Otto vs. Diesel Engines: Understanding Internal Combustion

Posted on Mar 26, 2025 in Technology

Otto Engine: Principles and Operation

Otto engines are internal combustion engines that use a mixture of gasoline and air. They feature an electric ignition system and intake and exhaust valves controlled by a distribution system. The Otto cycle consists of four strokes:

  • Ignition: At the end of the compression cycle, the air-fuel mixture is ignited by a spark plug.

Otto Engine Components

The Otto engine consists of:

  • A reciprocating piston
  • A connecting rod
  • A crankshaft

The piston moves within the cylinder between two points: Top Dead Center (TDC) and Bottom Dead Center (BDC). The distance between these points is the stroke length. Cylinder volume is calculated as:

π * (D2 / 4) * stroke

The compression ratio is calculated as:

(cylinder volume + combustion chamber volume) / combustion chamber volume

Theoretical Otto Cycle

  • Intake: The piston moves from TDC to BDC while the intake valve is open, filling the cylinder with air and fuel.
  • Compression: With both valves closed, the piston moves from BDC to TDC, compressing the air-fuel mixture in the combustion chamber.
  • Combustion and Expansion: The compressed mixture is ignited by the spark plug, causing rapid expansion that pushes the piston from TDC to BDC, converting chemical energy into mechanical energy.
  • Exhaust: The piston moves from BDC to TDC with the exhaust valve open, expelling the exhaust gases from the cylinder.

Valve Timing Diagram

A valve timing diagram graphically represents the moments when the valves open and close.

  • Exhaust Valve Opening (EVO): The exhaust valve opens slightly before the piston reaches BDC, allowing the high-pressure gases to escape.
  • Intake Valve Opening (IVO): The intake valve opens slightly before the piston reaches TDC, while the exhaust valve is still open. This overlap improves cylinder filling.
  • Exhaust Valve Closing (EVC): As the piston starts to move down, the exhaust valve closes, ensuring complete removal of waste gases.
  • Intake Valve Closing (IVC): As the piston descends during the intake stroke, the intake valve closes, maximizing the amount of air-fuel mixture entering the cylinder.

Diesel Engine: Principles and Operation

Diesel engines are internal combustion engines that intake only air during the intake stroke. Fuel is injected directly into the cylinder, and ignition occurs due to the high temperature of the compressed air. The Diesel cycle is a four-stroke cycle:

  • Intake
  • Compression
  • Injection
  • Exhaust

Diesel engines use diesel fuel, and fuel injection occurs in one or two stages. Ignition occurs due to the high air temperature (around 500°C) caused by the strong compression. The cylinder must be at maximum load for efficient combustion.

Theoretical Diesel Cycle

  • Intake: The piston moves from TDC to BDC with the intake valve open, drawing air into the cylinder.
  • Compression: The piston moves from BDC to TDC, compressing the air and raising its temperature.
  • Injection, Combustion, and Expansion: Fuel is injected into the hot, compressed air, causing it to ignite. The resulting expansion pushes the piston down.
  • Exhaust: The exhaust valve opens when the piston is at BDC, allowing exhaust gases to escape.

Otto vs. Diesel: Key Differences

  • Diesel: Maximum air intake, high temperature at the end of compression, auto-ignition of gases, combustion pressure of 70 to 90 bar.
  • Otto: Specific air-fuel mixture, spark ignition, rapid combustion at constant volume, maximum pressure of 30 to 40 bar.

Engine Distribution Systems

The distribution system includes all the components that control the opening and closing of the valves. These components include:

  • Camshaft
  • Sprocket
  • Transmission system
  • Valve guides
  • Valve seats
  • Springs
  • Rocker arms
  • Tappets

Distribution System Arrangements

  • OHV (Overhead Valve): Camshaft is located in the engine block.
  • OHC (Overhead Camshaft) and DOHC (Double Overhead Camshaft): Camshaft(s) are located in the cylinder head.

Crankshaft Forms and Firing Orders

Firing orders for various engine configurations:

  • 4-cylinder in-line: 1-3-4-2 or 1-2-4-3
  • Horizontally opposed 4-cylinder: 1-4-3-2
  • 5-cylinder: 1-2-4-5-3 (144° interval)
  • 6-cylinder: 1-5-3-6-2-4 or 1-2-4-6-5-3 (120° interval)
  • 6-cylinder in V: 1-3-6-5-4-2
  • 8-cylinder in V: 1-5-4-8-6-3-7-2 (90°)
  • 12-cylinder in W: 1-12-5-8-3-10-6-7-2-11-4-9 (60°)