Saturday, June 4, 2011

Cylinder block and crankcase design

Fig 4.21
There are a number of variations in crankcase design. As previously indicated, some cylinder blocks have a skirt, but others do not have a skirt below the crankshaft’s centre-line. The skirt also has a bearing on the design of the oil pan and the main bearing caps.
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.
Fig 4.22
This cylinder block for a V-6 engine has a skirt that extends below the crankshaft. The main bearings have the usual two bolts at the top of each cap, but they also have side bolts. The side bolts pass through the skirt of the crankcase from the outside and are threaded into the main bearing caps. This increases the rigidity of the bearing caps.
In-line engine
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.

Engine vibration and balance
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.
2. Power impulses. The pistons deliver power to the crankshaft as impulses and this causes a type of rotary vibration in the crankshaft.

3. Reciprocating parts. The pistons, in particular, produce an inertia force at the top and bottom of their strokes. This causes up-and-down vibrations in an engine.

4. Resonance. Vibrations can be transmitted between parts and amplified, even though the parts may not be directly connected.

Fig 4.23 (a,b and c)
Balancing rotating parts
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.
Figure 4.23(c) has a balance mass added. This is equal to, and directly opposite, the other mass. The centrifugal force from both masses will now balance and will not be noticeable. This is the effect achieved by having balance weights on a crankshaft.

See Balancing of power impulses

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