From: henry@ginger.sri.com (Henry "Credible" Pasternack)
Newsgroups: rec.motorcycles
Subject: How a motorcycle works.
Date: 29 Nov 89 18:49:01 GMT

Chapter 1: The Engine.

The engine has many components, each of which is required for the proper functioning of the whole. When the crankshaft spins, the offset lobes carrying the journals set up harmonic vibrations in the cases and cylinders. These vibrations cause air to be sucked out of the combustions chambers, past the rings which act like one-way valves in much the same manner as the reed valve in a diaphragm pump. The resulting vacuum causes the intake and exhaust valves to open (due to the pressure differential) allowing mixture to be drawn into the cylinder on the carb side, and spent combustion fumes on the exhaust side. The momentum of the inrushing gas pushes the pistons down, in what is known as the "intake stroke". The connecting rods prevent the pistons from falling out of the cylinders and into the cases, or from hitting the combustion chamber sides.

After the chamber is full, the pressure differential holding open the valves is relieved, and the valves shut. The mixture begins to cool slightly, producing a reverse pressure gradient, owing to the large amount of air pumped into the crankcase. Along with the vibratory energy supplied by the crankshaft, the pistons begin to move back towards the tops of the cylinders in what is known as the "compression stroke". As the density of the cylinder gases increases, the air and gasoline molecules are forced together more and more. Because they are already quite hot from spent gases which were taken in during the intake cycle, their kinetic energy is high, and many collisions occur. At a critical point, the air and gas molecules suddenly combine violently. The "combustion" causes the mixture to implode, and cool rapidly as the kinetic and thermal energy is absorbed. This makes the gas pressure drop, and the pistons are sucked strongly towards the top of he cylinders. The resulting unbalanced acceleration produces a driving force which is resonant with the vibration of the crankshaft, reinforcing its motion.

Because of the momentum imparted to the pistons during the implosion of the fuel/air mixture, they are now moving quite rapidly. As they are constrained by the connecting rods, they rebound at the top of the compression stroke, before hitting the cylinder head. The wrist pins and rod bearings are made of very hard metal, so the collision is very nearly elastic, and no energy is lost. Consequently, the pistons rebound, and begin traveling downward with an equal, but opposite velocity. The cylinder volumes thus begin to increase, and the gas pressure drops even lower than it was after implosion. This is called the "power stroke" because the motion of the pistons at this point is more powerful than it is at any other time in the cycle.

This extreme cylinder vacuum causes the intake valves to open, because the intake ports are maintained at atmospheric pressure. The exhaust valves, however, stay closed because of a vacuum which has developed in the headers. The cause of this vacuum is several fold: First, a rarefaction was caused during the intake stroke when hot exhaust gases were sucked into the cylinders. Second, the heat sinking effect of the long mufflers (whose job it is to cool the exhaust in order to most efficiently extract spent implosion products) has caused the exhaust to cool during the intake/compression cycle. Third, the acoustical resonance of the pipes reinforces the vacuum pulses at certain frequencies. This is why bikes have "powerbands".

When the intake valves open, fresh mixture rushes into the cylinders, immediately neutralizing the vacuum due to implosion. This allows the pistons to move full speed to the bottoms of the cylinders where they again rebound off the connecting rods and begin traveling upwards. The gas is squeezed and pressure rises, creating a superheated condition in the spent gases. The pressure is relieved by the piston rings and vented into the crankcase, where the gases are safely relieved to the exhaust pipe by the crankcase breather and exhaust gas recirculation system. This is called the "exhaust stroke".

Most engines are of the type known as "Diesel", named after Dr. Mercedes Diesel, who invented the internal combustion engine in Germany at the turn of the century. Diesels have the advantage of being able to run on relatively crude fuel. They last a long time because they are simple. There is another kind of engine, called the "Otto" engine, which burns gasoline. The "Otto" engine has several extra parts, namely a camshaft, ignition system, and spark plugs. The camshaft has eccentric lobes which are acted upon by the movement of the valves, causing the shaft to spin. The spinning shaft is used for timing the ignition system which periodically sends a burst of high voltage to the spark plugs located in the top of each cylinder. The spark comes at the end of the exhaust stroke, adding additional heat to the last portions of the spent implosion gases. This improves exhaust scavenging, greatly increasing the power and efficiency of the engine.

