ML20043F655

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Training Material for E-111 Emergency Diesel Generator Course, Chapter 4 (3-16), Combustion Air, Fuel & Exhaust Systems
ML20043F655
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Issue date: 02/12/2020
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Gary Callaway
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Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-1 of 48 USNRC HRTD 4.0 COMBUSTION

AIR, FUEL, AND EXHAUST SYSTEMS This chapter presents the basic principles of combustion air, diesel fuel, and exhaust systems, and their inter-relationship.

Learning Objectives:

As a result of this lesson, 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 fuel systems, and state the purpose of each.
3. Identify major components of a diesel engine fuel system and understand the purpose of each.
4. List the functions that must be performed by the components of a diesel engine fuel injection system.
5. Describe the construction and operation of a typical diesel engine fuel injection pump.
6. Describe the construction and operation of a typical diesel engine fuel injection nozzle.
7. Describe the construction and operation of a unit type diesel engine fuel injector.
8. Describe how the governor operates to control fuel delivery to each cylinder.
9. Describe the exhaust system functions, construction, and operation.

NOTE: Text, illustrations, and examples in this chapter are based on typical nuclear plant applications and, therefore, may not apply to a specific system or design.

4.1 Introduction to the Air and Fuel Systems In Chapter 2, we discussed the three elements of combustion: fuel, oxygen, and heat. The power developed by the diesel engine is directly related to the amount of fuel it can burn efficiently. In this chapter, we discuss the inter-relationship of these elements in the development of power and the manufacturers rating of a diesel engine.

4.1.1 Air and Fuel for Combustion 4.1.1.1 Composition of Air Air is composed primarily of nitrogen and oxygen. By weight, 76% of the air is nitrogen (N2) while 23% is oxygen (02). The remaining 1% is made up of other substances such as carbon dioxide, carbon monoxide, water vapor, dust, and other materials and gases. Of this mixture, only the oxygen is required for combustion.

4.1.1.2 Composition of Fuel Fuel oil is mostly two elements, hydrogen and carbon and, therefore, is classified as a hydrocarbon fuel. Its composition by weight is approximately 15 % hydrogen (H2) and 85

% carbon (C). Fuel oil also contains trace amounts of sulfur, organic compounds, and other substances. Paraffin-based fuel oil has the empirical chemical formula CnH2n+ 2 Fuel oil specifications are normally established by the engine manufacturer,

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-2 of 48 USNRC HRTD and incorporated into plant Technical Specifications (TS). Generic fuel oil specifications include the following:

  • Cetane Number: 40 (Minimum)
  • Total Sulfur: 15ppm (Maximum)
  • Viscosity: 32-40SUS @ 100ºF
  • Ash Content: 0.02% (by Weight)
  • Heating Value: 18,190 BTU/lb Minimum
  • Cloud Point: above 40oF NOTE: EPA lowered the maximum Sulfur content of Diesel Fuels in steps, from 3000ppm (0.3%) to 500ppm (0.05%), then 15ppm (0.0015%). Chapter 13 will cover some potential issues associated with Ultra-Low Sulfur (ULS) fuel, all of which appear to be readily manageable by NPP licensees.

4.1.2 Combustion Chemistry 4.1.2.1 Combustion with Theoretical Air Assume we mix a specific amount of fuel with exactly the amount of air required for complete combustion (theoretical air). This is based on the diesel engine operating at full load for optimum combustion heat. In this mixture, each carbon (C) atom and each hydrogen (H) atom will be in contact with the required number of oxygen (O) atoms for complete combustion. Then, the maximum conversion of chemical energy to thermal energy would occur. Under these ideal conditions, the following stoichiometric (chemically correct) reaction will occur.

2CnH2n + 2 + (3n + 1)O2 2nCO2 + (2n + 2)H20 Actually, not every atom of fuel mixes with exactly the correct number of oxygen atoms.

This is the result of incomplete mixing, residual exhaust gases in the cylinder, and the presence of various contaminants in the air. Under these imperfect conditions, complete combustion cannot occur.

4.1.2.2 Combustion with Excess Air In order to compensate for these less than perfect conditions, and to ensure that nearly complete combustion does occur, diesel engines are designed to operate with 15 to 25 % excess air.

4.1.2.3 Air to Power Relationship Assuming all other factors are equal, the relationship of air flow to the power developed by a diesel engine is as follows:

  • The greater the mass of air entering the engine, the more oxygen will be available to support combustion.
  • More oxygen available for combustion, means more fuel can be injected into the cylinder and still burn efficiently.
  • The more fuel which can be burned efficiently, the more usable power the engine can develop.

4.1.3 The Combustion Process We will begin this section by examining the series of events which occur in the cylinder as the fuel oil is sprayed into the heated air

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-3 of 48 USNRC HRTD charge. This fuel charge goes through the four phases listed below as combustion occurs.

Refer to Figure 4-1 and points C, D, and E on Figure 4-2 for the following discussion.

4.1.3.1 Delay Period A delay period occurs from the time of initial fuel injection to when actual ignition takes place. The delay period consists of two parts. First, the physical delay, which is the time it takes the fuel to atomize, mix with the air charge, and vaporize, thereby creating a combustible mixture of air and fuel. Second, the chemical delay is the localized pre-flame oxidation caused by the catalytic effect of high temperature wall surfaces and hot residual exhaust particles. These localized regions can reach 1000ºF to 2000ºF.

4.1.3.2 Rapid Combustion After the delay period, there is rapid combustion of the fuel which entered the cylinder during the delay period.

