AE 1005-AUTOMOTIVE ENGINES Unit 1
UNIT I -ENGINE CONSTRUCTION AND OPERATION (9 hours) Four stroke SI and CI engines - Working principle function, materials, constructional details of engine components- Valve timing diagram - Firing order and its significance - relative merits and demerits of SI and CI engines Two stroke engine construction and operation. Comparison of fourstroke and two-stroke engine operation.
Engine Classifications 1. Types of ignition (a) Spark Ignition (SI) An SI engine starts the combustion process in each cycle by use of a spark plug. (b) Compression Ignition (CI) The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression. 2. Engine cycle (a) Four-stroke cycle A four-stroke cycle has four piston movements over two engine revolutions for each cycle. (b) Two-stroke cycle: A two-stroke cycle has two piston movements over one revolution for each cycle. 3. Valve location (a) Valves in head (Overhead valve), also called I Head engine. (b) Valves in block (flat head), also called L Head engine. Some historic engines with valves in block had the intake valve on one side of the cylinder and the exhaust valve on the other side. These were called T Head engines. (c) One valve in head (usually intake) and one in block, also called F Head Engine; this is much less common.
4. Basic Design a. Reciprocating Engine has one or more cylinders in which pistons reciprocate back and forth. b. Rotary Engine is made of a block (stator) built around a large non-concentric rotor and crankshaft. The combustion chambers are built into the non-rotating block 5. Position and number of cylinders of reciprocating engines a. Single Cylinder Engine has one cylinder and piston connected to the crankshaft. b. In-Line Cylinders are positioned in a straight line, one behind the other along the length of the crankshaft. c. V Engine Two banks of cylinders at an angle with each other along a single crankshaft, allowing for a shorter engine block. The angle between the banks of cylinders can be anywhere from 15 to 120 with 60-90. d. Opposed Cylinder Engine: Two banks of cylinders opposite to each other on a single crankshaft (a V engine with 180 deg V). These are common on small aircraft and some automobiles with an even number of cylinders from two to eight or more. e. W engine: Engines of two different cylinder arrangements have been classified as W engines. They are not common, but some race cars of 1930 s and some luxury cars of the 1990s had such engines either with 12 cylinders or 18 cylinders. Another type of W engine is the modern 16 cylinder engine made for the Bugattiautomobile (W16). f. Opposed piston engine Two pistons in each cylinder with the combustion chamber in the center between the pistons. g. Radial engine: Engines with pistons positioned in a circular plane around a circular crankshaft
6. Air Intake Process (a) Naturally Aspirated: No intake air pressure boosts system. (b) Super charged: Intake air pressure increased with the compressor driven off of the engine crankshaft. (c) Turbo charged: Intake air pressure increased with the turbine compressor driven by the engine exhaust gases. (d) Crankcase compressed 7. Method of fuel input for spark ignition engines (a) Carbureted: A device for mixing air and fuel to facilitate the combustion process (b) Multipoint port fuel injection: One or more injectors at each cylinder intake. (c) Throttle body fuel injection: Injectors upstream in intake manifold. (d) Gasoline direct injection: Injectors mounted in combustion chambers with injection directly into cylinders. 8. Method of fuel input for compression ignition engines (a) Direct injection: Fuel injected into main combustion chamber. (b) Indirect injection: Fuel injected into secondary combustion chamber. (c) Homogeneous charge compression ignition: Some fuel added during intake stroke.
9. Fuel used (a) Gasoline (b) Diesel oil or Fuel oil (c) Gas, Natural gas, Methane (d) Alcohol-Ethyl, Methyl (e) Dual fuel: There are a number of engines that use a combination of two or more fuels. Some, Usually large, CI engines use a combination of natural gas and diesel fuel. These are attractive in developing third world countries because of the high cost of the diesel fuel. Combined gasoline alcohol fuels are becoming more common as an alternative to straight gasoline automobile engine fuel. (f) Gasohol: Common fuel consisting of 90% gasoline and 10% alcohol. 10. Application (a) Automobile, Locomotive, Stationery, Marine, Aircraft, Small, Portable, chain saw, model airplane. 11. Type of cooling (a) Air cooled (b) Liquid cooled, Water-cooled.
