Theory of Engine Operation - Delmar Learning

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Shop Manual Chapter x, page xx Preignition is an explosion in the combustion chamber resulting from the air-fuel mixture igniting prior to the spark being delivered from the ignition system. compression ratio of an engine by performing some machining operations or changing piston designs. Increasing the compression ratio may increase the power output of the engine, but the higher compression ratio increases the compression temperature. This can cause preignition and detonation. To counteract this tendency, higher octane gasoline must be used. Most gas automotive engines have a compression ratio between 8:1 (expressed as eight to one) and 10:1 and develop cylinder pressures during compression of between 125 and 200 psi. Diesel engines typically have compression ratios between 17:1 and 22:1. They can produce up to 600 psi of compression pressure. The usable compression ratio of an engine is determined by the following factors: • The temperature at which the fuel will ignite. • The temperature of the air charge entering the engine. • The density of the air charge. • Combustion chamber design. Calculating Compression Ratio When calculating the total volume above the piston, the combustion chamber in the cylinder head must also be considered. The formula for calculating compression ratio is: CR 5 (VBDC 1 VCH)/(VTDC 1 VCH) where CR 5 compression ratio VBDC 5 volume above the piston at BDC VCH 5 combustion chamber volume in the cylinder head VTDC 5 volume above the piston at TDC Example: Volume above the piston at BDC 5 56 cubic inches Combustion chamber volume 5 4.5 cubic inches Volume above the piston at TDC 5 1.5 cubic inches The compression ratio would be: 56 1 4.5 5 10:1 1.5 1 4.5 Horsepower and Torque Torque is a mathematical expression for rotating or twisting force around a pivot point. As the pistons are forced downward, this pressure is applied to a crankshaft that rotates. The crankshaft 34 Volume before compression: 480 cc BDC Compression ratio: 8:1 Volume after compression: 60 cc TDC Figure 2-24 The compression ratio is a measurement of the amount the air fuel mixture will be compressed.
transmits this torque to the drivetrain and ultimately to the drive wheels. Horsepower is the rate at which an engine produces torque. To convert terms of force applied in a straight line to force applied rotationally, the formula is: torque 5 force 3 radius If a 10-pound force is applied to a wrench one foot in length, 10 foot-pounds (ft.-lb.) of torque is produced. If the same 10-pound force is applied to a wrench two feet in length, the torque produced is 20 ft.-lb. (Figure 2-25). Horsepower can be determined once the torque output of an engine at a given rpm is known. Use the following formula: Horsepower 5 torque – rpm/5,252 Torque and horsepower will peak at some point in the rpm range (Figure 2-26). This graph is a mathematical representation of the relationship of torque and horsepower in one engine. The graph indicates that this engine’s torque peaks at about 1,700 rpm, while brake horsepower peaks at about 3,500 rpm. The third line in the graph represents the amount of horsepower required to overcome internal resistances or friction in the engine. The amount of brake horsepower is less than indicated horsepower due to this and other factors. The Society of Automotive Engineers (SAE) standards for measuring horsepower include ratings for gross and net horsepower. Gross horsepower expresses the maximum power developed by the engine without additional accessories operating. These accessories include the air cleaner and filter, cooling fan, charging system, and exhaust system. Net horsepower expresses the amount of horsepower the engine develops as installed in the vehicle. Net horsepower is about 20 percent lower than gross horsepower. Most manufacturers express horsepower as SAE net horsepower. For example, if the torque of the engine is measured at the following values, a graph can be plotted to show the relationship between torque and horsepower (Figure 2-27): Torque output @ 1,500 rpm 5 105 lb.-ft. Torque output @ 2,800 rpm 5 110 lb.-ft. Torque exerted on bolt 1 Foot radius Torque = 1 foot x 10 pounds = 10 pound-feet 2 Feet radius Torque = 2 feet x 10 pounds = 20 pound-feet Figure 2-25 Example of how torque can be increased. Force 10 pounds Force 10 pounds 35 Brake horsepower is the usable power produced by an engine. It is measured on a dynamometer using a brake to load the engine. Indicated horsepower is the amount of horsepower the engine can theoretically produce.
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 and 8: energy exerted on top of the piston
Page 9 and 10: molecules become less dense and the
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: Most engine manufacturers designate
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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