29-09-2016, 10:58 AM
1456742437-unit1.pdf (Size: 516.71 KB / Downloads: 34)
INTRODUCTION
A generating station in which diesel engine is used as the prime mover for the
generation of electrical energy is known as diesel power station.
In a diesel power station, diesel engine is used as the prime mover. The diesel burns
inside the engine and the products of this combustion act as the working fluid to produce
mechanical energy. The diesel engine drives alternator which converts mechanical
energy into electrical energy. As the generation cost is considerable due to high price of
diesel, therefore, such power stations are only used to produce small power.
Although steam power stations and hydro-electric plants are invariably used to generate
bulk power at cheaper costs, yet diesel power stations are finding favour at places where
demand of power is less, sufficient quantity of coal and water is not available and the
transportation facilities are inadequate. This plants are also standby sets for continuity of
supply to important points such as hospitals, radio stations, cinema houses and telephone
exchanges.
Advantages
(a) The design and layout of the plant are quite simple.
(b) It occupies less space as the number and size of the auxiliaries is small.
© It can be located at any place.
(d) It can be started quickly and it can pickup load in a short time.
(e) There are no standby losses.
(f) It requires less quantity of water for cooling.
(g) The overall cost is much less than that of steam power station of same
capacity.
(h) The thermal efficiency of the plant is higher than that of a steam power
station.
(i) It requires less operating staff.
Disadvantages
(a) The plant has high running charges as the fuel (diesel) used is costly.
(b) The plant doesn’t work satisfactorily under overload conditions for a longer
period.
© The plant can only generate small power.
(d) The cost of lubrication is generally high.
(e) The maintenances charges are generally high
Objectives
After studying this unit, you should be able to
understand about diesel engine power plant,
explain fuel injection system and its functions, and
describe various injection schemes.
4.2 ESSENTIAL ELEMENTS OF DIESEL
POWER PLANT
Fuel Supply System
It consists of storage tank, strainers, fuel transfer pump and all day fuel tank. The
fuel oil is supplied at the plant site by rail or road. The oil is stored in the storage
tank. From the storage tank, oil is pumped to smaller all day tank at daily or short
intervals. From this tank, fuel oil is passed through strainers to remove suspended
impurities. The clean oil is injected into the engine by fuel injection pump.
Air Intake System
This system supplies necessary air to the engine for fuel combustion. It consists of
pipes for the supply of fresh air to the engine manifold. Filters are provided to
remove dust particles from air which may act as abrasive in the engine cylinder.
Because a diesel engine requires close tolerances to achieve its compression ratio,
and because most diesel engines are either turbocharged or supercharged, the air
entering the engine must be clean, free of debris, and as cool as possible. Also, to
improve a turbocharged or supercharged engine’s efficiency, the compressed air
must be cooled after being compressed. The air intake system is designed to
perform these tasks. Air intake systems are usually one of two types, wet or dry. In
a wet filter intake system, as shown in the Figure 4.1, the air is sucked or bubbled
through a housing that holds a bath of oil such that the dirt in the air is removed by
the oil in the filter. The air then flows through a screen-type material to ensure any
entrained oil is removed from the air. In a dry filter system, paper, cloth, or a metal
screen material is used to catch and trap dirt before it enters the engine. In addition
to cleaning the air, the intake system is usually designed to intake fresh air from as
far away from the engine as practicable, usually just outside of the engine’s
building or enclosure. This provides the engine with a supply of air that has not
been heated by the engine’s own waste heat. The reason for ensuring that an
engine's air supply is as cool as possible is that cool air is denser than hot air. This
means that, per unit volume, cool air has more oxygen than hot air.
Thus, cool air provides more oxygen per cylinder charge than less dense, hot air.
More oxygen means a more efficient fuel burn and more power.
After being filtered, the air is routed by the intake system into the engine's intake
manifold or air box. The manifold or air box is the component that directs the
fresh air to each of the engine’s intake valves or ports. If the engine is
turbocharged or supercharged, the fresh air will be compressed with a blower and
possibly cooled before entering the intake manifold or air box. The intake system
also serves to reduce the air flow noise.
