CHAPTER 1. INTRODUCTION
Before the steam engine was invented, all of the physically demanding jobs like construction, agriculture, shipping, and even traveling, were done by strong animals or human beings themselves. The invention of the steam engine prompted the Industrial Revolution, at which time human beings started using automated machines to reduce human work load and increase job efficiency. In 1705 Thomas Newcomen invented the first version of the steam engine, which is also called atmospheric engine. The Fig in right hand side (Fig. 1) shows Newcomen steam engine. From this design, water (blue) is boiled and vaporized into steam (pink), which pushes the closed right valve (red) open (green). The steam pushes the piston to move up, which causes the pressure inside the cylinder to decrease. Gravity will push the water from the upper tank to open the left valve, and splash the water into the cylinder to cool steam. The steam inside the cylinder therefore is condensed, which turns the cylinder vacuum and sucks back the piston. The descending piston shuts two valves and finishes one cycle. The Newcomen Steam Engine was only used to pump water out of mines at that time. In 1769, James Watt improved the function of the steam engine and made it practical in the real world, which is why most people still think Watt invented the steam engine. James Watt’s steam engine is designed so that water goes into a high temperature boiler, is boiled and vaporized, and turns into high pressure steam. This steam pushes the piston, generating a forward and backward motion. Because the combustion room is located outside the engine, the steam engine is also called the external combustion engine.
According to the physics rule of motion, when an object is in static status it needs a larger force to overcome friction. When the object starts moving, the needed driving force becomes smaller and smaller, and the speed becomes faster and faster. Therefore, to move the piston in a steam engine from static position, very high pressure must be generated to push the piston. When the piston starts moving, the pressure decreases, because it is released from the exhaust by the movement of the piston, before it can be compressed into high pressure air. At low speed, the engine creates high pressure steam to push the piston, while at high speed, the steam pressure becomes low. That’s why the old steam powered locomotives start very slowly, but still can reach a very high speed. The steam engine is very efficient at generating power based on the physics rule of motion; however, it takes awhile before the machine can reach its highest efficiency. Another drawback is that the steam engine occupies too much space. Therefore, scientists tried to develop an engine with smaller size, but that can instantly generate the power needed. The internal combustion engine, which has been used for most machinery including vehicles, was invented. Several kinds of internal combustion engines have been widely used for vehicles, for example, in the two-stroke combustion cycle, four-stroke combustion cycle, and rotary engines. The first engine to use a four-stroke combustion cycle successfully was built in 1867 by N. A. Otto. The design of the internal combustion engine is much more complicated than the steam engine, however. All internal combustion engines need to go through the following procedures to finish the combustion cycle: intake, compression, combustion, and exhaust. First, the piston moves downward and at the same time gasoline is injected into the cylinder through inlet valve. Second, the piston moves upward and compresses the air. Third, the compressed air is fired and moves the piston downward again. Finally, the fired air is exhausted through exhaust valve and moves the piston upward again. While fired once every two cycles for a four-stroke cycle internal combustion engine, a two-stroke combustion cycle internal engine is fired once per cycle. The internal combustion design can instantly convert the power generated by the explosion of burning fuel into high pressure air to push the piston. Unlike the steam engine, for an internal combustion engine to move the piston faster and faster, more and more fuel is needed to generate higher pressure. In other words, for an internal combustion engine, high pressure is needed to keep the piston running at a high speed, while at low speed, only low pressure is necessary. This is just opposite to the function of the steam engine.
Even though it solves the dimension and slow start issues of the steam engine, the internal combustion engine generates another serious problem. When the piston is running at high speed, the pressure needed is also high, which violates the physics rule of motion. Running an engine at high speed with high pressure is not efficient, and also decreases the engine life. To solve this problem, the transmission system was invented.
To transfer engine power efficiently, the gear ratio between the engine and wheels plays a very important role. When we use a screwdriver, the portion we hold has a larger diameter, while the portion contacting with the screw has smaller diameter. This design makes users use less force to unscrew a screw while applying force on a larger diameter portion of the screw driver. Therefore, attaching a smaller gear to the engine side and connecting it to a larger gear to deliver power to wheels helps overcome friction when moving a static vehicle.

The figure 2 shows that the large gear of the wheels needs less force to drive it. However, it also shows that when the engine gear turns one circle, the wheel gear only turns about one half. The car won’t run as fast as possible.



