INTERNAL COMBUSTION ENGINE
Introduction
The invention and eventual development of internal combustion engines marked a significant stage in the evolution of engines since it provided a way for industrialists and engineers to build an engine that would suit various applications. Unlike the bulky and messy steam engines, internal combustion engines opened the possibilities of creating smaller, but more efficient engines that can be mounted on different types of machineries. The most common applications of which are in transport vehicles such as the engines used in automobiles, trucks, vessels and planes. Moreover, internal combustion engines are also used in non-mobile applications such as in the small scale generation of electricity and in industrial utilization. There are three types of internal combustion engines, namely, the spark ignition or gasoline engine; the diesel; and the gas turbine. These engines are collectively known as internal combustion engines, primarily because they convert the chemical energy of fuel to mechanical energy by burning it in the combustion chamber that is an integral part of the machine. This paper, however, will only limit its discussion on diesel and gasoline engines. With almost similar utilities, diesel and gasoline engines share the same basic principles that govern internal combustion engines. But apart from their similarities, they also have significant differences that make each type of engine unique from the other.
The first internal combustion engine was conceptualized by Abbe Hautefeuille and Christian Huygens in the 1680s. Their concept was to use the explosive action of igniting the gun power in order to run an engine that will be used in pumping water. Their crude concept, however, did not make it past experimentation. More than a century later, engineers regained interest in building an internal combustion engine that resulted in several unsuccessful attempts. One of which was that of Robert Street who, in 1794, was the first to conceptualize an engine that uses the action of the piston to create a combustion cycle. His untimely death is believed to have “retarded the development of the internal-combustion engine half a century, as all of the features mentioned are necessary to the highly efficient engines of today”. Another notable attempt was that of the Swiss inventor, Francois Isaac de Rivaz, who used a mixture of hydrogen and oxygen to make an internal combustion engine that he used to power the first internal combustion automobile (Ratiu, 2003, p.146). None of these attempts, however, were fairly successful, except that of the Belgian engineer, Jean Joseph Étienne Lenoir, who invented the first functional internal combustion engine in 1858 (Ratiu, 2003, p. 146). His engine first used coal gas as fuel and, later on, petroleum. The first use of gasoline as fuel has been attributed to Siegfried Marcus, an Austrian engineer. Using gasoline as fuel, the German engineer, Nikolaus August Otto, further improved the gasoline-powered internal combustion engine when he invented the four-stroke engine in 1876 (Ratiu, 2003, p.146). Also known as the Otto cycle, the four-stroke engine became the precursor of all modern gasoline engines. In the same year when the four stroke engine was patented, Dougald Clerk also invented the two stroke engine (Ratiu, 2003, p.147). The development of the diesel engine did not emerge until the German engineer, Rudolf Diesel, came up with a design of a combustion engine that uses compression to ignite a fuel instead of a spark. Diesel worked on his design in the 1890s and was able to produce an engine that operated on different types of fuel at higher fuel efficiency than that of a gasoline engine (Introduction to Diesel Engines, n.d., p.17). The diesel engine has just recently been developed relative to the gasoline engine. However, this engine type gradually dominated heavy duty applications.
Basic Principles of How an Internal Combustion Engine Works
As mentioned briefly in the introduction, the principle behind an internal combustion engine is to convert the potential chemical energy of fuels into mechanical energy in order to produce work (Pulkrabek, n.d., p.1). As its name suggests, the internal combustion engine harnesses the fuel’s chemical energy by burning it inside a combustion chamber. From the basic parts of an internal combustion engine shown below, it can be imagined that the rapid burning of the fuel after it enters the cylinder through the intake valve creates an explosion that forces the piston to run back and forth the chamber or cylinder. The piston in turn, is connected to a piston arm or connecting rod that is in turn attached to a crank shaft so that every time the piston completes one cycle, the crankshaft also completes one revolution.
Figure 1. Basic parts of an internal combustion engine.
An internal combustion engine, whether it runs on gasoline or diesel, can either be a two-stroke or four-stroke engine. The four stroke engine works in four cycles namely the intake stroke; the compression stroke; the power stroke; and the exhaust stroke. The two stroke engine, on the other hand, operates in two cycles only. As illustrated in figure 3, the intake and exhaust openings are sealed by the piston as it compresses the gas on its upward stroke. As the fuel explodes, the piston is pushed into its downward stroke, simultaneously taking in fuel in the cylinder while pushing out burned fuel at the exhaust. Internal combustion engines can also be classified according to the number and orientation of their cylinders. As shown in Figure 4, internal combustion engines can be classified as single cylinder, in-line or straight, V-engine, opposed cylinder, W-engine, opposed piston and radial (Pulkrabek, n.d., p.10).
Figure 2. Four-stroke engine cycle (Introduction to Diesel Engines, n.d., p.12).
Figure 3. Two-stroke engine cycle.
Figure 4. Classification of internal combustion engines according to cylinder orientation (a. single cylinder; b. in-line or straight; c. V engine; d. opposed cylinder; e. W engine; f. opposed piston; and g. radial.) (Pulkrabek, n.d., p.10).
