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 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.
The timing of the camshaft is all operated through the cambelt or timing belt which is synchronised with the movement of the crankshaft. 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.
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. 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.
This is the job of the two main oil seals, one at the front and one at the rear. The rear main seal is installed between the rear main journal and the flywheel. It is commonly a synthetic rubber lip-seal. The seal is pressed into a recess between the engine block and the oil pan. The seal has a shaped lip which is held tight against the crankshaft by a spring called a garter spring.
A failed oil seal is a major problem because they are adjacent to main journals which receive, and need, a good supply of pressurised oil. Combined with the spinning of the crankshaft, this leads to rapid loss of engine oil through any breach of an oil seal. The front oil seal is similar to the rear, although its failure is less catastrophic and it is more easily accessed.
The front oil seal will be behind the pulleys and timing gear. An oil seal itself is a cheap part, but accessing it requires a lot of labour in removing the transmission, clutch, flywheel and possibly the crankshaft. The basic crankshaft shown above is from an inline 4-cylinder engine. Other crankshaft designs will depend on the engine layout. This is a topic covered in more detail in the article about engine layout. But one point of note is that in V-shaped and W engines two connecting rods may share a single rod journal.
Some typical crankshaft layouts are shown below. A V6 crankshaft is somewhat specialist because it requires rod journals to be split to maintain an even firing interval.
This necessitates the rod journals being split, or splayed, in what is known as a split-pin or split-journal design. The crankshaft, being very sturdy, is a reliable component and crankshaft failures are rare unless an engine is subject to extreme conditions. Without enough oil pressure, the crankshaft journals will make contact with the bearing surfaces gradually increasing the clearance and worsening the oil pressure.
Taken to an extreme, this can lead to destroyed bearings and cause serious engine damage. Where journals are worn to below their service limits, or are no longer perfectly round, they will need to be ground as described below. Constant forces on the crankshaft can lead to fatigue fractures, usually to be found on the fillet where the journals join to the web.
A smooth radius of this fillet is critical to avoid weak spots leading to fatigue cracks. A crankshaft can be inspected for cracks using magnafluxing. Journals wear over time. They may develop a rough surface, or become out-of-round or tapered.
In these cases their surface can be restored in a process called crankshaft grinding. When a crankshaft is ground its journals will be reduced in diameter and so oversized, thicker bearings will need to be installed.
The cylinder capacity can be increased by moving the pistons over a longer stroke. The stroke of an engine is determined by the crank radius, being the distance of the rod journals from the main journals. A crankshaft with a larger crank radius will produce a longer stroke and a greater cylinder capacity - this is known as a stroker crankshaft. The crankshaft or crank is connected to the pistons by connecting rods conrods , which have a bearing at each end. The end connected to the crank is bigger, because the bearing it holds has to fit around the crank, which has to be chunky to resist a multitude of forces.
The bits of the crank which hold bearings are called journals - and there are two types. The rod journals are, you guessed it, the bits the conrods attach to these are also known as crank pins. Then there are the webs. These are the lumpy bits which look relatively unmachined compared to the rest of the crankshaft. Their job is to try and balance out the forces created by the pistons flying up and down and the crank itself rotating. The forces the crank has to withstand are enormous.
Then there are the forces created by piston acceleration. Though modern pistons and the associated gubbins of wrist pin, small end bearing etc are light, the speeds they reach are so high that this deceleration, and the associated forces on the crank, are large.
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