Inventions can change the direction of the world. Among the inventions that helped to shape the movement of people and goods and increase commerce is the compression engine. Rudolf Christian Karl Diesel was born in 1858 and is credited with the invention of the first ‘Diesel engine in the 1890’s. Although the first engines were large and the efficiency was low, they were the first to successfully utilize compression ignition making them ideal many industrial and transport applications.
What is a Diesel Engine?
The adoption of four-stroke theory in internal combustion led to the development of different engines using the same principles but utilizing different processes. As such, the diesel engine also known as the combustion ignition engine is an internal combustion engine that utilizes air that has been compressed to a high temperature to ignite the fuel that is injected into the cylinder at high pressure. Compared to gasoline engines where a mixture of fuel and air is compressed in the cylinder and a spark used to ignite the mixture, diesel engines are more fuel efficient than a gasoline powered engine. Consequently, any engine that uses the principles of combustion ignition is known as a diesel engine.
How Diesel Engines Work
Although diesel engines are similar in operation to a gasoline-powered engine in that they follow the four cycle’s operation pattern, they follow different thermodynamic cycles. In diesel engines, their operation follows the diesel cycle which is a thermodynamic cycle that estimates the pressure and volume of the combustion chamber in diesel engine and assumes a constant pressure during the heat addition or initial combustion phase of the engine.
First (Induction or Intake) stroke
During the initial phase of combustion or first stroke in a diesel engine, various processes take place. The downward movement of the piston creates a partial vacuum in the cylinder. As a result, the pressure difference created by the downward movement of the piston pulls air into the cylinder (Reif, 17). As such, the filling of air in the cylinder due to the downward movement of the piston completes this stroke, and the intake valve closes.
Second (Combustion) Stroke
During this phase, the piston moves up and in the process compresses the air in the cylinder which makes its temperature to rise. As it rises, a measured amount of fuel is injected into the cylinder in spray form, and combustion occurs due to the high temperature of the air thus generating heat and forcing the piston downwards (Reif, 17). In diesel engines, the lack of fuel in the cylinder at the start of the second stroke means that there is no auto-ignition. During this stroke, different compression ratios are used ranging from 12:1 to 22:1 (Wright, 224). As such, the compression ratios are higher than in gasoline engines.
Third (Power) Stroke
When the pressurized fuel molecules are injected into the cylinder and come into to contact with the heated pressurized air, they ignite and vaporize resulting in combustion and generate energy in the process. The generated energy forces the piston downwards from the Top Dead Centre [TDC] position towards the Bottom Dead Centre [BDC]. In addition, during this stroke, both the inlet and outlet valves remain in the closed position (Reif, 17). Consequently, the positive energy generated by the downward thrust of the piston is captured through an array of mechanical parts and channeled to where it’s needed to drive something.
Fourth (Exhaust) Stroke
As a result of the fuel combustion process in the cylinder, by-products in the form of heated and pressurized gases are generated. Among the many byproducts of the combustion, are gaseous residues of unburnt fuel, carbon monoxide [CO], carbon dioxide [C2O], various oxides of nitrogen and sulfur, and hydrogen chloride [HC]. However, as the piston begins to ascend toward the TDC, the exhaust valve opens while the intake valve remains closed and the exhaust fumes are expelled from the cylinder (Reif, 17). Once the exhaust fumes have been removed, the exhaust valve closes which will allow the intake valve to open at the beginning of a new cycle of strokes.
Parts of a Diesel Engine
Engines are made up of many parts that are housed in the engine block. Each of the parts in a diesel engine serve a specific purpose and are necessary to ensure the engine performs well. As such, the basic components of a diesel engine include; cylinder block, piston, piston rings, piston pin, connecting rod, crankshaft, cylinder head, Intake valve, exhaust valve, fuel injector, injector pump among others.
The cylinder block or engine block as it is commonly referred is the structure that holds the cylinders and some or all the other associated structures like galleries for coolant, crankshaft housing, exhaust passages, and cylinder sleeves among others (Duffy, 661). It is the base from which the entire engine is constructed.
As the name suggests, the cylinder head is the end cover of the cylinders and forms the combustion chamber (Duffy, 661). It allows for the function of intake and exhaust of gases from the cylinders while also allowing coolant to enter the engine.
Piston, Piston Rings, and Piston Pin
The piston is a cylindrical part that fits tightly in the cylinder and moves up and down the cylinder. It is the part that is responsible for the compression of air in the cylinder and its actions aid in intake and exhaust of gases from the cylinder. To ensure there are no leakages of gases in the combustion chamber, pistons are fitted with split rings known as piston rings which go into specially made groves on the outer part of the piston (Duffy, 661). While the piston pin attaches the piston to the connecting rod.
Inlet and Exhaust Valves
These valves operate based on pressure to move and either allow air into the combustion chamber or expel gases from the chamber (Duffy, 661). Their proper function ensures that the combustion chamber operates at the optimal pressure ensuring consistent power output.
The connecting rod connects the piston to the crank shaft forming a mechanism that converts the up and down motion of the piston into the rotating kind of the crankshaft (Duffy, 661). As such, it acts as a motion transmitting engine part.
The crank shaft is the part of the engine that is responsible for converting the reciprocating motion of the pistons into rotational motion with the aid of the connecting rod (Duffy, 661). As such, the engines output power is converted to rotary motion that can be used in different applications.
Duffy, James E. Auto Body Repair Technology. Cengage Learning, 2016.
Reif, Konrad. Diesel Engine Management: Systems and Components. Springer Fachmedien Wiesbaden, 2014.
Wright, Gus. Fundamentals of Medium/heavy Duty Diesel Engines. Jones & Bartlett Learning, 2017.
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