- - AGRICULTURAL CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAL MECHANICS (CLF2850) Unit Title: TYPES OF ENGINES _____________________________________________________________________________ (CLF2852) Topic: ENGINE OPERATING Time Year(s) PRINCIPLES 6 Hours 1 / 2 / 3 / 4 _____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (R-8) - Identify all components of the ignition systems and discuss their function. (R-9) - Test, repair, and/or replace components of the ignition system. Special Materials and Equipment: Ignition parts and pieces from several different ignition systems; test equipment, new and failed ignition parts to test; one-cylinder engine for students to trouble shoot problems References: Cooper, Elmer L. (1987). AGRICULTURAL MECHANICS: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Jacobs, C. O., & Harrell, W. R. (1983). AGRICULTURAL POWER AND MACHINERY. New York: McGraw-Hill. Evaluation: Quiz by instructor and evaluation of trouble shooting techniques by instructor. TOPIC PRESENTATION: OPERATING PRINCIPLES A. The purpose of the engine is to convert chemical, electrical, and heat energy into mechanical energy. This conversion is accomplished through the synchronization of many different engine systems. Each of the systems will be discussed in detail in CLF2854. However, it is important to understand the operating principles of the engine before each of these systems can be understood. In this lesson, we will concentrate on the ignition principle which provides the actual mechanical power to the vehicle. The ignition system on today's gasoline powered tractors and equipment has not changed a great deal from tractors of the past. One reason for this is that the diesel or compression ignition engine has dominated the farm power industry for more than 30 years. However, the spark ignition principle is still found on a lot of power equipment and is the object of a great deal of equipment maintenance and repair. B. Spark Ignition Principle - The need to ignite a fuel charge at a precise moment in an internal combustion engine is an important factor in engine operation. With all spark ignition engines, the ability to control the timing, duration, and voltage of the electrical charge is an important part of efficient high performance power generation. The electric spark is an efficient means of accomplishing this. 1. Spark Ignition Components a. Battery, 6 or 12 volt, Direct Current, wet cell, lead/acid type chemical current producer. ..................2 Volts per cell (electrolyte : : chamber) : : - + ........Vent (hydrogen gas - explosive!!) |~~-~~~~~+~~~| |+ - + - + - |..........Plates (positive and negative) |+ - + - + - | lead and lead peroxide |+ - + - + - | |+ - + - + - |......Electrolyte (sulfuric acid & water) |+ - + - + - | |+ - + - + - | |+ - + - + - | |+ - + - + - |..........Case (plastic, acid proof) |+ - + - + - | |+ - + - + - | ~~~~~~~~~~~~~~ b. Distributor - Mechanically driven, electrical rotary switch used to sequence the ignition spark to the correct cylinder according to the firing order. 1) The firing order is the sequence in which cylinders provide a power stroke. This 4-cylinder engine could be either 1,4,3,2 or 1,2,3,4. P _____ _____ O | | | | W | 2 | <--- Pistons ---> | 4 | E |_____| |_____| __R__ | _____ | | | | | | | | 1 | | | 3 | | |_____| | |_____| | || <---Engine | /~~~\ | /~~~\ || front | //~~~\\ | //~~~\\ || ------\ | /----// \\---- | /----// \\---|| ------\\ | //---// \\---\\ | //---// \---|| \\~~~ // Crankshaft \\~~~ // || ~~~ ~~~ Flywheel----->|| || 2) Base - The housing for other ignition components such as points, rotor, condenser, and the advance mechanism 3) Movable base for quick adjustment of ignition timing to control when spark actually occurs (normally at 4 - 6 degrees Before Top Dead Center, BTDC) c. Coil - A series of windings which allows the battery voltage to be stepped up to 20,000 - 50,000 volts so the spark can jump the gap at the spark plug. 1) Primary Winding - A large diameter wire with a short series of coils which is charged with battery voltage when the points are closed (grounded). 2) Secondary Winding - A small diameter wire with many more windings (100 times the primary) wound very close to the primary coils. The voltage is stepped up by an electromagnetic phenomenon called induced current. This stepped up voltage is then sent to the spark plug at the exact time the points open. d. Points - A mechanical, on/off switch which completes the electrical circuit in the primary windings. (On most recent automotive applications, the points have been replaced with an electrical switch called a diode which performs the same function.) The function of the points is to: 1) Time the engine firing order correctly. 