ML20043F673
| ML20043F673 | |
| 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: ML20043F673 (50) | |
Text
Chapter 2 INTRODUCTION TO DIESEL ENGINES
Learning Objectives As a result of this chapter, you will be able to:
- 1. Explain the basic energy conversion process in the operation of a diesel engine.
- 2. Differentiate between the operating principles of 4-stroke cycle and 2-stroke cycle diesel engines.
- 3. Identify selected basic terms applicable to the design and operation of a diesel engine.
- 4. Describe the basic heat/power balance which exists during the operation of a diesel engine.
Energy Conversion Processes in the Diesel Engine First: Chemical energy in the fuel injected into the cylinders is converted to thermal energy by a process called combustion.
During this process, carbon and hydrogen in the fuel chemically combine with oxygen in the air to create heat and light energy plus carbon dioxide and water as the primary waste products.
Energy Conversion Processes in the Diesel Engine Second: This thermal energy in the form of heated combustion gasses is converted into mechanical energy by expansive cooling of the gasses. This creates the forces on the pistons, connecting rods and crankshaft.
These components convert the linear motion of pistons into rotational motion and output shaft horsepower.
Energy Conversion Processes in the Diesel Engine Third: The generator attached to the engine output shaft converts rotational mechanical energy into electrical energy by a process involving electromagnetic induction.
These processes are illustrated in Figures 2-1 through 2-6 and 9-2, next.
- The electrical generator will be covered in more detail in Chapter 9.
Figure 2-1 Fixed Volume Figure 2-2 Fixed Volume Plus Heat Figure 2-3 Addition of Piston Figure 2-4 Addition of Piston + Heat FIGURE 2-5 Addition of Crankshaft FIGURE 2-6 Addition of Crankshaft plus Heat FIGURE 9-2 Simple Generator (Alternating Current)
The combustion process must be repeatable in proper timed sequence for each cylinder to produce useful work:
A cylinder is charged with fresh air.
The fresh air must be heated above fuel ignition temperature. That is accomplished by compressing the air.
A controlled amount of fuel is injected into the heated air volume at the proper time.
Combustion occurs, creating the power stroke for the piston -- mechanical energy.
This mechanical energy provides rotational mechanical power to drive the load (generator),
as well as energy to activate components and systems for the next cycle of the cylinder.
Burnt gases must be removed.
The configuration and sequence of events that occur in a 4-stroke cycle engine are illustrated in Figures 2-7 through 2-12.
Figure 2-7 Cycle Configuration Figure 2-8 Intake of Air Charge Figure 2-9 Compression of Air Charge Figure 2-10 Injection of Fuel Figure 2-11 Development of Power Figure 2-12 Exhaust of Spent Gas Diesel engine operation is based upon the ideal air standard cycle illustrated in Figure 2-13 A through C. These are plots of cylinder pressures and volumes during the diesel cycle.
DIESEL CYCLE Figure 2-13 Air Standard Diesel Cycle
DIESEL CYCLE Figure 2-13A Combined Cycle - Intermediate Diesel Cycle
DIESEL CYCLE Figure 2-13B OTTO CYCLE - Modern Diesel Engines use this Cycle
The cyclic sequence of events that occur during the operation of a 4-stroke cycle diesel engine are illustrated in Figure 2-14. This is a plot of cylinder pressure at the different crank angle positions in which the diesel cycle occurs.
Figure 2-14 PRESSURE VS. STROKE DIAGRAM
Basic differences between a 2-stroke and 4-stroke cycle engine:
A 2-stroke cycle engine develops power output for each engine revolution (2 piston strokes).
A 4-stroke cycle engine develops power output for each 2 engine revolutions (4 piston strokes).
Four-stroke cycle EDG engines have both intake and exhaust valves, mounted in the cylinder head.
Two dominant 2-stroke engines The 2-stroke OP engine has no cylinder heads nor valves. Both intake air and exhaust are through ports in the cylinder liner.
The 2-stroke cycle EMD engine has intake air ports through the lower cylinder liner and 4 exhaust valves mounted in the cylinder head.
Two-stroke cycle engines used to power the EDG units at nuclear power stations are not operated as naturally aspirated engines.
Cylinder exhaust scavenging air and cylinder combustion air are pumped in. A blower and/or a turbocharger are used for this.
The configuration and sequence of events that occur in an EMD 2-stroke cycle engine are illustrated in Figures 2-15 through 2-18.
Figure 2-15 Cylinder Configuration of 2-Stroke Cycle EMD Engine
Figure 2-16 Intake Event
Figure 2-17 Compression Event
Figure 2-18 Combustion and Power Stroke
The configuration and sequence of events that occur in a Fairbanks Morse opposed piston 2-stroke cycle engine are illustrated in Figures 2-19 through 2-22.