Next chapter: The Transmission.

Chapter 2: The Transmission.

In Chapter 1, we described the complex machinery making up the motorcycle engine. In this chapter, we discuss a mechanism for controlling the engine output. This device is known as the "transmission."

Because the motorcycle engine is a resonant system, the amplitude of its oscillations is proportional to the amount of energy stored in its moving parts. This value is equal to the time-integral of the energy produced in the engine minus that dissipated, i.e., the cumulative storage. If the stored energy should become too great, the magnitude of the crankshaft oscillations will grow without bounds, and the engine will be destroyed. Thus, some means must be provided to dissipate excess kinetic energy and keep the engine oscillations within normal operating limits. This energy-dampening function is provided by the transmission.

The transmission is constructed in two sections, the input section, which is connected to the engine crankshaft, and the output section, which is connected to the rear wheel. The rear wheel connection is a relative innovation, having first appeared on the race circuit during the mid-seventies. It improves the efficiency of the engine by providing road speed and engine load feedback to the power dampener system. The connection is typically made via a chain and sprocket arrangement. This layout has the advantage of being inexpensive, but is vulnerable to dirt, requires constant lubrication, suffers from backlash and tensioning problems, and requires periodic replacement. A superior method is to couple the rear wheel to the transmission via an enclosed drive shaft. Rear-wheel "shaft drive" is primarily found on expensive European touring bikes, but has lost favor in racing applications, where longevity is not a concern, because of its higher weight.

The transmission operates by dissipating the excess energy created by the engine. The rate of energy dissipation varies with the crankshaft oscillation speed, the road speed, and the transmission dissipation factor. The vast majority of transmissions are of the manual selection type, and contain four to six vaned dissipation impellors. A ratio selection lever, called the "gear shift" is used to slide impellors from the input shaft to the output shaft and back.

(The term "gear shift" is really an anachronism, left over from a time when low-cost farm tractor motors were directly coupled to the drive wheels using variable gear sets. Because the tractor loads were so high, and the engines were so weak, power dissipation was unnecessary. However, it was necessary for the farmer to physically exchange gear sets to match the tractor speed to varying terrain or applications. This was accomplished by stopping the tractor and unscrewing one gear set in order to replace it with another, known as "shifting gears." In recent times, only one manufacturer has attempted to build a passenger vehicle with a true gear shift transmission. The Audi 5000 passenger car, with its so-called "automatic" gear shift, contained a frighteningly complex mechanism for shifting gearsets without user intervention. Unfortunately, without a dampening transmission, the Audi power delivery was unpredictable, resulting in unintended accelerations. After several accidents occurred, Audi was forced to retrofit the 5000 with a standard impellor-type dampener.)

As the motorcycle moves, the rear wheel coupling causes the transmission output shaft, and the impellors attached to it, to spin. The impellors are bathed in transmission oil, which fills the inside of the transmission case. The spinning of the impellors causes the fluid to spin as well, so that as the bike speeds up, the fluid spin increases in proportion. At the same time, the engine oscillations cause the transmission input shaft, and its impellors, to spin. As the speed of the input shaft exceeds that of the output shaft, the input impellors experience drag in the dissipation fluid, resulting in the production of heat. The rate of heat production is equal to the rate of engine energy dissipation. So much energy is dissipated that the transmission and engine cases become quite warm. This heat loss is the major source of inefficiency in modern motorcycle powerplants.

The most sophisticated motorcycles have their transmission and engine components in a shared case, with a single oil bath performing the lubrication and power dissipation functions. A portion of the heat developed by the transmission is absorbed by the super-cold fuel-air implosion products, resulting in much higher specific power output. This heat supplements the energy supplied by the Coulomb environmental thermal extraction unit ("cooling system") described elsewhere in this journal. Earlier designs have the transmission in a separate case. Because the thermal conductivity between the cases is so poor, transmission temperatures are much higher in such setups. Thus, these motorcycles require separate, higher viscosity oil in the transmission.

