Range to diameter ratio

This is a very important issue in the engine design stage, which is the ratio of the piston stroke to the piston diameter, also called the stroke-diameter ratio.
Generally, high-speed engines use large pistons with short strokes, and low-speed engines use small pistons with large strokes.
However, the selection of the range-diameter ratio must consider the design purpose of the engine and the goal of performance enhancement.

The influence of piston stroke on engine performance

With the engine speed constant and the stroke increasing, the average piston speed will increase proportionally.
The average piston speed is an important parameter that characterizes the degree of engine strengthening.
The problems caused by the increase in the average piston speed are:
1. The friction loss increases, and the mechanical efficiency decreases; the thermal load of the piston assembly increases, and high temperature resistance and high load capacity oil are required;
2. The inertial force increases, and the mechanical vibration and noise increase;
3. As the air inlet and exhaust flow rate increases, the air inlet resistance becomes larger, which will reduce the inflation efficiency.

The influence of piston diameter on engine performance

A large piston diameter will increase the thermal load of engine cylinders, pistons, cylinder heads, valves and other parts.
Gasoline engines are limited by knocking, and the cylinder diameter generally does not exceed 100mm. There is no lower limit for the cylinder diameter of a gasoline engine.
The cylinder diameter of automotive diesel engines is generally between 80 and 160 mm.

The choice of path to diameter ratio

The range-to-diameter ratio of high-speed gasoline engines is generally between 0.7 and 1.0.
The range-to-diameter ratio of diesel engines is generally between 1.05 and 1.2. The compression ratio of the diesel engine is high and the piston stroke is longer.