ML20043F675
| ML20043F675 | |
| Person / Time | |
|---|---|
| Issue date: | 02/12/2020 |
| From: | Office of the Chief Human Capital Officer, Woodard Corp |
| To: | |
| Gary Callaway | |
| Shared Package | |
| ML20043F634 | List:
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| References | |
| Download: ML20043F675 (58) | |
Text
Chapter 4 COMBUSTION AIR, FUEL, AND EXHAUST SYSTEMS
Learning Objectives As a result of this chapter, you will be able to:
- 1. Describe the relationship between the intake air charge and fuel delivery in development of power in a diesel engine.
- 2. Identify major components of the diesel engine air intake and exhaust systems and state the purpose of each.
- 3. Identify major components of a diesel engine fuel system and state the purpose of each.
Learning Objectives (continued)
- 4. Identify functions that must be performed by components of the fuel injection system
- 5. Describe the basic construction of a typical diesel engine fuel injection pump and explain its operation.
- 6. Describe the basic construction of a typical diesel engine fuel injection nozzle and explain its operation.
Learning Objectives (continued)
- 7. Describe the basic construction of a unit-type fuel injector and explain its operation.
- 8. Explain how the governor functions to control fuel delivery to the cylinders of a diesel engine.
- 9. Describe the exhaust system functions, construction, and operation.
Elements of Combustion in a Diesel Engine Hydrocarbon fuel oil Meets or exceeds engine manufactures specs (including heat content and paraffin limits).
Atmospheric air Approximately 23% oxygen, 76% nitrogen.
Heat Sufficient to ignite the fuel oil mist injected into air heated by rapid compression to 1000oF.
Fuel Oil Specifications The engine manufacturer normally provides the specifications for the fuel to be used in his EDGs. A typical spec may include the following elements:
Cetane Number 40 (min)
Total Sulfur 15ppm (ULS max; LS was 500ppm)
Organic Chlorides 20 ppm (total max)
Viscosity 32-40 SUS @ 100 F Ash Content 0.02% (by weight)
Heating Value 18,190 BTU/lb (min)
Cloud Point 40 F (wax formation)
Other Fuel Properties Typical Fuel Chemical Analysis: CnH(2n+2) 15% hydrogen to 85% carbon For perfect (stoichiometric) combustion:
2CnH(2n+2) + (3n + 1) O2 2nCO2 + (2n + 2) H2O The instructor will briefly discuss Ultra-low Sulfur Diesel fuels. These have minor potential to impact EDGs in NPPs. Biodiesel fuel oil is of more concern.
Both will be discussed in Chapter 13, Case Studies.
The amount of fuel that can be burned depends on how much air (oxygen) is available in the cylinder to chemically react with fuel.
Natural aspiration provides less air.
Blower scavenging provides more air.
Turbocharging with aftercooling provides the most air.
When the three elements required for combustion are met:
The hydrocarbon fuel chemically reacts with the oxygen in the air Heat and light energy are created.
Carbon dioxide and water are the major waste products.
If other elements are in the fuel (e.g., sulfur or ash),
other products will be in the exhaust. Also, nitrogen in air tends to produce oxides of Nitrogen (NO, NO2).
Combustion (continued)
Combustion occurs in an engine cylinder as fuel is sprayed into the heated air charge. A flow diagram detailing activities occurring during fuel injection and combustion is shown in Figure 4-1.
Figure 4-2 illustrates a complete diesel engine cycle.
Figure 4-1 Combustion Activity
Figure 4-2 Diesel Cycle Pressure vs. Stroke
The fuel charge goes through four phases as combustion occurs.
Delay period - Start of injection until fuel ignitionboth physical and chemical delays.
Rapid combustion - After fuel ignition.
Continued combustion - Injection continues (not instantaneous), fuel keeps burning rapidly.
After burning (after injection stops) - Time for combustion of fuel not yet burned.
Intake Air System:
Must supply cool, clean air At the proper pressure At reduced noise levels Must provide excess air to assure complete combustion - usually 15 to 25%
Must provide sufficient air to scavenge exhaust gasses and cool the cylinder during the intake valve/port and exhaust valve/port overlap time.
(Excess air may be 100% or more.)
Intake Air System (continued)
Incoming air charge must be clean and free of abrasive particles and contaminating gasses.
Incoming air should be free of detrimental vibrations, pulses, and noise. (Silencers and flexible connections used.)
This system and its components are illustrated in Figures 4-3 through 4-8.
Figure 4-3 Basic Intake and Exhaust System Figure 4-4 Dry Type Air Filter
Figure 4-5 Oil Bath Air Filter
Figure 4-6 Blower/Supercharger Figure 4-7 Exhaust Driven Turbocharger Figure 4-8 Intake Air and Exhaust Flow
Intake Air System:
Should have large, short intake piping with minimum bends to reduce resistance to flow.
