Crankshaft and camshaft: what they do
The crankshaft turns each piston's straight-line push into rotation. The camshaft, driven from the crankshaft at half its speed in a four-stroke engine, opens and closes the valves at the right moment in each cycle.
How it works
Each connecting rod links a piston to an offset journal on the crankshaft. Combustion pressure pushes the piston down, the offset journal swings around the main axis, and the shaft rotates. Main bearings support the shaft in the block; rod bearings sit between rod and journal on a film of pressurised oil.
Why the camshaft runs at half speed
A four-stroke cycle — intake, compression, power, exhaust — takes two crankshaft revolutions. Each valve opens once per cycle, so the camshaft turns once for every two crank turns. A belt, chain or gear set keeps the two in time.
Where the power goes
Rotation leaves the rear of the crankshaft through the flywheel or flexplate into the clutch or torque converter, then the transmission, driveshaft and axle. The same chain applies to a passenger car and a Class 8 truck; the parts are sized very differently.
Symptoms and look-alikes
Knocking that changes with engine speed, low oil pressure and metal in the oil can point to bearing wear, but also to oil supply problems, wrong oil or other internal faults. Timing problems can resemble ignition or fuel faults. Symptoms overlap between systems. Confirm with a qualified technician and the manufacturer's service information before replacing parts.
Common questions
- Is a crankshaft the same as a camshaft?
- No. The crankshaft carries the engine's output; the camshaft only operates the valves (and in some engines, fuel pumps or injectors).
- Do diesel truck engines use the same idea?
- Yes, the basic piston-rod-crank arrangement is the same. Modern diesels differ in materials, loads, and fuel and valve control, which this page doesn't cover.
Sources & review status
- Motor Service's Automotive Encyclopedia, Jud Purvis and William K. Toboldt, 3rd edition, The Goodheart-Willcox Co., Chicago, c. 1958 (copyright dates 1954, 1956, 1958). PDF pp. 17–20, 43–48. Supports: Four-stroke cycle; crankshaft, camshaft, bearing and valve-train principles. Historic source.
- Dyke's Automobile and Gasoline Engine Encyclopedia, A. L. Dyke, 12th edition, second run, A. L. Dyke, St. Louis, Missouri, c. 1920. PDF pp. 25–29. Supports: Power path from engine through transmission and drivetrain to the wheels. Historic source.
The historic books describe 1920s and 1950s engines. Modern variable valve timing, overhead-cam layouts and electronic controls are not covered by them.
Original Repairius explanation. Not yet reviewed by a qualified technician; not a substitute for the maker's service information.