Internal Combustion Engines Study Notes

Internal Combustion Engines Study Notes

General Information

• Instructor: Kelvin Kofi Akroway
• Position: Assistant Lecturer
• Department: Mechanical Engineering
• Contact Email: kkakroway@umat.edu.gh

Introduction

• Heat Engine:
  • Definition: A machine that converts heat energy supplied to it into mechanical work.
  • The heat energy is supplied by burning fuel.

Classification of Heat Engines

• Internal Combustion Engines (IC Engines):
  • Combustion occurs inside the engine cylinder.
  • Examples: Diesel Engines, Petrol Engines, Gas Engines.
• External Combustion Engines (EC Engines):
  • Combustion occurs outside the working cylinder.
  • Examples: Steam Engines, Steam Turbines.

Classification of Internal Combustion Engines (IC Engines)

1. Cycle of Operation (No of Strokes per Cycle):
   • Two Stroke Cycle Engines
   • Four Stroke Cycle Engines
2. Thermodynamic Cycle or Method of Heat Addition:
   • Otto Cycle Engines (Combustion at constant volume)
   • Diesel Cycle Engines (Combustion at constant pressure)
   • Semi Diesel Engines (Dual combustion engines)
3. Types of Fuel Used:
   • Petrol Engines
   • Diesel Engines
   • Gas Engines
4. Ignition Method:
   • Spark Ignition (SI)
   • Compression Ignition (CI)
5. Cooling System:
   • Air Cooled Engines
   • Water Cooled Engines
6. Valves Location:
   • L head (Side valve) engine
   • T Head (Side valve) engine
   • I head (Overhead valve) engine
   • F head (Overhead inlet and side exhaust) engine.

Internal Combustion Engine Terminology

1. Bore: Inside diameter of the cylinder.
2. Top Dead Center (TDC): The extreme position reached by the piston at the top of the cylinder.
3. Bottom Dead Center (BDC): The extreme position reached by the piston at the bottom of the cylinder.
4. Stroke: The nominal distance the piston travels between TDC and BDC.
5. Compression Ratio (r): The ratio of maximum cylinder volume to clearance volume.
6. Cylinder Volume (v): The sum of swept volume and clearance volume, represented as:
   v = Vs + Vc
7. Swept Volume (Vs): The volume generated by the movement of the piston from one dead center to the other.
8. Clearance Volume (Vc): The space in the cylinder when the piston is at TDC.

Main Components of IC Engine

• Cylinder Block:

  • Main block of the engine containing accurately finished cylinders.
  • Houses crank, camshaft, piston, and other engine parts.
  • In water-cooled engines, equipped with water jackets for cooling.
  • Materials: Grey cast iron, aluminium alloys.
  • Generally made of a single casting.

• Cylinder Head:

  • Bolted to the cylinder block with studs.
  • Equipped with water jackets for cooling.
  • Made from cast iron or aluminum alloys.
  • Typically made in a single piece.

• Cylinder Liners:

  • Fitted into the cylinder bore, providing a wear-resisting surface.
  • Classified into:
  • Wet Liners: Surrounded by cooling water.
  • Dry Liners: Have metal-to-metal contact with the cylinder block, without contact with cooling water.
  • Material considerations: Must withstand abrasive wear and corrosion.
  • Commonly used materials: Chromium plated mild steel tubes.

• Crankcase:

  • Can be cast integral with the cylinder block or separately attached.
  • Materials: Cast iron, aluminium alloys, or alloy steels.

• Oil Pan or Oil Sump:

  • The bottom part of the engine containing lubricating oil.
  • Includes a drain plug for oil drainage.

• Piston:

  • Functions as a gas-tight seal to keep gases inside the cylinder.
  • Transmits the force of explosion to the crankshaft via the connecting rod.
  • Materials: Cast iron, aluminium alloys, chrome-nickel alloys, nickel-iron alloys, cast steel.

• Piston Rings:

  • Inserted in grooves on the piston, categorized into:
  • Compression Rings: Seal high-pressure gases.
  • Oil Rings: Control oil lubrication and return excess oil to the sump.
  • Materials: Typically made from wear-resistant materials including alloy steel.

• Connecting Rod:

  • Connects the piston and crankshaft, transmitting explosion force to the crankshaft.
  • Features bearings at both ends.
  • Small end connects to the piston via a gudgeon pin.
  • Must withstand heavy thrusts, requiring strength and rigidity.
  • Materials: Plain carbon steel, aluminium alloys, nickel alloy steels.

• Crank Shaft:

  • Main rotating shaft of the engine; power is obtained from it.
  • Converts the reciprocating motion of the piston into rotary motion.
  • Held in position by main bearings.
  • Materials: Billet steel, carbon steel, nickel-chrome, and other heat-treated alloy steels.

• Camshaft:

  • Contains multiple cams used to convert rotary motion into linear motion.
  • The number of cams corresponds to the number of valves in the engine.
  • Drives the fuel pump and controls the opening and closing of engine valves.