Types of Reciprocating Engines
V Engine
WankelRotary Piston Engine
COMPONENTS OF A FOUR STROKE CYCLE, DOHC PISTON ENGINE
OPERATION OF A FOUR STROKE ENGINE
COMPONENTS OF A FOUR CYLINDER ENGINE
Engine layouts
V Engine Crankshaft, Connecting Rod and Piston assembly
Radial Engine
A Fully Assembled Engine
Dismantled Engine
CYLINDER BLOCK
CYLINDER BLOCK
Cylinder block Thecylinder blockor engine block is a machined casting containing cylindrically bored holes for the pistonsof a multi-cylinder reciprocating internal combustion engine. It is a complex part at the heart of an engine, with adaptionsto attach thecylinder head, crankcase, engine mounts, drive housing and engine ancillaries, with passages for coolants and lubricants. Engine blocks are usually made from cast iron or, in modern engines, aluminium and magnesium
CRANK CASE
Crankcaseis the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder block. It protects the crankshaft andconnecting rods from foreign objects. In afour-stroke engine, the crankcase is filled mainly with air and oil, and is sealed off from the fuel/air mixture by the pistons. Intwo-stroke gasoline engines, the crankcase is sealed and is used as a pressurization chamber for the fuel/air mixture. As the piston rises, it pushes out exhaust gases and produces a partial vacuumin the crankcase which aspirates fuel and air. As the piston travels downward, the fuel/air charge is pushed from the crankcase and into the cylinder. In two-stroke gasoline engines, the crankcase does not contain engine oilbecause oil is mixed with the fuel, and the mixture provides lubrication for the cylinder walls, crankshaft andconnecting rod bearings.
CYLINDER HEAD
OIL PAN
CRANKSHAFT
Crankshaft is the main rotating shaft running the length of the engine. The crankshaft is supported bymain bearings. Portions of the shaft are offset to form throws to which theconnecting rodsare attached. As the Pistonsmove up and down, theconnecting rodsmove the crankshaft around. The turning motion of the crankshaft is transmitted to the Transmission and eventually to the driving wheels.
PISTON
Pistons Constructed of aluminum alloy Parts include top, ring grooves, ring lands, skirt, and piston pin boss Cooling fins on the bottom help the oil carry heat away from the piston top
Piston must be made of a material that meets the following requirements : Low Thermal expansion. The coefficient of thermal expansion must be low. It is best to use the same material for both pistons and cylinders. High heat conductivity. Low specific gravity (to decrease inertia duringhigh speed operation). Sufficient strength and large abrasion resistance even at high temperatures. Easy to cast
Alumuniumalloys is currently used because they satisfy all of the above requirements. Specialcast ironis used as well. A piston made of specialcast ironhas the same coefficient of thermal expansion as the cylinder, but tends to be heavy. Alumuniumalloys has a larger coefficient of thermal expansion than iron, but has high heat conductivity, therefore the temperature of the piston head can be lowered. However, alumuniumalloy has a weak point (poor lubricating oil retention). For this reason, pistons are usually plated with lead to eliminate this shortcoming. Seizure can be prevented by lead plating. Some pistons have a specialcast ironring carrier that is cast into the top ring groove to prevent abrasion. A piston usually tin plated to improve initial breaking in performance and to prevent rusting.
Thermal Problem of Pistons The strength and hardness of the alumuniumalloy used for manufacturing pistons will suddenly decrease when temperature exceeds 400 o C. As a result, abrasion and cracking will begin to occur. When Lo-Ex alloy is used, the piston head cavity temperature is designed to be 300-330 o C and the bottom of the top of ring groove is designed to below 230-250 o C. The overheating of piston can be prevented by various methods. For example the cooling efficiency can be raised to lower the temperature of the cylinder liner. The thermal flow type shape (dome shape that promotes the flow of heat from the top of the piston to the ring) can be adopted for the back of pistons so that the piston temperature will be even. Pistons can also be oil cooled.
Clearance between piston and cylinder When the piston is installed in the cylinder, there must be a specified clearance between them. Insufficient clearance will cause seizure due to thermal expansion, while excessive clearance will lead to compression leakage, inefficient heat radiation by the piston, over-consumption of lubricating oil, and piston slap.
Measurement of piston dimensions A piston is designed to maintain an even clearance with the cylinder during operation when thermal expansion is taken into consideration. Therefore the dimensions of the piston in the cold stage are supposed to be smaller than in the operating state by the amount of thermal expansion that takes place. The upper part of the piston is heated more than the lower part. Therefore its diameter is the smallest and the top and increases toward the bottom. In other words, a piston has conical shape.
Ovality Since heat is transmitted through the ribs that connect the bosses of the piston head and the piston pin, the ribs and bosses are heated more than the other parts. This mean that the expansion in the axial direction of the piston is larger. Therefore the diameter in the pin direction is smaller than the diameter in the perpendicular direction. (this called Ovality) Acast ironpiston is exactly round.