Exhaust System
This system leads the engine exhaust gas outside the building and discharges it
into atmosphere. A silencer is usually incorporated in the system to reduce the
noise level.
The exhaust system of a diesel engine performs three functions. First, the exhaust
system routes the spent combustion gasses away from the engine, where they are
diluted by the atmosphere. This keeps the area around the engine habitable.
Second, the exhaust system confines and routes the gases to the turbocharger, if
used. Third, the exhaust system allows mufflers to be used to reduce the engine
noise.
Cooling System
The heat released by the burning of fuel in the engine cylinder is partially
converted into work. The remainder part of the heat passes through the cylinder
wall, piston, rings etc. and may cause damage to system. In order to keep the
temperature of the engine parts within the safe operating limits, cooling is
provided. The cooling system consists of a water source, pump and cooling
towers. The pump circulates water through cylinder and head jacket. The water
takes away heat form the engine and it becomes hot. The hot water is cooled by
cooling towers and re circulated for cooling.
Lubricating System
The system minimises the wear of rubbing surfaces of the engine. It comprises of
lubricating oil tank, pump, filter and oil cooler. The lubrication oil is drawn from
the lubricating oil tank by the pump and is passed through filter to remove
impurities .The clean lubrication oil is delivered to the points which require
lubrication. The oil coolers incorporated in the system keep the temperature of the
oil low
An internal combustion engine would not run for even a few minutes if the
moving parts were allowed to make metal-to-metal contact. The heat generated
due to the tremendous amounts of friction would melt the metals, leading to the
destruction of the engine. To prevent this, all moving parts ride on a thin film of
oil that is pumped between all the moving parts of the engine. The oil serves two
purposes. One purpose is to lubricate the bearing surfaces. The other purpose is to
cool the bearings by absorbing the friction- generated heat. The flow of oil to the
moving parts is accomplished by the engine's internal lubricating system.
Oil is accumulated and stored in the engine's oil pan where one or more oil pumps
take suction and pump the oil through one or more oil filters as shown in the
figure. The filters clean the oil and remove any metal that the oil has picked up
due to wear. The cleaned oil then flows up into the engine's oil galleries. A
pressure relief valve(s) maintains oil pressure in the galleries and returns oil to the
oil pan upon high pressure. The oil galleries distribute the oil to all the bearing
surfaces in the engine. Once the oil has cooled and lubricated the bearing surfaces,
it flows out of the bearing and gravity-flows back into the oil pan. In medium to
large diesel engines, the oil is also cooled before being distributed into the block.
This is accomplished by either internal or external oil cooler. The lubrication
system also supplies oil to the engine’s governor.
Engine Starting System
This is an arrangement to rotate the engine initially, while starting, until firing
starts and the unit runs with its own power. Small sets are started manually by
handles but for larger units, compressed air is used for starting. In the latter case,
air at high pressure is admitted to a few of the cylinders, making them to act as
reciprocating air motors to turn over the engine shaft. The fuel is admitted to the
remaining cylinders which makes the engine to start under its own power.
Starting Circuits
Diesel engines have as many different types of starting circuits as there are
types, sizes, and manufacturers of diesel engines. Commonly, they can be
started by air motors, electric motors, hydraulic motors, and manually. The
123
Diesel Engine Power Plant start circuit can be a simple manual start pushbutton, or a complex auto-start
circuit. But in almost all cases the following events must occur for the
starting engine to start.
(a) The start signal is sent to the starting motor. The air, electric,
or hydraulic motor, will engage the engine’s flywheel.
(b) The starting motor will crank the engine. The starting motor
will spin the engine at a high enough rpm to allow the engine’s
compression to ignite the fuel and start the engine running.
© The engine will then accelerate to idle speed. When the starter
motor is overdriven by the running motor it will disengage the
flywheel.
Because a diesel engine relies on compression heat to ignite the fuel, a cold
engine can rob enough heat from the gasses that the compressed air falls
below the ignition temperature of the fuel. To help overcome this condition,
some engines (usually small to medium sized engines) have glow plugs.
