Piston
A piston is a cylindrical mechanical component that reciprocates within a cylinder to transfer force from an expanding gas or fluid to a crankshaft (in engines) or to compress/displace a fluid (in pumps and compressors). The piston forms a movable seal with the cylinder walls through piston rings, enabling controlled pressure differentials. Pistons are the heart of internal combustion engines, steam engines, hydraulic actuators, and pneumatic cylinders.
Key Formula
F = P * A = P * (pi * D^2) / 4
LaTeX: F = P \times A = P \times \frac{\pi D^2}{4}
| Symbol | Meaning | Unit |
|---|---|---|
| F | Force exerted by gas pressure on piston | N |
| P | Gas pressure in cylinder | Pa |
| A | Cross-sectional area of piston face | m² |
| D | Bore diameter (internal cylinder diameter) | m |
Worked Example
Problem
A four-stroke petrol engine has a bore diameter of 80 mm. At peak combustion, the cylinder pressure is 6 MPa. Calculate the maximum force exerted on the piston.
Solution
Step 1: Convert diameter to metres. D = 80 mm = 0.08 m Step 2: Calculate piston area. A = π × D² / 4 = π × (0.08)² / 4 = π × 0.0064 / 4 = 5.027 × 10⁻³ m² Step 3: Calculate force. F = P × A = 6 × 10⁶ × 5.027 × 10⁻³ = 30,159 N
Answer
Maximum force on piston = 30.2 kN
Piston Types in Different Engine and Machine Applications
| Application | Piston Material | Stroke Type | Operating Pressure | Key Feature |
|---|---|---|---|---|
| Petrol/Diesel Engine | Aluminium alloy | Reciprocating | 5–15 MPa (peak) | Piston rings for sealing |
| Steam Engine | Cast iron / Steel | Reciprocating | 0.5–3 MPa | Double-acting (steam both sides) |
| Hydraulic Cylinder | Steel with chrome | Reciprocating | 10–35 MPa | No piston rings; lip seals |
| Pneumatic Actuator | Aluminium / Plastic | Reciprocating | 0.4–1 MPa | Light-weight, fast response |
| Axial Piston Pump | Hardened steel | Axial reciprocating | 20–40 MPa | Variable displacement |
Interactive Tools
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Related Terms
Flywheel
A flywheel is a rotating mechanical device that stores rotational kinetic energy by virtue of its high moment of inertia, acting as an energy reservoir that resists changes in rotational speed. It smooths out fluctuations in power delivery from reciprocating engines (such as internal combustion engines) by absorbing energy during power strokes and releasing it during non-power strokes. Flywheels are used in punch presses, steam engines, automotive engines, and modern grid-scale energy storage systems.
Cam Mechanism
A cam mechanism is a higher kinematic pair consisting of a specially shaped rotating or translating element (the cam) that imparts a prescribed, non-uniform motion to a follower through direct contact. By designing the cam profile, engineers can generate virtually any desired follower displacement, velocity, and acceleration profile, making cam mechanisms indispensable in internal combustion engine valvetrains, automatic screw machines, textile machinery, and printing presses.
Four-Bar Linkage
A four-bar linkage is the simplest closed-loop kinematic mechanism consisting of four rigid links connected by four revolute (pin) joints, with one link fixed as the frame (ground link). It converts rotary input motion from a crank into complex output motions of the follower link, enabling a vast range of mechanical paths and oscillations. Four-bar linkages are fundamental in machine design, robotics, prosthetics, automotive suspensions, and mechanical toys due to their simplicity and versatility.
From Italian "pistone" (large pestle), derived from Latin "pinsere" (to pound, to stamp). The piston is one of the oldest mechanical devices, used in ancient bellows and pumps; its modern form in steam engines was developed by Thomas Newcomen (1712) and subsequently perfected by James Watt.