52 Questions
During which stage of combustion in a spark-ignition engine does the piston move downward, creating a vacuum?
Intake Stroke
Which stage of combustion in an SI engine is characterized by the generation of an electric spark from a spark plug?
Ignition Stage
What is the main purpose of the Compression Stroke in a spark-ignition engine?
To compress the air-fuel mixture
In a four-stroke SI engine, how many strokes are involved in completing one cycle?
Four strokes
What is the role of turbulence in the stages of combustion in an SI engine?
To influence the design of the combustion chamber
How does a two-stroke SI engine differ from a four-stroke SI engine in terms of stroke count for a complete cycle?
A two-stroke has fewer strokes than a four-stroke.
What is the consequence of the charge reaching its autoignition temperature and remaining up to the time of preflame reactions?
Knocking combustion
What initiates the combustion process in a Spark Ignition (SI) engine?
Spark plug
What causes detonation in a Spark Ignition (SI) engine?
Spontaneous and rapid ignition
What effect does autoignition have on engine components?
Structural damage
How can the phenomenon of knocking be detected scientifically?
Pressure transducer
What is the consequence of unburned charge failing to reach its autoignition temperature?
Complete combustion
How does knocking combustion impact engine vibration frequency?
Increases it
What is the desired quality for Spark Ignition (SI) engine fuels to avoid or inhibit detonation?
$\text{High autoignition temperature and long ignition lag}$
What is the result of two flame fronts colliding in an engine experiencing knocking combustion?
Severe pressure pulse generation
What is the characteristic sound associated with detonation in a Spark Ignition (SI) engine?
Knocking or pinging sound
What is the primary purpose of the octane rating of a fuel in an internal combustion engine?
Measuring the fuel's resistance to knocking
Which factor is NOT a key requirement for combustion chambers in Spark Ignition (SI) engines?
Exhaust Emissions
What is the primary environmental impact of detonations mentioned in the text?
Air and water pollution
What is the main function of the combustion chamber in a Spark Ignition (SI) engine?
To facilitate the combustion process
How does a higher octane rating in fuel benefit an engine?
By improving resistance to knocking
Why is it important to use fuel with the recommended octane rating for your vehicle?
To optimize engine performance and longevity
In what context are measures taken to minimize collateral damage and ensure safety during detonations?
Controlled demolitions
What aspect of the combustion chamber design helps in achieving better mixing and combustion in an engine?
Shape and size
What is the impact of excessive heat in a combustion chamber on engine performance?
It causes engine damage and reduced efficiency
Which factor does NOT influence the compression ratio of an engine according to the text?
Size of the spark plug
What factor in a spark-ignition (SI) engine can increase the likelihood of detonation by promoting turbulence and hot spots?
Poorly Designed Combustion Chamber
Which factor can contribute to detonation in a spark-ignition (SI) engine by causing premature ignition?
Carbon Deposits
What can help reduce the chances of detonation in spark-ignition (SI) engines by adjusting ignition timing to prevent spontaneous ignition?
Optimizing Air-Fuel Mixture
In spark-ignition (SI) engines, detonation can occur if the air-fuel mixture ignites spontaneously before what event?
Spark Plug Fires
Which factor contributes to detonation in spark-ignition (SI) engines by increasing combustion temperatures due to inadequate cooling?
Engine Overheating
Using lower octane fuels in spark-ignition (SI) engines can lead to detonation due to what reason?
Low Octane Fuel Resistance
What effect of detonation in spark-ignition (SI) engines can cause significant damage to structures and materials?
Shock Waves Production
What is one way to prevent detonation in spark-ignition (SI) engines related to the use of recommended fuel?
Optimizing Air-Fuel Mixture
What factor in spark-ignition (SI) engines may lead to detonation by increasing combustion temperatures due to higher air to fuel ratio?
Lean Air-Fuel Mixture
What is a potential consequence of detonation in spark-ignition (SI) engines that can cause injury and fatalities?
Blast Pressure Creation
What is one of the main disadvantages of T-Head combustion chambers?
Requires two camshafts
Which type of combustion chamber has a design that aims to reduce knocking tendency by shortening the effective flame travel length?
I-Head Type
In the I-Head Type combustion chamber, where are both valves located?
Cylinder head
Modern F-Head engines have the ________ valve in the cylinder head and the ________ valve in the cylinder block.
inlet; exhaust
Which type of combustion chamber has a characteristic of less surface to volume ratio, leading to less heat loss?
I-Head Type
What is the purpose of placing the spark plug in the centre of the effective combustion space in Ricardo's turbulent head design?
Reduce flame travel length
Which type of valve arrangement is described as a compromise between L-Head and I-Head types?
F-Head Type
What is a main advantage of the I-Head Type combustion chamber over the L-Head type?
Less flame travel length
What effect does increasing turbulence have on the knocking tendency in spark-ignition engines?
Decreases the knocking tendency
How does an increase in engine speed impact the flame speed and preflame reactions?
Increases flame speed and reduces preflame reactions
Why is a larger engine more prone to knocking compared to a smaller engine?
