TRADE OF HEAVY VEHICLE MECHANIC
PHASE 2
Module 3
Engine
UNIT: 1 & 3
Engine Components and Principles and Cylinder Head
Module 3 – Unit 1 & 3Engine Components and Principles and Cylinder Head
Table of Contents
1. Learning Outcome
1.1Key Learning Points
2.The Four Stroke Engine Cycle
2.1Basic 4-stroke principles
2.24 Stroke Engine Cycle
3.Cylinder Blocks
3.1Engine configurations
3.2Cylinder block
3.3Cylinder block construction
3.4Multi-Cylinder Engines
3.5Cylinder sleeves
3.6Grey iron
4.Cylinder Head Construction
4.1Cylinder heads
4.2Cylinder head design
4.3Intake and exhaust passages
4.4Gaskets and oil seals
4.5Head gaskets
4.6Turbulence
5.Camshafts & Related Systems
5.1Valves
5.2Valve seats
5.3Valve seats in cylinder heads
5.4Valve rotation
5.5Valve stem oil seals
5.6Intake valves
5.7Valve trains
5.8Valve-timing diagram
5.9Variable valve timing
5.10Camshafts & drives
5.11Understanding power and torque
5.12Overhead camshaft
5.13Cam lobes
5.14Timing belts & chains
5.15Timing belts & tensioners
5.16 Gear and Belt Drives
6.Pistons and Internal Engine Components
6.1Pistons
6.2Piston rings
6.3Connecting rod
6.4Compression ratio
6.5Compression ratio Calculation
6.6Testing compression pressure
Conclusion:
7.Crankshaft and Assembly
7.1Crankshaft
7.2Engine bearings
7.3Flywheel
7.4Exhaust systems
7.5Air cleaners
7.6EFI air cleaners
7.7Intake manifolds
7.8Intake air heating
7.9Forced induction
7.10Volumetric efficiency
7.11Exhaust manifold
7.12Catalytic converters
7.13Flexible connections
7.14Thermal expansion
7.15Silencer Box
7.16Superchargers
7.17Intercoolers
7.18Back-pressure
Heavy Vehicle Mechanic Phase 2Revision 2.0 December 2013
Module 3 – Unit 1 & 3Engine Components and Principles and Cylinder Head
1. Learning Outcome
By the end of this unit each apprentice will be able to:
Unit 1 - Engine Components and Principles
- Describe the principles of operation of a 4 cylinder, 4 stroke in-line engine, petrol and diesel
- Dismantle an engine and identify the main components and their functions
- State the basic materials used in their manufacture
- Re-assemble the engine using torque procedures
- Describe the operating principles of a basic two stroke engine - petrol and diesel
- Describe the valve operating mechanisms fitted to modern engines
- Identify the firing orders of engines having various camshaft arrangements
- Locate and interpret engine specifications from various vehicle workshop manuals
- Calculate the compression ratio of a cylinder
- Define "force" and the units in which force is measured
- Distinguish between tensile, compressive and shear force
- Describe what is meant by the terms, "power" and "torque"
Unit 3 – Cylinder Head
- Dismantle a cylinder head (4 cylinder inline engine bench unit), lap in the valves and re-assemble the head
- Identify the correct procedure and sequence to torque a cylinder head and adjust the valve clearances to manufacturers specifications on an operational engine
- Perform a compression test on an operational engine and analyse the results obtained with manufacturer’s specification to evaluate engine condition
- State the function and operating principles of a cylinder head and its components
1.1Key Learning Points
Unit 1 - Engine Components and Principles
- Dismantling and assembling procedure for a 4 cylinder engine
- Use of special tools for dismantling and assembling engines
- Principles of operation of 4 stroke and 2 stroke cycles, (diesel/petrol)
- Comparisons of 4 stroke and 2 stroke cycles, principles ofcombustion of diesel engines
- Terminology used e.g. TDC, BDC, compression, etc
- Names and functions of major components
- Relevant data located and applied from workshop manuals
- Firing orders, stroke chart identification
- Reciprocating and rotary motion, i.e. piston and crankshaftmovement
- Valve operating mechanisms, i.e. gear, chain, toothed belt
- Cylinder pairing arrangements
- Valve timing diagram, valve lead, lag and overlap
- Methods of valve clearance adjustment
- Combustion process: heat, gas laws and related principles
