An engine is the component of a machine that is responsible for the powering action to enable the machine do the required work. Thus, the role of an engine is to transform input energy into the required output and at the desired rate. A car engine transforms the chemical energy stored in form fuel to kinetic energy through a number of processes. Different machines or vehicles have different types of engines, for instance, petrol and diesel engines, four and two-stroke engines, and internal and external combustion engines. However, all these types of engines work on the same basic principle of pressure changes. The goal of this paper is to explain how an engine works from the perspective of a four-stroke engine.
Parts of an Engine
It is important to understand the basic parts of an engine in order to be able to picture out how an engine works. These components include the engine block, combustion chamber, cylinder head, piston, camshaft, valves, and sparkplug. The engine block or sometimes referred to as cylinder block forms the foundation on which the engine system is built. The engine cylinders are always grooved on the engine block. The combustion chamber is the region within which air-fuel mixture is ignited and burned to produce energy (Gupta, 2012). The combustion chamber is found within the cylinder just between the cylinder head and the piston. The cylinders act as the walls of this chamber while the piston and cylinder heads serve as the floor and ceiling respectively. Cylinder heads are small round indentation castings on the cylinders that designed to create room for combustion at the top of the chamber. The cylinder head hosts other engine elements such as valves, fuel injectors, and spark plugs.
The piston is a cylindrical piece of metal casting that is located inside the cylinder. The pistons provide a surface over which combustion occurs and transmits the energy generated during combustion to the output shaft through to the crankshaft and the connecting rod (Rajput, 2005). The role of a crankshaft is to convert the linear motion generated during combustion to a rotary motion. This is achieved through the slider-crank mechanism design of the components. The crankshaft is positioned on the crankcase. The camshaft is a special shaft that is designed to control the engine vales. Valves are openings that control the flow of fuel, air and combust gases in and out of the combustion chamber. The fuel injector is responsible for fuel injection, while the spark plug is designed to produce the spark required to start the combustion process. It is, however, important to note that spark plugs are only found in spark ignition engines and not in compression engines.
Figure 1 Parts of an engine
Intake and Compression Strokes
The intake stroke is the process through which air-fuel mixture is introduced into the combustion chamber. The intake process is initiated by a decrease in pressure inside the cylinder. As Bennett (2014) explains, the piston passes through from the top dead center (TDC) to the bottom dead center creating (TDC), a region of very low pressure is created inside the cylinder (combustion chamber). Air-fuel mixture then flows into the chamber due to the pressure difference. It is important to note compression ignition engines are not designed to receive air-fuel mixture from the carburetor but rather air and fuel get mixed inside the combustion chamber (Gicquel, 2012).The inlet valves remain open but the exhaust valves are closed during this stroke. In compression stroke, the piston is moved from the BDC to the TDC. Pressure builds up during this process because the piston narrows down the volume of the combustion chamber and both the exhaust and inlet valves stay closed during this process, trapping air-fuel mixture. The compressed air-fuel mixture known as charge nears ignition as the piston approaches the TDC.
Ignition, Power, and Exhaust Strokes
The ignition stroke is the process by which compressed air-fuel mixture begins to burn. The process is normally initiated by sparks from the sparkplug for spark ignition engines. However, compression ignition engines get ignited from the heat generated during the compression process (Ganesan, 2012). The ignition process has a very little effect on pressure. As the charge burns, it begins to expand forcing the piston to move back to BDC from the TDC. This is the power stroke. Pressure rises slightly during this process. The movement of the piston is then transferred to the crankshaft through the connecting rod. The crankshaft, connecting rod and piston are designed in such a manner that they form a mechanism that transforms the linear motion generated during the power stroke into a rotary motion. The combustion process ceases as the piston approaches the BDC. The exhausted valves open at the end of the power stroke. By design, the exhaust valves begin to open as the as the piston begins to make a return journey to the TDC. According to Mavrigian, (2014), the piston moves toward the TDC creates some sufficient pressure that helps to expel the gases. From exhaust stroke, the intake stroke is again initiated and the process is repeated.
Figure 2. The Four Strokes
The entire working process of an engine is based on pressure changes. The intake stroke is a considered as a low-pressure stroke because it is basically an expansion process that creates more room but without any other material to fill created space, air or fuel jets into the cylinder. The compression and combustion processes are all associated with high pressure. The ignition process does not affect pressure changes while the exhaust stroke is associated with low pressure.
Bennett, S. (2014). Modern Diesel Technology: Diesel Engines. Cengage Learning.
Ganesan, V. (2012). Internal Combustion Engines. McGraw Hill Education (India) Pvt Ltd.
Gicquel, R. (2012). Energy systems: a new approach to engineering thermodynamics. CRC Press.
Gupta, H. N. (2012). Fundamentals of Internal Combustion Engines. PHI Learning Pvt. Ltd.
Mavrigian, M. (2014). Performance Exhaust Systems: How to Design, Fabricate, and Install. Renniks Publications.
Rajput, R. K. (2005). A Text Book of Internal Combustion Engines. Laxmi Publications
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