4.1.3.3 Continued Combustion As fuel continues to be injected, normal combustion

occurs, increasing the temperature and pressure of the heated gases within the cylinder. Normal peak temperatures for such combustion range from 3500ºF to 4500ºF.

4.1.3.4 After-burning After the injection is completed, there is an after-burning period where any remaining fuel combines with the remaining oxygen to essentially complete the burning process.

4.2 The Intake Air System 4.2.1 Intake Air System Requirements In order for the engine to operate efficiently and reliably, the intake air system must provide the following:

4.2.1.1 Sufficient Air Quantity The system must supply a sufficient quantity of air to each cylinder to support complete combustion under maximum load. This typically means 15 25% excess air.

For 2-stroke cycle engines, additional air must also be provided to ensure proper scavenging of exhaust gases from cylinders.

4.2.1.2 Clean Air The incoming air charge must be clean.

That is, it must be free of abrasive particles which would damage the engines internal parts. Another type of contamination would be engine exhaust recirculation to the air intake, which would lower Oxygen content.

4.2.1.3 Cool Air The mass of air which can be contained in a specific volume is dependent on air density.

As air temperature increases its density decreases (if pressure remains constant).

Air that is too warm will not provide sufficient oxygen to support complete combustion.

Potential causes of that are recirculation of EDG room cooling air to the engine intake or, if the engine normally draws its air from the EDG room, failure of room cooling fans.

4.2.1.4 Reduced Noise Levels

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-4 of 48 USNRC HRTD Due to the cyclical action of the engine, pressure pulsations tend to develop in the flow of the incoming air charge. These pressure pulsations cause an increased noise level within the diesel space. They also create vibrations in the intake air piping, which can lead to damage and component or system failure. Flexible connections are usually provided to permit thermal expansion and isolate engine vibrations from the piping.

4.2.2 The Typical Intake Air System The intake air system shown in Figure 4-3 (the right hand section) displays the components which would be found in a typical nuclear plant application.

4.2.2.1 Intake Air Filter The intake air filter removes particulate suspended in the air prior to entering the system. It also helps to remove any excess moisture in the air and may reduce the noise level of the incoming air charge.

Various types of air filters are used.

Sometimes, two types will be combined for a single application. The two most common types, dry and oil bath, are discussed in detail below.

  • Dry Type Filters (Figure 4-4) use a porous, fibrous cloth or paper type media. As air passes through the media airborne particulates are captured and held, allowing only clean air to reach the engine. Periodically the media becomes restricted due to a buildup of the particulate. Since this restriction then reduces the air flow to the engine, the media or elements must be replaced.
  • Oil Bath Air Filters are very effective at removing particulate from the incoming air charge. The oil bath air filter shown in Figure 4-5 consists of a cylindrical housing, internal air piping, an oil reservoir, a wire mesh filter, and various baffles.

Incoming air enters the housing and travels downward toward the oil reservoir. As the air reaches the oil pool its forced to change direction 180o. This sharp change in direction causes the heavier particles to sling out from the air and become trapped by the oil. The air and lighter particles pick up some of the oil and carry it upward into the wire mesh. The oil and lighter particulate become trapped by the wire mesh allowing only clean air to pass through to the engine.

Periodic cleaning of the oil reservoir and wire mesh is the only maintenance needed.

4.2.2.2 Intake Air Silencer The intake air silencer consists of a plenum type housing which may include chambers and baffles to reduce or dampen the pulsations which have developed in the incoming air flow. Often, some form of sound deadening material may be included.

In some installations, the intake air silencer is incorporated into the intake air filter.

4.2.2.3 Intake Air Piping The intake air piping makes the physical connection between the intake air filter and silencer. This piping should be as short as possible and as large in diameter as practical so as to provide a minimal resistance to the air flow. Sharp bends and fittings should be kept to a minimum to ensure free air flow to the engine.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-5 of 48 USNRC HRTD 4.2.3 Blowers and Turbochargers As discussed previously, the power an engine can develop depends on the amount of air available for combustion. By mechanically increasing the air flow into the engine, the power produced by that engine will be increased. Two devices are commonly used on diesel engines to increase the quantity of air into the cylinders.

4.2.3.1 Blower/Supercharger The blower (Figure 4-6), sometimes called a supercharger, is a positive displacement air pump which delivers air to the engine. It consists of a pair of helical rotors inside a housing. As the rotors turn they draw air in through the intake air piping, force it through the blower housing and discharge it under pressure into the intake air manifold.

The blower is mechanically driven by the engine accessory drive gear train, so the quantity of air delivered by the blower is a function of engine rpm. These units require power to operate but the power gained from increased air flow more than offsets the power needed to operate the blower.

4.2.3.2 Turbocharger (Figure 4-7)

The turbocharger also provides an increase in intake air flow to the engine with a corresponding increase in power output.

Unlike the blower, the turbocharger uses a centrifugal compressor. The impeller is surrounded by a scroll-shaped compressor housing, much like a centrifugal pump. The impeller is attached to the turbine shaft. A turbine wheel is attached to the other end of the turbine shaft. This turbine wheel is encased in a turbine housing connected to the compressor housing by the bearing housing. The inlet of the turbine housing is connected to the outlet of the engine exhaust manifold.

Hot exhaust gases, which contain a substantial amount of thermal energy, enter the turbine housing, pass through a set of stationary blades, and are directed against blades of the turbine wheel. As these gases pass through the turbine wheel, they expand and cool, thereby releasing energy to rotate the turbine wheel and its shaft, which also rotates the compressor connected to it.

