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Questions and Answers
What is the ideal property of gasoline that relates to its ability to mix readily with air?
What is the ideal property of gasoline that relates to its ability to mix readily with air?
Which testing method specifically characterizes the volatility of gasoline?
Which testing method specifically characterizes the volatility of gasoline?
What characteristic of gasoline primarily affects cold starting in vehicles during winter?
What characteristic of gasoline primarily affects cold starting in vehicles during winter?
Which of the following effects on the engine is associated with tail end volatility of gasoline?
Which of the following effects on the engine is associated with tail end volatility of gasoline?
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What does the distillation curve for gasoline measure?
What does the distillation curve for gasoline measure?
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Which of the following factors does NOT influence the preparation of a fresh fuel mixture?
Which of the following factors does NOT influence the preparation of a fresh fuel mixture?
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What is a major disadvantage of using solid fuels?
What is a major disadvantage of using solid fuels?
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Which type of fuel is described as the most promising among alternatives?
Which type of fuel is described as the most promising among alternatives?
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Which of the following is NOT listed as a general requirement for engine fuels?
Which of the following is NOT listed as a general requirement for engine fuels?
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How does the time consumed by the engine to run for 1 degree relate to fuel preparation?
How does the time consumed by the engine to run for 1 degree relate to fuel preparation?
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Which hydrocarbon category is characterized by a chain structure and is saturated?
Which hydrocarbon category is characterized by a chain structure and is saturated?
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What is the primary structural feature of aromatics that differentiates them from other hydrocarbon categories?
What is the primary structural feature of aromatics that differentiates them from other hydrocarbon categories?
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In terms of antiknock characteristics, which hydrocarbon category is the least effective for use in SI engines?
In terms of antiknock characteristics, which hydrocarbon category is the least effective for use in SI engines?
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Which characteristic do olefins possess that makes them undesirable when stored over time?
Which characteristic do olefins possess that makes them undesirable when stored over time?
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What trend is observed in the boiling point of hydrocarbons as the number of carbon atoms in their structure increases?
What trend is observed in the boiling point of hydrocarbons as the number of carbon atoms in their structure increases?
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Study Notes
Fuel and Combustion
- Fuel is any substance that produces heat when burned with air
- The chemical reaction releasing heat energy is rapid
- Mixture preparation is time-consuming and depends on:
- Fuel type
- Preparation method (used for mixture)
- Method for introducing mixture into combustion chamber
Types of Fuels
- Fuels are categorized into four main groups:
- Solid (coal, wood, oil shale)
- Liquid (gasoline, alcohol, diesel)
- Gas (hydrogen, natural gas, LPG)
- Dual (Liquid-Liquid, Liquid-Gas, Gas-Gas)
Remarks
- Solid fuels are less common due to handling, storage, and ash disposal issues
- Gaseous fuels are the most promising alternative
- They burn cleanly and efficiently
- Eliminate start and distribution problems
- Form good homogeneous mixtures with air
- Handling and storage problems still exist
General Requirements
- Sufficient energy content
- Easy to handle/store
- Economically available in large quantities
- Free from hazards
- No chemical reactions with engine components
- Easy to mix with air for combustion
- Produces fewer pollutants when burned with air
- Low tendency to form deposits
- Good combustion qualities
Specific Requirements
- Engine consumes time to prepare and mix fuel. This puts pressure and time constraints on various systems (e.g. fuel, ignition system). This limits further fuel requirements
Detailed Combustion Properties
- Combustion duration is as short as possible, to maximize energy release
- Properties must be maintained throughout fuel use
- Combustion products should be ecologically and biologically harmless
Petroleum Structure
- Petroleum is a mixture of hydrocarbons with sulfur and impurities
- Hydrocarbons are categorized into four groups:
- Paraffin (CnH2n+2) - Chain structure, Saturated, Stable
- Olefins (CnH2n) - Chain structure, Unsaturated, Unstable
- Naphthene (CnH2n) - Ring structure, Saturated, Stable
- Aromatics (CnH2n-6) - Ring structure, Unsaturated, Unstable
Paraffin
- Series begins with Methane (CH4)
- Straight-chain molecular structure
- Names end in "ane"
- Examples include Methane, Propane, Heptane
Olefins
- Straight-chain hydrocarbons with double-bonded carbon atoms
- Names end in "ene" (single double bond), or "adiene" (two double bonds)
- Tend to form gummy deposits when oxidized
Naphthene
- Formula similar to olefins, but with different properties
- Cyclic/ring structure
- Saturated
Aromatics
- Ring structure with benzene molecules (C6H6) as the central structure
General Remarks on Fuel Types
- Normal Paraffin exhibits the poorest antiknock characteristics in SI engines, aromatics are best
- Antiknock characteristics of Paraffin can be improved by increasing carbon atoms and compactness
