Automotive Power Unit Study Notes

Automotive Power Unit

Mixture Formation

Introduction
  • Overview of Topics Related to Mixture Formation
    • Fuel
    • Otto Engine - Carburation
    • Otto Engine - Injection
    • Otto Engine - LPG/CNG/LNG
    • Diesel Engine - Injection
    • Diesel Engine - Common Rail
    • Otto Engine Exhaust Emissions
    • Diesel Engine Exhaust Emissions
Fundamental Concepts of Mixture Formation
  • Oxygen Requirement:
    • Oxygen is necessary for combustion and is found in the surrounding air.
    • Mixture Formation Definition:
      • A homogeneous mixture must be formed for optimal combustion, characterized by complete combustion and minimal emissions.
  • Mixture Types:
    • Homogeneous Mixture:
      • Applicable in conventional carbureted engines and engines with indirect gasoline injection.
    • Heterogeneous Mixture:
      • Common in modern engines, particularly those with direct gasoline injection or diesel engines.
  • Combustion Space Parameters:
    • Air ratio λ varies from rich (λ ≈ 1) to lean (λ ≈ 10), targeting complete combustion and clean exhaust emissions.
      • Complete Combustion Criteria:
      • All chemically bound energy is converted into heat or work.
      • Exhaust gases are clean: no CO, CH, or soot emissions.
      • Ideal air ratio for complete combustion: λ ≥ 1.
Conversion of Liquid Fuels
  • Vaporization Requirement:
    • Liquid fuels must be vaporized before mixing with air for complete combustion.
    • Gas engines are more efficient due to better combustion characteristics compared to liquid fuels.
    • Boiling Points:
      • Gasoline: boiling range approx. 30°C to 200°C.
      • Diesel: boiling range approx. 170°C to 400°C, resulting in complex mixture formation.
  • Mixture Formation Methods:
    1. Internal Mixture Formation
    2. External Mixture Formation
Internal and External Mixture Formation
Internal Mixture Formation
  • Occurs within the combustion chamber (cylinder)
  • Typically involves air intake followed by fuel injection to create a combustible mixture.
  • Characteristics:
    • Used in diesel engines and Otto engines with direct injection.
    • Involves a richer mixture for effective combustion.
External Mixture Formation
  • Traditional Otto Engine:
    • Fuel and air mixed outside the cylinder (manifold).
    • Results in a homogeneous mixture, characteristic of carbureted engines.
Fuel Chemistry and Specifications
  • Fuel is derived from petroleum distillation.
  • Composition:
    • Hydrocarbons within the boiling range 40°C - 200°C yield petrol and super petrol.
    • Specific heating value for gasoline: Ho = 40.1 - 41.9 MJ/kg.
    • Super petrol has a higher amount of anti-knock hydrocarbons than regular petrol.
  • Refining Processes:
    • Heavy fractions undergo cracking to produce lighter hydrocarbons, enhancing gasoline quality and performance.
Specific Heating Values and Density
  • Important for different motor fuels:
    • Diesel: 42.9 - 43.1 MJ/kg
    • Gasoline: 40.1 - 41.9 MJ/kg
  • Mixture Value Hm:
    • For optimal engine performance, the mixture value of the fuel-air mixture must be considered.
    • Comparable values for liquid fuels: Hm ≈ 3.5 - 3.7 MJ/kg.
  • Fuel density parameters for Otto engines limited to 720 - 775 kg/m³ as per the NEN 228 standard, affecting overall fuel quality.
Exhaust Components and Their Relevance
Harmful Fuel Properties
  • Sulfur Content:
    • Must be minimized in gasoline to prevent negative effects on fuel performance, including:
      • Adverse effects on anti-knock properties, catalyst damage, deposits, and emissions.
Composition of Gasoline
  • Composed of hydrocarbons:
    • Pure Hydrocarbons:
      1. Paraffins/Alkanes: CnH2n
      2. Olefins/Alkenes: CnH2n, CnH2n-2.
      3. Naphthenes: CnH2n.
      4. Aromatics: Ring-shaped hydrocarbons (Example: Benzene C6H6).
  • Oxygenated Hydrocarbons:
    • Include alcohols (methanol, ethanol) and ethers (MTBE) for improved performance and properties.
Fuel Additives and Their Roles
  • Used to enhance fuel properties (ignition, emissions reduction) while typically comprising less than 1% of total fuel volume.
    • Types of Additives:
      • Detergents, corrosion inhibitors, and oxidation stabilizers.
Combustion Characteristics and Knock Resistance
Essential Parameters
  • Fuel must have a low ignition tendency for Otto engines:
    • Knock Resistance:
      • An indicator of fuel quality; higher octane indicates better performance.
      • Octane number standard reference: iso-octane = 100, normal heptane = 0.
  • Testing Methods for Octane Number:
    1. Research Octane Number (RON).
    2. Motor Octane Number (MON).
    3. Road Octane Number (RoON).
Volatility of Gasoline
  • High volatility necessary for cold starts and complete combustion;
    • Volatility limits to prevent vapor lock and starting difficulties in engines due to thermodynamic properties and environmental conditions.
    • Key Attributes:
      • Boiling range, vapor pressure, and vapor-liquid ratio significantly influence engine performance.
Mixture Formation and Control Techniques
  • Description of the air ratio λ and its role in mixture formation:
    • λ ratio indicates the ratio of actual to theoretical air for complete combustion.
    • Operating Points: λ < 1 (rich mixture), λ = 1 (stoichiometric), λ > 1 (lean mixture).
  • Practical values for air ratios are specified for both gasoline and diesel engines across various loads.
    • Objective: Achieve complete combustion with minimal emissions.
Fuel Injection Systems and Types
Spark Plug Characteristics
  • Selection of electrodes and optimization of spark characteristics critical for efficient ignition.
Types of Spark Plugs
  • Various designs available (electrode shapes, materials, etc.).
  • Important Characteristics of Spark Plugs:
    • Electrical, thermal, chemical, and mechanical properties must meet high standards.
Diesel Engine Characteristics
Overview of Diesel Engines
  • Diesel engines rely on internal mixture formation and conceptually differ from Otto engines in combustion mechanisms.
Fuel Specifications
  • Diesel fuel characteristics emphasizing high cetane number, viscosity, and low aromatic content are reviewed.
Mixture Formation Dynamics in Diesel Engines
  • The key stages of the diesel combustion process are articulated, indicating the importance of ignition delay and combustion phases.
  • Pressure Measurements:
    • Highlighting the role of injection pressure and characteristics in driving fuel volatility and combustion efficiency.
Summary of Critical Takeaways
  • The importance of maintaining conducive conditions for fuel efficiency, performance, and minimal emissions is emphasized across combustion processes.
  • Injection Control Systems:
    • Various systems reviewed for effective management and emission control in diesel and Otto engines.
Emission Control Systems
  • The role of EGR, SCR, and DPF systems in mitigating harmful emissions is outlined, providing details on their operational mechanisms and influence on fuel economy.
Computational Requirements for Engine Control
  • Engine management systems outlined detailing inputs, outputs, and how ECU integration can lead to optimized performance.
Conclusion
  • Connecting the significance of mixture formation with performance, combustion efficiency, and emissions management in the context of modern automotive engineering.