- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURE MECHANICS (CLF2900) Unit Title: WORK AND POWER ____________________________________________________________________________ (CLF2910) Topic: DEFINITIONS AND Time Year(s) TERMINOLOGY 5 Hours 1 / 2 / 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (S-1) - Define energy, force, pressure, friction, work, and power. (S-3) - Describe the concept efficiency of energy conversion and give examples of energy loss. (T-1) - Define linear motion and rotational motion. (T-2) - Explain the relationship of friction to the transmission of power . Special Materials and Equipment: None References: Jacobs, C. O., & Harrell, W. R. (1983). AGRICULTURAL POWER AND MACHINERY. New York: McGraw-Hill. Smith, H. P., & Lambert, H. W. (1976). FARM MACHINERY AND EQUIPMENT. (6th ed.) New York: McGraw-Hill. Evaluation: Quiz by instructor and evaluation in discussion and lab work. TOPIC PRESENTATION: DEFINITIONS AND TERMINOLOGY A. Work 1. Work is the specific application of energy to move something somewhere. a. It is measured in terms of force and a distance moved. 2. Energy is the capacity to do work. Forms of energy include: a. Potential energy 1) Energy which is not active, but is stored up a) A coiled spring b) A ball at the top of a hill b. Kinetic energy 1) Resulting from a body in motion. 2) Proportional to the mass of the body and velocity squared 3) Noise energy - movement of air particles to produce sound c. Heat energy 1) Produced from friction or energy conversion d. Force energy 1) Produces or alters movement of another mass e. Electrical energy 1) Produced from movement of electrons through a conductor f. Embodied energy (inherent property of the matter) 1) Fuels energy 2) Chemical energy B. Power 1. Power is the rate of doing work or expending energy. a. Power is measured in terms of force, distance and time. 1) Pulling a force over a distance during a period of time 2) Lifting a weight a distance during a period of time 3) Providing torque at rotational speed 4) Providing an amount of flowing fluid at a certain pressure 2. Types of Power a. Fluid 1) High pressure hydraulic fluid to run motors, pressurize a hydraulic ram, etc. b. Engine 1) Brake Horsepower a) Brake Horsepower is the maximum amount of power available at the engine b) The term is commonly used to designate the power rating of stationary engine power units. 2) PTO Horsepower a) Power measured at the PTO shaft b) The term is commonly used to designate the sustainable power rating of 2-wheel drive and many 4-wheel drive tractors c) Typically 10 to 15% less than engine horsepower 3) Drawbar Horsepower a) Pulling power of the tractor developed by way of the tires or tracks b) Dependent upon soil surface, type of tire (radial or bias ply), tire inflation pressure, tractor weighting, type of hitch, etc. c) Commonly used to designate the power rating of both crawler and wheel-type tractors c. Electrical 1) As provided by a power company via electrical outlets 2) As generated by a portable motor-generator set to, for example, drive an AC/DC arc welding set-up C. Terms Associated with Work and Power 1. Friction is the resistance to motion which is set up when two moving surfaces come in contact with each other. It is dependent on the weight of the objects and the coefficient of friction of the two surfaces in contact. Rough surfaces increase resistance while smooth surfaces (for example, Teflon) decrease resistance. Friction usually produces heat as a by-product which can be either useful or detrimental in machines and power transmission. a. The transmission of power requires the movement of energy from one point to another point. This requires mechanical devices which absorb or use up part of the energy which is being transferred. Some of the efficiency of a power system is consumed by friction. b. Friction is utilized to slow or stop motion; when this happens heat is generated. Examples: brakes, clutches, and bearings which are poorly lubricated. c. Friction is useful in power generation. One example is the generation of heat in a diesel engine to allow combustion. When air is compressed, the molecules of air rub against each other and the friction produces heat. In a diesel engine this can result in an air temperature of 1000 degrees F. or more. 2. Pressure is defined as force per unit area; its units are pounds per square inch (psi) or pounds per square foot. In metric units it is kilopascals (1 psi = 6.895 kPa). Pressure is used in hydraulic and pneumatic machines to develop force and motion. 