Air is drawn into the center (inducer) of the compressor wheel. Rotation of the compressor wheel throws the air radially, by centrifugal force, increasing its velocity (kinetic energy). As the air exits the compressor wheel at high velocity, it enters the plenum-like compressor housing. The velocity of the air is suddenly decreased resulting in a sharp increase in the pressure in the compressor housing. This increase in pressure increases the density of the air flow entering the engine cylinders.

4.2.3.3 Blower vs Turbocharger There are significant advantages in using a turbocharger rather than or in addition to a mechanically driven blower.

1. Since the turbocharger is powered by the heat of the exhaust gases, it does not require power directly from the engine. It does present a restriction to exhaust gas flow and so creates a back-pressure in the exhaust manifold. However, the overall result is a greater increase in engine power output than would be attained with an engine driven blower.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-6 of 48 USNRC HRTD

2. The increased mass of intake air provided by the turbocharger is determined by the heat energy of the exhaust gases leaving the engine. The temperature and mass flow of the exhaust gases are functions of the quantity of fuel being burned in the cylinders, which is in turn a function of the load applied to the engine. As the load on the engine increases, the quantity of fuel burned increases. This increase in heat input to the engine increases the heat energy of the exhaust and therefore the amount of energy driving the turbocharger. The result is an increase in the mass of air entering the cylinders and a subsequent increase in engine power which gives the turbocharger a unique load-following ability not available with a blower.

Some engines, such as the Fairbanks-Morse opposed piston engine, efficiently combine a turbocharger and a blower. See Figure 4-8.

The EMD engine turbocharger is initially engine-gear driven during startup to provide exhaust scavenging until hot exhaust gases drive the turbocharger faster than the gear.

4.2.3.4 Intercoolers - Aftercoolers Turbochargers, while increasing the flow of intake air to the cylinders, also increase the temperature of the air. This increase in air temperature reduces the density of the air charge and therefore the amount of oxygen available for combustion.

To compensate for this reduction in density, most turbocharged diesel engines utilize heat exchangers known as intercoolers or aftercoolers. These are air-to-jacket-water heat exchangers located between the discharge of the turbocharger and the air intake manifold.

They reduce the temperature of the intake air charge by transferring excess heat from the air charge to the engine jacket water cooling system.

The water for cooling the Intercooler-aftercooler will be discussed further in Chapter 6, Engine Cooling Systems.

4.2.3.5 Diesel Engine Ratings The quantity, quality, and temperature of ambient and combustion air directly affect engine performance. Therefore, engine manufacturers rate and de-rate their engine based upon a set of standard conditions. A typical basis for ratings for one manufacturer is illustrated by Figure 4-9.

4.3 The Diesel Engine Fuel System Once the cylinder has been charged with air and the air compressed, raising its temperature above the ignition point for the fuel oil, a metered quantity of fuel is sprayed into the cylinder and combustion occurs.

The diesel engine fuel system can be divided into three separate but interdependent subsystems. Each system must perform reliably and efficiently in order to support the operation of the engine.

4.3.1 Fuel Oil Storage and Transfer System (Figure 4-11) 4.3.1.1 Fuel Oil Storage Tank Typically the fuel oil storage tank is sized to provide the engine with a specified (e.g. 5-or 7-day) fuel supply when operating at full

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-7 of 48 USNRC HRTD power. The required size of the tank depends on the rate of fuel consumption for the engine, at the specified heat value of the fuel. EMD and OP engines consume approximately 4,500 gallons per day.

Larger Cooper and Pielstick engines may consume around 10,000 gallons per day!

Fuel oil storage tanks usually incorporate a low point or sump for the collection and removal of water and sediment. This low point can be pumped out periodically to remove moisture and heavy contaminants.

In determining fuel available and accessible in a tank, consideration must be given to the amount of fuel between the pump suction level and the lowest level before tank refill.

4.3.1.2 Fuel Oil Transfer Pumps Fuel oil transfer pumps are fractional horsepower pumps that supply fuel to elevated day tanks that supply a positive pressure fuel head in sufficient quantity for initial starting and operation of the EDG.

Fuel oil transfer pumps may be placed in one of three locations, depending on the site-specific design.

  • The pumps may be submerged below the fuel oil level in the fuel oil storage tank. While ensuring a positive suction head for the pumps, this configuration makes pump maintenance somewhat difficult.
  • A second option involves locating the pumps in a pit, placing the suction connection below the fuel level in the storage tank. Pump access is improved but still less than ideal.
  • A better approach involves placing the pumps in the diesel room at floor level while placing jet pumps (ejectors) inside the fuel oil storage tank (as shown in Figure 4-11). These pumps do not draw suction directly from the storage tank but from the fuel oil day tank and they discharge to the nozzle of the jet pumps.

The venturi action of the jet pumps transfers the fuel oil into the fuel oil day tank. This method allows for easy maintenance of the transfer pumps, and the jet pumps require no maintenance as they have no moving parts to wear out.

4.3.2 Fuel Oil Supply System (Figure 4-12) 4.3.2.1 Fuel Oil Day Tank The day tank stores a limited amount of fuel at a location near the diesel engine. It may be located in the diesel room itself or in a room adjacent to the diesel room. The size of the tank depends on its location and on any governing codes or standards, including applicable technical specifications.

The tank is usually positioned so the fuel level is above the suction of the fuel oil supply pumps, to ensure a positive flow into the pumps. Automatic level switches in the day tank activate the fuel oil transfer pumps to ensure the level of the fuel in the tank is kept above a specified minimum.

