Most of the variations are designed to produce a stiffer engine structure. These designs also allow the use of lighter materials or thinner sections. Having a more rigid cylinder block assembly also helps to reduce engine vibrations.
The two-part engine block assembly shown previously in Figure 4.5 is made of aluminium alloy and is in two parts. The parts are split along the crankshaft centre-line. There arc no main bearing caps. The cylinder block carries the upper halves of the bearings in the normal way and the block base carries the lower halves of the bearings.
This arrangement provides a stiff crankcase and well supported bearings. As well, there is a cast aluminium alloy oil pan fitted to the underside of the block base and this adds to the rigidity of the engine.
Main bearing caps
The main bearing caps of many engines are secured by two bolts as shown previously in Figure 4. 17. However, they can also be secured by four or more bolts and Figure 4.21 is one example.
Figure 4.22 shows part of a cylinder block and crankshaft assembly for an in-line engine. This cylinder block does not have a skirt. Each of the main bearing caps is secured by four bolts -- two through the top of the cap and one on each side. There is also a support brace on top of the bearing caps that is held down by the cap bolts.
The oil pan that is bolted to the underside of the cylinder block is of cast aluminium alloy. The side bolts pass through the sides of the oil pan and are threaded into the sides of the bearing caps. The engine block, main bearing caps, support brace and oil pan are all bolted together to form a rigid assembly.
There are four main causes of vibration in an engine. Engine designers have to consider these and arrange for them to be balanced or reduced in some way. Briefly, the causes of vibration are as follows:
1. Rotating parts. Centrifugal force acts on all parts that rotate. Parts such as the crankshaft, flywheel and clutch must be balanced.
|Fig 4.23 (a,b and c)|
Figure 4.23 shows, simply, how rotating parts such as a crankshaft are affected by centrifugal force.
Figure 4.23(a) shows a section through a shaft that is in balance. Centrifugal force will act on the shaft when it rotates but, because it has no heavy spots, there will be no noticeable effect.
Figure 4.23(b) has a mass added to the shaft and so an unbalanced condition has been created. Centrifugal force will pull the mass outwards as the shaft rotates. Centrifugal force will increase rapidly as the speed of rotation is increased. This is the effect produced by the crank-pins of a crankshaft.