- Method and formula for calculating engine compression ratio
- Define force and its SI units of measurement
- Define torque and its SI units of measurement
- Define power and its SI units of measurement
- The relationship between engine power and torque, (linear andreciprocating motion)
- Location of timing marks (diesel/petrol) and their significance
Unit 3– Cylinder Head
- Dismantling and assembling procedures for a cylinder head
- Marking and matching of parts for assembly
- Correct use of special tools for removing and dismantling head
- Methods of valve clearance adjustment, importance of correct clearance
- Angles of valve face and seat, seating surfaces, flow clearance of 30º and 45º seats
- Compression: ratio, pressure and their measurements
- Factors affecting compression: leaks, adjustment, wear
- Compression test (dry and wet). Related hazards (ignition and fuel)
- Definition of linear expansion as applying to valves and head
- Operation of automatic tappet adjusters
- Function of valve stem seals. Smokey exhaust
- Torque procedure and sequences as applying to various cylinder heads
- Use of stretch head bolts, precautions, replacement
- Operation of valve rotators
- Method used to cut valve seats
You may also wish to review the information in the separate booklet covering related “Maths and Science” forthis unit.
2.The Four Stroke Engine Cycle
2.1Basic 4-stroke principles
This is a cylinder for a 4-stroke Diesel engine. The first step is to get the air into the cylinder. Air enters through an inlet port that is opened and closed by an inlet valve. This is called Intake. Next is compression. The piston compresses the air, which in turn heats the air. Fuel is then injected into the cylinder in a very fine spray. The heat from the compressed air ignites the fuel and it burns. This burning is called combustion.
The burning gases expand rapidly, and push the piston down the cylinder until it reaches bottom dead centre. The reciprocating action of the piston turns into the rotary motion of the crankshaft. The crankshaft forces the piston back up the cylinder, pushing leftover gases out past an exhaust valve. And everything is back where it started; ready to repeat the whole process.
The whole process is a cycle. A new charge enters and is ignited. Combustion occurs; expanding gases drive the piston down and turn the crankshaft which pushes the piston back up the cylinder. How they happen can change but they are always there. In one 4-stroke cycle, the crankshaft does 2 revolutions. In those 2 revolutions how many strokes does the piston make? It does 4 strokes. Out of those 4 strokes how many actually produce energy? In one 4-stroke cycle, only 1 stroke out of 4 delivers new energy to turn the crankshaft.
2.24 Stroke Engine Cycle
What is a stroke? It’s the movement of the piston from TDC (top dead centre) to BDC (bottom dead centre), OR BDC to TDC. A 4-stroke engine has the following "strokes", intake, compression, power, and exhaust.
A 4-stroke Diesel Engine uses "internal" combustion, meaning that the heat that causes the air in the cylinder to expand is generated "internally". (A steam engine is actually an "external combustion engine" as its heat source is outside the cylinder!)Those 4 strokes must include- Intake, Compression, IGNITION, Power & Exhaust. Let’s look at a simplified model. Note that the valves are ONLY open during their respective strokes, IE: intake valve open ONLY during the intake stroke, exhaust valve only during the exhaust stroke. Both are CLOSED during compression and power!
The intake stroke starts with the exhaust valve closed, the inlet valve opening, and the piston at its highest point, top dead centre.
It starts to move down, increasing the volume above the top of the piston. This makes pressure inside the cylinder lower than the pressure outside. This higher outside air pressure forces the air into the cylinder. The piston reaches bottom dead centre, the inlet valve closes, and the intake stroke ends.
