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What is a crankshaft and what does it do?

What is a crankshaft and what does it do?

What does a crankshaft do?

An engine crankshaft runs inside the bottom end of a car's motor and converts the vertical movement of the pistons into horizontal rotational movement, which ultimately drives the wheels via the gearbox.

Crankshafts have existed for hundreds of years, having been used in water mills and sawmills. They eventually became prominent in paddle boats, turning the energy from steam-powered pistons into rotational energy for the paddle wheels.

In today's cars, the crankshaft consists of evenly spaced ‘throws’ (there are four in a four-cylinder engine, as shown below), which are attached to the bottom of the pistons by connecting rods. These ‘throws’ are offset from the axis of the crankshaft, which is what creates the rotational energy.

What is a crankshaft

The crankshaft is attached to the engine by large bearings at either end. It connects to the flywheel, and through that, the clutch.

When the clutch is engaged, the rotational energy of the crankshaft is transmitted through the gearbox and on through the differential to the driveshafts, which are attached to the wheels, hence creating the car’s ability to move.

Camshaft vs crankshaft

A camshaft is positioned near the top of engine and is driven by the crankshaft via a timing chain or timing belt.

The camshaft (there can be up to four per engine, although the crankshaft diagram below shows two) features cam lobes along its length that work with the valve train (which comprises the pushrod, valve lifter, valve spring, valve and tappet or rocker arm) to facilitate the entry of air and fuel into the combustion chamber and the egress of exhaust gases following the power stroke part of the internal combustion engine cycle.

The camshaft's rotating cam lobes open and close the engine valves. The lobes vary in size and shape from engine to engine to govern the duration a valve opens for and by how much. The more valves there are, the more air and fuel and can be taken in and the more exhaust gases can escape, which increases engine power.

The engine crankshaft is positioned near the bottom end of the engine (as shown in the diagram below) and is attached to the pistons via connecting roads – the downward movement of the pistons as a result of the combustion process causes the crankshaft to rotate.

Camshaft vs crankshaft diagram

What goes wrong with crankshafts?

A crankshaft is an extremely complex and finely honed piece of engineering that doesn’t only consist of the crank throws. Key parts include crank pins, oil passages, the keyway, main journals and the flywheel mounting flange.

But the engine crank also comprises numerous in-built weights and balances that are designed to keep vibration to a minimum as it rotates.

Any such vibration could be magnified and would cause damage to the bearings the crankshaft is mounted in, as well as the conrods and pistons.

Before that happens, though, something called a crankshaft position sensor should detect any faults.

A crankshaft position sensor (shown below) is mounted on the engine block and detects the rotational speed and position of the crankshaft. The crankshaft sensor sends this information to the vehicle's ECU, which makes adjustments to optimise the operation of the engine.

However, sometimes corrections can't be made and if the crankshaft sensor detects a fault, the ECU will either stop the engine from starting in the first place or will put it into 'limp home' mode, where you won't be able to rev the car past a certain point.

You'll also see a 'check engine' light on the dashboard, informing you that the car needs immediate attention.

Crankshaft position sensor

How does a supercharger work with a crankshaft?

In some cars that are equipped with a supercharger, a pulley is attached to the end of the crankshaft, which is then attached to the supercharger by a belt.

When the engine is running the crankshaft rotates rapidly, driving the pulley, which moves the belt, and then the pulley on the end of the supercharger.

This then operates the turbine in the supercharger, drawing in air and boosting the power the engine produces.