53 Questions
What is the first stage of combustion in a CI engine?
Ignition delay period
During which period in a CI engine is some fuel already admitted but has not yet ignited?
Ignition delay period
What does the ignition delay period influence in a CI engine?
Combustion rate and knocking
What is the significance of the ignition delay period on engine performance?
Influences combustion rate
When does the ignition delay period end in a CI engine's combustion process?
When pressure-time curve separates from motoring curve
What is defined as the time between fuel injection and actual burning in a CI engine?
Ignition delay period
What does point a represent in the context of ignition delay?
The time of injection
Which type of fuel typically has a high physical delay?
Heavy viscous fuel
What is the main factor that greatly reduces the physical delay during ignition?
High injection pressures
During which delay period does the rate of heat-release reach its maximum?
Rapid combustion period
What is essential for achieving efficient and clean combustion in a diesel engine?
Optimizing the compression ratio
Which design factor directly affects the duration of the delay period in a compression-ignition engine?
Compression ratio
Why does the minimum autoignition temperature of a fuel decrease with an increase in compression ratio?
Increased density of compressed air
What is the main role of the delay period in a diesel engine?
To allow sufficient time for fuel-air mixture homogenization.
What factor is NOT important in affecting the delay period?
Fuel type
What factor contributes to diesel knock due to the ignition of fuel in localized areas of the combustion chamber?
Injection Timing
How does an increase in engine speed affect the delay period in a diesel engine?
Decreases the delay period in crank angle degrees
What is the primary cause of diesel knock in a compression-ignition engine?
Localized Ignition of Fuel
How does an increase in engine output affect the air-fuel ratio and delay period?
Increases air-fuel ratio and decreases delay period
What contributes to diesel knock related to injector issues in a compression-ignition engine?
Uneven Combustion
Why is the Cetane number important in evaluating diesel fuel?
Evaluates combustion characteristics
How does high quality diesel fuel help in reducing diesel knock?
Reduces knocking by enhancing combustion
'Diesel knock' can be described as what type of noise during the combustion process in a CI engine?
'Metallic' sound
'Diesel knock' is characterized by what occurrence in the engine cylinder?
'Hammer-like noise'
'Cetane rating' focuses on evaluating the ignition quality of diesel fuel in comparison to what parameter for gasoline engines?
'Octane number'
What is the relationship between the Cetane number and ignition delay?
Higher Cetane number results in a shorter ignition delay
Why is achieving a high Cetane number essential for diesel engines?
To reduce particulate matter emissions
How do high-Cetane fuels contribute to emissions control?
By reducing particulate matter in emissions
What role does fuel quality play in determining the Cetane number?
It significantly influences the Cetane number
What issue can arise from extremely high Cetane numbers in diesel fuels?
Increased emissions of nitrogen oxides
Why is efficient mixing of air and fuel crucial in the combustion chamber of a CI engine?
To achieve a uniform combustion process
Why does a CI engine require a high compression ratio?
To improve thermal efficiency
Why must the combustion chamber be robustly constructed in a CI engine?
To withstand high pressures and temperatures
How do the shape and geometry of the combustion chamber influence combustion in a CI engine?
They impact heat distribution and heat balance
Why is it important for the combustion chamber design to be compatible with the engine's ignition system?
To ensure proper ignition
What is the purpose of generating turbulence in the combustion chamber of internal combustion engines?
To promote efficient mixing of air and fuel
Which design element plays a critical role in shaping the combustion process within the combustion chamber?
Piston bowl shape
What is the primary function of a pre-chamber design in internal combustion engines?
To induce turbulence in the main chamber
How do modern engines utilize direct injection to enhance combustion efficiency?
By improving atomization and mixing of fuel
Which element is crucial in dissipating heat effectively within the combustion chamber to prevent engine damage?
Cooling passages material
In what way does squish contribute to enhancing turbulence in the combustion chamber?
By forcing the mixture into the combustion zone
How does a piston bowl design influence turbulence within the combustion chamber?
By significantly influencing turbulence
What role does jet and direct injection play in promoting turbulence in modern engines?
Promoting high-velocity fuel jets for better atomization
What is the purpose of Bowl-in-piston designs in diesel engines?
To create controlled turbulence during the compression and combustion processes
How do Variable Valve Timing (VVT) systems influence combustion efficiency?
By optimizing valve events to create desirable turbulence levels in the combustion chamber
What is the main function of a Variable Geometry Intake System?
To control airflow patterns and create turbulence for improved combustion
How does Turbulent Jet Ignition (TJI) technology contribute to combustion in engines?
By using high-velocity jets to induce turbulence and initiate combustion
What is the purpose of Exhaust Gas Recirculation (EGR) in engines?
