- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAl MECHANICS (CLF2850) Unit Title: TYPES OF ENGINES _____________________________________________________________________________ (CLF2851) Topic: ENGINE TYPES AND Time Year(s) OPERATING CYCLES 2 Hours 1 / 2 / 3 / 4 _____________________________________________________________________________ Topic Objectives: Upon completion of this unit the student will be able to: Learning Outcomes #: (R-1) - Distinguish among the different types of engines. (R-3) - Understand the basic terminology used in the study of engines. (R-4) - Identify the operating principles of two- and four- stroke engines. Special Materials and Equipment: Sample engines of both the two-cycle and four-cycle type; a list of terms for these types of engines, and all component parts of a two-stroke cycle engine. References: Jacobs, C. D., & Harrell, W. R. (1983). AGRICULTURAL POWER AND MACHINERY. New York: McGraw-Hill. Evaluation: Quiz by instructor and evaluation of lab on Supplemental Worksheet #1 TOPIC PRESENTATION: ENGINE TYPES AND OPERATING CYCLES A. History of the Internal Combustion Engine 1. Nikolaus Otto, a German engineer, developed the internal combustion engine concept in 1867. a. He produced a "free-piston" engine powered by igniting a charge of gas and air under a piston in a vertical cylinder which forced the piston up. The down stroke was provided by gravity and atmospheric pressure. b. Otto utilized the ideas of Beau de Rochas (French) who in 1862 published his theory stating the conditions necessary for maximum economy for an internal combustion engine. Beau de Rochas suggested that each of the following factors must be present for the efficient working of an internal combustion engine: 1) Maximum cylinder volume with minimum cooling surface. 2) Maximum rapidity of expansion 3) Maximum ratio of expansion 4) Maximum pressure of the ignited charge c. Beau de Rochas also stated the basic sequence of operations necessary to attain this economy: 1) Suction during the complete out-stroke of piston 2) Compression during the next in-stroke of piston 3) Ignition of the fuel/air charge at dead center of the piston and expansion during the resultant out-stroke 4) Forcing out of burned gases on the final in-stroke 3. Although Nikolaus Otto is credited with this important development in the internal combustion engine, many other scientists and engineers were at work developing refinements and changes to the early engine. a. In 1861, G. Schmidt proposed using a charge pump to compress a fresh mixture of fuel and air to three atmospheres before it entered the cylinder, thereby gaining greater expansion. b. In 1860, J.J.E. Lenoir marketed an engine which ran on illuminating gas. It was essentially a converted steam engine. B. Internal combustion engines can be classified either according to the type of fuel used to power them or by the cycle rotation of the engine. There are currently four different classifications of internal combustion engines that are in use today: l. Gasoline 2. Diesel 3. Two-stroke 4. Four-stroke C. Engine Cycles (All four cycles take place in all engine types.) l. Intake a. Diesel engines intake air only into cylinders. b. Gasoline engines intake both air and fuel into cylinders. 2. Compression a. Diesel engines compress air only. b. Gasoline engines compress both air and fuel within the cylinders. 3. Ignition a. Diesel engines ignite the air-fuel mixture spontaneously, due to the heat of compression. b. Gasoline engines ignite the fuel-air mixture with a spark from the spark plug. 4. Exhaust a. Both diesel engines and gasoline engines remove products of combustion (exhaust) by the upward movement of the piston. b. Some diesel engines remove exhaust with a blower in addition to removing it with the upward motion of the piston. D. Basic Design Concept 1. Two-stroke Cycle of Operation a. A two-stroke engine utilizes one revolution of the crankshaft between each power impulse. b. This engine completes all four of the parts of the Otto cycle in only one revolution of the crankshaft. 1) Intake and compression occur primarily during the up stroke (vertical cylinder orientation). 2) Power and exhaust occur primarily during the down stroke (vertical cylinder orientation). 3) Some overlap of the intake and exhaust parts of the cycle occur on a ported two-stroke engine such as a small chainsaw or leaf blower. ____Ignition Spark ! Head...._______!_______ | | _ | Cylinder |_______/ | ---------> _| _______ | ----------> Exhaust | . _/\________ | \_| -------> Out | || Piston || | || + || | ||___________|| PORTED TWO-STROKE CYCLE Carburetor | | \ \ ENGINE / / \ \ \____|~~~~~~~|_ / / \ \ Air + \ ] +=====+ [ In ____/===/===\_/ ] [ / \===+===/ \ \ / \ \ Crankcase / \ / \______________/ 2. Four-stroke Cycle of Operation a. The four-stroke engine utilizes two revolutions of the crankshaft between power impulses. b. This engine provides an individual piston stroke (up or down) to accomplish the complete Otto cycle. 1) Intake - down stroke of piston 2) Compression - up stroke of piston 3) Power - down stroke of piston 4) Exhaust - up stroke of piston c. The four-stroke engine must utilize a valve system in order to allow gases to enter the cylinder and escape to the atmosphere. 1) The intake valve provides an opening for incoming gases to enter the cylinder and must be timed correctly for the Otto cycle operation (usually open approximately 215 degrees of crankshaft rotation). 2) The exhaust valve opens to allow burned exhaust gases to escape to the atmosphere (usually open approximately 235 degrees of crankshaft rotation); it is also timed to correctly open in the Otto cycle. 3. All engines function through changes in pressure throughout the engine. a. Atmospheric pressure (14.7 psi at sea level) forces air through the carburetor into the engine when the pressure inside the cylinder is less than the atmospheric pressure. 1) To illustrate this imagine a tube with a smaller opening (called an orifice) positioned in the middle of the tube. In order to get movement of the air through the tube, the pressure must be different on one side of the orifice compared to the other side. See below: ------------------------___--------------------------- Air at 14.7 psi ----------> Air at 9.0 psi = air flow ________________________---___________________________ ------------------------___--------------------------- Air at 14.7 psi = Air at 14.7 psi = no air flow ________________________---___________________________ 2) The greater the difference in pressure the faster the air will try to move to the lower pressure area. For example, less restriction in the exhaust system will improve air flow through the carburetor. __________________________________________________________ ACTIVITY: 1. Observe sample engines of the two- and four-stroke cycle type for performance and special characteristics. __________________________________________________________ SUPPLEMENTAL WORKSHEET #1 TWO AND FOUR STROKE CYCLE NAME ________________________________ DATE ________________________________ CLASS ________________________________ A. Identify ten (10) terms which are new to you and develop a good definition for each term. 1. ______________________________________________________________________ 2. ______________________________________________________________________ 3. ______________________________________________________________________ 4. ______________________________________________________________________ 5. ______________________________________________________________________ 6. ______________________________________________________________________ 7. ______________________________________________________________________ 8. ______________________________________________________________________ 9. ______________________________________________________________________ 10. ______________________________________________________________________ B. Diagram the two-stroke cycle and the four-stroke cycle principles of internal combustion and show the differences between them. TWO-STROKE | FOUR-STROKE _______________________________|________________________________________ | | | | | | | | | | | | | | | | | | 7/15/91 YNJ/tf #%&C