How Piston Design Affects Combustion

The terms “four-cylinder” and “six-cylinder” are rather common in the automotive space, but what do they mean? The “cylinder” is actually the space in which a piston travels (a tube that is part of the engine block), and the number of cylinders in an engine corresponds to the number of pistons.

Each piston travels up and down in its own cylinder and a wrist pin travels through the piston through the top of a very strong I-beam connecting rod that is at the other end clamped (with a lubricated bearing) to one of the throws on the crankshaft, which is connected to the drivetrain where gears multiply the engine’s torque as the vehicle is driven. As the pistons repeat their four stroke cycle, one of the four strokes in each piston’s cycle uses the energy created during combustion to spin the crankshaft. Every cylinder handles an equal part of the job when it comes to driving the crank. The more cylinders an engine has, the smoother the power transfer from the pistons to the crank.

The more cylinders an engine has, the smoother the power transfer from the pistons to the crank.

–Richard McCuistian, ASE Certified Master Automobile Technician

The design and shape of pistons have evolved over the years, and this has definitely affected the combustion process. But how?

How Do Pistons Work?

Before we answer that, it’s important to know how pistons work. As an engine runs, pistons move up and down in each cylinder, triggered by the combustion in the combustion chamber above each piston.

Upon reaching each “dead center” point, each piston stops, reverses direction and then accelerates suddenly. Just before top dead center on the compression stroke, the spark plug is timed to fire, beginning the process where the hydrocarbon fuel and oxygen in the air unite, creating tremendous heat. Most of the air in the cylinder happens to be nitrogen, which is inert, but is superheated by the combustion event and expands, pushing the piston down against the crankshaft journal connected to that piston via the rod and its bearings.

The piston’s head or crown takes the brunt of the initial force and pressure once the combustion process begins. Thanks to the constant and quick directional changes, the piston pin areas are also under a lot of force. Thermal expansion also happens because of all the heat that gets transferred from the piston’s head to its body, and the piston pin area in particular gets affected.

Types of Piston Shapes

Pistons come in a variety of shapes and designs to deal with all of the pressure placed on them in the engine.

Elliptical

Pistons that take on an elliptical shape (also called cam ground pistons) are better at conforming to the changing dimensions of the cylinder bore. These oval-shaped pistons start off elliptical when cold and turn more circular as the engine reaches operating temperature. This improves the seal around the piston and increases combustion efficiency.

Tapered

The head shapes of tapered pistons have a small diameter that widens down the pistons’ bodies. This tapered shape is meant to compensate for thermal growth and expansion. The amount of heat applied to the piston head can sometimes cause it to expand. The piston’s tapered design allows it to move freely in the cylinder despite this change.

Barrel-Shaped Skirt

The barrel-shaped piston skirts allow for a smoother transition as the pistons change directions. Each piston rolls into the cylinder wall at the end of a stroke, right when it switches directions. This piston shape helps reduce noise and side loading on the piston skirt while also spreading the force of the directional change over a larger surface area.

Offset

There are piston designs where the piston pin is offset from the center of the piston. As a matter of fact, just about all pistons are built this way. This is why pistons have a notch and arrow, or an “F” for “front” cast or cut into the piston so the builder won’t put the pistons in backward. 

This design makes pistons quieter by reducing piston wobble. If you put the pistons in backward, you’ll get an engine knock. The offset wrist/piston pin allows the piston to move in a true linear fashion in the cylinder bore.

The Evolution and Effects of Piston Design

Pistons have evolved alongside engines over the years, becoming shorter and lighter with smaller skirts. Many are now made from aluminum alloys with more silicon, making them more heat resistant and less prone to thermal expansion.

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Pro Tip: Many pistons are coated with graphite above each skirt to reduce friction, and modern engines are built with low tension piston rings and many have dished pistons crowns.

The shape of piston tops, which are also known as crowns, have also changed over the years. While these parts used to be on the flat side, more modern piston tops now have bowl shapes. This has its own effects on combustion. For the most part, these bowl-shaped pistons are found in diesel engines, but they’re becoming more common in gasoline engines with direct fuel injection.

Because the piston’s crown is shaped like a bowl, it can control the movement of both the fuel and the air as the piston ascends before ignition for the compression stroke. This creates an air and fuel vortex inside the poston bowl before combustion or compression occurs, improving the mixture and creating more efficient combustion. As a result, the engine has more power to work with. The bowls can also be shaped to optimize fuel economy.

Overall, pistons continue to evolve to keep up with engine developments. Thanks to the rising popularity of direct injection in gasoline engines, we might just see even more unique piston designs hit the market sooner rather than later.