Lecture Notes: Petrol vs. Diesel Engines

Overview

Both petrol and diesel engines are internal combustion engines with similar anatomy but differ in their combustion methods.
Petrol Engine: Also known as spark ignition engines, uses a spark plug to initiate combustion.
Diesel Engine: Also known as compression ignition engines, uses air compression to ignite the fuel.

Components: Cylinder block, cylinder head, camshafts, valves, pistons, crankshaft, connecting rods.
*Ignition System: Older vs Newer Vehicles:
- Older: Distributor and igniter
- Newer: Distributorless with coil packs or coil-on-plug systems
*Spark Plug Operation:
- Creates spark by ionizing gases with high voltage (>20,000 volts)
- Causes a current arc that raises the heat, initiating combustion
- Initiates deflagration, a subsonic combustion process transferring heat layer by layer

Components: Lack spark plugs and ignition coils; relies only on compression.
*Combustion Process:
- Compresses air to high temperatures to ignite diesel fuel
- Fuel introduced late in the compression stroke
- Uses direct injection with injectors to introduce a mist of fuel
- Ignition delay before rapid combustion causing characteristic knocking sound
- Two combustion stages: Uncontrolled (premixed) and controlled (post-injection)

Diesel: 4500-5000 RPM (less due to stronger, heavier internals and longer stroke)
Petrol: 6000-9000 RPM (higher due to spark ignition and lighter components)

*Diesel Engines:
- Higher torque due to longer stroke and more compression
- Lower RPM due to compression ignition limitations
- More efficient (40-45% thermal efficiency)
- Longer lifespan due to lower RPMs and more robust internals
*Petrol Engines:
- Higher horsepower due to higher RPM
- Thermal efficiency between 20-35%
- Lower torque but wider power band
- More prone to knocking, hence lower compression ratios

Fuel Efficiency:
- Diesels use leaner air-fuel ratios leading to higher efficiency
- Petrol engines can run rich, especially forced induction (turbocharged/supercharged)
Emissions:
- Diesels emit less CO2, but more NOx and soot particles
- Modern diesels with complex emission controls (filters, DEF) are cleaner
Cost & Maintenance:
- Diesels more expensive to produce and maintain
- Require significant mileage to be financially economical due to higher upfront costs

*Diesel Engines:
- Suitable for long-distance and heavy-duty applications
- Provide massive torque, efficient for long-term use
*Petrol Engines:
- Better for high speed and dynamic performance
- Provides better acceleration and driving dynamics

Petrol Engines:
- Preferred for tight, twisty roads and better sound
- Higher RPMs provide a better driving experience
Diesel Engines:
- Massive torque can be addictive
- Better for adventure and exploration due to fuel efficiency and range