Long versus short stroke engines: what are the differences? – Jalopnik

Just about every piston engine is defined by two basic measurements: bore and stroke. Bore is the diameter of the cylinder. The stroke is the distance the piston travels from bottom dead center to top dead center. Introduce a little math, including the number of cylinders, and those measurements eventually lead us to an engine’s total displacement.

When bore and stroke are equal, the engine is described as “square”. A long stroke is called an undersquare, meaning the stroke is significantly longer than the bore, and a short stroke is called an oversquare, meaning it has a bore larger than the stroke.

When comparing long and short stroke engines, and their associated advantages and disadvantages, it is useful to compare engines of similar displacement. That way we can talk about the changes in bore and stroke in relation to each other, and how those differences in engine design impact power, efficiency and packaging.

An increase in the distance a piston travels in the cylinder, also called the stroke length, also increases the stresses acting on the piston and crankshaft. The longer the stroke, the more inertia must be managed in the cylinder. This is one of the main reasons why long stroke engines are not designed for very high speeds.

A short stroke motor has less inertial stress. Because the piston has a smaller distance to travel, the acceleration of the piston is lower at the same speed. All other factors being equal, this means the engine can run at a higher RPM before stress becomes a limiting factor. Short-stroke engines have an advantage when engine manufacturers want to maximize power because power output is directly related to engine speed; higher speed means more horsepower. Some of your favorite high-revving supercars are probably short-stroked.

However, engine speed is not the complete picture. Stroke length also affects valve timing and combustion behavior, all of which determine performance.

The bottom of an engine, with its cylinders and crankshaft, therefore has to deal with inertial stresses. Then there’s the effect the bore has on the top end and on components like valves. Larger bore cylinders have more room for valves, which control air and fuel flow to the combustion chamber. Larger valves allow for greater airflow into the cylinder.

Short stroke engines benefit directly from this. Because their larger bores result in better breathing, they can create power at higher rpm. The opposite happens with long stroke engines. Because they have smaller bores, their valve sizes and resulting airflow are limited. Generally, it is this restricted airflow that causes long stroke engines to be limited in their power production.

There are also the effects that bore and stroke have on turbulence. Turbulence is basically the movement of the air-fuel mixture in the cylinder. Smaller bores and deeper chambers retain turbulence for longer and ensure faster combustion. Cylinders with larger bores and flatter chambers promote rapid decay of turbulence. To counteract this, an earlier ignition timing is needed, but that is also a double-edged sword, because the piston crown and cylinder head are exposed to heat for a longer period of time. This increase in exposure can lead to an increase in heat loss and a decrease in efficiency.

A persistent belief is that long stroke engines inherently produce more torque. That is not always true. Differences in bore and stroke result in pressure changes, as well as lengthening or shortening of the effective lever arm, and those effects cancel each other out if the displacement remains the same. The stroke length itself does not determine the torque.

What does differ is where the torque is produced. Ever heard of the term “low-end torque”? Long-stroke engines generally reach their maximum torque at lower speeds. Smaller bores and valves restrict breathing at high speed, but the combustion process is well suited to lower engine speeds and strong torque delivery at low speeds. This is why most people think of long-stroke engines as relaxed, low-speed performance. This is also part of the reason why semi-trucks don’t use V8 engines, because the low RPMs put less strain on the engine, increasing reliability and reducing operating costs.

Because bore and stroke vary, so does friction. A longer stroke increases the travel distance of the piston, which increases friction. By limiting the distance a piston has to travel, short-stroke engines have reduced friction and air resistance. Windage is the air resistance the piston faces as it moves in the cylinder and, like friction, can cause energy loss in the engine. Another way that short-stroke engines control energy loss is by using a smaller crankshaft stroke, which results in rotating components moving through a smaller arc.

Another issue to consider is cooling. If the surface area of ​​a piston is smaller, it is easier to cool. But as the bore increases, the distance that heat must travel from the piston crown to the cylinder wall also increases.

Long-stroke engines with smaller bores benefit from easier piston cooling and reduced heat concentration. There is clearly a balance between these two dimensions: bore and stroke. Each has its pros and cons, and automakers must design engines with all the science and math in mind.