Lab session No. 3:-

Study of Components, Systems & working of Diesel Engine Model

Preliminaries:- A diesel engine or a compression ignition engine is an internal combustion engine that uses the heat of compression to initiate ignition and burn the fuel that has been injected into the combustion chamber.

Working:- In compression-ignition engines, air alone is inducted into the cylinder. The fuel is injected directly into the engine cylinder just before the combustion process is required to start. Load control is achieved by varying the amount of fuel injected each cycle, the air flow at a given engine speed is essentially unchanged. The liquid fuel, usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip, atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure cylinder air. Since the air temperature and pressure are above the fuel's ignition point, spontaneous ignition of portions of the already-mixed fuel and air occurs after a delay period of a few crank angle degrees. The cylinder pressure increases as combustion of the fuel-air mixture occurs. The compression ratio of diesels is much higher than typical SI engine values, and is in the range 12 to 24, depending on the type of diesel engine and whether the engine is naturally aspirated or turbocharged. The valve timings used are similar to those of SI engines. Processes involve in the operation of 4-Stroke Diesel Engine are:

1. Induction stroke: With the inlet valve open and the exhaust valve closed, the piston moves away from the cylinder head. The outward movement of the piston will establish a depression in the cylinder, its magnitude depending on the ratio of the cross-sectional areas of the cylinder and the inlet port and on the speed at which the piston is moving. The pressure difference established between the inside and outside of the cylinder will induce air at atmospheric pressure to enter and fill up the cylinder. A maximum depression of maybe 0.15 bar below atmospheric pressure will occur at about one-third of the distance along the piston's outward stroke, while the overall average pressure in the cylinder might be 0.1 bar or even less.

2. Compression stroke: With both the inlet and the exhaust valves closed, the piston moves towards the cylinder head. During the compression stroke, the air charge initially at atmospheric pressure and temperature is reduced in volume until the cylinder pressure is raised to between 30 and 50 bar. This compression of the air generates heat which will increase the charge temperature to at least 600 C under normal running conditions.

3. Power stroke: With both the inlet and the exhaust valves closed and the piston

almost at the end of the compression stroke, diesel fuel oil is injected into the dense and heated air as a high-pressure spray of fine particles. Provided that they are properly atomized and distributed throughout the air charge, the heat of compression will then quickly vaporize and ignite the tiny droplets of liquid fuel. Within a very short time, the piston will have reached its innermost position and extensive burning then releases heat energy which is rapidly converted into pressure energy. Piston gets pushed away from the TDC & power is transmitted through connecting rod to the crankshaft which in turn develops the rotation energy.

4. Exhaust stroke: When the burning of the charge is near completion and the piston has reached the outermost position, the exhaust valve is opened. The piston then reverses its direction of motion and moves towards the cylinder head. The sudden opening of the exhaust valve towards the end of the power stroke will release the still burning products of combustion to the atmosphere. The pressure energy of the gases at this point will accelerate their expulsion from the cylinder.

Components of Diesel Model:- Following are the components of Diesel engine model:

1. Block: Body of engine containing the cylinders, made of cast iron or aluminum.

2. Overhead Camshaft: Rotating shaft used to push open valves at the proper time in the engine cycle, either directly or through mechanical or hydraulic.

3. Combustion chamber: The end of the cylinder between the head and the piston face where combustion occurs. The size of the combustion chamber continuously changes from a minimum volume when the piston is at TDC to a maximum when the piston is at BDC.

4. Connecting rod: Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small engines.

5. Connecting rod bearing: Bearing where connecting rod fastens to crankshaft.

6. Cooling Fan: It is a on the outside surfaces of cylinders and head of Diesel engine model. It cools the cylinders by conduction and convection.

7. Crankcase: Part of the engine block surrounding the rotating crankshaft. In many engines, the oil pan makes up part of the crankcase housing.

8. Crankshaft: Rotating shaft through which engine work output is supplied to external systems. The crankshaft is connected to the engine block with the main bearings. It is rotated by the reciprocating pistons through connecting rods connected to the crankshaft.

9. Cylinders: The circular cylinders in the engine block in which the pistons reciprocate back and forth.

10. Exhaust manifold: Piping system which carries exhaust gases away from the engine cylinders, usually made of cast iron.

11. Exhaust system: Flow system for removing exhaust gases from the cylinders.

12. Flywheel: Rotating mass with a large moment of inertia connected to the crankshaft of the engine. The purpose of the flywheel is to store energy and furnish a large angular momentum.