The transmission dissipation factor is controlled by the gear shift lever. In lower "gears", all of the dissipation impellors are slid onto the output shaft. This causes the transmission oil to spin most energetically. With only the drag due to the rotation of the input shaft itself, the crankshaft revs freely to very high energy levels. If the driver does not shift quickly to a higher "gear", the engine will be damaged. Shifting "up", impellor disks are slid in succession from the output shaft to the input shaft. Thus, motion of the output shaft results in less transmission oil spin. Simultaneously, the greater number of input impellors causes greater oil shear, increasing the drag, and the rate of engine power dissipation. This is why motorcycles accelerate most strongly in lower gears, where transmission dissipation is least.

As the motorcycle comes up to speed, a point is reached where the engine power production very nearly matches that required to overcome aerodynamic, tire, and other external sources of drag. At this point, the transmission input and output shafts move at approximately the same speed. The power dissipation is quite low, because little oil shear takes place between the input and output impellors. The remaining friction is between the moving oil and the transmission cases themselves. Modern design, has reduced this loss to less then a few percent of total engine power production.

Between the crankshaft and the transmission input shaft is a mechanical coupling called the "clutch". It is called this because it consists of a set of expanding fingers which grip the input shaft in much the same way as a bird clutches a branch. The user may decouple the clutch by actuating the clutch lever, causing the fingers to open slightly so that the shafts may spin independently. The clutch serves two purposes. First, it unloads the transmission during shifts so that the disks may be slid without damage. Second, it allows the driver to temporarily disconnect the engine from the transmission. In this condition, the engine revs increase without limit, maximizing available power. This is useful when maximum acceleration is required, or when starting out from a stop. Care must be taken that the clutch must is not held in so long that the crankshaft rev limit is exceeded. In the next chapter, we will describe the means by which engine power is coupled to the front and rear wheels, and the method for varying power delivery.

Chapter 3: Power Conversion Components

The conversion of engine oscillatory energy to driving force at the wheels is accomplished by a three-stage electro-hydro-mechanical system of great sophistication and complexity. This process is carried out by the frame and brakes, as shall be described in this chapter. As engine power outputs have increased, these two components have evolved considerably to accommodate a new generation of motorcycle powerplants. We shall examine the configuration of a typical modern power conversion system.

As the crankshaft spins, harmonic vibrations are created in the engine cases by the eccentric motion of its counterweights. These vibrations are a direct manifestation of the energy stored in the motion of the engine parts. Part of the energy, as we have seen, is dissipated by the transmission. The remainder is delivered to the brakes by the frame where it is converted into electromotive force and used to apply thrust to the wheels.

Energy transmission is the primary, but not the only function of the frame. In addition to coupling the engine oscillations to the brakes, the frame forms an important part of the motorcycle chassis, bearing the loads of the suspension components and the rider. Thus, frame design is a tradeoff between power, handling, and comfort. We will consider this compromise momentarily.

Early motorcycle engines had only one cylinder, thus creating the maximum amount of crankshaft imbalance, and vibratory output, per unit displacement. In order to provide more power, it was necessary to increase the amplitude of the engine vibrations to the point where crankshaft, crankcase, and frame longevity was seriously compromised. Rider comfort was similarly affected, and sales in motorcycles plummeted. It was out of this crisis that the first twins, and soon thereafter, multi-cylinder engines, were developed. By including two or more cylinders, peak vibratory amplitudes were lowered. The power output was maintained because the number of oscillation peaks per unit time rose in inverse proportion to the amplitude change. With numerous small crankshaft counterweights, structural loads were diminished, and engines could spin to higher redlines, increasing the amount of fuel imploded, and the specific power output. Rider complaints of buzziness and discomfort quickly vanished, and interest in motorcycling soared to new heights. Engine specific power output soared as well, quickly surpassing the once unthinkable 100 HP per liter mark.

The power pulses of the early big singles came at extremely long intervals, allowing each implosion to be felt by the rider (and leading to their being labeled "humpers" in reference to the lumbering rhythm of their pistons). Consequently, the power generation and transmission occurred at a very low frequency, explaining why early frames are so flexible and wobbly. Modern engines produce much higher frequency output pulses that are largely damped by frames designed to resonate at the firing rate of a big single. This is why contemporary design has emphasized extreme frame stiffness, driving resonant frequencies to much higher limits and significantly cutting absorptive losses.