Glow plugs are located in the cylinder head of the combustion chamber and
use electricity to heat up the electrode at the top of the glow plug. The heat
added by the glow plug is sufficient to help ignite the fuel in the cold
engine. Once the engine is running, the glow plugs are turned off and the
heat of combustion is sufficient to heat the block and keep the engine
running. Larger engines usually heat the block and/or have powerful starting
motors that are able to spin the engine long enough to allow the
compression heat to fire the engine. Some large engines use air start
manifolds that inject compressed air into the cylinders which rotates the
engine during the start sequence.
4.3 FUEL INJECTION SYSTEM
Fuel injection is a system for mixing fuel with air in an internal combustion engine. A
fuel injection system is designed and calibrated specifically for the type of fuel it will
handle. Most fuel injection systems are for diesel applications. With the advent of
electronic fuel injection (EFI), the diesel gasoline hardware has become similar. EFI’s
programmable firmware has permitted common hardware to be used with different fuels.
Carburetors were the predominant method used to meter fuel before the widespread use
of fuel injection. A variety of injection systems have existed since the earliest usage of
the internal combustion engine.
The primary difference between carburetors and fuel injection is that fuel injection
atomizes the fuel by forcibly pumping it through a small nozzle under high pressure,
while a carburetor relies on low pressure created by intake air rushing through it to add
the fuel to the air stream.
The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from a
pump or a pressure container farther back in the fuel supply.
Objectives
The functional objectives for fuel injection systems can vary. All share the central
task of supplying fuel to the combustion process, but it is a design decision how a
particular system will be optimized. There are several competing objectives such
as :
power output,
fuel efficiency,
emissions performance,
reliability,
smooth operation,
initial cost,
maintenance cost,
diagnostic capability, and
range of environmental operation.
Certain combinations of these goals are conflicting, and it is impractical for a
single engine control system to fully optimize all criteria simultaneously. In
practice, automotive engineers strive to best satisfy a customer's needs
competitively. The modern digital electronic fuel injection system is far more
capable at optimizing these competing objectives consistently than a carburetor.
Carburetors have the potential to atomize fuel better.
Benefits
Operational benefits include smoother and more dependable engine response
during quick throttle transitions, easier and more dependable engine starting,
better operation at extremely high or low ambient temperatures, increased
maintenance intervals, and increased fuel efficiency. On a more basic level, fuel
injection does away with the choke which on carburetor-equipped systems must be
operated when starting the engine from cold and then adjusted as the engine
warms up.
An engine’s air/fuel ratio must be precisely controlled under all operating
conditions to achieve the desired engine performance, emissions, and fuel
economy. Modern electronic fuel-injection systems meter fuel very accurately,
and use closed loop fuel-injection quantity-control based on a variety of feedback
signals from an oxygen sensor, a mass airflow (MAF) or manifold absolute
pressure (MAP) sensor, a throttle position (TPS), and at least one sensor on the
crankshaft and camshaft to monitor the engine's rotational position. Fuel injection
systems can react rapidly to changing inputs and control the amount of fuel
injected to match the engine's dynamic needs across a wide range of operating
conditions such as engine load, ambient air temperature, engine temperature, fuel
octane level, and atmospheric pressure.
A multipoint fuel injection system generally delivers a more accurate and equal
mass of fuel to each cylinder, thus improving the cylinder-to-cylinder distribution.
Exhaust emissions are cleaner because the more precise and accurate fuel
metering reduces the concentration of toxic combustion byproducts leaving the
engine, and because exhaust cleanup devices such as the catalytic converter can be
optimized to operate more efficiently since the exhaust is of consistent and
predictable composition.
Fuel injection generally increases engine fuel efficiency. With the improved
cylinder-to-cylinder fuel distribution, less fuel is needed for the same power
output. When cylinder-to-cylinder distribution is less than ideal, as is always the
case to some degree with a carburetor or throttle body fuel injection, some
cylinders receive excess fuel as a side effect of ensuring that all cylinders receive
sufficient fuel. Power output is asymmetrical with respect to air/fuel ratio; burning
extra fuel in the rich cylinders does not reduce power nearly as quickly as burning
too little fuel in the lean cylinders. However, rich-running cylinders are
undesirable from the standpoint of exhaust emissions, fuel efficiency, engine
wear, and engine oil contamination. Deviations from perfect air/fuel distribution,
however subtle, affect the emissions, by not letting the combustion events at the
chemically ideal (stoichiometric) air/fuel ratio. Grosser distribution problems
eventually begin to reduce efficiency, and the grossest distribution issues finally
affect power. Increasingly poorer air/fuel distribution affects emissions,
efficiency, and power, in that order. By optimizing the homogeneity of
cylinder-to-cylinder mixture distribution, all the cylinders approach their
maximum power potential and the engine's overall power output improves.
