Fuel and Combustion Overview

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