4.3.3.2 Fuel Oil Strainers (Figure 4-13)

These strainers, located between the fuel oil day tank and the suction of the fuel oil supply pumps, remove large particulate, sediment, and moisture from the fuel. They are usually of the duplex type incorporating a three-way valve which allows one strainer element to

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-8 of 48 USNRC HRTD be taken out of service for cleaning while the EDG remains operational. The strainer element is usually a fine wire mesh which can be removed periodically for cleaning.

Valves on duplex type strainers should be left in a position that uses only one of the strainer elements. That makes the other immediately available if needed (rather than both being plugged in case of a problem).

4.3.3.3 Fuel Oil Supply Pumps (Fig.4-14)

Two fuel oil supply pumps are normally provided. One is engine driven and is functions whenever the engine is running.

An electric-driven supply pump is also provided to ensure positive fuel flow during startup, before the engine-driven pump can provide sufficient pressure for engine run.

These pumps are usually positive displacement gear type pumps though some applications use screw type pumps. They supply fuel oil to the fuel header under a fairly low pressure (e.g. 45 psig).

4.3.3.4 Fuel Oil Filters (Figure 4-15)

These

filters, located between the discharge of the fuel oil supply pumps and the fuel oil manifold or header, remove any minute particles (e.g. 5 micron or as the engine manufacturer specifies) that may be in the diesel fuel.

These filters are normally of the duplex type with a three-way valve to allow for replacement of the elements while the engine remains in operation. The elements use a paper or fabric like media to trap these extremely small particles. Again, the valve handle should be in a position to use only one of the filter elements at a time.

4.4 Fuel Injection Systems The fuel injection system has the most precise and demanding job of the three systems. Regardless design used, the fuel injection system must perform each of the following functions:

  • The quantity of fuel delivered to each cylinder is metered to control the power produced by the engine.
  • Inject the fuel into the heated air charge at a time relative to the rotation of the crankshaft, which produces the desired combustion characteristics.
  • Inject the fuel at a rate which will ensure smooth, complete, efficient combustion.
  • The injection must begin and end quickly. This is to prevent uncontrolled distribution and poorly atomized fuel from entering the cylinder. Under these conditions, the fuel would not mix adequately with the oxygen in the cylinder which would waste fuel and produce soot and cause engine problems.
  • The fuel must be sufficiently atomized to provide optimum mixing of the fuel with the compressed air charge. The more effective the atomization, the more complete the combustion.
  • The fuel spray must be distributed evenly throughout the combustion space. This helps to ensure effective mixing and complete combustion.

Two basic types of fuel injection systems are commonly used on diesel engines. The

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-9 of 48 USNRC HRTD pump and nozzle type is a two-part system with an injection pump and a separate nozzle. The unit injector type combines the injection pump and nozzle into a single unit.

4.4.1 Fuel Injection Pump and Nozzle System (Figure 4-16)

In the pump and nozzle system, a fuel injection pump, operated by the engine camshaft, injects the fuel at the proper time and proper rate, and stops the delivery quickly to ensure

clean, efficient combustion. The injection nozzle, mounted in each cylinder head (on 4-stroke and 2-stroke conventional engines) or through the wall of the cylinder liner (on opposed piston engines), atomizes the fuel while distributing it evenly throughout the combustion space.

4.4.1.1 Injection Pump Construction (Figure 4-17)

The main components of the injection pump are the plunger and barrel. These two items are precision machined and fitted to ensure precise delivery of the fuel. Together, the plunger and barrel regulate the quantity of fuel entering the cylinder while establishing other key injection characteristics such as injection timing and injection rate.

The pump body is the main structural and pressure-retaining component of the pump.

It houses the plunger and barrel assembly.

A spur gear keyed to the plunger allows the plunger and its helix to be rotated for the purpose of metering the fuel. A fuel control rack, positioned by the engine governor, engages with the spur gear to provide a means for rotating the plunger from outside the body, thereby metering the fuel.

The return spring and spring retainer return the plunger to the non-delivery position, while keeping the cam follower in contact with the injection cam.

A delivery valve assembly at the discharge of the plunger acts as a spring-loaded check valve which prevents the back flow of fuel during non-delivery.

4.4.1.2 Injection Pump Operation Refer to the injection pump in Figure 4-17.

Though there are different pump designs, all of them are constant stroke, variable volume (as dictated by governor demand for more, or less, fuel). And they all depend on the fact liquids are nearly incompressible.

  • Principle of Operation - The plunger is precision-fitted to the barrel which has separate fill and spill ports. A single or double helix is cut into the end of the plunger. A slot/drilled passage connects the plungers delivery end to the helix.

The plunger has a mechanically constant stroke length established by the lift of the injection cam lobe. The fuel delivery is determined by "effective" stroke length, which is created by the indexing of fill and spill ports with the plunger helix. The effective stroke is that portion of the mechanical stroke where both the fill and spill ports are simultaneously blocked.

Whenever both ports are closed at the same time, fuel pressure builds up within the barrel and is then directed to the nozzle assembly. Rotation of the plunger within the barrel changes the relationship of the helix to the ports. This changes the length of the effective stroke and, thereby, the fuel quantity delivered.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-10 of 48 USNRC HRTD

  • Zero Fuel Delivery (Figure 4-18)- At zero delivery, the slot aligns with one of the ports so there is no time when both ports are covered simultaneously. Fuel moves back and forth through the slot and in and out of the spill port.
  • Engine Idling - With the plunger at the bottom of its stroke, fuel enters through the fill port to fill the barrel. As the plunger moves upward, both ports become blocked and fuel is delivered to the injection nozzle where it is sprayed into the cylinder.

Continued upward movement of the plunger delivers fuel until the helix uncovers the spill port at which time the injection ceases, and fuel passes through the slot and out the spill port.