Consider the following situation from Figure 3: the wheel gear has a smaller size, which needs more force to move it while the car is static.
It won’t even be possible to move the car if the engine power is not large enough. However, when the engine gear turns 1 cycle, the wheel gear may turn 2, which makes the car run faster.

Based on the physics rule of motion, after the object starts moving, the driving force needed becomes smaller. Therefore, if the car can run on the large gear condition (Figure 2) when starting, but change to a small gear (Figure 3) when moving, that is, applying a large force when starting, but a small force when moving, this will makes the power transmission much more efficient.

Kinds of Transmission Systems Used For the Automobile:
The most common transmission systems that have been used for the automotive industry are:
• Manual transmission,
• Automatic transmission,
• Semi-automatic transmission,
• Continuously-variable transmission (C.V.T.).
Manual Transmission:
The first transmission invented was the manual transmission system. The driver needs to disengage the clutch to disconnect the power from the engine first, select the target gear, and engage the clutch again to perform the gear change. This will challenge a new driver. It always takes time for a new driver to get used to this skill.
Automatic Transmission:
An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid engaging/disengaging clutch during gear change. A completed gear set, called planetary gears, is used to perform gear ratio change instead of selecting gear manually. A driver no longer needs to worry about gear selection during driving. It makes driving a car much easier, especially for a disabled or new driver. However, the indirect gear contact of the torque converter causes power loss during power transmission, and the complicated planetary gear structure makes the transmission heavy and easily broken.

Semi-Automatic Transmission:
A semi-automatic transmission tries to combine the advantages of the manual and automatic transmission systems, but avoid their disadvantages. However, the complicated design of the semi-automatic transmission is still under development, and the price is not cheap. It is only used for some luxury or sports cars currently.
Continuously Variable Transmission (C.V.T.):-
The Continuously Variable Transmission (C.V.T.) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios. The other mechanical transmissions described above only allow a few different gear ratios to be selected, but this type of transmission essentially has an infinite number of ratios available within a finite range. It provides even better fuel economy if the engine is constantly made run at a single speed. This transmission is capable of a better user experience, without the rise and fall in speed of an engine, and the jerk felt when changing gears.

CHAPTER 2. MANUAL TRANSMISSION SYSTEM
Manual transmissions also referred as stick shift transmission or just ‘stick', 'straight drive', or standard transmission because you need to use the transmission stick every time you change the gears. To perform the gear shift, the transmission system must first be disengaged from the engine. After the target gear is selected, the transmission and engine are engaged with each other again to perform the power transmission. Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission.


Fig: The transmission system delivers the engine power to wheels.

The main components of manual transmission are:
1. Clutch
2. Gear box
3. U- joint
4. Shafts
5. Differential gear box





Clutch:
Clutch is a device which is used in the transmission system of automobile to engage and disengage the engine to the transmission or gear box. It is located between the transmission and the engine. When the clutch is engaged, the power flows from the engine to the rear wheels in a rear-wheel-drive transmission and the vehicle moves. When the clutch is disengaged, the power is not transmitted from the engine to the rear wheels and vehicle stops even if engine is running.
It works on the principle of friction. When two friction surfaces are brought in contact with each other and they are united due to the friction between them. If one is revolved the other will also revolve. The friction depends upon the surface area contact. The friction surfaces are so designed that the driven member initially slips on driving member when initially pressure is applied. As pressure increases the driven member is brought gradually to speed the driving member.
The three main parts of clutch are:
1. Driving member
2. Driven member
3. Operating member
The driving member consists of a flywheel mounted on the engine crank shaft. The flywheel is bolted to cover which carries a pressure plate or driving disc, pressure springs and releasing levers. Thus the entire assembly of flywheel and cover rotates all the times. The clutch housing and the cover provided with openings dissipate the heat generated by friction during the clutch operation.
The driving member consists of a disc or plate called clutch plate. It is free to slide length wise on the splines of the clutch shaft. It carries friction materials on both of its surfaces when it is gripped between the flywheel and the pressure plate; it rotates the clutch shaft through splines.
The operating members consists of a foot pedal, linkage, release or throw-out bearing, release levers and springs necessary to ensure the proper operation of the clutch.
Now the driving member in an automobile is flywheel mounted on crank shaft, the driven member is the pressure plate mounted on transmission or gear box input shaft. Friction surfaces or clutch plates is placed between two members.

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