125
A fuel Diesel Engine Power Plant -injected engine often produces more power than an equivalent carbureted
engine. Fuel injection alone does not necessarily increase an engine's maximum
potential output. Increased airflow is needed to burn more fuel, which in turn
releases more energy and produces more power. The combustion process converts
the fuel's chemical energy into heat energy, whether the fuel is supplied by fuel
injectors or a carburetor. However, airflow is often improved with fuel injection,
the components of which allow more design freedom to improve the air’s path into
the engine. In contrast, a carburetor's mounting options are limited because it is
larger, it must be carefully oriented with respect to gravity, and it must be
equidistant from each of the engine's cylinders to the maximum practicable degree.
These design constraints generally compromise airflow into the engine.
Furthermore, a carburetor relies on a restrictive venturi to create a local air
pressure difference, which forces the fuel into the air stream. The flow loss caused
by the venturi, however, is small compared to other flow losses in the induction
system. In a well-designed carburetor induction system, the venturi is not a
significant airflow restriction.
4.3.1 Basic Function
The process of determining the necessary amount of fuel, and its delivery into the
engine, are known as fuel metering. Early injection systems used mechanical methods to
meter fuel (non electronic or mechanical fuel injection). Modern systems are nearly all
electronic, and use an electronic solenoid (the injector) to inject the fuel. An electronic
engine control unit calculates the mass of fuel to inject.
Modern fuel injection schemes follow much the same setup. There is a mass airflow
sensor or manifold absolute pressure sensor at the intake, typically mounted either in the
air tube feeding from the air filter box to the throttle body, or mounted directly to the
throttle body itself. The mass airflow sensor does exactly what its name implies; it
senses the mass of the air that flows past it, giving the computer an accurate idea of how
much air is entering the engine. The next component in line is the Throttle Body. The
throttle body has a throttle position sensor mounted onto it, typically on the butterfly
valve of the throttle body. The throttle position sensor (TPS) reports to the computer the
position of the throttle butterfly valve, which is used to calculate the load upon the
engine. The fuel system consists of a fuel pump (typically mounted in-tank), a fuel
pressure regulator, fuel lines (composed of either high strength plastic, metal, or
reinforced rubber), a fuel rail that the injectors connect to, and the fuel injector(s). There
is a coolant temperature sensor that reports the engine temperature, which the engine
uses to calculate the proper fuel ratio required. In sequential fuel injection systems there
is a camshaft position sensor to determine which fuel injector to fire.
The fuel injector acts as the fuel-dispensing nozzle. It injects liquid fuel directly into the
engine's air stream. In almost all cases this requires an external pump. The pump and
injector are only two of several components in a complete fuel injection system.
An EFI system requires several peripheral components in addition to the injector(s), in
order to duplicate all the functions of a carburetor. A point worth noting during times of
fuel metering repair is that early EFI systems are prone to diagnostic ambiguity. A single
carburetor replacement can accomplish what might require numerous repair attempts to
identify which one of the several EFI system components is malfunctioning. Newer EFI
systems can be very easy to diagnose due to the increased ability to monitor the realtime
data streams from the individual sensors.
Typical EFI Components
Animated cut through diagram of a typical fuel injector
Injectors
Fuel Pump
Fuel Pressure Regulator
126
Power Plant Engineering ECM – Engine Control Module; includes a digital computer and circuitry to
communicate with sensors and control outputs
Wiring Harness
Various Sensors (Some of the sensors required are listed here)
Crank/Cam Position (Hall effect sensor)
Airflow (MAF sensor)
Exhaust Gas Oxygen (Oxygen sensor, EGO sensor, UEGO sensor)