Agricultural Mechanics 8208-B Diesel Fuel Systems PDF

# AGRICULTURAL MECHANICS 8208-B ## OPERATION AND SERVICE OF DIESEL FUEL SYSTEMS Diesel fuel is the fuel of choice to power modern agricultural equipment. Diesel is less expensive than gasoline, has a higher power potential than gasoline or L.P. gas, a lower volatility rating, and is the safest an...

# AGRICULTURAL MECHANICS 8208-B ## OPERATION AND SERVICE OF DIESEL FUEL SYSTEMS Diesel fuel is the fuel of choice to power modern agricultural equipment. Diesel is less expensive than gasoline, has a higher power potential than gasoline or L.P. gas, a lower volatility rating, and is the safest and easiest to handle and store. A major disadvantage to using diesel fuel is that diesel engines are more expensive to build because of the precision fuel system and more rugged design. Diesel engines have the following differences from gasoline or L.P. gas engines: - The fuel is injected into the combustion chamber under high pressures. - They use the heat from compressed air, rather than a spark to ignite the fuel. - The quality and type of fuel is different. - Air entering the engine is not controlled by a throttle, and therefore the engine operates with an excess of air. Diesel engines can operate on either the two-stroke cycle or four-stroke cycle. ## Comparing the Qualities of Different Agricultural Fuels | Factor | Gasoline | LP - gas | Diesel Fuel | |---|---|---|---| | Fuel economy | Fair | Good | Excellent | | Maintenance level | Fair | Good | Good | | Weight / Hp ratio | Low | Low | Excellent | | Cold - starting qualities | Good | Fair | Fair | | Rate of acceleration | Good | Good | Fair | | Suitable for continuous duty | Fair | Good | Good | | Motor oil contamination | Fair | Least | Low | The fuel which is injected into the combustion chambers is delivered at pressures between 2000 and 5000 psi. To create these pressures, diesel engines use a very precise fuel system which requires very close tolerances between the moving parts of a high-pressure pump. Each component of a diesel fuel system must function properly in order for the engine to operate, and for efficiency and power output to be maintained. For additional information on the operation of diesel engines, refer to IMS topic #8203-A. ## PARTS OF A DIESEL FUEL SYSTEM All diesel engines are fuel injected, however the components and design of the fuel system will vary according to the manufacturer and type of equipment. The most common type of injection systems used in diesel engines is the direct injection system, which injects the fuel directly into the cylinder or combustion chamber. (Refer to topic #8208-A) - **Fuel tank** - stores fuel on the equipment. - **Fuel lines** - carry fuel from the fuel tank to the cylinder. - **Fuel transfer pump** - pumps fuel from the fuel tank, through the fuel filters, and to the injector pump. - **Fuel filters** - remove foreign particles and water from the fuel before it enters the injector pump. - **Injector pump** - high pressure pump which meters and delivers fuel to the injectors. - **Injector nozzles** - atomize and spray the fuel into the combustion chamber. Diesel fuel systems are designed to operate under two pressure levels. The transfer pump operates under low pressure, approximately 4 - 10 psi, to move fuel from the fuel tank, through the fuel filters, to the injector pump. The injector pump, also called a jerk pump, increases the pressure of the fuel to approximately 2,000 - 5,000 psi and delivers it to the injectors. There are two basic types of injector pumps. The distribution type injector pump has one set of plungers which work together to supply fuel to all of the injectors. An in-line type injector pump has one individual plunger for each injector. ## Distributor Injection Pumps Distributor injection pumps, which are used on some types of agricultural equipment, normally use one pump to deliver fuel to the injectors. This type of pump uses a drive shaft to rotate the distributor rotor inside the hydraulic head. The distributor rotor has cylinder bores which contain plungers that rotates inside of a stationary cam ring which has one lobe for each cylinder in the engine. The plungers are moved towards each other at the same time by the lobes on the cam ring to create enough pressure to pump the fuel and activate the injectors. As the rotor rotates inside of the hydraulic head, its discharge passages line up with charging ports in the hydraulic head which lead to individual injectors. A metering valve, which is controlled by governor, determines the amount of fuel which is allowed to enter the pump. Distributor pumps may also have automatic speed and load advance systems which control the timing and amount of fuel delivered to the injectors. Although distributor pumps are most commonly used on agricultural equipment with small to medium horsepower engines, the use of in-line pumps has increased in recent years in large horsepower applications. ## In-Line Injection Pumps In-line injection pumps use an individual plunger for each cylinder, however all of the plungers are contained in one housing. This type of pump is commonly used on engines of 125 horsepower or greater, since they are capable of delivering larger quantities of fuel. In-line pumps use plungers which are contained in barrels to pressurize the fuel. The barrel has either one or two openings, called ports, which allow fuel to enter the barrel and be pressurized. In-line pumps use a camshaft, instead of a cam ring, to move the plungers inside of the barrels. As the