Theory of Engine Operation - Delmar Learning

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the piston as it is pushed downward to turn the crankshaft. This is an example of how engine efficiency is decreased. Friction may occur in solids, liquids, and gases. The airflow over a vehicle causes friction. The manufacturers carefully engineer their vehicles to be aerodynamic. This means that the resistance to motion from friction between the vehicle and the air is minimized. This is carefully calculated and measured as the coefficient of draft (Cd). Unfortunately, there are conditions and vehicles that can lose up to half of their mechanical energy to aerodynamic drag. Behavior of Liquids and Gases Molecular Energy Electrons are in constant motion around the nucleus of atoms or molecules. Kinetic energy is always occurring. The kinetic energy of liquids, gases, and solids changes in response to temperature. When temperature decreases, so does kinetic energy. As temperature increases, so does kinetic energy. Solids have the slowest reaction to temperature, while gases have the quickest response to changes in temperature. We know this theory already through our discussion of combustion. As temperature increases within the combustion chamber, the gases rapidly expand and the increasing kinetic energy is used to create thermal energy that is converted into mechanical energy. Temperature The volume of liquids, gases, and solids increases with temperature. It is the increased volume of gases on top of the pistons that produces the force to push the piston downward. When filling engine fluids, such as tire air pressure and coolant, we must be aware of the effects of temperature on volume. It is easy to overfill a coolant reservoir if the coolant is at ambient temperature and you top the fluid off to the maximum level. As the fluid heats up, it may expand beyond the capacity of the reservoir and develop enough pressure to blow the cap’s pressure relief valve. Pressure and Compressibility Pressure is always applied equally to all surfaces of a container. If there is a leak in the container, the pressure will leak out and equalize with the atmospheric pressure. Gases and liquids both flow, so they are classified as fluids. But there are distinct and significant differences between the characteristics of gases and liquids. Gases can be compressed; the molecules squeeze tighter together and form greater pressure. You can easily fill a tire with excess pressure. Luckily, you can further compress the gases in a combustion chamber to produce more power. Liquids, on the other hand, cannot be compressed. A liquid fills a chamber, and you cannot add more liquid; you can only increase pressure. Brake and clutch hydraulic systems work with liquids. When you depress the clutch or brake pedals, you move the liquid, under pressure to exert force on a piston. This force produces mechanical work from the brake caliper piston or the clutch hydraulic lever. If there is a leak in the system, air can enter. Air is a gas and is compressible. Air in a hydraulic system will cause a lack of pressure and force. Pressure and Vacuum Air is a gas with weight and mass. It is layered high above the surface of the earth and exerts a pressure on the earth’s surface. This pressure is called atmospheric pressure. A column of air extending from the earth’s surface at seal level to the outer edge of the earth’s atmosphere weighs 14.7 pounds. We say that atmospheric pressure is 14.7 pounds at sea level. This pressure changes as we climb in altitude. If we are 14,000 feet above sea level in the mountains, that column of air no longer weighs 14.7 lbs; atmospheric pressure decreases. Also, as temperature increases, the 22
molecules become less dense and therefore weigh less; atmospheric pressure decreases. These changes in atmospheric pressure caused by altitude and temperature are important for the PCM to know about in order to deliver the correct amount of fuel for the actual amount of air in the cylinder. When air pressure is greater than atmospheric pressure, it is a positive pressure. When air pressure is lower than atmospheric, it is a negative pressure or vacuum. The engine creates vacuum as the engine valves are closed and the piston moves from the top of the cylinder to the bottom of the cylinder. The volume of the chamber has increased, but no more air has been added to fill the space; pressure is reduced and a vacuum is formed. Vacuum is typically measured in inches of mercury vacuum ("hg vacuum). At atmospheric pressure, there are 0 "hg vacuum. When there is a total absence of air or O absolute pressure (no atmospheric pressure at all), a perfect vacuum can be measured at 29.9 "hg vacuum. An engine develops significant vacuum during its intake strokes as the pistons move downward and the valves begin to close. Liquids, solids, and gases move from an area of high pressure to an area of low pressure. Automotive engines rely on engine vacuum and this principle to fill the combustion chambers with air to support combustion. When the throttle is opened, atmospheric pressure flows rapidly into the low pressure area of the cylinder to fill it with air. An engine that cannot develop adequate vacuum will not pull in enough air to make maximum power. A modern, fine-running engine will typically develop about 16–18 "hg vacuum at idle. Turbocharged and supercharged engines increase this pressure difference before and after the throttle plate to help fill the combustion chambers fully under all conditions. The engine still develops the same amount of vacuum, but the supercharger or turbocharger delivers roughly 8–15 psi of pressure over atmospheric behind the throttle. When the throttle opens, the greater pressure difference increases airflow into the cylinders. Engine Operation Most automotive and truck engines are four-stroke cycle engines. A stroke is the movement of the piston from one end of its travel to the other; for example, if the piston is at the top of its travel and then is moved to the bottom of its travel, one stroke has occurred. Another stroke occurs when the piston is moved from the bottom of its travel to the top again. A cycle is a sequence that is repeated. In the four-stroke engine, four strokes are required to complete one powerproducing cycle. The internal combustion engine must draw in an air-fuel mixture, compress the mixture, ignite the mixture, then expel the exhaust. This is accomplished in four piston strokes (Figure 2-7). The first stroke of the cycle is the intake stroke (see Figure 2-7A). As the piston moves down from top dead center (TDC), the intake valve is opened so the vaporized air-fuel mixture can be pushed into the cylinder by atmospheric pressure. As the piston moves downward in its stroke, a vacuum is created (low pressure). Since high pressure moves toward low pressure, the air-fuel mixture is pushed past the open intake valve and into the cylinder. After the piston reaches bottom dead center (BDC), the intake valve is closed, and the stroke is completed. Closing the intake valve after BDC allows an additional amount of air-fuel mixture to enter the cylinder. Even though the piston is at the end of its stroke and no more vacuum is created, the additional mixture enters the cylinder because it weighs more than air alone. The momentum of air flowing into the cylinder also helps pull more air in after the piston has reached BDC. The compression stroke begins as the piston starts its travel back to TDC (see Figure 2-7B). The intake and exhaust valves are both closed, trapping the air-fuel mixture in the combustion chamber. The movement of the piston toward TDC compresses the mixture. As the molecules of the mixture are pressed tightly together, they begin to heat. When the piston reaches TDC, the mixture is fully compressed and highly combustible. Compressing the mixture provides for better burning and for intense combustion. The ignition system induces a spark at exactly the right time 23 Shop Manual Chapter x, page xx The stroke is the amount of piston travel from TDC to BDC measured in inches or millimeters. A cycle is a complete sequence of events. Top dead center (TDC) is a term used to indicate that the piston is at the very top of its stroke. Bottom dead center (BDC) is a term used to indicate that the piston is at the very bottom of its stroke.
Page 1 and 2: Theory of Engine Operation Upon com
Page 3 and 4: Figure 2-2 This engine cutaway show
Page 5 and 6: Figure 2-6 A camshaft. belt, and is
Page 7: energy exerted on top of the piston
Page 11 and 12: Engine Classifications Engine class
Page 13 and 14: Exhaust camshaft 2. Overhead cam (O
Page 15 and 16: second vibration occurs when all pi
Page 17 and 18: Plug Engine Displacement Retainer F
Page 19 and 20: Most engine manufacturers designate
Page 21 and 22: transmits this torque to the drivet
Page 23 and 24: Torque output @ 3,500 rpm 5 115 lb.
Page 25 and 26: Transfer port Most gasoline two-str
Page 27 and 28: Intake valve rich mixture Precombus
Page 29 and 30: Figure 2-33 The VIN number can be s
Page 31 and 32: Figure 2-37 Additional markings ind
Page 33: Multiple Choice 1. Increasing the c

Theory of Engine Operation - Delmar Learning

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