2) When the points are closed a magnetic field exixts in the coil which covers both the primary and secondary windings. When the points open, the magnetic field collapses and a current is induced in the secondary winding. This high voltage, induced current is directed to the spark plug. e. Condenser - An electrical "sponge" which absorbs the electrical current and prevents the points from arcing and burning; this increases the point life. f. Spark Plug - An insulated electrode that threads into the combustion chamber; it conducts the electric spark at the proper time which ignites the air fuel mixture. 1) The insulated electrode connects to the secondary high voltage wire. 2) The grounded electrode connects to the spark plug base and completes the circuit when the spark jumps the gap. 2. Magneto - In addition to having all the component parts of the distributor, the magneto also has a current generating unit which replaces the battery. a. The magneto was used a great deal in the early days of the tractor and automotive engine when hand cranking to start the vehicle was accepted practice. b. Today the most common use of the electric ignition magneto is in small engine applications (1 to 15 HP). 3. Advance Mechanism - Normally two different types which work together to maintain the correct ignition advance at all engine RPM's and load conditions. a. Mechanical Advance - These consist of counterweights which are attached to the breaker plate which advances the point cam, thus the spark when centrifugal force throws the counterweight out. The faster the shaft spins the more the spark is advanced. b. Vacuum Advance - This is operated by the intake manifold vacuum and will react more quickly than the mechanical advance mechanism to advance the ignition for quick changes in engine RPM as in acceleration. C. The other type of engine, and the one most used in agriculture today, is known as a compression ignition or, more commonly, a diesel engine. 1. The compression ignition principle of fuel combustion was first demonstrated by a German, Rudolf Diesel, in the late 1800's. a. When air is compressed the friction of the molecules rubbing together creates heat. b. If the air is squeezed tight enough, the temperature reached can be high enough to ignite a fuel when it is introduced into the hot air mass. 1) Compressed air temperatures in today's diesel engines will reach 1000 degrees F. or more. 2) Diesel fuel spontaneously ignites at about 650 degrees F. when sprayed or atomized into the cylinder combustion chamber. c. Compression ratio is a ratio of the volume in the cylinder with the piston at BDC (bottom dead center) compared to the volume in the cylinder with the piston at TDC (top dead center). 1) Diesel engines have compression ratios of from 16:1 to 22:1. 2) Gasoline engines (spark ignition) have ratios of from 7:1 to 11:1. __________________________________________________________ ACTIVITY: 1. Select a group of ten parts from the diesel system and lay out for students to identify. 2. Using Supplemental Worksheet #2, assist the students in completing the different lab assignments. 3. Demonstrate an injector test as outlined on Supplemental Worksheet #2. ___________________________________________________________ SUPPLEMENTAL WORKSHEET #2 COMPRESSION IGNITION ENGINE NAME ___________________________________ DATE ___________________________________ CLASS ___________________________________ A. Explain the differences between a compression ignition and a spark ignition engine. B. Identify the ten (10) parts of a compression ignition engine laid out on the work bench. 1. _____________________________ 2. _____________________________ 3. _____________________________ 4. _____________________________ 5. _____________________________ 6. _____________________________ 7. _____________________________ 8. _____________________________ 9. _____________________________ 10. _____________________________ C. After checking the manufacturer's instructions, use the injector tester (if available) to perform the following injector tests and record the results. (Be very careful not to let fuel under high pressure penetrate the skin.) 1. Leakage Test - Slowly pump tester up to "pop off" pressure point and check for fuel drips or other leaks. Pass ________ Fail_______ . 2. "Pop off" Pressure Setting Point - Pump up test pressure to the point where the first injection occurs and record the pressure. _______psi. 3. Rapidly pump injector tester to deliver several quick injection shots. Listen for a characteristic "chatter" of the nozzle which indicates the correct function of the nozzle valve. _________ yes/no. (Note: This test will not work on some newer nozzles.) 4. Leak Back Test - Pump the nozzle pressure up to just below the pop off point and hold. As the pressure leaks back, note the rate of leakage and compare with the specifications for the nozzle. Leak Back Allowance ______________ Nozzle Actual __________________ . 5. In order to check the spray pattern delivery and uniformity, use a piece of newspaper or other clean paper and hold it just below the nozzle while injecting one or two strokes. 7/15/91 YNJ/tf #%&C