Explain in detail the working principle of the piston-connecting rod assembly

The piston-connecting rod assembly is the core power transmission unit of the crankshaft-connecting rod mechanism. Its primary function is to convert the reciprocating linear motion of the piston into the rotary motion of the crankshaft, while reversely transmitting the crankshaft’s inertial force to complete the intake, compression, and exhaust strokes. It serves as the key executive component enabling the engine’s energy conversion.

Its working principle shall be elaborated in conjunction with the operating cycle of a four-stroke engine, as follows:

I. Composition of the Piston-Connecting Rod Assembly and Interconnection of Components

The piston-connecting rod assembly consists of the piston, piston rings, piston pin, connecting rod, connecting rod bearings, and connecting rod bolts. The interconnection logic of these components forms the foundation for motion conversion:

Piston: It directly withstands combustion pressure, with its crown forming part of the combustion chamber and its skirt fitting precisely against the cylinder wall. A piston pin bore runs through the piston, hinging to the small end of the connecting rod via the piston pin.

Piston Pin: Adopting a fully floating or semi-floating connection, it can rotate slightly within the piston pin bore and the bushing of the connecting rod small end to reduce frictional wear, while transmitting the linear thrust of the piston.

Connecting Rod: Rod-shaped in structure, its small end connects to the piston via the piston pin, and its big end articulates with the crankshaft’s connecting rod journal via connecting rod bearings. It acts as the "bridge" linking linear motion and rotary motion.

Connecting Rod Bearings: Inlaid inside the big end of the connecting rod, they provide a sliding friction pair for the articulation between the connecting rod and crankshaft, while storing engine oil to ensure lubrication.

Piston Rings: Divided into compression rings and oil control rings. Compression rings seal the gap between the cylinder and piston to prevent mixture leakage and combustion gas blow-by into the crankcase; oil control rings scrape off excess engine oil from the cylinder wall to avoid oil entering the combustion chamber for combustion.

II. Operating Process in the Four-Stroke Cycle

Taking a four-stroke gasoline engine as an example, the piston-connecting rod assembly completes a cycle of "force transmission → motion conversion → inertial return stroke" across four strokes:

Power Stroke (Core Stroke for Power Output)

The spark plug ignites the air-fuel mixture inside the cylinder, generating instantaneous burst pressure of hundreds of kilopascals that acts vertically downward on the piston crown.Driven by this pressure, the piston moves downward along the cylinder wall in accelerated linear motion.The linear thrust of the piston is transmitted to the small end of the connecting rod via the piston pin, forcing the connecting rod to rotate around the crankshaft’s connecting rod journal. The big end of the connecting rod performs circular motion around the crankshaft journal, pulling the crankshaft to rotate around the main journal. This converts the pressure energy of combustion into the rotational mechanical energy of the crankshaft, which drives the vehicle or powers other accessories.

Exhaust Stroke (Inertia-Driven Return Stroke)

After the power stroke, the crankshaft continues to rotate relying on the inertia stored in the flywheel.The rotation of the crankshaft’s connecting rod journal drives the movement of the connecting rod big end, and the small end of the connecting rod pulls the piston upward to move linearly along the cylinder wall.The upward-moving piston squeezes the exhaust gas inside the cylinder, which is then discharged out of the cylinder through the exhaust valve, completing the exhaust stroke.

Intake Stroke (Inertia-Driven Suction Stroke)

The inertial rotation of the crankshaft continues to pull the piston downward in linear motion via the connecting rod.The downward movement of the piston creates negative pressure inside the cylinder, drawing outside air into the cylinder through the intake valve and completing the intake stroke.

Compression Stroke (Inertia-Driven Compression Stroke)

The inertial rotation of the crankshaft pushes the piston upward in linear motion via the connecting rod.The upward-moving piston compresses the air-fuel mixture inside the cylinder, raising its temperature and pressure to prepare for the next power stroke.

III. Key Kinematic Characteristics and Technical Requirements

Complexity of Motion

During operation, the small end of the connecting rod performs reciprocating linear motion with the piston, while the big end executes rotary motion with the crankshaft. The rod body of the connecting rod undergoes planar complex motion. This results in uneven force distribution and wear across different parts of the connecting rod, which is why connecting rods are manufactured from high-strength forged alloy steel.

Precision of Fit Clearances

The fit clearance between the piston and cylinder wall must be controlled within 0.02–0.05 mm. Excessively large clearance can cause gas leakage and increased oil consumption; excessively small clearance may lead to piston seizure due to thermal expansion.The clearances between the piston pin and connecting rod small end bushing, as well as between the connecting rod bearings and crankshaft journals, must ensure that oil can form a lubricating film, while preventing impact noise caused by excessive clearances.

Dynamic Balance Requirements

The piston-connecting rod assembly is a high-speed reciprocating component. Uneven mass distribution will generate massive reciprocating inertial forces, causing increased engine vibration and noise, and even damage to crankshaft bearings. Therefore, the piston-connecting rod assemblies must be weighed and matched before assembly, ensuring that the mass difference between assemblies of each cylinder in the same engine does not exceed 5 grams.