Because the internal-combustion engine has zero torque at zero rpm, it must perform its complete operating cycle continuously to make useful torque. It has to be spinning.
This is a problem because as our journey begins, the vehicle is sitting still. How do we couple an engine, spinning at several hundred or a few thousand rpm, to a load that is sitting still?
This is no problem for an electric motor or a steam engine, both of which can exert torque from zero rpm.
In the earliest days of the motorcycle, this problem was sidestepped by adding an engine to an ordinary bicycle. The engine was coupled directly to the rear wheel by belt and was started by vigorous pedaling by the operator. Being pushed through its operating cycle and provided with fuel and ignition, the engine eventually starts and takes over the driving of the vehicle. When the vehicle stops, so must the engine.
This cumbersome arrangement was tolerated for quite a while after cars had so-called "free-engine clutches," devices in the driveline that allowed engine and rear wheel to be coupled or uncoupled at will.
Meanwhile, Ferdinand Porsche, working for the Lohner Co., had developed his "Mixte" drive for autos in 1900. Each drive wheel had an electric motor built into its hub, while the vehicle's engine drove a dynamo that charged a battery set. (Does this sound familiar?) To drive away, the operator sent battery power to the electric wheel motors, and the machine smoothly accelerated.
Dr. Hermann Foettinger, working at the Vulcan shipyard in 1905, conceived the fluid coupling, in which a pump accelerates fluid (typically oil, but in some cases water) against the vanes of a turbine wheel attached to the load. At low rpm of the driving pump, little torque is generated at the turbine because the slow-moving fluid exerts little pressure against its vanes. This allows the driving engine to idle. To accelerate the load, the operator simply throttles up the engine, increasing the torque transfer until the load (such as a 1948 DeSoto) begins to move.
Of course, we know that the friction clutch became the device that made
both autos and motorcycles into useful transportation rather than mere curiosities. A typical modern motorcycle clutch consists of a stack of thin rings or discs, every other one of which is faced with some kind of friction material. The plain discs have teeth on their IDs that engage a similarly toothed inner hub, which drives the transmission. The friction discs have teeth on their ODs that engage an outer hub, driven by the engine. One or more springs compress this stack of discs, pressing the friction and plain discs together. A means is provided by which to vary the spring pressure between maximum and zero.
With the engine running, we "lift the clutch," which means that we reduce clutch spring pressure to zero so that it cannot transmit torque. Now, we select first gear and begin to allow spring pressure to press the spinning and stationary clutch discs together lightly. This slight torque load causes the engine rpm to drop, so we simultaneously throttle up a bit to compensate. The driven discs of the clutch are now receiving enough torque to begin to turn the load—to drive the rear wheel. Because we are experienced operators, we do this smoothly and continuously such that the machine accelerates.
Skilled motorcyclists like this process because it's familiar. They like the sound of the engine, accelerating in each gear, its rpm dropping slightly at each upshift. When Honda demonstrated to focus groups the hydrostatic drive on one of its ATVs (it was later used as the transmission of the DN-01 motorcycle), they were confused by the steady rpm of the engine and the seamlessly smooth acceleration of the vehicle. This transmission allowed the engine to operate constantly at its rpm of maximum torque so that vehicle acceleration was also maximum. This cannot be achieved with any number of fixed transmission ratios, which force the engine off of its torque peak to regions of lower torque as it accelerates across a range of rpm in each gear.
But people didn't like the sound. It was unfamiliar and, therefore, wrong, even though constant engine operation at peak torque gave greater acceleration on that particular vehicle than multi-speed shifting. Honda engineers therefore offered a "six-speed mode" in which the hydrostatic drive mimicked the less-efficient operation of a fixed-ratio transmission— and its traditional sound.
Motorcyclists have also generally rejected any form of automatic transmission, such as those with a fluid torque converter or infinitely variable belt drive. Sadly for the traditionalists, change is coming. One form of such change is the likely wider adoption of dual-clutch transmissions. Each of a pair of clutches controls alternate ratios, so that power is never interrupted during acceleration. Operation of DCTs is automated (imagine having multiple clutch levers on the left handlebar!), so they are, formally, "automatic."
Some traditionalists will hate this; for them, clutching and shifting are one of the "manly arts" to be forever defended against in any form of automatic shifting transmissions, which they contemptuously dismiss as "slush-boxes." Change isn't easy for any of us, but we don't huddle together preserving manual desk calculators in a world of computers.
In the U.S., diesel locomotive engines drive AC generators, which, in turn, send power to electric traction motors. In Europe, a separate tradition exists in which the diesel drives the wheels through a multi-element hydraulic torque converter. A variety of power transmission schemes are contemplated for future hybrid autos—and perhaps for motorcycles, as well.
My point is that a great variety of couplings between engine and load are possible, and we must keep our minds open to the advantages they may bring: greater performance, efficiency or versatility. The sound of a traditional motorcycle accelerating excites us because we grew up with it—it is our "machine poetry." But after 1944, 450-mph piston-engine aviators had to accept the unfamiliar roar of 600-mph jets, too. □
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