Camshafts And Crankshafts Explained The Simple Way

These two shafts are intrinsically linked and are vital components to any four-stroke engine. Here's everything you need to know!

Remind me later
Camshafts And Crankshafts Explained The Simple Way - General Mechanical Advice

Engines, transmissions and power transfer is a complicated grouping of gears, shafts and rods that make internal combustion one of mankind’s greatest inventions. The efficiency of IC has come a long way since 1876, with much to be said for two different types of shaft that are perfectly designed to help initiate the engine cycle and to transmit the torque created by the combustion down the line to the transmission. These are the camshaft and crankshaft respectively, so let’s jump into what they are and what their essential role is within a car’s drivetrain.

Camshaft

Camshafts And Crankshafts Explained The Simple Way - General Mechanical Advice

Camshafts are made from cast iron or steel and can be mostly found in the head of an engine, nestled above the cylinders. They are generally found in two orientations:

SOHC (Single Over-Head Cam)
DOHC (Dual Over-Head Cam)

Running along the shaft are lobes which are manufactured to sit at different angles. These lobes are positioned in such a way that – when the camshaft is rotated – they come into contact with rocker arms that then open the engine valves. The lobes themselves are egg-shaped, with the ‘pointy’ end contacting the rocker arms, opening the valves at specific times within the engine cycle. This allows the air/fuel mixture to enter the cylinder and then the exhaust gases to leave the cylinder at the required time. The valves themselves are spring-loaded, meaning that once the lobe has done its job of opening the valve, it naturally closes as the spring becomes uncompressed.

This CAD render shows the camshaft and its lobes (green) and their relationship with the rocker arms (red) and the valves (grey)
This CAD render shows the camshaft and its lobes (green) and their relationship with the rocker arms (red) and the valves (grey)

The timing of the camshaft is all operated through the cambelt (or timing belt) which is synchronised with the movement of the crankshaft. This means that the timing of the valves opening is bang on with the engine’s cycle, avoiding any valve or cylinder damage due to mistiming.

While a SOHC system has a camshaft that completes both the intake and exhaust stroke valve movements, a DOHC system has two camshafts above each bank of cylinders – an intake camshaft and an exhaust camshaft. So in an inline four-cylinder engine with a SOHC, there would simply be a single camshaft in the engine’s head. But in a V8 with SOHC, there would be two camshafts overall (one on each side of the V).

5 MB
Here you can see the lobes of dual over-head camshafts opening the engine valves on each stroke

The most extreme camshafts come in the shape of those featured on the Bugatti Veyron. With a W16 to keep in shape, the Veyron employs a quad-cam setup with a total of 64 lobes. This allows the camshafts to open all 64 valves that are present within the leviathan of an engine, designed to the ultimate of tolerances to keep the Bugatti’s powerplant running just right.

Crankshafts

The beautiful crankshaft from the W16 Veyron
The beautiful crankshaft from the W16 Veyron

Crankshafts are generally made from steel and sit below the cylinders and pistons in the engine block. Their job is to convert the vertical movement of the pistons into a rotation to be transferred through to the flywheel and then the transmission. The crankshaft has crank pins along its length that line up horizontally with the pistons above and form the ‘stepped’ orientation of the shaft itself.

The crank pins are dimensioned and positioned to allow for each cylinder to travel from Top Dead Centre to Bottom Dead Centre and back, transmitting that reciprocating movement into the shaft’s rotation. The connection between the pistons and the crankpins is through connecting rods, whose ‘big ends’ connect onto the crank pins.

5 MB
A crankshaft from a four-cylinder engine in action

The rotation of the crankshaft is then transferred through to the flywheel which sits at the end of the shaft to balance it in case of irregular engine pulses and to complete the torque conversion from the internal combustion occurring in the cylinders.

A DOHC and crankshaft in their respective positions
A DOHC and crankshaft in their respective positions

Despite the effective design of the crankshaft (which has existed for centuries), most of an engine’s power losses occur in the crankshaft area, be it through heat, vibration, noise and friction. The multi-directional nature of the forces being applied to the crankshaft from the pistons means that the art of balancing a crankshaft can also be extremely difficult which is why engineers try to minimise a crankshaft’s length as much as possible. This is the big reason why the V8 engine took over from an engine configuration like an in-line eight, due to its relatively small and controllable crankshaft setup that stops any unwanted flexing occurring.

The relationship between the camshaft and crankshaft is extremely vital for the integration of a car’s drivetrain and should definitely not be underestimated. They essentially start and end the engine cycle - from inlet stroke to exhaust stroke - keeping the different mechanical processes of each cycle in perfect harmony through their belted connection. They may seem just like machined billets of steel, but they form one of the most essential partnerships with a vehicle’s powertrain.