How Motorcycles W·o·r·k 6

HOW motorcycles W·O·R·K 6

ENGINE CONFIGURATIONS

STEVE ANDERSON

INDIVIDUALLY, THE BASIC components of any motorcycle engine are quite simple. But when these pistons, cylinders and crankshafts are strung together in multiples, a collection of simple parts can end up comprising a very complicated engine.

Even a one-cylinder engine, the most basic configuration, can offer many variations and numerous levels of complexity. For starters, the engine can be two-stroke or fourstroke. If it’s a two-stroke, there is further classification based on the type of intake system: piston port, reed valve or rotary valve.

For a fourstroke, similar categories describe the mechanisms that actuate the valves: pushrodoperated overhead valves, single overhead cam or dual overhead cams. Engine designers usually start with one of these single-cylinder archetypes and, through the wonders of parts multiplication, spin off multi-cylinder derivatives.

Dividing a given displacement into more cylinders pays off in many ways. First, everything else being equal, the more cylinders an engine of a certain displacement has, the more power it can make. That’s largely due to the higher operating rpm and increased valve area a Multi allows. Second, and often more important, multi-cylinder engines can be designed so the shaking forces created by pistons changing speed and direction cancel one another; the forces generated in one direction by one piston are balanced by the forces created in the opposite direction by another piston. So with two cylinders or more, vibration can be drastically reduced from single-cylinder levels. Finally, another type of smoothness can be achieved through cylinder multiplication: smoothness of power flow. A four-stroke Single, for example, generates power in a brief burst as the piston descends on the power stroke, then depends on substantial crankshaft flywheel-effect to average this burst over the remainder of the 720degree operating cycle. Gearbox loads with a Single are high, and the large flywheel mass robs engine power during acceleration. With a Multi, less flywheel is required because a greater number of smaller power bursts are combined during each cycle, reducing peak loads in the process.

Of course, the word “Multi” itself doesn’t fully describe an engine that has two or more cylinders. There are V and flat and inline configurations, from two cylinders on up, some with their crankshafts running side-toside, others with them placed foreand-aft. Each has its advantages and disadvantages, its own compelling reasons for use on a certain type of motorcycle. So, to make those reasons more apparent to you, we present the following pictorial encyclopedia of common motorcycle engine types (and a few that are not so common), each with a brief description of its particular appeal. 0