Understanding Key Engine Components: Pistons, Crankshafts, Flywheels, and Combustion Technology

Are pistons and crankshafts the most "tired" parts of an engine? 

Once the engine is running, the "head" of the piston is subjected to high temperatures and pressures while moving up and down at high speed. The working environment is extremely harsh, and the piston is often considered the "heart" of the engine. This is why the material and manufacturing precision of the piston are held to very high standards.

The crankshaft, which is driven by the pistons, also faces considerable challenges, as it must rotate at high speeds. It performs thousands of rotations per minute, bearing the critical responsibility of driving the oil pump, generator, air conditioning compressor, camshaft, and other mechanisms. As the central transfer shaft of engine power, the crankshaft is designed to be exceptionally strong and durable.

How is linear motion converted into rotational motion?

The piston moves up and down in a straight line within the cylinder, but the engine must output rotational force to drive the wheels. This conversion is achieved through the structure of the crankshaft. The connecting rod journal (the shaft the connecting rod attaches to) and the main journal (the crankshaft's primary axis) are not aligned but offset. This offset arrangement allows linear motion to be converted into rotational motion.

This mechanism is similar to pedaling a bicycle. The two feet act as two adjacent pistons, the pedals as the connecting rod journals, and the central crankset as the crankshaft’s main journal. When one foot pushes down (similar to a piston performing work or intake), the other foot rises (representing compression or exhaust). This cyclical movement converts linear motion into rotational motion.

Why is the engine flywheel so big?

Of the four strokes of a piston—intake, compression, power, and exhaust—only the power stroke generates energy. The other three strokes require energy to proceed smoothly. The flywheel stores kinetic energy from the engine to help the crankshaft maintain smooth rotation throughout all four strokes.

The flywheel is relatively large to ensure sufficient kinetic energy storage, enabling steady engine operation. This principle is similar to a gyroscope toy: once spun, it can maintain its motion for a long time.

Engine displacement and compression ratio

The volume swept by the piston as it moves from the top dead center to the bottom dead center is called the cylinder displacement. The total displacement of all cylinders in an engine is known as the engine displacement, typically measured in liters (L). For example, cars may have displacements of 1.6L, 2.0L, or 2.4L. While the exact volumes may not equal a whole number (e.g., 1998mL or 2397mL), they are approximated as 2.0L or 2.4L.

The compression ratio measures the degree to which the air-fuel mixture is compressed in the cylinder. It is the ratio of the total cylinder volume to the combustion chamber volume. Compressing the mixture increases its combustibility, improving engine performance and efficiency.

What is variable displacement, and how is it achieved?

Variable displacement engines adjust the number of active cylinders based on power needs. For instance, a 6-cylinder engine may operate in 3-cylinder, 4-cylinder, or full 6-cylinder mode. This reduces fuel consumption during low-power conditions like idling while maintaining performance when needed.

This is achieved by controlling the intake valves and oil circuits to deactivate specific cylinders. For example, the Volkswagen TSI EA211 engine uses electromagnetic controllers and a camshaft with a spiral groove sleeve to activate or deactivate cylinders as needed.

What is direct injection, and what are its advantages?

Traditional engines inject fuel into the intake manifold, where it mixes with air before entering the cylinder. However, this process can result in fuel adsorption on the manifold walls and uneven mixing. In contrast, direct injection sprays fuel directly into the cylinder, where it mixes with air. The engine control unit (ECU) precisely controls the injection timing and quantity, enhancing atomization and mixing efficiency.

This technology, initially developed for diesel engines, improves fuel economy and power performance. It has been widely adopted in cars from Volkswagen, BMW, Mercedes-Benz, and General Motors.