There is generally a limit of about 6" H2O.
Turbochargers increase air quantity, but also raise its temperature. Intercoolers remove this heat, thereby increasing air density to burn more fuel.
Diesel Engine Ratings The quantity, quality, and temperature of ambient and combustion air directly affect engine performance and ratings.
A typical basis for rating and de-rating diesel engines is shown in Figure 4-9.
Figure 4-9 Typical Basis for Generator Ratings: DEMA Rating Combustion Exhaust System Designed to efficiently remove exhaust gases from engine cylinders, to minimize dilution of the new charge of fresh combustion air.
Must have low resistance to flow to minimize engine pumping losses. Resistance typically limited to 10 to 12" of H2O.
The left side of Figure 4-3 illustrates a typical exhaust system.
Figure 4-3 (Repeated) Basic Intake and Exhaust System The combustion process creates a large volume of high energy noise pulses to consider in:
Design of exhaust manifolds, and the exhaust input into the turbocharger.
Design of mufflers to reduce ambient noise levels (a personnel matter). A typical exhaust muffler is illustrated in Figure 4-10.
Figure 4-10 Exhaust Gas Muffler Other Exhaust Design Considerations:
The need for jacket water cooling at the cylinder exhaust belts.
Design and rating of exhaust piping to outside atmosphere. Important to avoid re-circulation into combustion air intake.
Exhaust back pressure relief valve to permit continuing EDG operation if blockage occurs.
Good place for a break!
The Diesel Fuel Oil System:
Must supply each cylinder with fuel oil that:
Meets or exceeds manufacturers specs, Is clean and dry, Is injected at the proper time In the proper amount, At the proper pressure, and Finely atomized
The fuel oil system is divided into three separate but interdependent systems:
Fuel oil storage and transfer to the day tank Must store sufficient fuel for extended period of operation and pump to the "day tank" for use This system is illustrated in Figure 4-11 Fuel supply to the engine fuel header, from the day tank. This includes final filtration Fuel Injection into the Cylinder via the Injection Pump and Nozzle assemblies
Figure 4-11 Fuel Oil Storage and Transfer System The Fuel Oil Supply System This system is illustrated in Figure 4-12.
Provides a day tank supply with positive pressure head to the engine driven fuel pump.
Has a positive displacement fuel pump to pressurize the header. (Figure 4-14)
Includes strainers and filters to assure clean fuel. (Figures 4-13 and 4-15)
Figure 4-12 Fuel Oil Supply to Engine Fuel Header Figure 4-13 Fuel Oil Strainer
Figure 4-14 Figure 4-15 Fuel Oil Supply Pump Fuel Oil Filter
The Fuel Injection System must:
Meter the quantity of fuel delivered to each cylinder, to control power output.
Atomize and inject the fuel into the cylinder's heated air charge in proper sequence, timing.
Inject fuel at a rate for smooth, complete, efficient combustion.
The system is illustrated in Figure 4-16.
Figure 4-16 Fuel Injection Pump and Nozzle System Separate individual cylinder fuel injection pumps and injection nozzles are typical for most engines. A typical fuel injection pump is illustrated in Figure 4-17.
Figure 4-17 FM OP Engine Injection Pump Construction
Operation of the fuel injection pump to control the amount of fuel to be injected is illustrated in Figures 4-18 through 4-20.
Figure 4-18 Zero Fuel Delivery
Figure 4-19 Figure 4-20 Low Power Full Power
The fuel injection nozzle has to atomize and evenly distribute the fuel as it sprays it into the combustion space. Figure 4-21 is a nozzle cutaway, showing its component parts.
Figures 4-22 and 4-23 depict nozzle spray patterns.
Figure 4-21 Injection Nozzle
Figure 4-22 Injection Nozzle Spray Pattern Figure 4-23 Pintle Nozzle
The EMD engines utilize a unit type fuel injector in which the fuel injection pump and nozzle are in a single assembly. Figure 4-24 is a unit type injector cutaway, showing its component parts.
Figure 4-24 Unit Type Fuel Injector
Fuel pump and injector operation are illustrated in Figures 4-25 through 4-28.
Figure 4-25 Injector Operation Figure 4-26 Zero Fuel Delivery
Figure 4-27 Low Power Figure 4-28 Full Power
Static injector timing is established by the relationship between the engine camshaft lobes and the fuel injection pumps plungers. Slight changes in timing are accomplished by the shape of the helix in the fuel pump assembly.
The engine governor output shaft rotation drives the individual cylinder fuel pump racks, controlling the amount of fuel delivered to cylinders. The Governor will be discussed in detail in Chapter 8.
END OF CHAPTER 4