PARTS OF A PISTON
DIFFERENT TYPES OF PISTON
Piston ring
Piston ring Apiston ringis an open-ended ring that fits into a groove on the outer diameter of a piston in a reciprocating engine The three main functions of piston rings in engines are: 1. Sealing the combustion/expansion chamber. 2. Supporting heat transfer from the piston to the cylinder wall. 3. Regulating engine oil consumption. Most automotive pistons have three rings: The top two while also controlling oil are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings). Typically, top ring and oil control rings will be coated withchromium, ornitrided, possibly plasma sprayedor have a PVD (physical vapourdeposit) ceramic coating. For enhanced scuff resistance and further improved wear, most moderndiesel engines have top rings coated with a modified chromium coating
FITTING A PISTON RING
RING TYPES
PISTON PIN Gudgeonpin orwrist pin is that which connects the pistonto the connecting rod and provides a bearing for the connecting rod topivotupon as the piston moves The gudgeon pin is typically aforgedshort hollow rod made of a steel alloy of high strength and hardness
Circlip Acirclip(a combination of 'circle' and 'clip ), orsnap ringis a type offastenerconsisting of a semi-flexible metal ring with open ends which can be snapped into place, into amachinedgrooveon a dowel pin or other part to permitrotationbut to prevent lateral movement.
CONNECTING ROD
CONNECTING ROD In a reciprocatingpiston engine, the connecting rodconnects thepistonto the crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion.
CAMSHAFT
CAMSHAFT The camshaft is used to operatepoppet valves. It then consists of a cylindrical rod running the length of the cylinder bankwith a number of oblonglobesprotruding from it, one for each valve. The cams force the valves open by pressing on the valve, or on some intermediate mechanism as they rotate. Chilled iron castings: this is a good choice for high volume production. A chilled iron camshaft has a resistance against wear because the camshaft lobes have been chilled, generally making them harder. Billet Steel: When a high quality camshaft is required, engine builders and camshaft manufacturers choose to make the camshaft from steel billet. This method is also used for low volume production. This is a much more time consuming process, and is generally more expensive than other methods. However the finished product is far superior.
CAMSHAFT
DOHC
CAM LOBE
Rocker arm Rocker arm is a reciprocating lever that conveys radial movement from the camlobe into linear movement at the poppet valveto open it.
Valves Four-strokeIC engines employ valves to control the flow of fuel and air into the combustion chamber and exhaust gases out of the cylinder. Two-strokeengines use ports in the cylinder bore, covered and uncovered by the piston. However, special types of valves are used.
Poppet valves Poppet valvesare the most common and get their name from the popping open and close during operation. Intake valves are chrome steel and are cooled by the incoming air and fuel mixture. Exhaust valves are also alloy steel but are often filled with metallic sodium for cooling. Valve faces may be coated with Stelliteto reduce wear and corrosion. Stellitealloy is a range ofcobalt-chromium alloy designed forwearresistance. It may also contain tungstenormolybdenumand a small but important amount of carbon.
Why exhaust valves are small? The exhaust valves open against pressure within the cylinder at the end of the working stroke where the pressure is considerably higher. Further more, the pressure of the exhaust gases assists, once the valve is open, in expelling the gasses through the open valve. Because of this consideration it is usual to find that exhaust valves are designed to be of a smaller diameter than the inlet valves. Being smaller also assists with keeping them cool which is important as exhaust valves often give rise to thermal problems.
Valve Rotation Both the inlet and exhaust valve seats get damaged during the operation and from time to time they have to be reconditioned by grinding-in the valves. This is required often for the exhaust valves because they operate at higher temperatures and the exhaust gases contain carbon particles which get trapped under the valve seat and cause pitting. The life of an exhaust valve between reconditioning can be extended if the thermal loads to which it is subjected can be evenly distributed around the valve. This is accomplished by the rotating the valves slowly as the engine is working.
FOUR STROKE ENGINE
FOUR STROKE ENGINE The four stroke engine was first demonstrated by NikolausOtto in 1876, hence the cycle of operation is called as the Otto cycle The technically correct term is Four Stroke (Cycle) Engine The four stroke engine is the most common type of engine used nowadays It powers almost all 2 wheelers, cars and trucks
The four strokes of the cycle are 1. Intake or Inlet 2. Compression 3. Power or Expansion 4. Exhaust Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.
Four Stroke Engine
Actual Otto Cycle
Ideal and Actual Valve Timing Diagram (4S SI Engine)
Ideal Diesel Cycle
Actual Diesel Cycle
Actual Valve Timing Diagram (4S CI Engine)
Operation Single Cylinder
Operation Multi Cylinder
Four Stroke Petrol Engine
IntakeStroke Air-fuel mixture or Air is introduced to fill the combustion chamber. Piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture or air through the open intake valve into the cylinder to fill the low pressure area created by the piston movement.