Both intake and exhaust valves stay closed as the piston leaves bottom dead centre. The piston moves up, squeezing the air into a smaller and smaller volume, which compresses it. That causes the air charge temperature to rise, and that makes ignition easier and combustion (burning of fuel) more complete.
Just before the piston reaches top dead centre, the next key event occurs - Ignition. The air expanding in the cylinder pushes the piston down the cylinder. This is the Power stroke that drives the engine. The piston now moves from bottom dead centre to top dead centre. The exhaust valve opens, and the piston pushes out the leftover gases. Let’s look at a complete 4-stroke cycle:
1Intake - takes air charge into the cylinder.
2Compression - squeezes the air-fuel mixture into a smaller and smaller volume.
3Ignition - the air under pressure is ignited.
4Power - burning, expanding gases push the piston down creating a power stroke that turns the crankshaft.
5Exhaust - the piston moves upward, forcing burned gases from the chamber.
3.Cylinder Blocks
3.1Engine configurations
The way engine cylinders are arranged is called the engine configuration. Tilting an engine reduces its height. This can reduce the height of the bonnet as well, which allows a more streamlined body shape.
Tilting can be carried to an extreme by laying the engine completely on its side. It is then called a flat engine. This greatly reduces engine height.
As the number of cylinders increases, the length of the block and the crankshaft can become a problem. One way to avoid this is with a V configuration. This design makes the engine block and the crankshaft shorter, and more rigid.
In vehicle applications, the number of cylinders can vary, usually from 4, up to 12. Common angles between the banks of cylinders are 90 degrees and 60 degrees.
V-type engines are wider than inline engines, and may also be lower.
Horizontally-opposed engines have 2 banks of cylinders, 180 degrees apart, on opposite sides of the crankshaft. A useful design when little vertical space is available. It is shorter than a comparable in-line engine but wider than a V-type.
Rotary engines use a rotor in housing, instead of a piston in a cylinder. This provides a very compact power unit.
3.2Cylinder block
The cylinder block is the largest part of the engine. Its upper section carries the cylinders and pistons. Normally, the lower section forms the crankcase, and supports the crankshaft. It can be cast in one piece from grey iron. Or it can be alloyed with other metals like nickel or chromium.
The iron casting process begins by making up the shapes of what will become water jackets and cylinders as sand cores which are fitted into moulds. This stops these parts becoming solid iron during casting.
Molten iron is poured into sand moulds that are formed by patterns in the shape of the block.
After casting, core sand is removed through holes in the sides and ends, leaving spaces for the cooling and lubricant passages. These holes are sealed with core or welsh plugs. The casting is then machined. Cylinders are bored and finished, surfaces smoothed, holes drilled and threads cut.
All cylinder blocks are made with ribs, webs and fillets to provide rigidity but also keep weight to a minimum.
3.3Cylinder block construction
As more manufacturers try to make vehicles lighter and more fuel efficient, more and more engine blocks are being cast from aluminum.
A block made of aluminum alloy is lighter than if it were made of cast iron. So if two engines are generating the same power, the alloy version would have a better weight-to-power ratio than the cast iron version.
Aluminum alloy blocks are made by various casting processes, including pressure casting. Another method is gravity casting, where the molten metal is poured into molds.
Cast iron liners are usually used in the cylinders of aluminum blocks, and sometimes in cast-iron blocks. Some sleeves are cast into the block.. Grooves on the outside form a key that stops any movement in the cylinder. They also increase surface area to assist heat transfer from the sleeve to the block.
Some blocks don’t need liners. They can be made of wear resistant material that makes a hard-wearing surface for the pistons and piston rings. Or the cylinder bore may have some sort of surface treatment to make it hard-wearing.
