Agricultural Mechanics PDF 8203-A Principles of Engine Operation

# Agricultural Mechanics 8203-A ## Principles of Engine Operation ### Introduction * An engine converts heat energy into mechanical energy. * Early engines used steam and atmospheric pressure to pump water. * Development of internal combustion engines began in the mid-1800s. * Early engines were...

# Agricultural Mechanics 8203-A ## Principles of Engine Operation ### Introduction * An engine converts heat energy into mechanical energy. * Early engines used steam and atmospheric pressure to pump water. * Development of internal combustion engines began in the mid-1800s. * Early engines were inefficient and bulky. * Problems to overcome in the development of early engines included: size, weight, igniting the fuel charge in the combustion chamber, friction, internal heat, and providing a seal between the piston and cylinder walls. * Petroleum provided new fuel and lubricant possibilities. * The first internal combustion four-cycle engine with practical commercial applications was invented in 1876 by Nikolaus Otto. * The four-stroke cycle engine was often referred to as the "Otto cycle engine." * Rudolf Diesel patented the first "diesel" engine in the early 1890s. ### Basic Parts and Operation of an Engine * Several things are needed for an engine to convert heat energy into usable mechanical energy: * Some type of combustible fuel. * Air for oxygen. * Heat or a source of fire to start combustion. * Means of compressing the mixture, containing the combustion in an enclosed area, and converting the heat of combustion into reciprocating and rotary motion. * Means for the series of events to be repeated over and over again in a sequence. * This series of events is called a "cycle" and includes: 1. Filling a cylinder with a combustible mixture. 2. Compressing the mixture into a smaller area. 3. Igniting the mixture, allowing it to expand and produce power. 4. Removing the burnt gases from the cylinder. * This sequence of events in the cycle is known as: 1. Intake 2. Compression 3. Power 4. Exhaust * This cycle can occur simply with a cylinder, piston, connecting rod, and crankshaft. * Agricultural applications range from small single cylinder gasoline engines to large horsepower multicylinder stationary diesel engines. ### Identification and Function of Engine Parts * **Major Stationary Parts** * The cylinder block is the main housing of the engine and provides a surface for other parts and accessories to be attached. * The cylinder block contains the main bearing journals in which precision main bearings are fitted and the crankshaft rotates. * The block is usually made of cast iron or aluminum alloys. * The block contains the cylinder(s) in which combustion takes place. * The cylinder block also contains internal water jackets or passages around the cylinder(s) for coolant to circulate. * Cylinders are either hollow tubes machined to precise specifications directly into the cylinder block housing, or replaceable machined "sleeves or liners" that are inserted into the cylinder block. * Cylinders are the main part of the combustion chambers and provide the smooth surfaces in which the pistons slide up and down. * Replaceable cylinder sleeves may be dry or wet. `Dry sleeves` are thin replaceable wear surfaces that fit into already completed cylinder walls and are not exposed to the coolant. `Wet sleeves` are thicker and not only form replaceable wear surfaces, but the cylinder walls themselves which becomes part of the water jacket. * This allows the coolant to flow closer to the hot combustion chambers to remove the heat faster. * The oil pan is bolted to the bottom of the cylinder block and serves as a reservoir for the lubrication oil. The oil pan and lower section of the cylinder block are often referred to as the engine crankcase. * The cylinder head is the "cap" that is bolted to the top of the cylinder block. * The cylinder head encloses the upper end of the cylinders to form the combustion chamber. * The cylinder head contains holes above each cylinder in which the spark plugs or diesel injectors are inserted. * The cylinder head may also contain internal water passages for coolant to circulate for cooling purposes. * Depending upon engine design, the head may contain the intake and exhaust ports and valves. * **Major Rotating Parts** * The crankshaft is the main shaft of the engine. * The crankshaft changes the reciprocating action of the connecting rods and pistons into rotary motion. * The crankshaft is made of strong cast or forged steel and contains precision machined bearing surfaces called journals on which the crankshaft rotates. * Offsets or throws also contain precision journals to which the connecting rods are connected. * There is a throw for each cylinder in a multicylinder engine with a counter weight opposite it. * The throws are positioned around the crankshaft so that they counter balance each other as the crankshaft rotates and evenly time the power strokes. * The flywheel is attached to the end of the crankshaft. * The flywheel provides a heavy circular weight for momentum or inertia which smooths out the engine power impulses as each cylinder "fires." * The flywheel also provides a surface to attach the clutch and for a starter drive to engage. * The camshaft is a smaller rotating shaft that contains