25-06-2013, 04:42 PM
DIESEL ENGINES OPERATION AND DIAGNOSIS PPT
DIESEL ENGINES.ppt (Size: 5.12 MB / Downloads: 453)
DIESEL ENGINES
In 1892, a German engineer named Rudolf Diesel perfected the compression-ignition engine that bears his name.
The diesel engine uses heat created by compression to ignite the fuel, so it requires no spark ignition system.
The diesel engine requires compression ratios of 16:1 and higher.
Incoming air is compressed until its temperature reaches about 1,000°F (540°C).
This is called heat of compression.
As the piston reaches the top of its compression stroke, fuel is injected into the cylinder, where it is ignited by the hot air
DIESEL ENGINE CONSTRUCTION
Diesel engines must be constructed heavier than gasoline engines because of the tremendous pressures that are created in the cylinders during operation.
The torque output of a diesel engine is often double or more than the same size gasoline powered engines.
FIGURE 4-5 The common rail on a Cummins diesel engine. A highpressure pump (up to 30,000 PSI) is used to supply diesel fuel to this common rail, which has cubes running to each injector. Note the thick cylinder walls and heavy-duty construction.
FUEL TANK AND LIFT PUMP
A fuel tank used on a vehicle equipped with a diesel engine differs from the one used with a gasoline engine in several ways, including:
A larger filler neck for diesel fuel.
No evaporative emission control devices or charcoal (carbon) canister.
The diesel fuel is drawn from the fuel tank by a lift pump and delivers the fuel to the injection pump.
Between the fuel tank and the lift pump is a water-fuel separator.
Water is heavier than diesel fuel and sinks to the bottom of the separator.
DIESEL INJECTOR NOZZLES
Diesel injector nozzles are spring-loaded closed valves that spray fuel directly into the combustion chamber or precombustion chamber.
Injector nozzles are threaded into the cylinder head, one for each cylinder, and are replaceable as an assembly.
The top of the injector nozzle has many holes to deliver an atomized spray of diesel fuel into the cylinder.
Parts of a diesel injector nozzle include:
Heat shield.
Injector body.
Diesel injector needle valve.
Injector pressure chamber.
GLOW PLUGS
Glow plugs are always used in diesel engines equipped with a precombustion chamber and may be used in direct injection diesel engines to aid starting.
A glow plug is a heating element that uses 12 volts from the battery and aids in the starting of a cold engine.
As the temperature of the glow plug increases, the resistance of the heating element inside increases, thereby reducing the current in amperes needed by the glow plugs.
ENGINE-DRIVEN VACUUM PUMP
Because a diesel engine is unthrottled, it creates very little vacuum in the intake manifold.
Several engine and vehicle components operate using vacuum, such as the exhaust gas recirculation (EGR) valve and the heating and ventilation blend and air doors.
Most diesels used in cars and light trucks are equipped with an engine-driven vacuum pump to supply the vacuum for these components.
DIESEL FUEL
Diesel fuel must meet an entirely different set of standards than gasoline.
The fuel in a diesel engine is not ignited with a spark, but is ignited by the heat generated by high compression.
All diesel fuel must be clean, be able to flow at low temperatures, and be of the proper cetane rating.
Cleanliness.
Low-temperature fluidity.
Cetane number.
DIESEL FUEL SPECIFIC GRAVITY TESTING
The density of diesel fuel should be tested whenever there is a driveability concern.
The density or specific gravity of diesel fuel is measured in units of API gravity.
API gravity is an arbitrary scale expressing the gravity or density of liquid petroleum products devised jointly by the American Petroleum Institute and the National Bureau of Standards.
DIESEL FUEL HEATERS
Diesel fuel heaters help prevent power loss and stalling in cold weather.
The heater is placed in the fuel line between the tank and the primary filter.
Some coolant heaters are thermostatically controlled, which allows fuel to bypass the heater once it has reached operating temperature.
ACCELERATOR PEDAL POSITION SENSOR
Some light truck diesel engines are equipped with an electronic throttle to control the amount of fuel injected into the engine.
Because a diesel engine does not use a throttle in the air intake, the only way to control engine speed is by controlling the amount of fuel being injected into the cylinders.
Instead of a mechanical link from the accelerator pedal to the diesel injection pump, a throttle-by-wire system uses an accelerator pedal position sensor.
To ensure safety, it consists of three separate sensors that change in voltage as the accelerator pedal is depressed.