The plunger completes its strokes, returning to the bottom of its stroke where it is again filled with fuel.

  • Low Power - At low power, the plunger is rotated as shown in Figure 4-19. With the plunger at the bottom of its stroke, the barrel again fills with fuel. Upward movement of the plunger closes off both ports. Fuel continues to be delivered until the helix uncovers the spill port, stopping the fuel injection. Rotation of the plunger has increased the effective length of the stroke and therefore the amount of fuel injected into the cylinder.
  • Full Power - At full load, maximum fuel delivery is required. The plunger is now rotated as shown in Figure 4-20. With the plunger at the bottom of its stroke, the barrel is again filled with fuel.

Upward movement of the plunger blocks off both ports and begins the delivery of fuel into the combustion chamber. With the plunger in the full fuel position, the ports are covered for the greatest amount of time with the maximum amount of fuel entering the cylinder.

As the helix uncovers the spill port, fuel delivery stops just as before. The plunger then completes its stroke and returns to bottom position to be refilled.

4.4.2 Fuel Injection Nozzles (Figure 4-21)

The fuel injection nozzle has the job of atomizing the fuel as it enters the combustion space and distributing the fuel evenly for efficient combustion.

4.4.2.1 Nozzle Body or Housing The housing or body is the main structural and pressure retaining component of the assembly. Fuel supply and return lines connect to the upper end of the nozzle body while at the lower end is the nozzle spray tip.

4.4.2.2 Nozzle Spray Tip (Figure 4-22)

The spray tip actually enters the combustion space. A series of small holes in the tip atomize the fuel as it passes through. The size of the holes determines the degree of atomization while the number of holes and their angle distribute the fuel evenly and correctly throughout the combustion chamber.

4.4.2.3 Pintle Nozzle (Figure 4-23)

Some engines, such as the Fairbanks-Morse opposed piston, use a single-hole,

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-11 of 48 USNRC HRTD "pintle" type nozzle. Here, a small plunger or pintle passes through the single hole. As fuel is delivered, the pintle lifts up creating a cone-shaped fuel spray which atomizes and distributes the fuel for efficient combustion.

4.4.2.4 Nozzle Valve Assembly The nozzle valve assembly consists of a needle valve in a fitted, lapped cylinder. It may be part of the spray tip, or a separate unit. Its function is to prevent combustion gases from entering the nozzle assembly.

The nozzle valve is held in the seated position by the nozzle spring assembly.

4.4.2.5 Nozzle Spring Assembly A spring seat directs the force of the spring against the upper tip of the nozzle valve.

The force of the spring acting on the nozzle valve sets the pressure of the fuel being injected into the cylinder. The spring force is adjustable by either adjusting a screw or changing the thickness of the shim pack at the upper end of the spring.

4.4.3 Injection Nozzle Operation (Fig. 4-21)

The fuel delivered by the injection pump passes through a heavy walled fuel pipe to the nozzle assembly. This fuel is directed through internal passages in the nozzle body to the nozzle valve near its seat.

The high pressure fuel creates an upward force against the needle valve. When the force of the fuel is sufficient to overcome the spring force, the nozzle valve unseats, and fuel is injected into the combustion space.

As soon as fuel delivery stops, the sudden drop in pressure allows the nozzle valve to quickly seat, stopping the injection.

4.4.4 Unit Type Fuel Injectors The unit type injectors used on the EMD 2-stroke cycle engine combine the injection pump and injection nozzle into a single unit installed in each cylinder head. With this type injector, high pressure fuel lines and their potential for leakage are eliminated.

The main components of the unit type injector are the matched and lapped plunger and bushing assembly as shown in Figure 4-24. This plunger uses an upper and lower helix to control injection timing and duration with a T-shaped drilled passage to bypass the fuel. The bushing has an upper port and a lower port positioned 180 to each other.

The body and nut form the structural portion of the injector. The rack gear engages with the spur gear, which is indexed to the plunger. The spur gear causes the plunger to rotate while allowing it to move freely up and down.

A follower, actuated by the injector lobe on the engine cam lift, connects to the end of the plunger. The follower spring returns the plunger to its upper most position when the cam in on its base circle. A check valve located below the plunger and bushing prevents the back flow of fuel during the upward stroke of the plunger.

A spring-loaded needle valve is located in the injector spray tip. The spring holds the valve seated while establishing the injection pressure.

4.4.5 Injector Operation (Figure 4-25)

The fuel pump supplies fuel to the unit injector at low pressure, about 50 psi.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-12 of 48 USNRC HRTD As with the injection pump discussed previously, fuel injection occurs whenever both ports (upper and lower for unit injectors) are closed simultaneously.

Figure 4-25 illustrates the following. With the plunger at the top of its stroke, fuel enters through the lower port to fill the cavity below the plunger. The fuel also travels upward through a drilled passage in the plunger and bypasses out the upper port.

Downward plunger movement closes the lower port but allows fuel to bypass out the upper port. As soon as the upper port is closed by the helix on the plunger, injection begins. Fuel is delivered as long as both ports are closed.

Injection stops when the lower helix uncovers the lower port and fuel begins to bypass the plunger and out the lower port into the fuel return passages of the injector body. The plunger continues downward to the end of its mechanical stroke.

4.4.5.1 Zero Fuel Delivery (Figure 4-26)

With the plunger in its upper-most position, fuel passes through the lower port to fill the bushing. As the plunger moves downward, the plunger blocks off the lower port, and fuel bypasses through the drilled passage.

When the engine is to be shut down, the fuel control racks are moved to the zero fuel position. This indexes the plunger helix to the ports as shown in Figure 4-26. In this position, there is no time during the stroke when both ports are simultaneously blocked. Fuel simply bypasses into the return passages in the body of the injector, and none is delivered to the cylinder.

