Steam Power and Internal Combustion Engines: Principles and Operation
Cycle of Steam
The majority of installations for power generation from burning fuel (thermal) are steam-based, and these plants work according to variations of the Rankine thermodynamic cycle.
Plants usually utilize water steam as the working fluid and operate by burning solid, liquid, or gaseous fuel. Its main components are illustrated in Figure 3.1, where one can identify:
- Steam turbine
- Condenser
- Feed pump
- Boiler (steam generator)
- Chimney
- Electric Generator
- Cooling System (cooling tower)
As seen in Figure 3.1, a steam plant can be divided into four major subsystems:
Subsystem A:
- Function: Converts heat into mechanical work. Constitutes the thermodynamic cycle itself.
Subsystem B:
- Function: Supplies heat to subsystem A by vaporizing the water in the boiler. Operates on a high-temperature thermal reservoir.
- Heat source: Burning fossil fuel or, in nuclear plants, a controlled nuclear reaction.
Subsystem C:
- Operates in a low-temperature heat reservoir, absorbing heat rejected by subsystem A.
- Steam leaving the turbine condenses in the condenser, where cooling water tubes are present. The water is then recirculated.
- Challenge: Access to sufficient cooling water and pollution control for both fossil fuel and nuclear plants.
Subsystem D:
- Converts the mechanical work produced at the turbine shaft into electricity or uses it for direct activation of a load (compressor, pump, etc.).
Generation of Electricity
Electric generators convert mechanical energy from a prime mover into electrical energy. This conversion occurs through electromagnetic phenomena, primarily described by Faraday’s Law-Lenz, which states that a voltage is induced when a winding is subjected to a variable magnetic field.
Faraday’s Law-Lenz: e = -N dΦ/dt
Where: N is the number of turns, Φ is the magnetic flux, and dΦ/dt is the rate of change of flux.
This variation can be in time or space, resulting in the following situations:
- Stationary conductor, moving magnetic field.
- Stationary conductor, variable magnetic field.
- Moving conductor, moving magnetic field.
- Moving conductor, variable magnetic field.
These situations create relative motion between the field and conductor, inducing voltage. Figure 3.2.1 illustrates this.
In another scenario, a loop rotates within a magnetic field.
Internal Combustion Engine Principles
Overview
Invention: 19th Century
Important Aspects: Approximately 50% efficiency and impact on pollutant emissions
Concept of Heat Engine
Heat engines transform thermal energy from fuel combustion into useful mechanical energy. Most use air and O2 as the oxidizer. The working fluid evolves cyclically, transmitting energy to mechanical parts. In jet engines, fluid momentum powers the vehicle. Work is done by varying pressure and volume created by heat.
Types of Machines:
Volumetric Machines: Fluid evolves in a pulsating cavity.
- Reciprocating: Active fluid moves inside a cylinder with variable volume, transmitting energy through a moving piston (connecting rod-crankshaft).
- Rotary: Volume varies between the rotor and housing (Wankel engine).
Classification of Thermal Machines
External Combustion Engine: Fluid does not participate in combustion; heat transfer occurs through a heat exchanger.
Internal Combustion Engine: Fuel burns inside the engine; fluid is activated by a fuel-air mixture.
Dynamic Machines (Turbo Engines): Fluid flows continuously, delivering energy to blades or airfoils. Kinetic energy can propel the vehicle (jet engine; some energy used for compressors).
Classification of Motors
Cycles: 4-stroke (Otto, Diesel, mixed, Miller), 2-stroke (spark and gasoline)
Ignition: Spark ignition (SI), Compression ignition (CI – Diesel)
Valve Placement: Side, head, rotating
Fuels: Gasoline, flex-fuel, alcohol, coal
Charging: Naturally aspirated, supercharged
Injection: Direct or indirect
Ignition Type: Magneto or electronic
Cooling: Liquid or air
Cylinder Geometry: Inline, opposed, V, W, radial (star)
Motor Principle and Operation
Increased pressure from fuel-air mixture combustion drives the crankshaft rotation. The engine consists of a piston-cylinder assembly connected to a crankshaft via connecting rods. Piston reciprocation rotates the crankshaft. A flywheel stores kinetic energy.
Terminology:
- TDC: Top Dead Center
- BDC: Bottom Dead Center
- Stroke: Distance between TDC and BDC
- Crank Radius: Half of the stroke
Engine characteristics relate to bore (D) and stroke. In some engines, stroke may be greater than bore. Keeping cylinder volume constant, increasing bore and decreasing stroke (or vice versa) affects piston linear velocity. For a given rotational speed, piston velocity changes.
Swept Volume: Volume displaced by the piston from TDC to BDC (cylinder volume).
Clearance Volume: Volume above the piston at TDC (combustion chamber volume).
Compression Ratio: (Swept Volume + Clearance Volume) / Clearance Volume
Gasoline engines: Compression ratio 8:1 to 11:1. Alcohol can allow higher ratios. Compression increases pressure and temperature.
SI Engine (Spark Ignition)
Ignition is initiated by a high-voltage spark. The intake valve opens, allowing fuel-air mixture intake. The exhaust valve (E) releases exhaust gases.
Four-Stroke Cycle:
Intake: Intake valve opens, piston moves down, drawing in fuel-air mixture (TDC to BDC).
Compression: Both valves closed, piston moves up, compressing the mixture, increasing temperature and turbulence (BDC to TDC). Complete combustion is desired, minimizing pollutants. Final temperature should be below auto-ignition temperature.
Expansion: Spark ignites the compressed mixture. High pressure drives the piston down (TDC to BDC), providing power.
Exhaust: Exhaust valve opens, high-speed exhaust gases exit. Piston moves up, expelling remaining gases (BDC to TDC). Valve closes.
CI Engine (Compression Ignition – Diesel)
No pre-mixed fuel-air intake. No spark ignition. Air is compressed, reaching high temperature. Fuel (diesel) is injected into the hot, compressed air, igniting spontaneously. High-pressure injection system is required for precise fuel delivery and smooth combustion.
Four-Stroke Diesel Cycle:
Intake: Piston moves down, intake valve opens, fresh air intake (TDC to BDC).
Compression: Both valves closed, piston moves up, compressing air (BDC to TDC).
Combustion/Expansion: Fuel injected near TDC, ignites spontaneously. Injection continues during piston descent (power stroke).
Exhaust: Exhaust valve opens, burned gases escape. Piston moves up, expelling gases (BDC to TDC). Diesel engines require higher compression ratios (almost double gasoline) for high temperatures needed for ignition.