When the cylinders, block and crankcase are all cast together, it is called a mono-block construction. A horizontally-opposed block has a split crankcase. The two engine blocks are joined together by the flanges of the crankcase.
In air-cooled engines, the cylinders are usually made as separate parts, and then bolted to the same crankcase. Each cylinder has cooling fins. They’re often machined to give uniform thickness and allow free flow of air.
3.4Multi-Cylinder Engines
The power developed by an engine can be increased by:
- Enlarging the cylinder or
- Increasing the number of cylinders.
A single large cylinder would seem the most popular choice since there are fewer parts to manufacture and maintain, but the disadvantages far outweighs the advantages. A large single cylinder engine needs a heavy flywheel to carry the piston over its idle strokes and to smooth out the torque fluctuations. Due to the fact that there is only one power stroke per two revolutions of the crankshaft, the piston and con-rod would also be heavier which means the engine speed would be limited and acceleration slow. As a result the power output-weight ratio would be low.
A multi-cylinder engine of the same cubic capacity and weight would have more power strokes per revolution. Lighter pistons, con-rods and flywheel which means that the torque would be smoother, better balance engine, higher engine speed and more power output.
Commercial vehicle engines are usually four/six or eight cylinder four stroke engines arranged inline depending on the required pulling power. Other variations are the `V-type` configuration .
3.5Cylinder sleeves
Cylinder sleeves are used in engine blocks to provide a hard-wearing material for pistons and piston rings. The block can be made of one kind of iron that’s light and easy to cast, while the sleeve uses another kind that is better able to stand up to wear and tear.
There are three main types of sleeves - dry, flanged dry, and wet.
The dry sleeve can be cast in or pressed into a new block, or used to recondition badly-worn or damaged cylinders that can’t easily be re-bored. It’s a pressed fit in its bore in the cylinder blocks. Its wall is about 2mm thick. Its outer surface is in contact with the block for its full length. Its top finishes flush with the top of the block and can hardly be seen. Once in place, dry sleeves become a permanent part of the cylinder block.
A flanged, dry sleeve is like a normal dry sleeve, but a flange at the top fits into a recess in the surface of the engine block. It’s not a tight fit and it can be replaced if it’s worn.
With a wet sleeve, the outer surface is part of the water-jacket around the cylinder. It’s called wet because it has coolant against its outer surface. This helps speed up heat transfer between the sleeve and coolant. The sleeve is sealed at the top to prevent coolant leaks. This stops coolant entering the combustion chamber, and the bottom of the crankcase. A flange at the top of the sleeve fits into a recess in the block. The lower end has 1 or 2 neoprene sealing rings.
The walls on wet sleeves are thicker than on dry sleeves. They don’t have the same support from the block as dry sleeves so they depend on their wall thickness to stop distortion.
In diesel engines, vibration caused by combustion can cause cavitation. This damage appears similar to corrosion and it can eventually destroy the cylinder.
3.6Grey iron
Grey iron is a form of cast iron. There are many different kinds of cast iron, depending on the particular materials they contain.
Grey iron is a cast iron that contains carbon in the form of graphite, plus silicon, manganese and phosphorus. The fractured surface of a cast iron with graphite appears grey, hence the name. It is brittle and cannot absorb shocks. It resists heat and corrosion, and can be cast into many different shapes. It is used for many components.
4.Cylinder Head Construction
4.1Cylinder heads
The cylinder head bolts onto the top of the cylinder block where it forms the top of the combustion chamber.
In-line engines of light vehicles have just one cylinder head for all the cylinders. Larger in-line engines can have 2 or more.
V-type and horizontally-opposed engines have a separate cylinder head for each bank of cylinders.
Just as with engine blocks, cylinder heads can be made of cast iron, or aluminum alloy. A head made of aluminum alloy is lighter than if it were made of cast iron. Aluminum also conducts heat away more quickly than iron. So with an aluminum-alloy head, the heat of combustion can be conducted away into the coolant more quickly.