4.4.5.2 Low Power (Figure 4-27)

At idle or low power, the bushing is filled while the plunger is at its upper most point of travel. Downward movement of the plunger blocks off the lower port, and the fuel bypasses the plunger and exits through the upper port until the upper helix closes the upper port. Injection begins as soon as the upper port is covered and continues until the lower helix passes the lower port allowing the fuel to bypass into the return passages of the injector body.

The beginning of the injection is determined by the upper helix while the ending of the injection is regulated by the lower helix. As in the other injectors, the effective stroke is the distance the plunger travels when both ports are blocked.

4.5.5.3 Full Fuel (Figure 4-28)

For maximum fuel delivery, the plunger is indexed as shown in Figure 4-28. With the bushing full, injection begins as the upper helix blocks the upper port. Downward movement of the plunger delivers fuel until the lower helix uncovers the lower port.

In this orientation, the effective stroke length is at its maximum. Maximum fuel is delivered leading to maximum power output for the engine.

4.4.5.4 Needle Valve Action (Figure 4-24)

The high pressure fuel is directed through internal passages to the needle valve. The force of fuel acting on the needle valve causes the valve to overcome the spring force and inject fuel into the combustion space.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-13 of 48 USNRC HRTD 4.4.5.5 Injector Timing Static injector timing is established by the relationship between the injector plunger and the fuel injection lobe on the engine camshaft. During engine operation, timing must change slightly according to the load on the engine. This incremental timing is accomplished by the shape of the helix on the plunger. As the engine load increases, the helix closes off the upper port earlier in the timing sequence which gives the cylinder more time to complete the combustion process. Helix design is very important and will vary for different engines, as well as for different applications of the same engine (e.g., nuclear, rail, marine).

4.5 The Exhaust System Just as the intake air system is designed to efficiently supply fresh air to the cylinders, the exhaust system is designed to efficiently remove burned gases out of the cylinders.

To maximize the amount of air available for combustion, the exhaust system must minimize the amount of exhaust gases remaining in the cylinders. The basic configuration of a diesel engine exhaust system is shown in Figure 4-3 (the left half).

4.5.1 Exhaust System Requirements Following are the fundamental requirements of the engine exhaust system:

4.5.1.1 Minimal Resistance to Flow The exhaust system must be designed and constructed so that it presents a minimum resistance to flow. Engine horsepower ratings are based upon not exceeding a specified exhaust back pressure.

4.5.1.2 Reduce Noise Levels The high energy level and large volume of exhaust gas flow can create a substantial noise problem. The exhaust system must include some form of noise suppression device to reduce the noise level of the exhaust gases to an acceptable level.

4.5.1.3 Direct Exhaust Gas Flow The system must be designed and constructed in a manner which will direct the exhaust gases far enough away from the combustion air intake to prevent the cross-over of exhaust gases into the air intake, thereby contaminating the intake air and reducing the effectiveness of the combustion process.

4.5.2 Exhaust System Components 4.5.2.1 Exhaust Manifold or Header The exhaust manifold or header collects the exhaust gases at the cylinder heads or exhaust ports and directs the gas flow to the inlet of the turbocharger. Some exhaust manifolds or headers are water cooled which reduces the buildup of heat in the diesel engine space.

4.5.2.2 Exhaust Gas Muffler The exhaust muffler (Figure 4-10), reduces exhaust noise by dampening the pulsations resulting from the cyclical action of the engine. This is accomplished by passing the gases through a series of chambers and baffles causing a gradual expansion of the gases and a reduction in noise emission.

The chamber dimensions and shape may be tuned for anti-resonant action at engine pulse frequencies, further reducing noise.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-14 of 48 USNRC HRTD 4.5.2.3 Exhaust Relief Valve In nuclear applications where the exhaust gas silencer and/or piping are exposed to the atmosphere and unprotected, an automatic relief valve may be installed. This device functions similarly to a conventional safety valve. It may be either spring-loaded or weight loaded in the closed position; however, some are simply a thin sheet of metal (such as heavy aluminum foil) which will rupture at an excessive pressure.

Should the exhaust system downstream of the valve become damaged or clogged, the back-pressure in the exhaust will increase.

When the back pressure in the exhaust system exceeds a specified value, the relief valve will automatically open, or rupture, discharging the gases to the atmosphere and allowing the engine to continue to operate normally.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-15 of 48 USNRC HRTD Figure 4-1 Combustion Activity

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-16 of 48 USNRC HRTD Figure 4-2 Diesel Cycle - Pressure vs Stroke

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-17 of 48 USNRC HRTD Figure 4-3 Basic Intake and Exhaust System

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-18 of 48 USNRC HRTD Figure 4-4 Dry Type Air Filter

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-19 of 48 USNRC HRTD Figure 4-5 Oil Bath Air Filter

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-20 of 48 USNRC HRTD Figure 4-6 Blower/Supercharger

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-21 of 48 USNRC HRTD Figure 4-7 Exhaust Driven Turbocharger

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-22 of 48 USNRC HRTD Figure 4-8 Intake Air and Exhaust Flow

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-23 of 48 USNRC HRTD Figure 4-9 Typical Diesel Generator Basis for Ratings/DEMA Ratings

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-24 of 48 USNRC HRTD Figure 4-10 Exhaust Gas Muffler

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-25 of 48 USNRC HRTD Figure 4-11 Fuel Oil Storage and Transfer System

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-26 of 48 USNRC HRTD Figure 4-12 Fuel Oil Supply to Engine Fuel Header