To introduce controlled exhaust gas into the intake air to reduce NOx emissions
What is a characteristic of Direct-Injection Chambers in compression ignition engines?
They have minimum heat loss during compression due to a high surface area to volume ratio
What is a drawback of Shallow Depth Chambers in large engines?
High fuel-injection pressure required for operation
How does an Indirect-Injection Chamber differ from a Direct-Injection Chamber?
Indirect-Injection Chambers are divided into distinct compartments connected by restricted passages
What is the main function of a Swirl Chamber in an engine?
To create a strong rotary movement in the air inside the chamber before combustion
What is the significance of EGR (Exhaust Gas Recirculation) system when it comes to the composition of the air-fuel mixture?
It alters the composition by introducing exhaust gas into the intake air
Study Notes
Combustion Chamber Design in CI Engines
- The combustion chamber design must meet multiple requirements to achieve efficient, clean, and reliable combustion
- Key factors:
- Arrangement of fuel injectors and glow plugs
- Heat management
- Emission control
- Piston-bowl design
- Fuel injection system integration
- Adaptability to varied operating conditions
Methods of Generating Turbulence in Combustion Chamber
- Swirl: introducing a swirling motion to the incoming air-fuel mixture
- Tumble: creating a vertical or axial swirling motion within the combustion chamber
- Squish: designing the combustion chamber to promote a squishing or squeezing effect on the air-fuel mixture
- Jet and Direct Injection: using high-velocity fuel jets to create turbulence in the air
- Pre-Chamber Design: using a small chamber adjacent to the main combustion chamber
Ignition Delay Period
- The time between the start of fuel injection and the actual ignition of the fuel-air mixture
- Factors affecting delay period:
- Compression ratio
- Engine speed
- Output
- Fuel atomization
- Combustion chamber design
Period of Rapid Combustion
- The phase in which the pressure rise is rapid
- During this period, most of the fuel admitted would have evaporated and formed a combustible mixture with air
- The rate of heat release is maximum during this period
Types of Combustion Chambers for CI Engines
- Direct-Injection Chambers:
- Open combustion chamber with a flat cylinder head and a cavity in the piston crown
- Advantages: minimum heat loss, no cold starting problems, fine atomization
- Drawbacks: high fuel-injection pressure required, complex design of fuel injection pump
- Shallow Depth Chamber:
- Used for large engines running at low speeds
- Cavity diameter is very large, resulting in negligible squish
- Indirect-Injection Chambers:
- Swirl Chamber: a spherical-shaped chamber separated from the engine cylinder
- Divided combustion chamber with restricted passages
Stages of Combustion in CI Engines
-
Ignition Delay Period
-
Period of Rapid Combustion
-
Period of Controlled Combustion
-
Period of Afterburning### Cetane Number and Diesel Engine Performance
-
A higher Cetane number results in a shorter ignition delay, promoting quicker and more complete combustion in diesel engines.
-
Higher Cetane numbers lead to improved ignition characteristics, resulting in better cold-start performance, reduced engine noise, and lower emissions.
-
Engines running on high-Cetane fuels exhibit smoother operation, lower particulate matter, and nitrogen oxides, contributing to cleaner air and compliance with environmental regulations.
-
Refineries control the refining process to produce diesel fuels with specific Cetane ratings, and additives can enhance the Cetane number of lower-quality diesel fuels.
-
A higher Cetane number is generally desirable, but extremely high Cetane numbers may lead to faster combustion, causing issues like engine knock or increased emissions of nitrogen oxides.
Combustion Chamber Requirements for CI Engines
Efficient Combustion Chamber Design
- The combustion chamber must facilitate the thorough mixing of air and fuel to create a homogenous mixture for uniform and controlled combustion.
- A high compression ratio is required to elevate the air temperature within the combustion chamber, promoting better combustion efficiency and thermal efficiency.
Robust Combustion Chamber Construction
- The combustion chamber must be robustly constructed to withstand high pressures and temperatures associated with the combustion process.
- Durable materials and effective cooling mechanisms are employed to ensure the longevity and reliability of the combustion chamber.
Combustion Chamber Shape and Geometry
- The shape and geometry of the combustion chamber influence turbulence, swirl, and squish patterns during combustion, impacting combustion speed, heat distribution, and pollutant formation.
- Design considerations aim to achieve an optimum balance between these parameters for efficient and clean combustion.
Ignition System Compatibility
- The combustion chamber design should be compatible with the engine's ignition system.
Learn about the stages of combustion in Compression-Ignition (CI) engines, including the ignition delay period, rapid combustion, controlled combustion, and afterburning. Explore the details of each stage in this quiz.
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