Cylinder Offset Changes Everything

Let's imagine two engines made from the same parts. They have the same crankshaft, the same piston, the same wrist pin and the same connecting rod. The only difference between them is that this engine has the cylinder center offset from the crankshaft center whereas in this engine they are perfectly aligned.

If you observe these two engines in rotation you can see that they do the same thing. The crankshaft rotates, the piston reciprocates, the rod does it’s own thing. However this is nothing other than a superficial illusion, because this little offset actually changes everything. Despite being made from the same parts these two engines are fundamentally different. The cylinder offset impacts power and efficiency as well as engine size, it dramatically influences engine balance and it disrupts the length and duration of the piston strokes. And in this video we will explain how and why all of this happens and why many recent engines employ an offset cylinder configuration. We will start simple and then gradually increase the level of mind-bogglingness until you get completely sick and disgusted of looking at these two engines and close this video. Let’s see how long you can last.

So let’s start easy. Heres’ the first fact: The offset cylinder engine makes more power and is more efficient. Why? Let’s imagine that both pistons are just a bit past the start of the combustion event

So we have combustion pressure building inside the chamber resulting in massive forces pushing down on the piston. But the piston is obviously connected to the crankshaft via a wrist pin which means that the piston also pushes down on the rod and then the rod pushes down on the crankshaft. The problem lies in the fact that the rod is inclined at a certain angle. The downward force exerted on the rod is directed at the small end of the rod, meaning that this force is actually trying to spin the rod or flip it over if you will. As the rod tries to flip over it ends up pushing the piston against the cylinder wall which increases friction. The sharper the angle of the rod the harder the rod pushes the piston into the cylinder wall and the greater the friction. As you can see the offset cylinder allows us to noticeably reduce the angle of the rod which reduces friction. Reduced friction means that less of the energy generated by the engine gets wasted on friction which means that there’s more available to be converted into usable work a.k.a power.

Next up let’s talk about size. Yamaha on their website claim that an offset cylinder engine is more compact. Now this isn’t a lie per se, but this statement is only true under certain conditions. As you can see in our example both engines are of the same size. In fact the offset actually makes this engine wider overall. So why did Yamaha say this? Well they said it because the only other way to reduce cylinder friction is to make the connecting rod longer. As you can see if we make a zero offset engine with a longer rod the resulting rod angle ends up being noticeably reduced which leads to reduced friction, however the a longer rod obviously leads to a much taller engine. So basically what Yamaha is trying to say is that a zero cylinder offset engine can’t have reduced friction without being taller because of its increased rod length. So the offset cylinder engine is only more compact than when compared against a zero offset engine with a longer rod.

Now let’s address something that you have probably already noticed. The offset cylinder engine obviously has a longer stroke. How is this possible if the crankshaft is the same? As we know, an engine’s stroke or the distance the piston travels from top dead center to bottom dead center is determined by the length of the crankshaft throw or the distance between the center of the main journal and the rod journal. This distance is what determines the diameter of the imaginary circle drawn by the crankshaft during rotation and the distance between these two points on the circle ends up being the stroke of the piston.

On the offset engine bottom dead center is here. As you can see the line which connects the crankshaft with the piston is obviously longer in the case of the offset engine because this line is at an angle. Now what connects the piston to the crankshaft is obviously the connecting rod which means that in order to retain the same stroke the offset engine would have to employ a longer rod. If you keep the rod length the same it means that this line is forced to become shorter which pulls down the piston an additional distance which increases stroke.



00:00 Power and efficiency
03:31 Stroke length
05:35 Unequal strokes
15:06 Balance

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