More time required for the flame to travel
How does the combustion chamber shape influence the antiknock characteristics?
Shortens flame travel distance
What is the role of high turbulence in the combustion chamber?
Leads to better antiknock characteristics
Why is the spark plug centrally located in the combustion chamber of an engine?
To reduce flame travel distance and knocking tendency
What happens if the unburned mixture reaches its self-ignition temperature during preflame reactions?
The charge autoignites at various locations
Which of the following is a result of heat-release due to combustion in spark-ignition engines?
Compression of burned part of mixture
Study Notes
Stages of Combustion in SI Engines
- Intake Stroke: Piston moves downward, creating a vacuum that draws air-fuel mixture into the combustion chamber.
- Compression Stroke: Piston moves up, compressing the air-fuel mixture, increasing temperature and pressure.
- Ignition: Spark plug generates an electric spark, igniting the compressed air-fuel mixture, marking the beginning of combustion.
- Combustion (Power) Stroke: Ignited mixture rapidly burns, producing high-pressure and high-temperature gas, forcing the piston down and converting thermal energy to mechanical work.
- Exhaust Stroke: Piston moves up, pushing exhaust gases out of the combustion chamber and into the exhaust system.
Factors Influencing Combustion
- Turbulence: Depends on combustion chamber design and engine speed; affects autoignition temperature and preflame reactions.
Detonation in SI Engines
- Definition: Abnormal combustion of air-fuel mixture in the combustion chamber.
- Cause: High temperature, pressure, or hot spots in the combustion chamber.
- Effects: Uncontrolled rapid increase in pressure and temperature, distinctive knocking sound, damage to engine.
Octane Rating of Fuel
- Measure of fuel's resistance to knocking during combustion.
- Higher octane rating indicates greater resistance to knocking.
- Regular unleaded gasoline has an octane rating of 87, while premium gasoline has higher ratings (e.g., 91).
Requirements for Combustion Chambers in SI Engines
- Air-Fuel Mixture Formation: Proper mixing of air and fuel through carburetion or fuel injection.
- Ignition System Compatibility: Design supports ignition system, including spark plug location and orientation.
- Compression Ratio: Affects engine performance and efficiency.
- Cooling: Efficient heat dissipation prevents engine knocking and reduced efficiency.
- Shape and Size: Influences turbulence, swirl, and flame front propagation.
- Detonation Control: Design minimizes detonation likelihood.
- Emissions Control: Design reduces pollutant formation.
Factors Responsible for Detonation
- High Compression Ratio
- Advanced Ignition Timing
- Lean Air-Fuel Mixture
- High Engine Temperature
- Low Octane Fuel
- Carbon Deposits
- Poorly Designed Combustion Chamber
- Excessive Carbon Buildup
- Engine Overheating
- Incorrect Spark Plug Heat Range
Effect of Detonation on SI Engines
- Shock Waves
- Heat and Fire
- Blast Pressure
- Fragmentation
Combustion Chamber Types
-
T-Head Type: Early design with high knocking tendency.
-
L-Head Type: Modification of T-Head, with valve mechanisms on the same side of the cylinder.
-
I-Head Type (Overhead Valve): Superior design with valves on the cylinder head, reducing knocking tendency.
-
F-Head Type: Compromise between L-Head and I-Head types, with one valve in the cylinder head and one in the cylinder block.### Factors Affecting Knocking Tendency
-
Increased turbulence increases flame speed and reduces the time available for the end charge to attain autoignition conditions, thereby decreasing the tendency to knock.
-
Engine speed increases turbulence, flame speed, and reduces the time available for preflame reactions, resulting in reduced knocking tendency at higher speeds.
Flame Travel Distance and Engine Size
- Flame travel distance is affected by engine size, combustion chamber size, and spark plug position.
- A larger engine has a greater tendency for knocking due to the longer time required for the flame front to traverse the combustion chamber.
- Generally, a spark-ignition engine is limited to a size of about 150 mm bore.
Combustion Chamber Shape and Turbulence
- A more compact combustion chamber results in shorter flame travel and combustion time, leading to better antiknock characteristics.
- Combustion chambers are designed to be spherical to minimize the length of the flame travel for a given volume.
- High turbulence in the combustion chamber promotes a high combustion rate, reducing combustion time and knocking tendency.
Location of Spark Plug
- Central location of the spark plug in the combustion chamber minimizes flame travel, resulting in minimum knocking tendency.
- Using two or more spark plugs in large engines can also reduce flame travel.
The Phenomenon of Knock in SI Engines
- Combustion initiated by the spark plug spreads across the combustible mixture, forming a definite flame front.
- Heat release due to combustion increases the temperature and pressure of the burned part of the mixture, causing the unburned mixture to expand and compress adiabatically.
- If the temperature of the unburned mixture exceeds the self-ignition temperature of the fuel, spontaneous ignition or auto ignition occurs, resulting in knocking.
Explore the impact of increasing engine speed on turbulence, flame speed, and knocking tendency in internal combustion engines. Learn how higher speeds can reduce the likelihood of knocking by altering preflame reactions and flame travel distance.
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