Study Notes on Internal Combustion Engine & Gas Turbines

INTERNAL COMBUSTION ENGINE & GAS TURBINES

Module I - INTRODUCTION

Heat Engine

• Definition: A heat engine is a device that transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work.
• Types of Heat Engines:
  1. External Combustion Engine
  2. Internal Combustion Engine

External Combustion Engine

• Description: In this engine, the products of combustion of air and fuel transfer heat to a second fluid, which serves as the working fluid of the cycle.
• Examples:
  • In a steam engine or steam turbine plant, combustion heat generates steam, which is used in a piston engine (reciprocating engine) or turbine (rotary engine) for useful work.
  • In a closed cycle gas turbine, heat from combustion in an external furnace is transferred to gas, usually air, which acts as the working fluid of the cycle.

Internal Combustion Engine

• Description: In this engine, combustion of air and fuels takes place inside the cylinder, directly providing motive force.
• Classification of Internal Combustion Engines:
  1. According to Basic Engine Design:
  • a) Reciprocating Engine
  • b) Rotary Engine
  1. According to Type of Fuel Used:
  • a) Petrol Engine
  • b) Diesel Engine
  • c) Gas Engine (CNG, LPG)
  • d) Alcohol Engine (ethanol, methanol, etc)
  1. According to Number of Strokes per Cycle:
  • a) Four Stroke
  • b) Two Stroke
  1. According to Method of Igniting the Fuel:
  • a) Spark Ignition Engine
  • b) Compression Ignition Engine
  • c) Hot Spot Ignition Engine
  1. According to Working Cycle:
  • a) Otto Cycle (constant volume cycle)
  • b) Diesel Cycle (constant pressure cycle)
  • c) Dual Combustion Cycle (semi-diesel cycle)
  1. According to Fuel Supply and Mixture Preparation:
  • a) Carburetted Type (fuel supplied through carburettor)
  • b) Injection Type (fuel injected into inlet ports, inlet manifold, or directly into cylinder before ignition)
  1. According to Number of Cylinders:
  • a) Single Cylinder
  • b) Multi-Cylinder Engine
  1. Method of Cooling: Water-Cooled or Air-Cooled
  2. Speed of the Engine: Slow Speed, Medium Speed, and High Speed Engines
  3. Cylinder Arrangement: Vertical, Horizontal, Inline, V-Type, Radial, Opposed Cylinder Engine
  4. Valve Design and Location: Overhead (I head) and Side Valve (L head);
     • In two stroke engines: Cross scavenging, Loop scavenging, Uniflow scavenging.
  5. Method Governing: Hit and Miss Governed Engines, Quantitatively Governed Engines, and Qualitatively Governed Engines.
  6. Application: Automotive Engines for Land Transport, Marine Engines, Aircraft Engines, Industrial Engines, Prime Movers for Electrical Generators.

Comparison between External and Internal Combustion Engines

Main Components of Reciprocating IC Engines

1. Cylinder: Main part enclosing the piston, must withstand high pressure (>50 bar) and high temperature (>2000 °C). Constructed from cast iron, steel alloys, or aluminum alloys.
2. Cylinder Head: Covers top end of the cylinder; houses inlet/exhaust valves and spark plugs/injectors; includes a gasket (copper or asbestos) for airtight seal.
3. Piston: Converts energy from combustion to motion, typically made of aluminum alloy for heat conductivity and strength.
4. Piston Rings: Elastic seals to prevent leakage of gases or oil, made from steel alloys. Includes compression rings (top) and oil rings (bottom).
5. Connecting Rod: Transfers reciprocating motion to the crankshaft; constructed from special steel or aluminum alloys.
6. Crankshaft: Converts piston motion to rotary motion; made from high-strength steel.
7. Crankcase: Houses cylinder and crankshaft; acts as a sump for lubricating oil.
8. Flywheel: Maintains speed and stores excess energy during power stroke.

Terminology Used in IC Engines

1. Cylinder Bore (D): Inner diameter of working cylinder.
2. Piston Area (A): Cross-sectional area of the piston.
3. Stroke (L): Distance piston travels between two reversals of motion.
4. Dead Centre: Position of piston at stroke reversal (Bottom Dead Centre - BDC; Top Dead Centre - TDC).
5. Displacement Volume or Swept Volume (Vs): Volume generated by piston travel = $Vs = A imes L$.
6. Clearance Volume (V_c): Volume above piston at TDC.
7. Cylinder Volume (V): Total volume; $V = Vs + Vc$.
8. Compression Ratio (r): Ratio of cylinder volume to clearance volume.
9. Ignition System Types:
   • SI Engine: Otto Cycle operation, uses gasoline.
   • CI Engine: Diesel Cycle operation, uses diesel.