camshaft rotates, the lobes operate the individual plunger assemblies to pump high pressure fuel to the individual injectors. Each plunger has a spiral groove called a helix machined into its side. The position of the helix is controlled by the control rack, and determines the length of time the ports are open or closed. The control rack is connected to the throttle linkage and rotates the plungers inside of their barrels. As the throttle linkage is moved, all of the plungers are rotated at the same time. When the throttle is fully opened, the plungers are turned so that the helix will cover the fuel discharge port for the entire length of the plunger's effective stroke. This closes the fuel discharge port, and allows more fuel to be delivered to the injector. If the throttle linkage is closed, the plunger is turned so that the helix will not cover the discharge port for as long a period of time. This movement allows some of the fuel to be discharged before the plunger has completed its effective stroke and reduces the amount of fuel delivered to the injectors. (See Illustration). ## Diesel Pump Test Stands Diesel pumps are analyzed by using an injection pump test stand. Injection pump test stands are equipped to run six standard tests. These tests are the leakage test, vacuum test, transfer pump pressure test, fuel delivery test, idle adjustment test, and the torque control adjustment test. ## Fuel Lines The three types of fuel lines used in diesel fuel systems are: 1) heavy-weight lines, 2) medium-weight, and 3) light-weight lines. - **Heavy-weight lines** are used to transport high pressure fuel, up to 5,000 psi, between the injector pump and the injector nozzles. This type of line will have thick walls and a small inside diameter. The length of the heavy lines should be about the same for proper injection timing. - **Medium-weight lines** are used to transport fuel from the fuel tank, to the transfer pump, and to the injector pump. This type of line has thinner walls and a larger inside diameter, and is designed to handle pressures from 5 to 45 psi. - **Light-weight lines** are used to transfer fuel under little or no pressure. They are commonly used as fuel leak-off lines to carry fuel of less than 5 psi from the injectors and injector pump back to the tank. This is usually fuel which has been used to lubricate and cool the moving parts in the injector pump and injector nozzles. The fuel lines and connections should be periodically inspected for leaks, breaks, or loosening. Never inspect the lines for leaks with your bare hands. Highly pressurized fluids can penetrate deeply into flesh when sprayed from small openings, and cause a serious infection if not properly treated. Usually a leak or other failure can be easily identified by the presence of fresh fuel. However, if the exact location of the leak cannot be determined, a sheet of paper can be moved around the suspected area until fuel sprays on it. When replacing a line it is important to always use a line with the identical size, shape, inside diameter, and length as the one which has failed. The technical or service manual for each piece of equipment will provide the specifications and procedures for replacing and servicing fuel lines. ## Fuel Filters Since diesel fuels tend to be impure, and the working parts of the injection system have close tolerance fits, the filtering system of a diesel engine is very important. Most diesel fuel systems have a series of filters to clean diesel fuel and prevent the injection system from being damaged by foreign particles. A system will typically have three progressive stages of filters. These stages are: 1) a filter screen at the tank or transfer pump to remove large particles, 2) a primary filter which removes most of the small particles, and 3) a secondary filter to remove any very fine particles which pass through the primary filter. Most diesel filters will have a water trap which collects water and heavy sediments that can be drained periodical. Filters remove foreign particles by: 1) straining, 2) absorption, or 3) magnetic separation. - **Straining** is a mechanical method of filtering which uses a screen to trap particles larger than the holes in the screen. Screens can be made of wire for filtering larger particles, or paper or cloth for filtering finer particles. - **Absorption** is done by trapping foreign particles and some moisture in layers of a filtering material. Cotton waste, cellulose, felt, or woven yarn are some of the materials commonly used as filtering materials in this type of filter. - **Water** is usually removed from fuel by magnetic separation. This is done by treating a paper filter with chemicals that will cause the water to form droplets which will drop into the water trap. This type of filter is also designed to remove solid particles by using either the screening or absorption methods. Sometimes fuel conditioners are added to diesel fuel tanks. Diesel fuel conditioners contain chemicals that cause water to form droplets and settle out into the water trap. Filters can be arranged in series or parallel, depending upon the volume of fuel which is filtered. When filters are arranged in a series, all of the fuel must pass through the first filter before it can move to the next filter or part of the system. This system cleans the fuel better since the second filter can pick up dirt missed by the first filter. However parallel filters can move a larger volume of fuel, since only part of the fuel goes through each filter. Parallel filtering systems will have two