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-27 of 48 USNRC HRTD Figure 4-13 Fuel Oil Strainer

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-28 of 48 USNRC HRTD Figure 4-15 Fuel Oil Filter Figure 4-14 Fuel Oil Supply Pump

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-29 of 48 USNRC HRTD Figure 4-16 Pump and Nozzle System

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-30 of 48 USNRC HRTD Figure 4-17 FM OP Engine Injection Pump Construction

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-31 of 48 USNRC HRTD Figure 4-18 Zero Fuel Delivery

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-32 of 48 USNRC HRTD Figure 4-20 Full Power Figure 4-19 Low Power

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-33 of 48 USNRC HRTD Figure 4-21 Injection Nozzle

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-34 of 48 USNRC HRTD Figure 4-22 Injection Nozzle Spray Pattern

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-35 of 48 USNRC HRTD Figure 4-23 Pintle Nozzle

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-36 of 48 USNRC HRTD Figure 4-24 Unit Type Fuel Injector

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-37 of 48 USNRC HRTD Figure 4-25 Injector Operation

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-38 of 48 USNRC HRTD Figure 4-26 Zero Fuel Delivery

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-39 of 48 USNRC HRTD Figure 4-27 Low Power

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-40 of 48 USNRC HRTD Figure 4-28 Full Power

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-41 of 48 USNRC HRTD Figure 4-29 Turbocharger Cutaway Section

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-42 of 48 USNRC HRTD Figure 4-30 Casing Assemblies

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-43 of 48 USNRC HRTD Figure 4-31 Brown and Boveri Turbocharger

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-44 of 48 USNRC HRTD WALKAROUND SESSION 4 4.0 COMBUSTION AIR, FUEL, AND EXHAUST SYSTEMS Purpose This session's purpose is to complement the classroom instruction of Chapter 4.

Learning Objectives Upon completion of this lesson you will become familiar with:

1. The appearance and function of the air combustion system and its components.
2. The appearance and function of the fuel oil system and its components.
3. The appearance and function of the exhaust system and its components.

4.1 Combustion Air System The instructor will use the OP engine cutaway to illustrate and explain the combustion air system and the combustion air intake manifold that would connect to the supercharger, turbocharger, intake air piping, filters, and silencers. He will explain the flow of combustion air into each engine cylinder.

The instructor will use cutaway blower and its component parts to illustrate how they function to pressurize combustion air and scavenge exhaust gases.

The instructor will use the cutaway turbocharger and its component parts to illustrate their function in converting high temperature exhaust gases into pressurized engine intake combustion air.

The instructor will use the cutaway dry air filter and its components, as well as the cutaway oil bath air filter and its components, to illustrate their features and functions.

4.2 Fuel Oil System The OP engine cutaway will be used to illustrate the flow path of fuel oil in the engine.

The instructor will use the cutaway fuel oil transfer pump to illustrate its component parts and their functions.

The cutaway fuel supply pump will be used to illustrate component parts and their functions.

The instructor will use the ALCO engine to illustrate the following:

  • The governor-to-fuel-rack linkage including the individual adjustable linkage to each engine cylinder fuel injection pump fuel metering gear.
  • How each cylinder fuel injection pump stroke is controlled by a cam on the engine camshaft.

The instructor will use the dual cutaway fuel oil filter and the cutaway fuel oil strainer to illustrate the component parts of each and their functions.

The instructor will use a cutaway fuel injection pump to illustrate component parts and their functions in metering and supplying fuel to the injection nozzle.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-45 of 48 USNRC HRTD The instructor will use the cutaway fuel injection nozzle to illustrate component parts and their function in the injection process.

4.3 Exhaust System The instructor will use the OP engine cutaway to illustrate:

  • The flow path of exhaust gases from each cylinders exhaust ports to exhaust manifold.

The instructor will use the turbocharger cutaway to illustrate how hot exhaust gases drive the turbine to produce pressurized intake combustion air.

The instructor will use the muffler cutaway to illustrate how its baffles reduce noise.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-46 of 48 USNRC HRTD HANDS-ON SESSION 4A 4A.0 COMBUSTION AIR, TURBOCHARGERS, AND SCAVENGING BLOWERS Purpose This session's purpose is to complement the classroom instruction of Chapter 4.

Learning Objectives Upon completion of this lesson you will be able to:

  • Better understand instructor presentation of the cutaway scavenging air blower.

4A.1 Turbochargers The purpose of the turbocharger is to provide air to the engine for combustion.

Before turbocharging became common, most engines of the 4-stroke cycle design were naturally aspirated. That is, the air was breathed into the cylinder by the action of the piston sucking the air into the engine.

This resulted in the air in the cylinder being slightly below atmospheric pressure and as a result, the air was less dense and contained less oxygen than normal air.

Because the engine is dependent on the amount of oxygen to burn the fuel, the engines output was restricted. By using a turbocharger, more air (oxygen) is pumped into the cylinder at positive pressure.

Therefore, the engine is capable of burning more fuel and putting out more power.

The same general principle applies to the 2-stroke cycle engine as well. The normal scavenging air blower output capacity was restricted by the engine output horsepower to drive it. Turbocharging a 2-stroke cycle engine considerably increases the power output capability without an increase in the horsepower to drive the blower since the energy to drive the turbocharger is derived from waste energy in the engine exhaust.

While it does cost some energy for the engine to overcome the increased exhaust back pressure required to drive the turbocharger, the net effect of the increased combustion air supply to the engine far outweighs the slight increase in the horsepower required to overcome the increase in exhaust back pressure.