3. The prime efficiency factors in engines are: a. Mechanical efficiency takes into account all factors that take horsepower away from an engine, including friction HP loss. b. Volumetric efficiency is the limiting factor in an engine's maximum torque output. The amount of air an engine can take into the cylinder is limited by things such as the air cleaner and manifold as well as temperature and atmospheric pressure. c. Brake thermal efficiency (expressed as a percentage) is brake horsepower expressed in BTU's divided by the fuel heat input BTU's times 100. It takes into account all engine losses and can be used to compare heat energy of a fuel and the useable power it will produce. Different fuels have different specific heat values: 1. Gasoline = 120,000 BTU's per gal. 2. Diesel = 140,000 BTU's per gal. 3. L.P. Gas = 95,000 BTU's per gal. 4. Ethyl Alcohol = 84,000 BTU's per gal. 5. Methyl Alcohol = 61,000 BTU's per gal. 4. Energy Losses a. Heat Loss - The escape of heat in an engine and the heat loss from other bearings and parts in a machine contributes to a massive loss of fuel and an energy waste. A brake thermal efficiency of 25% is common in the normally aspirated gasoline engine. This percentage can be improved slightly with additional accessories such as turbochargers and intercoolers. On a diesel engine which does not have a throttle plate restricting the incoming air and which also injects its fuel into the combustion chamber, the percent can be 35. About two-thirds of the fuel put through an engine is wasted. b. Mechanical Loss - Mechanical losses in engines and other machines are due to operation of additional items which are necessary for the operation of the engine but which subtract from the total horsepower available for useful work. The following are some examples: 1) Fuel pump 6) Oil pump 2) Valves 7) Air conditioner compressor 3) Alternator or generator 8) Water pump 4) Cooling fan 9) Distributor 5) Hydraulic pump 10) Air blowers 5. Torque is a twisting force applied about the center of rotation or fulcrum. 6. Linear motion is movement along a straight line. When applied to an engine, the back-and-forth or up-and-down motion of the piston is called reciprocating motion. Reciprocating motion can be seen on many machines: slides, gears, valves, and numerous chain and belt adjustment mechanisms. 7. Rotary motion is the circular movement around a center pivot point. In an engine, the change from reciprocating motion to rotary motion is done by the crankshaft. All shafts, sprockets, gears, and pulleys are involved in rotary action in order to transmit their energy in power transmission. 8. Other Power Terms a. Inertia is the tendency of something to maintain its state of rest or motion. If we have water in a bucket and go to slide it across the table the initial movement of the bucket does not move the water. It will cause the water to slosh up the side of the bucket because the water tries to say in the original place. By the same token if something is moving and it is slowed or stopped there is a measurable force which tries to maintain the motion. b. Matter exists in the three different physical states: solid, liquid, or gases. 1) Solids have both a definite volume and a definite shape. 2) Liquids have a definite volume but not a definite shape (they assume the shape of their container). 3) Gases have neither a definite volume (they can be expanded or compressed), nor a definite shape. c. Weight is the action of the earth's gravitational pull on matter. The farther away from earth an object is the less it weighs. d. Mass is the measure of how much matter is in a body. The weight of a body changes the farther it is away from the earth, but the mass remains the same. 9. Useful Energy Conversions a. 1 Horsepowe = 2,545 BTU's heat energy per hour b. 1 Horsepower = 746 watts or .74565 kilowatts c. 1 Horsepower = 33,000 lbs. lifted 1 ft. in 1 min. d. 1 Horsepower = 550 lbs. lifted 1 ft. in 1 sec. e. 1 Horsepower (U.S.) = 1.01378 metric horsepower f. 1 Horsepower = .70685 BTU's per second g. 1 Horsepower = .17812 calorie per second D. Principles of Thermodynamics 1. First, energy is conserved. The state of matter can change from a liquid to a gas, but the amount of total matter does not change. Matter can be changed, but it cannot be destroyed. 2. Second, the process is irreversible. For instance, mechanical energy can be changed into heat energy, but heat energy can never be changed back completely into mechanical energy. The reason is that some of the heat energy becomes a part of the process. For example, if the fuel burned in a diesel engine produces horsepower and heat, the horsepower can be captured, but the heat is lost to the engine coolant, to the exhaust, and to the engine metal itself. 3. Charles' Law states that temperature changes on the mass of a gas result in direct changes to its pressure and volume. 4. Boyle's Law states that when the volume of a gas is decreased, (compressed), the pressure of the gas will go up in an inverse ratio, if the temperature remains the same. __________________________________________________________ ACTIVITY: 1. Record the efficiency of a one-cylinder engine by measuring the exact amount of fuel burned in one hour; compare that with the measured amount of work (accomplished by using a Prony brake device attached to the PTO end of the crankshaft). __________________________________________________________ 7/23/91 YNJ/JWR/sg #%&C