Unless you drive a rotary-powered Mazda, the characteristics of your gasoline or diesel engine are largely defined by its bore (the width or diameter of the cylinders) and stroke (the distance the piston travels within the cylinder).
But if you want to make more power, is it better to increase bore or stroke? Jason Fenske of Engineering Explained breaks it down in the accompanying video.
The short answer is that a bigger bore is generally the best way to get more power. It creates more space, allowing for bigger valve openings, which in turn can bring more fuel and air into the cylinder. This doesn’t work well at low rpm but does at high rpm. That plays well in to the other factor. A bigger bore with a shorter stroke also allows an engine to rev higher, which creates more horsepower.
Conversely, a long stroke is generally better for fuel efficiency, because it reduces surface area during combustion. With less surface area, there is less room for heat to escape, ensuring more of the energy from combustion is turned into useful work to push down the piston.
A small-bore, long-stroke engine also requires the flame to travel less distance during combustion, which means the burn duration is shorter. That allows combustion to do once again do more work and make the engine more efficient.
However, these are just generalizations. It’s possible for big-bore engines to be efficient, or for long-stroke engines to be powerful. But without looking at any other variables, there is a correlation between bore size and power, and between stroke length and efficiency.
Bore and stroke aren’t the only factors that go into the design of an engine, and that’s why these aren’t hard and fast rules. The mass of rotating parts and the use of turbocharging or supercharging can impact power output and efficiency.
Talking about an engine in isolation also only provides part of the picture. An engine’s performance is ultimately determined by the car it’s used in. The choice of transmission and the vehicle’s weight and aerodynamics also affect efficiency. At the same time, a powerful engine is pointless if that power can’t be put to the pavement.
For more depth, check out the video above. Like all EE videos, you’ll certainly expand your knowledge of automotive engineering.