Turbocharged engines are more efficient than blower-scavenged or naturally-aspirated engines, thus proving that turbo-charging assists the engines power output.

The turbocharger consists of five basic parts. See Figures 4-29 and 4-30.

1. The rotating assembly consisting of the turbine and the compressor mounted on a common shaft.
2. A center housing that holds the bearings that support the rotating assembly.
3. The compressor housing with its diffuser ring.
4. The exhaust casing which surrounds the turbine end of the rotating assembly.
5. The exhaust inlet casing with its nozzle ring to direct flow to the turbine blades.

On most of the turbochargers made by American manufacturers, the bearings are within the center housing and the lube oil supply is from the engine lube oil system. In

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-47 of 48 USNRC HRTD most turbochargers of European manufacture, the bearings are outboard of the turbine and compressor wheels and the oil is supplied from sumps within the turbocharger assembly and there is a self-contained lube oil pump. See Figure 4-31.

There are examples of these differing constructions available for students to study.

With directions from the instructor, students will disassemble a typical OP turbocharger by using the following procedure:

1. Remove the bolts holding the exhaust inlet casing to the exhaust housing and remove the exhaust inlet casing.
2. Remove the bolts holding the exhaust housing to the center (bearing) housing and remove the exhaust housing section.
3. Remove the bolts holding the air inlet casing to the air compressor housing and remove the inlet casing.
4. Remove the bolts holding the air compressor housing to the center housing and remove the compressor casing.
5. This leaves the rotating assembly supported by the center (bearing) housing. Remove the nut at the compressor end of the rotating assembly and use a puller for removing the compressor wheel from the rotating shaft. When the compressor wheel has been removed, then pull the shaft with the turbine wheel, from the bearing housing.
6. Inspect the bearings, including the thrust bearing surfaces.

While the turbocharger is disassembled, inspect nozzle ring for signs of damage and the presence of piston ring parts or other debris. Also inspect compressor wheel for damage, particularly the leading edges of the blades where air enters the compressor.

The turbocharger is reassembled in reverse order to that given above for disassembly.

However, it is necessary in the process to add or subtract shims to obtain the correct end clearance in the rotating assembly.

Also, take care in replacing the exhaust inlet casing and compressor inlet casing, to not damage the compressor wheel or the turbine wheel and nozzle ring.

Most turbocharged FM engines have an adapter section in the exhaust piping, just prior to the turbos exhaust inlet connection.

This section contains a conically shaped screen to keep engine parts from entering the turbocharger. Clearances between the end of the nozzle ring and the turbine blades are such that parts of piston rings that may have failed, for instance, will not pass through the turbine and may machine the turbine blades. The screens purpose is to trap these parts and prevent them from entering the turbocharger. There are small boxes attached to these transition sections in which this debris is collected. These should be inspected periodically to see that they are clear. If debris is found, the engine should be inspected to determine the source of the debris. The most likely source is pieces of broken piston rings so this section is often referred to as a ring catcher.

4A.2 Scavenging Air Blower Using the cutaway blower, the instructor will discuss its construction and operation. Its gear drive from the engine and mounting on the engine will be shown.

Emergency Diesel Generator Combustion Air, Fuel, and Exhaust Systems Rev 3/16 4-48 of 48 USNRC HRTD HANDS-ON SESSION 4B 4B.0 FUEL INJECTION PUMPS, FUEL INJECTION NOZZLES Purpose This session's purpose is to complement the classroom instruction of Chapter 4.

Learning Objectives Upon completion of this lesson you will be able to understand:

  • Disassembly/assembly and test of fuel injection pumps, and how they operate.
  • Disassembly/assembly and test of fuel injection nozzles, and how they operate.

4B.1 Injection Pumps and Nozzles The students, with the assistance and directions of the instructor, will dissemble a PC engine injection pump.

After disassembly, the pump parts will be examined and explained. This injection pump is similar to the OP engine injection pump, but much larger. The relationship of the injection pump to the cam shaft and tappet assembly will be explained and demonstrated.

4B.2 Injection Nozzle Assemblies The injection nozzle is a very important part of the engine. If the injection nozzle does not function properly, the fuel in not properly atomized into the cylinder and the fuel may not burn properly. This affects the operation of the engine by making exhaust temperatures higher than normal and fuel consumption higher than it ought to be.

The nozzle should open sharply when the fuel pressure reaches the specified point to open the nozzle and spray fuel. The nozzle should also close sharply, with no dribbling or leaking from the nozzle. Once it closes.

it should open at a specific pressure.

The purpose of this exercise is to disassemble several injection nozzles of various types, and inspect their parts. The nozzles are then reassembled and tested to see that they operate properly. The testing process is generally referred to as pop testing. Refer to nozzle illustrations, Figures 3-39 and 4-21 through 4-28.

At the instructors direction, disassemble assigned injector nozzle(s). The instructor will direct which parts are to be inspected and how an inspection is carried out.

Instruction will be given on reassembling the nozzles. Some nozzle assemblies have copper gaskets which must be annealed before being installed. Annealing is carried out by heating the gaskets until cherry red and quenching in water. The gasket must then be inspected to see that it was not damaged in the annealing process.

After nozzles are reassembled, attach the assembly to a nozzle test stand and pump fuel pressure up until the nozzle pops open.

It should open at or slightly above specified pressure and spray fuel in the form of a fine mist. The nozzle should not leak after it pops shut, nor while subsequently being pumped up again to recheck pop pressure.

CAUTION: Keep hands away from the nozzle tip when pop testing the nozzle. The fuel comes out of the nozzle at a high enough pressure to cause injury to hand or fingers in the spray path.

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