13. Fuel injector: A pressurized nozzle that sprays fuel into the cylinder of model.

14. Fuel pump: Mechanically driven pump to supply fuel from the fuel tank (reservoir) to the engine. It takes power from the crankshaft through the belt drives.

15. Glow plug: Small electrical resistance heater mounted inside the combustion chamber of diesel engine model, used to preheat the chamber enough so that combustion will occur when first starting a cold engine.

16. Head: The piece which closes the end of the cylinders, usually containing part of the clearance volume of the combustion chamber. The head is usually cast iron or aluminum, and bolts to the engine block.

17. Head Gasket: Gasket which serves as a sealant between the engine block and head where they bolt together.

18. Intake Manifold: Piping system which delivers incoming air to the cylinders usually made of cast metal.

19. Pulleys & Belt drives: They are used to transmit power to camshaft, fuel pump, water pump & oil pump.

20. Main Bearing: The bearings connected to the engine block in which the crankshaft rotates.

21. Oil Pan: Oil reservoir usually bolted to the bottom of the engine block, making up part of the crankcase. Acts as the oil sump for the engine.

22. Oil Pump: Rotary Pump used to distribute oil from the oil sump to required lubrication points. It is driven by the crankshaft through belt drives.

23. Piston: The cylindrical-shaped mass that reciprocates back and forth in the cylinder, transmitting the pressure forces in the combustion chamber to the rotating crankshaft.

24. Piston Rings: Metal rings that fit into circumferential grooves around the piston and form a sliding surface against the cylinder walls.

25. Wrist Pin: Pin fastening the connecting rod to the piston (also called the gudgeon pin).

26. Radiator: Liquid-to-air heat exchanger of honeycomb construction used to remove

heat from the engine coolant after the engine has been cooled. An engine-driven fan is

used to increase air flow through the radiator.

27. Starter: An electrical motor used to crank the engine in the start. Electrical power is provided by the battery.

28. Turbocharger: Turbine-compressor is used to compress incoming air into the engine. The turbine is powered by the exhaust flow of the engine and thus takes very little useful work from the engine.

29. Valves: Used to allow flow into and out of the cylinder at the proper time in the cycle.

30. Water jacket: System of liquid flow passages surrounding the cylinders, usually constructed as part of the engine block and head.

31. Water Pump: Pump used to circulate engine coolant through the engine and radiator. It is mechanically run off of the engine.

32. Air Filter: It cleans the incoming air by restricting the particles present in air.

33. Injection Lines: They are used to transport the fuel to the

fuel injectors.

Systems of Diesel engine Model

Indirect injection (IDI) system:- Indirect injection systems have a divided

combustion chamber, with some form of pre-chamber in which the fuel is injected, and a main chamber with the piston and valves. The purpose of a divided combustion chamber is to speed up the combustion process, in order to increase the engine output by increasing engine speed. There are two principal classes of this combustion system; pre-combustion chamber and swirl chamber. Pre-combustion chambers rely on turbulence to increase combustion speed and swirl chambers rely on an ordered air motion to raise combustion speed. Combustion chamber use heat-resistant inserts with a low thermal conductivity. The insert is quickly heated up by the combustion process, and then helps to reduce ignition delay. These combustion chambers are much less demanding on the fuel injection equipment. The fuel is injected and impinges on the combustion chamber insert, the jet breaks up and the fuel evaporates. During initial combustion the burning air/fuel mixture is ejected into the main chamber, so generating a lot of turbulence. This ensures rapid combustion in the main chamber without having to provide an ordered air motion during the induction stroke. Since these systems are very effective at mixing air and fuel, a large fraction of the air can be utilized, so giving a high bmep with low emissions of smoke. Glow Plugs: A glow plug is a heating device used to aid starting diesel engines. In cold weather, high speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition due to the higher surface-to-volume ratio. Pre-chambered engines make use of small electric heaters inside the pre-chambers called glow plug. The glow plug is a pencil-shaped piece of metal with a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and begins to emit light in the visible spectrum, hence the term glow plug.