or more filters, each handling a portion of the fuel passing through the fuel system. Regardless of which filtering system is used, the filters must be changed and the water traps drained at regular intervals as is outlined in the owner's manual. Failure to follow the service and maintenance procedures found in the owner's or operator's manual can result in decreased engine power, fuel efficiency, and serious damage to the fuel system. Additional information regarding filters can be found in IMS topic #8208-D. ## Injection Nozzles The injector nozzles are responsible for the atomizing and spraying of fuel into the combustion chambers of the engine. The basic parts of an injector nozzle are the: 1) body, 2) nozzle valve, 3) spray tip, and 4) pressure spring. Pressurized fuel enters the body of the injector, which is closed on one end by the nozzle valve. Tension from the pressure spring holds the nozzle valve shut until cracking pressure is reached. Cracking pressure is the amount of pressure required to push the valve off of its seat. Once the valve opens, fuel is sprayed through the spray tip into the combustion chamber. The amount of pressure on the fuel, and the size, number, and angle of the holes in the spray tip can be varied to obtain the desired spray pattern. Most of the injector nozzles used in modern engines are closed-type nozzles which only have pressure acting on one side of the nozzle valve. The two classes of closed-type nozzles are: 1) inward-opening nozzles and 2) outward-opening nozzles. - **Inward-opening nozzles** retract the nozzle valve into the spray tip once cracking pressure is reached to release the fuel. - **Outward-opening nozzles** open outward when cracking pressure is reached to deliver the fuel. Usually the cracking pressure for outward opening nozzles is lower than that required for inward opening nozzles. It is important to know which type of nozzles is recommended for use on an engine before replacing or servicing them. If the wrong type of injector nozzle is installed, engine power and fuel efficiency will be decreased, or the engine may not start. Equipment manufacturers recommend different injector nozzles for their various types of machinery, depending on the type of system used as well as the size and design of the engine used to power their equipment. **ALWAYS CONSULT THE OPERATOR'S OR TECHNICAL MANUAL BEFORE SERVICING OR REPAIRING INJECTOR NOZZLES OR THE INJECTION SYSTEM!!** ## STARTING AIDS FOR DIESEL ENGINES Diesels use the heat produced by compressing air to combust the fuel vapor in the combustion chamber. This can make diesels difficult to start in cold weather conditions when the outside temperature and the temperature of the engine parts are low enough to prevent combustion. Starting fluid or glow plugs can be used to compensate for this problem. Although there are many different types of starting fluids, ether is the most commonly used. As the engine is cranked, the fluid is either sprayed directly into the intake manifold from an aerosol can or by a special attachment on the engine. However, some equipment manufacturers do not recommend the use of starting fluids in their engines. Engines which are not designed to use starting fluids may have glow plugs installed in them. A glow plug is actually a small electric heater which is mounted beside the injection nozzle, with the tip extending into the combustion chamber. When an electrical current is supplied to the glow plugs, they glow "red hot" and heat the inside of the combustion chamber to allow combustion to occur. Starting fluid should never be used in combination with glow plugs to start an engine. Always consult the owner's or operator's manual for instructions on how to cold start the engine of a piece of equipment. ## Glossary of Terms - **Absorption** - mechanically filtering diesel fuel by passing the fuel through layers of a filtering material to trap foreign particles and some moisture. Cotton waste, cellulose, felt, and woven yarn are commonly used in absorption filters. - **Cracking pressure** - amount of pressure required to move the nozzle valve in a fuel injector off of its seat. - **Glow plug** - a starting aid consisting of a small electric heater mounted near the injection nozzle, used to heat the combustion chamber and allow combustion to occur. - **Magnetic separation** - chemically filtering the water from diesel fuel by passing the fuel through a treated paper filter. The chemicals in the filter cause the water to form droplets and fall into the water trap. - **Straining** - mechanically filtering diesel fuel by passing the fuel through a metal screen to trap the large particles. Screens can be made from wire, paper, or cloth. ## References - Fundamental of Machine Operation: Preventive Maintenance, Deere and Company, Moline, IL. - Fundamental of Machine Operation: Tractors, Deere and Company, Moline, IL. - Fundamental of Service: Fuels, Lubricants, and Coolants, Deere and Company, Moline, IL. - Fundamental of Service: Engines, Deere and Company, Moline, IL. - Jacobs and Harrell, Agricultural Power and Machinery, McGraw Hill, New York, NY - Promersbersberger, Priebe, and Bishop, Modern Farm Power, Reston Publishing, Reston, VA. ## Acknowledgements - Kirk Edney, Curriculum Specialist, Instructional Materials Service, revised and updated these materials - Vickie Marriott, Word Processor, Instructional Materials, Service, word processed and formatted this topic. - Christine Stetter, Artist, Instructional Materials Service, produced the illustrations for this topic.