diameter and stroke

 

Today I am going to talk to you about the diameter and the stroke in the engines. It is a very hackneyed topic by the connoisseurs and those who are not so much, and everyone has heard of short-stroke and long-stroke engines, but what is that?

Well, within the internal combustion engines the pistons rise and fall within the cylinder moved by the explosions that are produced in it. The problem is that the engines turn so fast that the dimensions of the cylinder and pistons become of paramount importance. At the moderate speed of 3.000 rpm that we so often see in our cars, the piston goes up and down 50 times, per second!

Therefore, for the same displacement (volume inside the cylinder), you can choose a narrow and long cylinder or a wide and short one. As the piston moves, it generates enormous inertia, and that is where dimensions come in. More or less it is said that a normal engine can reach a piston linear speed of up to about 20 m/s (in competition they are close to 30 m/s). Linear velocity is the stroke of the engine (how far the piston can travel in the cylinder from the lowest to the highest point) multiplied by the engine speed (how many times it climbs that distance in a given time). Well, in order not to exceed those figures that are imposed on us by materials and reliability, when designing an engine the first thing we have to know is what we expect from it. If we want it to be sporty and go up a lot of laps, we will use a short race so that the linear speed is lower and thus it moves more easily at high speed. In addition, the cylinder will have a larger diameter, with larger valves that once again favor breathing at high speed.

It seems that the long-stroke engine is an ugly duckling, but it also has its advantages, since it makes for more torque at low speeds, which is the one that is normally used more, so we will have to take this into account when choosing. Motors with a smaller stroke than diameter are called supersquare.

Still, the engine in a Lamborghini Gallardo is long-stroke and goes up to 8000 rpm, so there are exceptions…

By the way, the diagram is of the engine of a 24 Lancia D1953, an 6mm x 88mm (bore per stroke) V90. It is also an exception, as Italian engines, and more of that type and era tend to short runs generally.