Combustion in diesel and gasoline engines represents the fundamental process where fuel is converted into energy through controlled burning. Despite both being internal combustion engines, diesel and gasoline engines differ in their combustion processes due to varying fuel properties and operating principles.

1. COMBUSTION PHENOMENA
IN DIESEL AND GASOLINE
ENGINES
BY
ER. T. AYISHA NAZIBA, DR. D. RAMESH, DR. S. PUGALENDHI

2. DIESEL ENGINE COMBUSTION
Compression Ignition (CI) Process
• Air Compression: During the compression stroke, air is compressed within the
cylinder to high pressures (typically 14-25 bar in modern diesel engines).
• Heat Generation: The compression of air raises its temperature significantly (up to
700-900°C), causing rapid heating.
• Fuel Injection: Near the end of the compression stroke, diesel fuel is injected directly
into the highly compressed, hot air.
• Autoignition: The high temperature and pressure cause the injected fuel to
autoignite, initiating combustion without the need for a spark plug.
• Combustion Development: The burning fuel generates high-pressure gases that force
the piston down, performing work on the crankshaft.

4. COMBUSTION CHARACTERISTICS
• Laminar Burning: Diesel combustion initially involves a diffusion-
controlled (or mixed-mode) combustion where the flame speed is
relatively slow.
• Ignition Delay: The period between start of fuel injection and start of
combustion is known as ignition delay, critical for controlling
combustion timing.
• Mixture Formation: Fuel is atomized into small droplets upon injection,
mixing with air to form a combustible mixture.
• Combustion Phases: Diesel combustion typically exhibits premixed
combustion followed by diffusion combustion, influencing pollutant
formation and engine efficiency.

5. EMISSION CONSIDERATIONS
• NOx Formation: Diesel engines produce higher levels of nitrogen oxides
(NOx) due to high combustion temperatures.
• Particulate Matter (PM): Diesel combustion can lead to the formation of
soot and particulate matter, which requires filtration systems to reduce
emissions.

6. GASOLINE ENGINE COMBUSTION
Spark Ignition (SI) Process
• Air-Fuel Mixture Formation: Air and gasoline (or petrol) are mixed externally
or internally within the intake system.
• Compression Stroke: The air-fuel mixture is compressed during the
compression stroke, raising its temperature.
• Spark Ignition: A spark plug ignites the compressed air-fuel mixture near
the end of the compression stroke.
• Flame Propagation: The flame front rapidly propagates through the
combustion chamber, burning the mixture.
• Expansion Stroke: The burning gases expand, forcing the piston down and
generating power.

8. COMBUSTION CHARACTERISTICS
• Homogeneous Charge: Gasoline engines typically use a
homogeneous air-fuel mixture for combustion.
• Flame Propagation: Gasoline combustion is characterized by
rapid flame propagation due to the presence of a well-mixed,
homogeneous charge.
• Knock Resistance: Gasoline engines require fuels with high
octane ratings to resist knock (premature combustion).

9. EMISSION CONSIDERATIONS
• NOx and Hydrocarbons: Gasoline engines also produce NOx
and hydrocarbon emissions, though typically at lower levels
compared to diesel engines.
• Catalytic Converters: Gasoline engines often incorporate
catalytic converters to reduce emissions of NOx, CO (carbon
monoxide), and hydrocarbons.

10. DIFFERENCES
• Fuel Injection vs. Carburetion: Diesel engines use direct fuel
injection, while gasoline engines historically used carburetors (now
mostly use fuel injection for better efficiency and emissions control).
• Combustion Timing: Diesel engines control combustion timing by
varying the start of fuel injection, whereas gasoline engines control
timing using spark timing.
• Emission Control: Both types of engines require emission control
systems, such as exhaust gas recirculation (EGR), selective catalytic
reduction (SCR), and particulate filters, to meet stringent
environmental regulations

11. THANK YOU