Thermochemical Treatments, Corrosion, and Engines
Thermochemical Treatments
These treatments aim to enhance hardness, wear resistance, and corrosion resistance.
Cementing
This process involves adding carbon to steel.
Carbonitriding
This process involves adding nitrogen and carbon to steel.
Nitriding
This process involves adding nitrogen to steel.
Corrosion
Corrosion is the progressive deterioration of a material due to an oxidizing agent. It typically occurs in moist environments, while oxidation occurs in hot and dry environments.
Types of Corrosion
Uniform Corrosion
When a metal is placed in an electrolyte (e.g., moist air), some areas act as the cathode and others as the anode, with these roles changing over time.
Galvanic Corrosion
This occurs when two different metals in contact are exposed to an electrolyte (e.g., water, moist air).
Differential Aeration Corrosion
In a metal with cracks or crevices, dirt and moisture can penetrate. The outer areas are more exposed to air and oxygen, while the inner areas are oxygen-deprived, leading to corrosion.
Pitting Corrosion
Oxidation in a small crack depletes oxygen beneath the surface, causing corrosion to deepen.
Intergranular Corrosion
A second phase precipitates at grain boundaries, creating a galvanic cell.
Erosion Corrosion
This involves the removal of the protective oxide layer on metals, often caused by the wear of a fluid flowing through a conductor.
Corrosion Protection
Types of Protection
Design
Selecting appropriate materials and ensuring a larger anode surface area than the cathode. Welding can help avoid cracks and crevices.
Inhibitors
Substances that react with oxygen, removing it or forming a protective coating on the material’s surface.
Protective Coatings
Isolating the anode and cathode. Thorough surface cleaning is essential before applying the coating.
Metal Coatings
Covering the material with metals.
Organic Coatings
These are paints.
Surface Reaction Coatings
Chemical transformation occurs through reaction with an external agent.
Anodic Protection (Passivation)
A surface reaction coating forms an adherent, waterproof film that prevents galvanic cell formation.
Cathodic Protection
Making the metal to be protected the cathode by forcing it to act as the anode. This can be achieved through sacrificial anodes or impressed current.
Material Selection
Using corrosion-resistant materials.
Engines
An engine transforms thermal energy into mechanical energy to produce work.
Engine Classification
Based on Combustion Location:
External Combustion Engines
Heat from burning fuel transfers to an intermediate fluid, which generates mechanical power via a reciprocating or rotary engine (e.g., steam engines, steam turbines).
Internal Combustion Engines
Expanding gases directly cause the movement of engine mechanisms (e.g., gasoline engines, diesel engines, gas turbines, turboprops).
Based on Mechanical Energy Generation:
Reciprocating Engines
The working fluid acts on pistons moving up and down.
Rotary Engines
The fluid acts on rotating pistons or turbines.
Jet Engines
The fluid generates thrust through the principle of action and reaction.
External Combustion Machines
Steam Engine
Water from the pump enters the boiler in its liquid phase at high pressure and temperature. The water absorbs heat from the fuel, raising its temperature to boiling point and producing saturated steam. The steam is superheated and then enters the cylinder or turbine, converting thermal energy into mechanical energy. The steam then condenses and returns to its liquid state. The liquefied vapor goes back to the pump, where its pressure increases before re-entering the boiler. In an open circuit, the steam is released into the atmosphere, bypassing the condenser.
Reciprocating Steam Engine
A steam engine uses a piston within a cylinder. The piston moves alternately due to steam from the boiler, converting linear motion into rotary motion via a rod-crank system and flywheel. A distributor connected to the flywheel moves horizontally above the cylinder, but in the opposite direction of the piston. When one area is in contact with the boiler, the other is connected to the condenser or atmosphere, and vice versa, resulting in continuous reciprocation.
Rotary Steam Engine (Turbine)
Steam expands in a turbine after passing through a nozzle, losing pressure and gaining speed. The steam strikes the turbine tangentially. The turbine’s runner has blades that absorb the steam’s energy, causing shaft rotation. Forces on the blades result from the steam’s impact and hydrodynamic action due to the blade shape, making the steam flow faster over the top surface than the bottom. This creates a pressure difference, causing the blade and wheel to spin.
Internal Combustion Engines
Rotary Engines (Gas Turbine)
Continuous fuel combustion produces expanding gases that act on a turbine. Key elements include a compressor, combustion chamber, and turbine.
Reciprocating Internal Combustion Engines
These engines convert thermal energy into mechanical energy using one or more pistons.
Function and Classification
The engine operates on a four-stroke or two-stroke cycle.
Four-Stroke Cycle
Intake
The descending piston creates a vacuum, drawing air or a fuel-air mixture into the cylinder through the open intake valve.
Compression
The intake valve closes, and the rising piston compresses the mixture.
Expansion
Fuel ignites, increasing pressure and pushing the piston down.
Exhaust
The exhaust valve opens, and the rising piston expels combustion gases.
Two-Stroke Cycle
First Stroke
Ignition occurs at top dead center (TDC). Expanding gases push the piston down, opening the exhaust port. The descending piston compresses the mixture in the crankcase, which then enters the cylinder, scavenging exhaust gases.
Second Stroke
The rising piston closes intake and exhaust ports, starting compression. Simultaneously, the intake port opens, drawing fluid into the crankcase.
Lubrication and Cooling
Lubrication reduces friction between moving parts. A pressurized oil circuit distributes oil from the sump. Cooling is achieved by air or water. Air cooling uses fins to increase surface area. Water cooling circulates water around cylinders and heads, then through a radiator cooled by airflow.
Refrigeration Equipment
Components include a compressor, condenser, evaporator, and expansion valve (or turbine). The compressor draws low-pressure vapor from the evaporator and compresses it. The condenser releases heat to the environment. The evaporator absorbs heat. The expansion valve reduces pressure, and the refrigerant expands fully in the evaporator.
Steam Cooling Systems
These systems use cryogenic fluids undergoing condensation and evaporation cycles to lower temperature. Evaporation absorbs heat, reducing temperature. Therefore, fluids with low boiling points are used.
Heat Pump
A heat pump operates similarly to a refrigerator, exchanging heat between hot and cold reservoirs.