(a) With reference to the combustion of fuel, explain EACH of the following terms:
(i) atomisation; (3)
(ii) penetration; (2)
(iii) compression ratio.(1)
(b) State the factors which influence the terms explained in part (a).(4)
Atomization in Combustion
Atomization is the process of breaking down a liquid fuel into a fine mist of tiny droplets. This is crucial for efficient combustion in engines and burners.
1. Atomization processes | Fundamentals – Lechler
2. Liquid Fuel Atomization and Combustion (Chapter 12) – Renewable Fuels – Cambridge University Press & Assessment
Why is Atomization Important?
- Increased Surface Area: Breaking the fuel into small droplets dramatically increases the surface area exposed to the oxidizer (usually air). 1. Liquid Fuel Atomization and Combustion (Chapter 12) – Renewable Fuels – Cambridge University Press & Assessment www.cambridge.org
- Faster Evaporation: The increased surface area allows for quicker evaporation of the liquid fuel, turning it into a vapor which can mix more readily with the air. 1. Evaporation – Wikipedia en.wikipedia.org
- Better Mixing: The finer the droplets, the better the mixing between the fuel and air, leading to a more homogeneous mixture.
- Improved Combustion Efficiency: A well-atomized fuel burns more completely, releasing more energy and producing fewer harmful emissions.
Methods of Atomization
Several methods are used to atomize liquid fuels:
- Pressure Atomization: High pressure forces the fuel through a small orifice, creating a fine spray.
- Air-Assist Atomization: A high-velocity air stream breaks up the liquid fuel as it exits a nozzle.
- Ultrasonic Atomization: High-frequency vibrations break the liquid into tiny droplets. 1. Ultrasonic atomization principle – AMAZEMET | Freedom In Metal Additive Manufacturing www.amazemet.com
In essence, atomization is a critical step in preparing liquid fuels for combustion. It ensures efficient energy release and minimizes pollutants.
1. Liquid Fuel Atomization and Combustion (Chapter 12) – Renewable Fuels – Cambridge University Press & Assessment
Penetration in Combustion
Penetration in the context of fuel combustion, particularly in diesel engines, refers to the distance the injected fuel spray travels into the combustion chamber before it starts to vaporize and mix with the air.
Factors Affecting Penetration
Several factors influence fuel penetration:
- Injection Pressure: Higher injection pressure results in greater penetration as the fuel is injected with more force. 1. Temporal Evolution of Split-Injected Fuel Spray at Elevated Chamber Pressures – MDPI www.mdpi.com
- Nozzle Geometry: The shape and size of the injector nozzle affect the spray pattern and penetration. 1. The Effect of Fuel Injector Nozzle Configuration on JP-8 Sprays at Diesel Engine Conditions apps.dtic.mil
- Fuel Properties: Fuel viscosity, density, and surface tension impact how far the fuel travels before breaking up.
- Ambient Conditions: Temperature and pressure within the combustion chamber can affect fuel vaporization and penetration.
Importance of Penetration
Optimal penetration is crucial for efficient combustion:
- Sufficient Mixing: Adequate penetration ensures proper mixing of fuel and air, leading to complete combustion.
- Ignition Delay: Penetration affects the ignition delay, which is the time between fuel injection and the start of combustion.
- Combustion Efficiency: Proper penetration contributes to higher combustion efficiency and lower emissions.
In essence, penetration is a key parameter in diesel engine combustion, influencing factors like ignition timing, combustion phasing, and overall engine performance.
Compression Ratio in Combustion
Compression ratio is a fundamental parameter in internal combustion engines. It’s defined as the ratio of the volume of the cylinder when the piston is at its lowest point (bottom dead center, or BDC) to the volume when it’s at its highest point (top dead center, or TDC).
1. Compression ratio – Wikipedia
2. How To Calculate Engine Compression Ratio And Displacement – JE Pistons
In simpler terms, it’s how much the air-fuel mixture is compressed before ignition.
1. COMPRESSION RATIO: HOW to CALCULATE, MODIFY and CHOOSE the BEST one – BOOST SCHOOL #10
How Does it Affect Combustion?
- Higher Compression:
- Increases temperature and pressure of the air-fuel mixture before ignition. 1. COMPRESSION RATIO: HOW to CALCULATE, MODIFY and CHOOSE the BEST one – BOOST SCHOOL #10 www.youtube.com
- Improves combustion efficiency by allowing for a more complete burn of the fuel. 1. COMPRESSION RATIO: HOW to CALCULATE, MODIFY and CHOOSE the BEST one – BOOST SCHOOL #10 www.youtube.com
- Increases engine power and torque. 1. Shows the graph between Torque and Compression ratio In Fig. 2 increase… – ResearchGate www.researchgate.net
- Lower Compression:
- Results in lower temperatures and pressures during combustion.
- Can lead to incomplete combustion and lower efficiency.
Factors Influencing Compression Ratio:
- Engine design: Cylinder head shape, piston shape, and combustion chamber volume.
- Fuel type: Higher compression ratios generally require higher octane fuels to prevent premature ignition (knocking). 1. Why are high compression ratios not used in sparkignition engines? – Vaia www.vaia.com
In essence, compression ratio is a critical factor in determining the performance and efficiency of an internal combustion engine.