Fuel Injectors: The most important part of the fuel injector is the nozzle. All nozzles have a needle that closes under a spring load when they are not spraying. Open nozzles are used much less than closed nozzles since, although they are less prone to blockage, they dribble. When an injector dribbles, combustion deposits build up on the injector, and the engine exhaust is likely to become smoky. In closed nozzles the needle-opening and needle-closing pressures are determined by the spring load and the projected area of the needle. The pressure to open the needle is greater than that required to maintain it open, since in the closed position the projected area of the needle is reduced by the seat contact area. The differential pressures are controlled by the relative needle diameter and seat diameter. A high needle-closing pressure is desirable since it maintains a high seat pressure, so giving a better seal. This is also desirable, since it keeps the nozzle holes free from blockages caused by decomposition of leaked fuel.

Fuel Tank

Strainer

Fuel Pump

Fuel Injector

Air Intake System:- It consists of fresh air induction system into the cylinders. It

includes Turbo-Charger, Air Filter, Intake manifold & intake valves. Because a diesel engine

requires close tolerances to achieve its compression ratio, and because most diesel engines

are either turbo charged or supercharged, the air entering the engine must be clean, free of

debris, and as cool as possible.

Turbo-charger: A turbocharger is a turbine driven forced induction device that makes an engine more efficient and produce more power for its size by forcing extra air into the combustion chamber. A turbocharged engine is more powerful and efficient than a naturally aspirated engine because the turbine forces more air, and proportionately more fuel, into the combustion chamber than atmospheric pressure alone. The key difference between a turbocharger and a conventional supercharger is that the latter is mechanically driven from the engine, often from a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine that is driven by the engine's exhaust gas. Compared to a mechanically-driven supercharger, turbochargers tend to be more efficient but less responsive.

Cooling System:- Diesel

engine model has a liquid

cooling system. The cooling

system consists of the engine's

water jacket, a thermostat, a

water pump, a radiator and

radiator cap, a cooling fan

(belt-driven), hoses, the heater

core, and usually an expansion

(overflow) tank.

Coolant Jackets: The water

jacket is a collection of

passages within the block and

head. These passages let the

coolant circulate around the

"hot spots" (valve seats and

guides, cylinder walls, combustion chamber, etc.) in order to cool them off. Liquid cooled

engines have passages for the liquid, or coolant, through the cylinder block and head. The

coolant has to have indirect contact with such engine parts as the combustion chamber, the

cylinder walls, and the valve seats and guides. Running through the passages in the engine

heats the coolant (it absorbs the heat from the engine parts), and going through the

radiator cools it. After getting "cool" again in the radiator, the coolant comes back through

the engine. This business continues as long as the engine is running, with the coolant

absorbing and removing the engine's heat, and the radiator cooling the coolant.

Thermostat*: The thermostat provides control for your engine's warm-up period.

Expansion Tank*: Cooling systems make use of a clear plastic container, which is

connected to the overflow tube from the radiator. This container provides extra storage

space for the coolant when it expands and is called the expansion, or overflow tank. It is

also known as the coolant reservoir, or overflow canister.

Radiator cap*: The radiator cap acts as more than just a "lid" for your radiator; it keeps

engine cool by sealing and pressurizing the coolant inside it.

Belt powered Cooling Fan: The reason the coolant goes into the radiator is to allow air to

pass through it and cool the coolant. This is done by an external forcing medium i.e. Fan.

Radiator Shroud*: The radiator shroud prevents the recirculation of air around the fan. It

is usually a plastic hood that encloses the fan to guide the air through the core, and stop it

from coming back around and through the fan again. It also protects you from the fan

blades.

Lubrication System:- When two solid surfaces are in contact in an engine, they will touch each other at the roughness high spots of the surfaces. The smoother the surfaces are machined (on a macroscopic level), the lower will be the surface high points (microscopic) and the less will be the average distance separating them. If one surface is moved relative to the other, the high points will come into contact and will resist motion (friction). Points of contact will become hot, sometimes to the point of trying to weld together. To greatly reduce resistance of surface-to-surface motion, lubricating oil is added to the space between the surfaces. Lubricating oil adheres to the solid surfaces, and when one surface moves relative to the other, oil is dragged along with the surface. The oil holds the surfaces apart and one surface hydraulically floats on the other surface. The only resistance to relative motion is the shearing of fluid layers between the surfaces, which is orders of magnitude less than that of dry surface motion. Three important characteristics are needed in a lubricating fluid:

It must adhere to the solid surfaces. 2. It must resist being squeezed out from between the surfaces, even under the

extreme forces experienced in an engine between some components. 3. It should not require excessive force to shear adjacent liquid layers. The property

that determines this is called viscosity

*Not included in Model

Lubrication

Oil Filter

Oil Pump

Oil sump &

Strainer