IntakeStroke The cylinder continues to fill slightly after BDC also as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. Depending on engine design. The intake valve then closes and the air-fuel mixture or air is sealed inside the cylinder.
CompressionStroke Trapped air-fuel mixture (called as charge) is compressed inside the cylinder. Compressionis the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves remain closed to ensure that the cylinder is sealed to provide compression. The flywheel helps to maintain the momentum necessary to compress the charge.
IGNITION -SI The spark plug initiates combustion at approximately 20 of crankshaft rotation before TDC by a spark. The combustion starts when the charge gets ignited. Combustionis the rapid chemical reaction in which a fuel chemically combines with oxygen in the mixture and releases energy in the form of heat. During combustion a flame spreads throughout the combustion chamber by a progressing flame front. Aflame frontis the boundary wall that separates the charge from the combustion by-products. The flame front progresses across the combustion chamber until the entire charge has burned.
Fuel Injection -CI With both the inlet and the exhaust valves closed and the piston about 23 degbtdc diesel is injected into the dense and heated air as a high-pressure spray of fine particles. Proper atomization and distribution of fuel throughout the air charge gets heated by the hot compressed air and quickly vaporizes and ignites the tiny droplets of fuel. By this time, the piston reaches TDC and extensive burning releases heat energy which is rapidly converted into pressure energy. Expansion pushes the piston away from the cylinder head.
PowerStroke Thepower strokeis the Stroke during which the hot expanding gases force the piston towards the BDC Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power Stroke, both valves remain closed.
Exhaust Stroke Theexhaust strokeoccurs when the burnt gases are expelled from the combustion chamber to the atmosphere. Piston reaches BDC during the end of power stroke the cylinder is filled with exhaust gases, the exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.
FIRING ORDER
CYLINDER NUMBERING Front of the engine is the part where thepulleysfor the accessories (alternatorand water pump) are, and rear of the engine is where theflywheel, through which the engine connects to thetransmission. The front of the engine may point towards the front, side or rear of the car. In mostrear-wheel drivecars, the engine islongitudinally mountedand the front of the engine also points to the front of the car. Infront-wheel drivecars with atransverse engine, the front of the engine usually points towards the righthand side of the car.
CYLINDER NUMBERING In front-wheel-drive cars withlongitudinally mountedengines, most often the front of the engine will point towards the front of the car, but some manufacturers (Saab,Citroën,Renault) have at times placed the engine 'backwards', with #1 towards the firewall.
CYLINDER NUMBERING V ENGINES In av engine, cylinder numbering varies among manufacturers. Generally, the most forward cylinder is numbered 1 Some manufacturers continue numbering along that bank first, so that one side of the engine would be 1-2- 3-4, and the opposite bank would be 5-6-7-8. Others will number the cylinders from front to back along the crankshaft, so one bank would be 1-3-5-7 and the other bank would be 2-4-6-8.
FIRING ORDER Thefiring order is the sequence of power delivery of each cylinder in a multi-cylinder reciprocating engine. This is achieved by spark plugssparking in a SI engine in the correct order, or by the sequence of fuel injection in aci engine. Choosing an appropriate firing order is critical to Minimise vibration To improve engine balance Achieve smooth running Long engine fatigue life User comfort Firing Order heavily influences crankshaft design.
FIRING ORDER 1-3-2 3 Cylinder Engine 1-3-4-2 Most Common Four Cylinder Engine 1-5-3-7-4-8-2-6 V8 Ferrari 1-6-5-10-2-7-3-8-4-9 V10
TWO STROKE ENGINE The second type of Internal Combustion Engine operates on the Two Stroke Cycle This engine was invented by Dugald Clerk (1854-1932), a British Engineer in the year 1880 Two stroke engine have no valves They don t have camshaft, cams, springs and other valve train elements
Two stroke engine Operation
TWO STROKE ENGINE The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston.
Intake The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston.the illustrated engine features a poppet intake valve, however many engines use a rotary valve incorporated into the crankshaft.
During the downward stroke the poppet valve is closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.
Transfer/Exhaust Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder.this expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Some of the fresh mixture is also expelled.
Compression The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake process is happening beneath the piston)
Power At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.
Since the two stroke engine fires on every revolution of the crankshaft, they are more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes them popular in light motorcycles, chainsaws, line trimmers, outboard motors, snowmobiles, and model airplanes. Unfortunately, two stroke engines are inefficient and pollutes heavily due to the amount of unburntfuel that escapes through the exhaust port.