Automotive systems today are a vital part of life all over the world, either by helping to produce, harvest and move food to distribution centers, by moving workers into the economic machine, or simply improving the quality of life by extending the range of movement of populations. To better understand the automotive system in special the common automobile, we can better understand physics, mechanics, chemistry, and how they apply in our lives.
Considering a car as a complete system
The Car. An entire system in its own right. The many uses of the automobile have given rise to many forms. The many makers of cars have each added their own style to these forms. Even car owners have done much to make even more variations. Form follows function, and the functions required of a car determine the design parameters and constraints on the car as a whole and every single part of its construction. Although many different cars have been created to do many different things, and some cars have been created to do many things themselves, across the wide diversity of car uses, shapes, and sizes, most of them have evolved into having very similar systems making up their construction. Imagine a Formula 1 race car parked next to a new Sport Utility Vehicle. The differences are immediately and strikingly obvious. Under the skin, deep within the arrangement of interconnected parts, the 2 vehicles still have quite a lot in common. Even specific systems such as the suspension, are dramatically different in appearance and construction, yet each performs the same functions on both vehicles. Both cars use a reciprocating combustion engine. They both feature hydraulically operated braking systems.
This book serves to explain the most common systems, and hopefully explore some of the rare and even odd systems that have been used, as well as diving into the new systems that are now arriving and are on the way. This book's objective is to present the forces and considerations at play behind the design and functionality of the parts of the system, and the interplay of each system with the others. Often it will be seen that a part of one system will have an equal role in yet another.
Introduction to the Engine
The engine is the most important part of any vehicle.
The modern automotive engine is quite a system in itself. Rather complicated in its entirety, it can also be broken down into a set of subsystems.
Before any discussion of the engine subsystems can begin, an understanding of the engine as a whole must be made. In our conventional sense, an automotive engine converts the chemical energy in gasoline into mechanical energy of moving a vehicle down the street. Gasoline is burned in the engine. In a process known as combustion, the atoms of the gasoline molecule are combined with atoms of the air molecule, and the result is new compounds and extra energy. The extra energy is used to propel the car.
This is an important consideration, because so many of the involved factors are based on the chemical reaction of fuel & air.
At this time it is important to point out the difference between a motor and an engine.
A motor uses energy. An engine converts energy. As a prime example of the difference, let us consider steam. A steam locomotive would be a steam engine. The locomotive burns coal or wood, and thereby converts the chemical energy of the fuel into heat. The heat turns water into steam, the pressure of the steam turns the drive wheels. However, a steam turbine would be a steam motor. The steam pressure is created in an external process. High pressure steam flows into the turbine, creating the mechanical energy.
This brings us to internal combustion engine. In the case of the steam locomotive, the combustion takes place in a burner and the heat from the burner is applied to a boiler. Steam exits the boiler and enters the mechanisms to turn the wheels, be it a turbine or reciprocating assembly. This could be called an external combustion engine, because the reaction of fuel and air takes place in the burner, and the conversion to mechanical energy takes place in the drive mechanism. In the internal combustion engine, the pressure from the combustion itself operates the mechanical parts that create motion.
Among internal combustion engines, there are several varieties. Different types of fuel have been successfully used. Most modern cars burn either diesel fuel or gasoline. There are also different methods to create motion from the combustion process. Gas turbines and rotary engines have been used in automobiles, along with the prevalent reciprocating engine. The reciprocating engine currently exists in two forms; 2-stroke (or 2-cycle) and 4-stroke (or 4-cycle). These names refer to the length of the combustion cycle within the combustion chamber. In a 2-stroke engine, the piston will move down (1) during the power/intake stroke, then up (2) during the exhaust/compression stroke. This 2-stroke cycle repeats while the engine is running.
In the 4-stroke engine, the piston moves down (1) for an intake stroke, then up (2) for a compression stroke. The piston then moves down again (3) forced by the power of combustion, during the power stroke. Finally, the piston moves up (4) in the exhaust stroke. At this point the 4 stroke cycle repeats while the engine is running.
Some history may be useful here in getting us to a useful understanding of the ICE as we know it today, however. The first known "atmospheric engine" (this term will be explained shortly) was created by Christian Huygen in the 1670s for King Louis XIV. The purpose of this "external combustion engine" was to carry water up a certain altitude from the Seine River to the Palace of Versailles's central pond, in Versailles, France. Although it never actually performed work, this prototype is crucial to ICE development.
The two terms which define his invention are "external combustion" and "atmospheric;" external combustion means that the fuel-energy conversion was occurring outside of the work-producing chamber and atmospheric means that the piston in this engine was exposed to atmospheric pressure. To put these in context, imagine a massive cylinder with a vertical piston and 3 main ports; 2 of these ports are horizontally extending through the chamber wall, separated by some vertical distance, with the third port being at the chamber bottom. The piston itself is attached at the open chamber top by a pulley to some arbitrarily-set device, with the back face of the piston exposed to atmospheric air pressure.
In this system, a body of water was boiled outside the main chamber (external combustion) and the steam was carried into the main chamber via the lower horizontal port, which would build cylinder pressure and force the piston vertically up until the higher horizontal port was reached. The steam pressure would then dump out into open atmosphere, the atmospheric pressure at the piston-back would force it down, excess water from cooling and condensation would exit out the bottom vertical port, and the pulley-attached device would have some usable working stroke (such as a water carrier from the river Seine up to the Palace of Versailles).
In the next ~90 years, several small but important events would occur. Huygen's associate Dennis Papin actually makes the atmospheric engine work. The Englishman Thomas Savory patented the use of atmospheric-style engines for removing water from coal mines. Thomas Newcomen developed a valved system around 1712 which improves system efficiency. Eventually, that valving system is automated. In the 1760s, however, a very important turning point takes place- James Watt made the steam engine a closed system by installing a condenser core to the steam in/out ports, thus recirculating the same water and cutting fuel usage by 75%.
The first real automobile was a 3-wheeled, steam-engine propelled carriage built by Nicolas Cugnot of France in the late 1760s. Self-propelled vehicles like these would be virtually non-existent for the next century.
Operation of the four cycle engine
As the four stroke engine is most commonly employed in modern automobiles, most information here will be derived from and apply to it. Two stroke engines operate under different principles.
The four strokes that make up one cycle are: (1) Induction,air/fuel mixture enters the cylinder (2) Compression,mixture is compressed (3) Ignition,the mixture is ignited (4) Exhaust,burnt gases are expelled
The drive train transfers power from the engine to the ground.
As important as starting, even more so at times; is stopping. All modern cars use a hydraulic brake system. Through the brake pedal, the operator pressurizes the brake fluid in the master cylinder. The pressurized fluid acts on pistons. These pistons apply pressure to a shoe or pad. The friction surface of a shoe acts on the friction surface of a drum. The friction surface of a pad acts on the friction surface of a disc. The friction converts kinetic energy of the moving parts into heat.
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