Frame stiffness has an adverse effect on handling and comfort because small road surface perturbations are transmitted directly to the rider. A stiff frame has a deliberately low energy absorption factor, causing extreme stress concentration at the various frame member joints. This is why early attempts at the construction of aluminum box-beam frames for road racing purposes were ultimately unsuccessful. As increasingly sophisticated suspension springs and dampeners were devised, the role of shock absorption in the frame was diminished. At the same time, new metallurgy and fabrication teqhniques have solved the longevity problem. So good are modern frame and suspension components that frame stiffness is no longer a significant design liability.

The next phase of power conversion is the hydraulic stage. Bolted securely to the frame are brake master cylinders which convert frame oscillatory forces into hydraulic compression waves. These pressure waves travel down hydraulic lines to the brake calipers where they cause the brake pads to spin with great vigor. Here the conversion to electromotive force takes place. The pads are in close proximity to the aluminum or steel rotors, which in turn are bolted firmly to the wheels. The pads are impregnated with a permanent magnetic material, and as they spin, eddy currents are induced in the disk rotors. The resulting counter-magnetic field creates a continuous propulsive force, in much the same fashion as an electric motor, save that the magnetic field is self-generated without the aid of stator or armature windings.

Early motorcycles used drum brakes and cable actuated brake "shoes". This configuration was only marginally effective for low-frequency application, and completely unuseable with modern high-frequency engines. Poor brakes largely explain the low power delivery and reliability problems that plagued the British singles and early twins.

Power modulation is accomplished by varying the spacing between the brake pads and rotors. This is controlled by a secondary hydraulic system connected to a hand lever for the front brake, and a foot lever at the rear. As the levers are squeezed (or depressed), the pads move closer to the rotors, and the power delivery increases, causing the bike to accelerate. This has the added advantage of providing a safety feature, for if the rider is thrown from the bike, the levers are released, and the brake pads retract to the idle power position. Pads gradually lose their magnetic field and have to be replaced periodically. With worn pads, the rider must squeeze the brake lever much harder to achieve a given power level, and maximum acceleration is reduced. As brake technology has improved, organic pads have given way to semi-metallic pads, which offer a much better combination of magnetic field strength, longevity, and electrical conductivity.

The primary hydraulic fluid must be changed periodically, as it absorbs moisture from the air over time. If the water content of the hydraulic circuit becomes too high, the fluid may boil, absorbing excess thermal energy directly from the engine. In this situation, the brake pads will overspin, causing the motorcycle to zoom wildly out of control. Should this happen, the only remedy is to hit the brake cut-off switch, which causes the pads to be ejected from the calipers, terminating power delivery.

The motorcycle must also have a mechnism for slowing down. This is accomplished at the rear wheel by fitting deceleration baffles into the transmission case inner walls. Actuated by a foot control, the deceleration baffles extend into the spinning transmission fluid, causing a sudden, significant increase in viscous drag. Braking is most effective in lower gears, when most impellors are engaged on the output shaft. This is why it is helpful to shift down when slowing the motorcycle.

The last miscellaneous control component is the throttle. This device is operated by a twistgrip control, and is used to vary the engine mixture to compensate for temperature, humidity, and altitude. It is possible to continuously tune the engine fuel-air mixture between the limits of optimum power delivery and optimum economy. Older motorcycles require the operator to develop a sense of the correct setting, while newer bikes incorporate a "fuel guage" which continuously indicates the fuel-air ratio and is labeled for optimum settings. It is also possible to use the throttle for power modulation, as was attempted in certain experimental motorcycles. The most notable of these was the Cagiva Paso 750, whose dual-downdraft "Weber" was the fruit of a concerted attempt to develop a continuously-variable-power induction system. The "Weber" project was a dismal failure, as it became obvious that it is impossible to provide correct mixture control over such a wide range of engine operating conditions. For the time being, therefore, the electro-hydro-mechanical conversion system is here to stay.