eccentric lobes positioned along and around it to push open the valves. * The camshaft must be "timed" to the crankshaft so the valves open and close at the correct time during the engine cycle. * This is possible by aligning special timing marks on the crankshaft and camshaft timing gears during engine assembly. * A camshaft contains precision machined journals and rotates in special cam bushings inside the engine block. * **Major Reciprocating Parts** * Pistons form the final part of the combustion chambers and are attached to the connecting rods by piston pins. * Pistons receive the force of the expanding heat energy during combustion and converts it to reciprocating mechanical motion by sliding up and down inside the cylinders. * Pistons are made of strong heat-resistant cast iron or aluminum alloys. * Pistons have grooves around them which are fitted with special sealing rings. * Piston rings prevent combustion and compression pressures from escaping into the crankcase, as well as, lubrication oil from the crankcase from entering the combustion chamber. * The head or top surface of a piston may be flat, convex, concave, or irregular. * Different shapes allow for different compression ratios and cause different swirling patterns as the fuel and air enter the combustion chamber and exhaust gases are forced out. * Connecting rods connect the pistons to the crankshaft. * Connecting rods convert reciprocating motion of the pistons to rotary motion of the crankshaft. * Connecting rods have precision bearing surfaces on both ends. * Valves open and close the intake and exhaust ports to the combustion chambers. * An exception is the two-stroke cycle engine which will be discussed later. * Valves move up and down in special precision machined valve guides. * Valves are also made of heat resistant alloys to prevent them from "burning" or distorting from extreme high temperatures. * Valves are held closed by springs and are pushed open by the cam lobe action from the rotating camshaft. * Depending upon engine design, the valve train may also contain valve lifters, (also called cam lifters or cam followers) push rods, and rocker arms. * Valves may be located in the cylinder block beside the cylinders, or in the cylinder head above the cylinders. ### Two and Four-Stroke Cycle Engines * **Four-Stroke Cycle Engine** * The crankshaft makes two complete revolutions for each cycle of events to occur. * The four-stroke cycle is also called the "Otto cycle" after the German inventor who first developed the four-stroke cycle engine. * The intake stroke moves the piston downward to the bottom of its stroke (Bottom Dead Center or BDC.) * A mixture of fuel and air (only air in the case of a diesel engine) is drawn into the cylinder past the open intake valve. * The exhaust valve is closed. * The compression stroke moves the piston back toward the top of its stroke (Top Dead Center or TDC) in the cylinder compressing the fuel/air mixture (air only in a diesel engine.) * The intake and exhaust valves are both closed. * The power stroke begins just as the piston starts back down. * Both the intake and exhaust valves remain closed. * In a gasoline or LP-gas engine, the compressed fuel/air mixture is ignited by a spark causing combustion in the cylinder. * In the case of a diesel engine, diesel fuel is injected directly into the cylinder just before or as the piston reaches TDC and starts the power stroke. * The heat from the compressed air ignites the fine mist of diesel fuel as it is sprayed into the cylinder. * The combustion pushes the piston down converting heat energy into mechanical energy. * The piston and crankshaft convert the reciprocating motion into rotary motion. * The exhaust stroke starts as the piston reaches BDC again. * The exhaust valve opens and the piston starts back to the top of the cylinder on the exhaust stroke pushing the exhaust gases out past the exhaust valve. * As the piston nears TDC, the exhaust valve closes and the intake valve opens and the cycle starts over again. * It takes two complete revolutions of the crankshaft and four strokes of the piston to complete the "four-stroke cycle." * **Two-Stroke Cycle Engine** * The cycle of events is completed in one revolution of the crankshaft and two strokes of the piston. * The intake and exhaust occur during part of the compression and power strokes. * Two-stroke cycle engines may use: 1. flat reed valves. 2. valves similar to a four-stroke cycle engine. 3. ports or openings in the cylinder walls covered by the piston to open and close the intake and exhaust ports. ### Cylinder Arrangement * Multicylinder engines may be classified into three different types by cylinder arrangement. * **In-line**: All cylinders are placed in a straight line above the crankshaft. * **V-type**: Cylinders are located in two rows above the crankshaft in a V-arrangement. Every other cylinder is on the opposite side. * **Opposed**: Two rows of cylinders are on opposite sides and oppose each other. ### Principles of Combustion * **Combustion** means to burn. * Three things are needed to produce a fire or for combustion to occur: 1. Heat 2. Fuel 3. Oxygen * The three ingredients must be present in the correct proportions. * **Internal Combustion**: The combustion takes place inside a combustion chamber or cylinder. * **Fuel/air mixture**: Fuel must