Four-Stroke Engine Cycle

1. Suction Stroke (inlet valve open, exhaust valve closed): Fresh air-fuel mixture drawn into cylinder upon piston movement from TDC to BDC.
2. Compression Stroke (both valves closed): Mixture is compressed into clearance volume; ignited by spark.
3. Expansion Stroke (both valves closed): High pressure from burned gases drives piston down, producing power.
4. Exhaust Stroke (exhaust valve open, inlet valve closed): Burned gases expelled as piston moves from BDC to TDC.

Two-Stroke Engine Cycle

• Description: No piston stroke for suction and exhaust; suction accomplished by compressed air in crankcase or a blower, with exhaust products removed through ports.

Comparison of Four-Stroke vs. Two-Stroke Engine

Comparison of Spark Ignition (SI) and Compression Ignition (CI) Engines

Valve Timing Diagram

• Definition: The valve timing diagram shows the timing for when the inlet and exhaust valves open and close relative to the piston position.
• Factors affecting valve timing:
  • Inlet valve opens 12-30° CA before TDC, closes 10-60° CA after TDC.
  • Exhaust valve opens 25-55° CA before BDC, closes 10-30° CA after TDC.

Port Timing Diagram (2-Stroke Engine)

• Description: Illustrates the timing of the opening and closing of the ports in a two-stroke engine during its operation.

Otto Cycle Overview

• Description: The thermodynamic cycle involving reversible adiabatic compression and expansion, with heat addition at constant volume.
• Key Equations:
  • Heat supplied: $qs = Cv(T3 - T2)$
  • Compression ratio and thermal efficiency defined based on changes in state variables through the cycle.

Diesel Cycle Overview

• Description: Thermodynamic cycle for CI engines involving reversible adiabatic compression and heat addition at constant pressure.
• Key Equations:
  • Heat supplied: $Q1 = Cp(T3 - T2)$
  • Key thermodynamic parameters are related to cycle performance, efficiency, and device reactions.

Fuels & Fuel Injection

• Definition: Fuels suitable for IC engines should promote fast chemical reaction during combustion.
• Types of Fuels:
  1. Hydrocarbons derived from crude petroleum.
  2. Alternative Fuels including:
  • Alcohols (Methanol, Ethanol)
  • Natural Gas (Methane)
  • LPG (Propane, Butane)
  • Hydrogen

Classification of Petroleum Fuels:

• Liquid Hydrocarbons: Mixtures primarily composed of hydrocarbons.
  • Paraffins: General form $CnH{2n+2}$, includes straight-chain and branched isomers; high number of hydrogen atoms, low density.
  • Naphthenes: Saturated hydrocarbons (cycloparaffins) with stable molecular structure.
  • Olefins: With double bonds, gaining priority in SI-engine applications.
  • Aromatics: Conjugated structures with lower performance relative to IC engines.

Refinery Processes

• Distillation: Separation of crude oil into various fractions based on boiling points; e.g., produces gasoline, jet fuel, diesel from specified temperature ranges.

Cracking Processes

• Types of Cracking: Thermal and catalytic cracking to break down heavier hydrocarbons into lighter, usable fuels.

Alternative Fuels

1. Alcohols:
   • Includes Methanol, Ethanol, with different characteristics and efficiency metrics.
   • Methanol's RON and problems of compatibility, toxicity, and energy content balance.
2. Biodiesel: Produced via transesterification of vegetable oils or fats, offering better lubrication properties but potential challenges during cold weather.
3. Biogas: Produced from organic waste decomposition, also having fuel characteristics beneficial for CI and SI engines.
4. Hydrogen: Produced via various methods, high energy content and potential uses in several applications.
5. Natural Gas: Mainly consists of Methane with several uses across energy sectors, including in IC engines either as CNG or LNG.
6. LPG: Widely used as a clean alternative in vehicles and heating solutions.

Engine Testing and Performance

• Evaluative Metrics:
  • Specific Fuel Consumption
  • Brake Mean Effective Pressure
  • Specific Power Output
  • Specific Weight
  • Emission levels (smoke density, exhaust gas analysis)

Engine Cooling System

• Purpose & Requirements: Maintain optimum temperatures to prevent engine failures caused by excessive heat or cold, protect components, and enhance overall operational efficiency.
• Types: Air cooled vs. water cooled systems with various configurations.

Lubrication System

• Purpose: Prevent wear of moving parts, efficient cooling, sealing gaps, and cleaning engine components.
• Types: Mist lubrication, wet sump lubrication, and dry sump lubrication systems.

Conclusion

These principles form the basis of understanding internal combustion engines and their operational characteristics, from design elements to performance metrics; thus, crucial for mechanical engineering and automotive applications.