Classification of Petrol Engines

1. Two Stroke Cycle Petrol Engines
2. Four Stroke Cycle Petrol Engines

Four Stroke Cycle Petrol Engines

Construction

• Consists of a piston reciprocating inside a cylinder, connected to the crankshaft via connecting rod and crank.
• Mounts inlet and exhaust valves on the cylinder head.
• A spark plug is provided for ignition.
• Fuel used: Petrol.

Working

1. Suction Stroke:

   • Piston moves from TDC to BDC.
   • Inlet valve opened, exhaust valve closed.
   • Pressure inside the cylinder falls below atmospheric pressure, drawing in air-fuel mixture.

2. Compression Stroke:

   • Piston rises from BDC to TDC.
   • Both inlet and exhaust valves closed, compressing air-fuel mixture within the cylinder.

3. Working Stroke (Power Stroke):

   • Burning gases expand, exerting force on the piston, pushing it from TDC to BDC.
   • This motion converts into rotary motion for the crankshaft.
   • Both valves remain closed.

4. Exhaust Stroke:

   • Piston moves upward from BDC.
   • Exhaust valve opened, inlet valve closed, expelling burnt gases outside.
   • Exhaust valve closes shortly after TDC, while the inlet valve opens slightly before TDC, starting a new cycle.

Key Parameters

• Compression Ratio: Ranges from 5 to 8.
• Pressure at End of Compression: Approximately 6 to 12 bar.
• Temperature at End of Compression: Reaches between 250°C to 350°C.

Four Stroke Diesel Engine

Construction

• Similar piston and crankshaft setup to petrol engines.
• Fuel injector mounted on the cylinder head.
• Fuel used: Diesel.

Working

1. Suction Stroke:

   • Piston moves from TDC to BDC, sucking in fresh air.
   • Inlet valve opened, exhaust valve closed.

2. Compression Stroke:

   • Piston moves from BDC to TDC, compressing air.
   • Both valves remain closed.

3. Working Stroke (Power Stroke):

   • Burning gases push the piston down from TDC to BDC, generating power.
   • Motion converted to rotary motion via connecting rod.
   • Both valves remain closed.

4. Exhaust Stroke:

   • Follows similar principles as the petrol engine exhaust stroke.

Two Stroke Cycle Petrol Engines

Construction

• Piston reciprocates inside a cylinder connecting to crankshaft via a connecting rod.
• Uses cylinder wall ports instead of valves: inlet, exhaust, and transfer ports.
• Piston crown designed to help with port opening/closing.

Working

1. First Stroke (Compression, Ignition, Inductance):

   • Piston moves up, compressing the air-fuel mixture.
   • At TDC, spark plug ignites the mixture.
   • Fresh mixture enters crankcase via the inlet port.

2. Second Stroke (Expansion and Crankcase Compression):

   • As gases expand, they push the piston down.
   • Fresh air-fuel mixture enters the cylinder through the transfer port during this stroke.

Two Stroke Cycle Diesel Engines

Construction

• Equipped with air supply for exhaust and fresh charge.
• Uses ports rather than valves and features a plate for admitting air.

Working

1. First Stroke (Compression and Inductance):

   • Similar process to petrol engines but with fuel injected into hot compressed air at TDC for ignition.

2. Second Stroke (Expansion and Transfer):

   • Expansion of gases pushes the piston down, generating work while new air enters through ports.

Scavenging

• Definition: The process of expelling burnt gases from the cylinder for the admission of fresh charge.
• Involves both exhaust and transfer ports being open momentarily to facilitate charge transfer without loss.

Comparison between SI and CI Engines (General Comparison)

Comparison of Four Stroke and Two Stroke Cycle Engines

Merits and Demerits

• Two Stroke Cycle Engine
  • Merits: Simpler design, lighter construction, uniform torque.
  • Demerits: Higher fuel consumption, more noise, poor scavenging.
• Four Stroke Cycle Engine
  • Merits: Better thermal efficiency, more effective scavenging.
  • Demerits: Heavier, more complicated design, non-uniform torque.

Specific Characteristics and Comparisons

• Compression Ratio:
  • Four Strokes: 14-22:1
  • Two Strokes: Lower with less efficiency.
• Fuel System:
  • Four Strokes utilize carburettors, while Two Strokes leverage direct scavenging with ports.
• Lubricants:
  • Four Strokes: Regular oils, maintaining better lubrication due to longer cycles.
  • Two Strokes: Likely higher oil consumption due to lack of a dedicated lubrication method.

Maintenance and Disassembly of IC Engine

Steps to Disassemble an Engine:

1. Disconnect battery cables.
2. Remove hood and air cleaner.
3. Label all wires and vacuum lines.
4. Drain coolant and oil.
5. Remove radiator, distributor, and spark plug wiring.
6. Remove clutch parts and coolant pump; mount engine on a stand.
7. Remove the oil pan and valve covers.

Summary

• Understanding internal combustion engines involves recognizing the differences in construction, operation, fuel types, and applications between various engine types.
• Maintenance and disassembly involve methodical processes requiring attention to detail and understanding of engine components and their functions.