I’ve always been fascinated by the fact that many manufacturers cannot make up their minds about the number of cylinders to specify for any particular displacement class. In the 2,0-litre class, many manufacturers have moved from four-cylinder engines to six cylinders and back again to four. What are the advantages of each layout?
SIMON MACFARLANE – Port Alfred
Four-cylinder engines tend to be lighter than sixes. They also cost less to build because the manufacture and assembly of the parts take less time. They use less fuel because the frictional losses are less. This is especially true of the piston-ring-to-cylinder-bore interface where most of the frictional losses occur. In addition, the bigger bores make such an engine wider and smaller numbers of cylinders tend to make the engine rougher. On a modern four, this will mean that double engine-speed balance shafts will have to be used. These shafts have to be driven by a chain or gears, and mounted in bearings at each end. This construction will nullify some of the weight and low-friction advantage of the four. These engines usually have heavier ywheels so that they pick up speed more slowly. They’re also not as happy at speeds above 6 000 r/min as a six, with the result that slightly milder valve timing is normally applied so that a four tends to develop a higher maximum torque than a six but at a lower engine speed.
On a six-cylinder engine, most of the above advantages become disadvantages. For example, a six tends to weigh more but be narrower, unless it’s a V6. The major advantage of a straight- or at-six is that it is the smallest number of cylinders that are in perfect balance. This means that these engines are inherently smooth and do not need a balance shaft. The lighter pistons and ywheel usually result in an engine that thrives on revs. The valve timing is then chosen to deliver maximum torque at the higher engine speeds.
The V6 is a special case. Its major advantage is more compact dimensions but it’s not as smooth as other six-cylinder layouts. Some designs have a balance shaft but others do not, depending on the V-angle chosen for the cylinder block.
