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.

Petrol Engines (Spark Ignition)

• 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

Diesel Engines (Compression Ignition)

• 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)*

Key Differences in Engines

1. *Ignition Method:

• Petrol: Spark Plug
  • Diesel: Air Compression

2. *Fuel Injecting Timing:

• Petrol: During intake stroke
  • Diesel: Late in the compression stroke

3. *Injection Types:

• Petrol: Port and direct injection
  • Diesel: Almost always direct injection

4. *RPMs

• 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)

5. *Compression Ratios

• Diesel: High (15:1 to 23:1)
  • Petrol: Lower (11:1 to 14:1 for natural; 8.5:1 to 10:1 for forced induction)

Performance Characteristics

• *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

Environmental and Economic Factors

• 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

Practical Applications

• *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

Fun Factor

• 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

Conclusion

• Both engines have unique advantages and are suited for different scenarios.
• Understanding these will help in making informed choices for specific needs.