Figure 2-19 Cylinder Configuration of 2-Stroke Cycle Opposed Piston Engine
FIGURE 2-20 FIGURE 2-21 FIGURE 2-22 Intake Event Compression Event Combustion and Power Event
Understanding Engine Terminology:
Figures 2-23 through 2-25 illustrate and define some diesel engine terminology.
Figure 2-23 Cylinder Bore and Stroke
Cylinder Displacement The cylinder volume is determined by:
2 Displacement =
4 The displacement for the whole engine is the cylinder displacement multiplied by the number of cylinders.
Figure 2-24 Clearance Volume including piston cup or head recesses.
Va = Vb + Cyl Displ.
CR = (Vb + Disp)/Vb CR = 1 + (Disp/Vb)
Figure 2-25 Compression Ratio = Ratio of Volumes, where VB is the Clearance Volume
Engine Performance Output Brake Horsepower (BHP)
It represents the net power output of the engine at the crankshaft BHP = T N / 5252 T = torque in foot pounds N = engine speed in rpm Torque is usually measured with a Prony Brake or by other means (e.g., gen output converted to HP)
Indicated Horsepower (IHP)
IHP = PiLAN / 33,000 Pi = mean effective pressure in psi L = length of stroke in feet A = area of the piston in square inches N = number of power strokes per minute If L is in inches, the constant becomes 396,000 Area (square inches) L (inches) = Displacement of a cylinder. That multiplied by the number of cylinders gives the Engine Displacement (DISP).
Frictional Horsepower (FHP)
This is the power required to operate the engine components and move air into, and exhaust gasses out of, the engine.
The relationship between horsepowers is:
BHP = IHP - FHP or FHP = IHP - BHP
Torque (T)
This represents the twisting or turning effort of the engine.
T = 5252 x BHP / N where N = number of engine revolutions/minute Also T = BMEP x DISP / 75.4 where DISP is that per revolution.
Brake Mean Effective Pressure (BMEP)
This is a theoretical term. It represents the mean effective pressure in psi (pounds per square inch) for each power stroke at rated BHP for the 4-stroke and 2-stroke cycle engines respectively.
BMEP = 792,000 x BHP / (D x N) [4 cycle]
BMEP = 396,000 x BHP / (D x N) [2 cycle]
where:
D = DISP = total cylinder displacement of engine N = Number of revolutions per minute
Typical Engine Design Criteria There are some limits imposed by physical forces that limit the power output of an engine to ensure reliability and longevity. They are as follows:
- BMEP - Typical BMEP for 2-stroke engines run from 150 to 160 psi and for 4-stroke engines run from 300 to 320 psi.
- Peak Firing Pressure - 1200 to 1500 psi
- Piston Speed - 1500 to 2000 fpm (large engines) where Piston Speed = piston stroke (in) x 2 x rpm / 12 (OP engine at 900 rpm has a piston speed of 1500 fpm.)
Engine Efficiency Mechanical Efficiency em = BHP / IHP (.85-.90)
Thermal Efficiency et = energy output of crankshaft energy input of fuel oil (all in BTU units) 1 BHP = 2545 BTU/hr
Typical heat balance for a diesel engine (HEAT IN = HEAT OUT)
Power output (BHP) = 33 - 40%
Exhaust and radiation = 30 - 33%
Jacket water cooling = 10 - 15%
Lube oil cooler = 4 - 8%
Turbocharger aftercooler = 5 - 10%
Generator inefficiency = 3 - 5%
Radiation (engine to room) = 2%
Brake Specific Fuel Consumption (bsfc)
This is an indication of engine efficiency.
bsfc = fuel burned per hour / BHP Typical bsfc for modern engine is about .30 to
.325 lbs per bhp-hr.
Super charging with blowers and/or turbochargers Blowers are gear-driven from engine and require engine horsepower.
Turbochargers are powered from the energy in the engine exhaust and do not usually require engine horsepower. The EMD turbocharger requires engine horsepower during startup and at low power levels.
Typical times for events to occur in a 900 rpm, 2-stroke or 4-stroke diesel engine are shown in Figures 2-26 and 2-27. Note that all events in the 2-stroke cycle engine at 900 rpm must occur within 67 thousandths (.067) of a second.
FIGURE 2-26 Typical Times for Events 900 rpm Diesel Engines Figure 2-27A Typical Timing Diagrams - 4 Stroke Cycle Figure 2-27B Typical Timing Diagrams - OP Engine END OF CHAPTER 2