?

To solve the riddle: what we've got here is actually two in-line two-cylinder engines, one behind the other. We also know that in two-cylinder in-line four-strokes, the crank-webs are in the same position. The highlight is however, that these two engines can, if needs be, be quickly coupled through the shaft above and next to the crankshaft.

What we have here, is a very special kind of cylinder deactivation. When only two cylinders are in operation, it avoids any additional friction which would be caused by the second engine. It simply stands still. Now, why does the second engine need a valve deactivation, when it's been separated below anyway? So that the gases are only sent there where they are actually used.

The only other thing that you may have noticed, is the fairly complicated clutch on the auxiliary shaft. This in fact, is packed full of inventive talent. It's not enough, to simply couple the two engines with each other. In the engaged state, the crank mechanism of the one must be turned at exactly 180° to the other, so that the mass compensation can function at all.

This of course, cannot be quite as good as a genuine in-line four-cylinder. In this case, at least the 1st.-order inertia-force is bound to be somewhat more pronounced. Although, when one takes a closer look at the auxiliary shaft, one can see that this is also functions as a compensation shaft. This of course, is particularly important in the operation with only two cylinders, as can be seen in the above video about the Fiat-Twin.