be mixed with air which supplies the oxygen. * **Ignition System**: Some means is also needed to ignite the fuel/air mixture so that the mixture will burn evenly and produce usable mechanical power. * **Gasoline, LP, and Natural Gas Engines**: Use electrical spark ignition systems to provide a spark (heat) to ignite the fuel/air mixture inside the combustion chamber. * **Diesel Engines**: Utilize the heat from compression itself for ignition. * **Ways to make fuel burn faster once it is ignited:** * **Increase oxygen** * **Heat the fuel/air mixture** * **Break down fuel into smallest particles** * **Rate of Flame Propagation:** the rate at which the fuel burns when ignited. * **Detonation:** If the rate of propagation is too fast, part of the fuel/air mixture will explode too violently rather than an even, controlled burn. * **Compression Ratio:** the ratio of the volume in the cylinder when the piston is at BDC, to when it is at TDC. * **Compression Ratio and Fuel Type** * Gasoline engines: `8 to 1` * Diesel engines: `16 to 1` * **Compression Ratio and Engine Performance** * Higher compression ratios range from as low as `5 to 1` for a small single cylinder gasoline engine to `17 to 1` can be used in some high performance engines. * Higher compression ratios can lead to preignition which is when the fuel/air mixture in gasoline engines is "pre" and uncontrollably ignited from compression heat instead of "timed" ignition by the electrical spark system. * Factory LP-gas engines operate at around an `8.5 to 1` or higher compression ratio. * **Diesel Engines:** * More efficient fuel and can produce more power potential per gallon. * **Fuel/Air Ratios:** * The amount of fuel and air needed to ensure proper engine operation. * Normally expressed by weight, not by volume. * The ratio is approximately `1 to 15` or one part fuel to `15` parts air (air weighs approximately `1.25` ounces per cubic foot). * For comparison, this equals one gallon of gasoline fuel to approximately `9,000` gallons of air. ### Engine Piston Displacement * **Bore:** The diameter of a cylinder. * **Stroke:** The distance the piston travels from TDC to BDC and is dependent upon the "throw" of the crankshaft. * **Piston Displacement:** The volume of air the piston displaces or pushes out when moves from BDC to TDC. * **Formula for calculating piston displacement for one cylinder:** * PD = (3.1416 * bore * bore * stroke) / 4 * **Total Cubic Displacement:** Piston displacement for one cylinder multiplied by the number of cylinders. * **Units of Measure:** * cubic inches * cubic centimeters (C.C.) * cubic liter (C.L. or L) ### Naturally Aspirated vs Super or Turbocharged Engines * **Naturally Aspirated Engine:** Air or fuel/air mixture is forced into the cylinder by atmospheric pressure as a vacuum is created on the intake stroke. * **Supercharged or Turbocharged Engine:** A blower or turbine force air or fuel/air mixture into the cylinder during the intake stroke. ### Engine Lubrication and Cooling Systems * **Friction** produces heat. * **Lubrication** helps reduce friction. * **Cooling** removes excess heat. * **Thermal Dynamics:** Important in understanding the cooling and lubrication systems. * **Heat Travel:** Heat travels from an area of highest temperature to an area of lower temperature and seeks an equilibrium. * **Cooling and Lubrication System Demands:** As larger and more powerful engines are developed, the demands on the coolant and lubrication systems increase. * **Lubrication System Functions:** * Reduce friction between moving parts. * Absorb and dissipate heat. * Deaden the noise of the engine. * Clean and flush internal parts. * Provide a seal between the piston rings and cylinder walls. ### Measuring Power of an Engine * **Energy:** the ability or capacity to do work. * **Work:** the effect of a force in producing a change of position of an object against an opposite force. * **Power:** the amount of work accomplished in a given period of time. * **Horsepower:** A measure for comparing different engines. * **Torque:** The turning effect of force. * **Dynamometer:** An instrument used to measure torque and determine the power produced by an engine. * **Types of Dynamometers:** * Prony-brake dynamometers. * Hydraulic dynamometers. * **Ways to Express Horsepower:** * **Flywheel Horsepower** or **Engine Horsepower:** Measured by connecting a dynamometer directly to the crankshaft or flywheel. * **PTO Horsepower:** Measured at the power take off shaft of the tractor or implement. * **Drawbar Horsepower:** A measure of the pulling power an engine can produce when mounted in a moving machine. * **Rated or Advertised Horsepower:** The value used by manufacturers to indicate the engine and/or PTO horsepower an engine should produce under normal operating conditions. ### References * Fundamentals of Service: Engines, Deere and Company, Moline, IL. * Fundamentals of Machine Operation, Tractors, Deere and Company, Moline, IL. * Promersberger, Priebe, and Bishop, Modern Farm Power, Prentice-Hall, Reston, VA. * Parady and Turner, Understanding and Measuring Power, American Association For Vocational Instructional Materials, Athens, GA. * Jacobs and Harrell, Agricultural Power and Machinery, McGraw-Hill, New York, NY. * Toboldt, Johnson, and Olive, Automotive Encyclopedia. Goodheart-Willcox Inc. South Holland, IL.