Engine Combustion Dynamics Quiz

Combustion in Spark-Ignition Engines

• The combustion process is crucial in internal combustion engines, converting fuel's chemical energy into high-pressure, high-temperature gases.
• Efficient combustion is rapid and repeatable, minimizing pollutants and adhering to fuel regulations (anti-knock and volatility).
• The flame is the confined area where fuel oxidation and energy release occur, usually less than a millimeter thick.
• Fuel-air mixtures are well-mixed before combustion, aided by turbulence within the engine cylinder.
• Combustion begins with a spark to ignite the mixture.
• Flame propagation resembles a thin, wrinkled sheet through the premixed fuel-air mixture.
• Energy release starts slowly, peaks, and then decreases as combustion ends, reflecting changes in cylinder pressure.

Combustion Fundamentals in SI Engines

• High-energy sources ignite air-fuel mixtures, leading to self-sustaining reactions.
• Ignition systems (12-48V primary, 50,000V secondary circuit) use an inductor for increased voltage.
• Spark plugs initiate combustion with minimum energy required for a specific fuel-air mixture.
• Excessive energy generates plasma, leading to sustained flame propagation.
• Insufficient energy results in flame quenching, no reaction.

Flame Propagation

• Flame speed (or burning speed) describes the rate of premixed flame propagation.
• Laminar flame speed (SL) is the flame speed in laminar flow (no turbulence), a characteristic property of the unburned gases.
• Turbulent flame speed (ST) is significantly faster than laminar, increasing with turbulence.
• Flame zones include unburned mixture, pre-heat, conduction, and visible flame zones. Each zone has specific effects on the flame propagation process.

SI Engine Combustion Process

• A spark ignition engine operates under a certain pressure (motored pressure).
• Pressure rises above motored pressure (firing pressure) as the flame grows.
• Combustion characteristics (different cycles) vary due to local mixture composition and turbulence conditions.
• Combustion typically occurs close to top dead center (TDC).
• Combustion process can be divided into four phases: spark ignition, early flame development, flame propagation, and flame termination.
• Optimizing timing determines maximum brake torque(MBT).

Phases of Combustion

• Spark ignition: Ignition of the air-fuel mixture by the spark plug.
• Early flame development: Initial flame growth and propagation near the spark plug.
• Flame propagation: Flame expansion/spread throughout the combustion chamber.
• Flame termination: Completion of combustion and flame cooling/extinction

Abnormal Combustion

• Factors like fuel composition, engine design, and operating parameters can lead to abnormal combustion (knock and spontaneous ignition). -Knock is a phenomenon characterized by pressure oscillations causing a sharp metallic noise.
• Spontaneous ignition occurs where a fuel-air mixture ignites prematurely before the flame front reaches.

Analysis of Cylinder Pressure Data

• Cylinder pressure data (piezoelectric transducers) provide insights into the combustion process.
• Pressure changes based on cylinder volume, combustion, heat transfer, flow, and leakage.
• Measurements relate to combustion rate, provided models for other phenomena are considered.
• Accurate pressure data provides critical info about the combustion.

Heat Release Approach

• Heat transfer, crevices, and leakage effects impact the combustion process.
• The chamber content is considered as a single zone.
• First law of thermodynamics (energy balance) to approximate heat released.
• Cylinder temperature changes with combustion.

Combustion Process Characterization

• Parameters like flame development angle (θa), rapid burning angle (Δθ), and overall burning angle (Δθ) characterize the combustion process.
• Flame development angle represents the time from spark to significant fuel combustion.
• Rapid burning angle characterizes the bulk of combustion process.

Relation to Flame Propagation

• Flame propagation in a spark ignition engine is influenced by speed and composition of the charged air fuel mixture.
• The speed of the spread of flame front is important parameter in combustion.
• Flame speed, transport speed influence the propagation rate.

Dependence of Efficiencies on the Air-Fuel Ratio

• Internal thermal efficiency varies with air-fuel ratio, peaking at near stoichiometric values. -Engine performance benefits most when all engine components and systems operate optimally.
• Engine characteristics are typically graphed in terms of fuel conversion efficiency vs bmep.

WOT Performance Characteristic

• Full load (wide-open throttle) operating conditions produce maximum power and torque with associated fuel consumption characteristics.
• The ideal torque-speed characteristic is hyperbolic, going to infinity as the engine speed decreases to zero, and vice-versa.
• Operating performance characteristics can be depicted by a graph of fuel consumption vs bmep to assist in design optimization.

Power and Gearbox

• Maximum torque occurs at the speed where a tangent line from the origin meets the P-curve of the performance characteristic(power curve.
• Engine power and speed ranges are limited, this is why gearboxes are used for better coverage of this range of operation.

Abnormal Combustion and Fuel Properties

• Knock and spontaneous ignition are abnormal combustion phenomena. -Knock is a high-frequency pressure oscillation causing engine noise. -Spontaneous ignition occurs when fuel ignites prematurely near the end of the chamber.
• Fuel characteristics (like volatility and octane rating) affect these phenomena, making some fuels more resistant to knock than others.

Rapid Compression Machine

• The rapid compression machine (RCM) simulates combustion characteristics to study high-pressure, pre-ignition processes.
• The RCM measures pressure changes over time related to the combustion process.
• The RCM is used to determine the auto-ignition characteristics of different fuels.

Knock Fundamentals

• Knock occurs when fuel ignites prematurely in the combustion chamber.
• Autoignition is possible when fuel mixture reaches high temperature.
• Knock often happens in the end gas (closer to piston) area. -Knock occurs at different rates depending on the engine and fuel.

Mechanisms of Auto Ignition

• Auto-ignition is a chemical reaction where fuel ignites without an external ignition source, like a spark.
• Auto-ignition occurs because of high temperature and/or pressure conditions.
• Intermediate products (like hydro peroxides) are critical factors determining auto-ignition behavior.

Conventional Fuels from Crude Oil

• Crude oil is the primary source for transportation fuels like gasoline, and diesel fuels.
• Refinery processes like distillation, alkylation, and reforming convert crude oil into various usable fuel fractions.
• Gasoline, kerosene, diesel oils, and fuel oils are important fractions widely used globally.

Volatility of Liquid Fuels

• Volatility refers to a liquid's tendency to evaporate.
• Important for fuel systems in spark-ignition (SI) engines, as it affects fuel-air mixtures readiness when igniting.
• Volatility is an important parameter during the design of the engine (manifold, injection system, fuel properties/characteristics).

Fuel Factors

• Various hydrocarbon components respond differently to knock.
• Branched-chain hydrocarbons increase knock resistance.
• Knock rating is an important parameter during automotive engine design.

Motor Fuels and Knock Rating

• Knock resistance measured with octane number.
• Standard testing procedures measure research octane number (RON) and motor octane number (MON).
• Higher octane numbers mean higher knock resistance.

Knock Control Strategies

• Strategies for controlling knock include spark timing adjustments and fuel modifications.
• EGR (exhaust gas recirculation) and direct fuel injection are also used to decrease knock and optimize engine performance. -Engine management systems react to changes in operating conditions to allow for adjustments in operations.