- Paraffins are the best option for CI engines, aromatics are the worst
- Fuel boiling point is related to the number of atoms; fuels with fewer atoms evaporate easier
Refining Process
- Petroleum constituents have different boiling points, which is the basis for refining
- Refineries use processes to separate components based on boiling points
Fuels for Spark Ignition Engines
- Gasoline is used in ~99% of cars
- Lighter liquid petroleum fraction boiling up to 200°C
- Mixture of ~40+ hydrocarbons
- Compositions depend on crude oil and refining process
- Ideal gasoline requirements:
- Easily mixes with air
- Resistant to knocking
- Clean burning, avoiding corrosion
- Avoids gum formation
Fuel Volatility
- Volatility is the tendency to evaporate under given conditions
- Measured by plotting percentage vaporized against temperature (Distillation Curve)
- Different fuels have different volatility profiles (e.g., volatile gasoline, non-volatile gasoline, diesel fuel)
Measurement Process
- Several methods to measure fuel volatility exist
- Commonly used methods include ASTM distillation test and Reid vapor test
Effect on Engine Operation
- Fuel volatility affects a various engine characteristics like cold/hot startability, vapor lock, short trip economy, and more
Volatility Effects (Detailed)
- Front-end volatility affects cold/hot starting and vapor lock
- Mid-range volatility affects warm-up, acceleration, and short/long trip economy
- Tail-end volatility affects crankcase dilution, spark plug fouling, evaporative losses, and varnish/sludge deposits
Cold and Hot Starting
- Cold starting is difficult in winter due to insufficient fuel vaporization at low temperatures. Higher volatility fuels remedy this issue
- Hot starting (or Percolation) is difficulty restarting in hot weather due to excessive fuel evaporation, leading to a rich mixture. Low volatility fuels remedy this problem
Vapor Lock
- Excessive and rapid vapor formation in the fuel system restricts fuel supply, causing the fuel mixture to be very lean
- Requires proper fuel system design and fuels with low volatility
Other Effects
- Warm-up periods are dependent on mid-range volatility
- Engine acceleration and smoothness, combined with fuel economy, is sensitive to fuel volatility
- Short trip and long trip economics require good mid-range volatility and lower volatility for long trips, respectively
- Carburetor icing occurs when humidity and cold temperatures cause water condensation and freezing, which is aggravated and remedied using lower volatile fuels; a lower inlet manifold temperature is also an indicator that fuels have low volatility
Crankcase Dilution
- Some fuel passes through piston walls unevaporated , diluting crankcase oil; this reduces viscosity and causes lubricant displacement
- Crankcase dilution is avoided by proper venting and/or high-volatility fuels
Varnish and Sludge Deposits
- High-boiling-point hydrocarbons in the fuel tend to form varnish/sludge on piston rings, valves, and spark plugs
- Using fuels with reduced tail-end volatility decreases these issues
Gasoline Additives
- Used to enhance gasoline properties
- Anti-knocking agents
- Deposit modifiers
- Antioxidants
- Detergents
- Lubricants
- Metal deactivators
- Anti-rust agents
- Anti-icing agents
- Dyes
Knock Rating of SI Engine Fuels
- Multiple methods for rating knock resistance
- Highest useful compression ratio
- Octane number
- Sensitivity
- Performance number
Octane Number (ON)
- Determines fuel performance against a mixture of isooctane and n-heptane
- Higher ON means greater resistance to knocking
Research and Motor Octane Number
- Knocking tendencies vary for different engines and operating conditions.
- ON is determined using multiple testing conditions:
- Cooperative Fuel Research (CFR) engines measure rather severe (Motor ON) and moderate (Research ON) duty
Sensitivity
- Difference between RON (Research Octane Number) and the MON (Motor Octane Number)
- This difference in value is labeled sensitivity
Performance Number (PN)
- Used for fuels with knock resistance greater than n-heptane
- Measurement of knock resistance behavior, as a ratio of knock-limited IMEP
- Formula to calculate ON using PN: ON = 100 + (PN -100) / 3
CI Engine Fuels (Diesel Fuels)
- Petroleum fractions between kerosene and lubricating oils
- Crude source and refining method affect diesel fuel properties
- Important properties that affect diesel fuel characteristics include viscosity, volatility, ignition quality, and more
Viscosity
- Measures the internal friction or resistance to the flow of a fluid
- Measured using a Saybolt viscosimeter
- Higher viscosity indicates more time needed for flow
- Viscosity in diesel fuels affects fuel system flow and lubricating abilities
Volatility
- Indicated by the 90% distillation temperature
- Affects HC, NOx emissions, smoke density, and exhaust odor
- Higher volatility reduces HC emissions and increases NOx emissions
Ignition Quality
- Ability of diesel fuel to ignite under engine operating conditions
- Long ignition delay leads to engine knocking
- Short ignition delay may not allow adequate combustion
API Gravity and Specific Gravity
- Specific gravity compares the weight of a fuel to an equal volume of water
- API gravity is a more accurate representation for measuring fuel flow properties
- Calculated as API = 141.5 / (Specific gravity @ 15°c) - 131.5
Other Properties
- Heat of combustion, contamination, sulfur content, carbon residue (when burned in the absence of air, it indicates the tendency of the fuel to form carbon deposits), water and sediment content, ash content
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Description
This quiz covers essential concepts related to fuel and combustion, including types of fuels, their characteristics, and general requirements for effective combustion. Test your understanding of solid, liquid, and gaseous fuels, and discover why gaseous fuels are considered a promising alternative. Learn about the factors that influence mixture preparation and combustion efficiency.
