Physics is a branch of science concerned with different aspects of the physical world such as energy and matter. Physics encompasses wide scientific subjects such as electricity, heat, magnetism, sound, mechanics, light, and radiation among others. Modern technologies, such as digital devices and nano technologies are based on simple physics concepts. As a result, all engineering disciplines require a profound physics background. In the future, I plan to pursue a career in mechanical engineering in the automotive industry. This is a paper on the importance of physics in mechanical engineering.
As a mechanical engineer, one is required to clearly understand a wide range of topics in physics such as mechanics of machines, gas laws, motion, forces, and velocity. A mechanical engineer working in the automotive industry has to fully comprehend and internalize all the processes and stages involved in the operation of an internal combustion engine. Automotive engineers design and manufacture vehicle engines that convert the chemical energy in gasoline fuel to mechanical energy.
Operation of the internal combustion engine and the underlying physics principles
Internal combustion engines are the prime movers in vehicles. The engines operates through four stages: admission of a fuel-air mixture into the engine, compression of the mixture, ignition of the mixture with the subsequent combustion and expansion of the gaseous mixture, and the rotation of the crankshaft by the action of the expanding gases on the crankshaft. These stages are also termed as suction, compression, power, and exhaust stages (Glenn Research Center par.3).
The first operation in an internal combustion engine is the preparation of the air-fuel mixture in an engine part called a carburetor. The mixture must be optimized for complete combustion to avoid fuel wastage and clogging the engine with un-combusted hydrocarbons in the form of soot. To achieve this, the fuel is atomized and vaporized before mixing it homogenously with air. The nature of the mixture depends on the engine loading. There are four categories of engine loads; starting, accelerating, cruising, and idling (Glenn Research Center par.4). According to physics, atomization is done to increase the surface area of combustion to ensure speedy and complete combustion of the fuel. Also, altering the air fuel proportions changes the rate of combustion according to the required engine torque and speed. A high air proportion leads to rapid combustion and hence increased engine speed and reduced torque. Conversely, a low air proportion is characterized by slow combustion and reduced speed and high engine torque.
The prepared engine air-fuel mixture is sucked into the engine cylinder where it’s compressed and ignited by a spark plug in gasoline engines or by high pressures in diesel engines. During suction, the piston in the engine cylinder moves downwards thereby creating a partial vacuum which forces open an inlet valve and the air-fuel mixture is drawn into the cylinder. The piston then moves upwards compressing the gaseous mixture in the process. Ignition of the compressed mixture forces it to expand and the increased volume due to the heat and combustion products pushes the piston downwards thereby forcing it rotate the crankshaft. When the piston gets to the bottom of the cylinder, an exhaust valve opens and the piston is forced upwards by the inertial of the crankshaft and the action of adjacent pistons on the crankshaft. The piston them moves back to the top of the piston ready for another cycle to begin. In an internal combustion engine, the piston moves between two points in the cylinder, the bottom dead center and top dead center marks.
The compression stage in the engine cycle can be explained by the gas laws. When the gaseous mixture is compressed the volume reduces and the particles are more closely packed. After ignition and combustion, the gases gain heat energy and the particles’ haphazard movement is more pronounced. Also, the change in the chemical properties of the mixture gaseous mixture leads to increase in volume.
Pressure is inversely proportional to volume. When the gas is compressed, volume reduces and the pressure rises. Conversely, increasing the volume lowers the pressure. In the piston, increase in volume in the cylinder due to combustion forces the piston downwards in order to maintain the same pressure levels.
Also, physics dictates that pressure in fluids is equal in all directions. Therefore, when the exhaust valve is opened, the combustion products rushes out of the cylinder due to the differential pressure between the cylinder and the ambient. As a result, pressure in the cylinder is equal to the ambient when the exhaust valve is open.
Figure 1: Labeled diagram of a cylinder in an internal combustion engine. Source: Glenn Research Center.
The linear movement of the piston is converted to rotary motion by a crankshaft. Therefore, the torque and speed generate by the crankshaft depends action of the piston. The crankshaft is coupled to a gear train which transmits the rotary motion to the wheels of the vehicle (Glenn Research Center par. 6). A lot of mechanical knowledge is required in designing the engine parts right from the crankshaft to the wheels. The crankshaft itself has to be primed to rotate smoothly when acted upon by the cylinders in the engine.
Also, in depth knowledge of gears is required. The gears must have the right configurations so that the engine’s speed and torque falls below its power curve. The engine’s operation is a tradeoff between power and speed. When the speed is high, the power output is low. Conversely, high engine power can only be achieved and low speeds. The gears diameters and the number of teeth are determined by the required speeds and accompanying power.
Figure 2: Graph of torque against speed in an internal combustion engine.
Source: Simanaitis
Mechanical engineers also deal with other peripheral physical phenomenon such as friction and lubrication. In engineering, friction is beneficial and can also be harmful to engineering products with moving parts. To overcome the negative effects of friction such as wear and tear, lubrication is used. Friction is used in motor vehicles to disengage the gears from the engine during shifting. Also, friction between the tires and the ground offers a firm grip essential in movement and braking.
The discipline of mechanical engineering burrows heavily from physics. A strong background in physics is required for one to become a competent engineer. Physics offers rudimentary engineering principles which are then explored deeper in tertiary institutions. Modern engineering feats, which are sometimes operated by digital computers, are designed from a combination of simple physics principles. Therefore, the physics course has armed me with scientific principles which shall help me perform my duties as an engineer.
Works Cited
Simanaitis, Dennis. Transmission Talk. Simanaitis Says, 2013. Web. 25 June 2015.
Glenn Reseacrh Center. 4 Stroke Internal Combustion Engines. National Aeronautics and Space Administration, 5 May 2015. Web. 25 June 2015.