The Gasoline Engine
The gasoline engine is also known as the spark ignition engine because of the method used to ignite the fuel inside the cylinder. The typical gasoline engine uses a carburetor as its fuel delivery system. However, some new vehicles are now using fuel injection systems to regulate and provide a more efficient fuel delivery. The figure below is a typical gasoline engine. In a four-stroke gasoline engine, the cycle starts with the downward movement of the piston, which simultaneously opens the intake valve and draws in the mixture of air and gasoline gas. The next cycle is the compression wherein the piston moves upward. Both valves are closed during the compression cycle; thus the gasoline-air mixture is compressed by the piston head in the combustion chamber. Gasoline gas in the combustion chamber does not readily ignite because it has a lower compression. To ignite the gas, a gasoline engine uses a spark generated by a high-voltage electrical charge from the spark plug. The ignition of the compressed air-fuel mixture creates an explosion that pushes the piston downward. This stroke is called the power cycle and the power generated by this stroke is what drives the operation of the engine. The last stroke is the exhaust stroke. From the bottom of the cylinder, the piston rises back up again. This time, the intake valve is closed while the exhaust valve automatically opens as the piston pushes the burned fuel mixture out of the exhaust pipe. These strokes continue in a rapid cycle for as long as the engine is running.
Figure 5. Basic parts of a gasoline engine.
The Diesel Engine
A diesel engine does not differ much from a gasoline engine. Like a gasoline engine, a diesel engine is also composed of a combustion chamber with a cylinder and tight-fitting piston that is attached to a crankshaft. A diesel engine also has intake and exhaust valves whose opening is controlled by the camshaft. The major difference between a diesel engine and gasoline engine, however, lies in the process of how the fuel is being burned. Unlike the gasoline engine, a diesel engine does not rely on the electrical spark that comes out of the spark plug in order to ignite its diesel fuel. Rather, it relies on the heat generated from compressing the air in the combustion chamber. A diesel engine also follows the Otto cycle, but instead of creating a spark to generate its power cycle, the diesel engine injects fuel into the combustion chamber, which automatically ignites because of the extreme heat that is generated by the pressurized air inside the chamber.
Figure 6. Basic parts of a diesel engine.
Differences between Gasoline and Diesel Engine
Almost every component of a gasoline engine can also be found in a diesel engine, except that they differ in initiating combustion. Gasoline engines rely on spark to initiate combustion while diesel engines compresses air into an auto ignition temperature in order to initiate combustion. For the same reason, the gasoline engine has a carburetor or fuel injection system and a spark plug while the diesel engine only has a fuel injection pump. Since gasoline engines rely on the electrical spark to initiate ignition, it has a more complex electrical system as compared to a diesel engine. Diesel engines, on the other hand, have more complex fuel systems in order to regulate the amount of fuel that it sprays on the combustion chamber. By principle, gasoline engines start easily compared to diesel engines. Diesel engines, however, are designed with a glow plug in order to help the engine start by heating the combustion chamber. It is important that the gasoline engine’s compression is kept below the auto ignition pressure. When the compression is too high, gasoline engines will encounter premature ignition that results to engine knock. For the same reason, gasoline engines have lower compression ratios as compared to diesel engines. Most gasoline engines, for instance, have a compression ratio of 10:1, which means it compresses the air-fuel mixture in the cylinder at one-tenth of its original size (Fernando, n.d., p.27). Diesel engines, on the other hand, must have high compression ratios in order to automatically ignite its fuel. A typical diesel engine, therefore, has a compression ratio that ranges between 12:1 and 24:1 (Fernando, n.d., p.32). Gasoline engines are more fuel sensitive as compared to diesel engines. It also requires precise conditions in order to function efficiently. Diesel engines, on the other hand, are not as fuel sensitive as compared to gasoline engines. However, because of the high compression ratios required to make it work, most diesel engines have larger components than a gasoline engine of equal power. In terms of fuel efficiency, the diesel engine is also more efficient than the gasoline engine. The gasoline engine, for instance, has a thermal efficiency that ranges between 25 to 30 percent, which means that only 25 to 30 percent of the chemical energy found in gasoline is converted to mechanical energy (Fernando, n.d., p.30). Diesel engines, on the other hand, have a higher thermal efficiency, which ranges between 35% to 40% (Fernando, n.d., p.36).
Conclusion
Gasoline and diesel engines are internal combustion engines; that is, they burn fuel in a combustion chamber that is an integral part of the machine. Almost all of the machine components between a gasoline and diesel engine are identical except that they have different ignition principles. The gasoline engine, for instance, ignites its fuel through an electrical spark that is generated by the spark plug while the diesel engine ignites its fuel by raising the compression ratio of the air inside the combustion chamber. For the same reason, the gasoline engine is also referred as a spark ignition engine while the diesel engine is commonly referred as a compression ignition engine. Both engines have their own advantages and drawbacks. Diesel engines, for instance, are more fuel efficient than gasoline engines yet they are also costly to maintain. Gasoline engines, on the other hand, are lighter and are easier to maintain than diesel engines, despite their lower fuel efficiency compared to diesel engines. Diesel engines are also more suitable for heavy duty applications while the gasoline engines are more suitable in applications where speed is preferred.
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