Otto Engine: Characteristics, Combustion, and Components

Otto Engine Characteristics

The Otto engine is an internal combustion engine that utilizes a pre-mixed fuel-air mixture. Key characteristics include:

• Internal Combustion: Consumes a fuel-air mixture.
• Electric Ignition: Employs an electric system to ignite the compressed mixture.
• Valve Control: Intake and exhaust valves are precisely controlled by the engine's operating system.

Otto Cycle

The Otto cycle consists of four strokes, equivalent to two crankshaft revolutions:

1. Intake: Admission of the air-fuel mixture.
2. Compression and Ignition: Compression of the mixture followed by spark ignition.
3. Combustion and Expansion: Burning and expansion of the mixture, generating power.
4. Exhaust: Expulsion of combustion byproducts.

Fuel

The Otto engine typically uses petroleum-derived fuels with the following properties:

• Density: 0.71 to 0.76 kg/l at 15°C.
• Calorific Value: Approximately 44,000 kJ/kg.
• Volatility: Highly volatile.
• Octane Rating: Defines antiknock capability, indicating the fuel's resistance to self-ignition. Higher octane ratings reduce the risk of pre-ignition.

Mixture Preparation

Fuel must mix with air to provide the necessary oxygen for combustion. The stoichiometric air-fuel ratio for gasoline is approximately 14.7:1 (air to fuel) by mass. The mixture should be as homogeneous as possible.

Ignition

Combustion begins with a spark just before the piston reaches Top Dead Center (TDC). This is known as ignition advance, which compensates for the time it takes for the flame to propagate and generate peak pressure at TDC.

Engine Operation

Load Regulation

Engine load is regulated by adjusting the quantity of the fuel-air mixture admitted into the cylinder, while maintaining a relatively constant air-fuel ratio. This is controlled by the throttle valve in the intake manifold, operated by the accelerator pedal.

Engine Components

The Otto engine comprises:

• A reciprocating piston.
• A connecting rod and crankshaft system, converting linear motion to rotary motion.
• Valves and a spark plug located in the cylinder head.
• An engine block housing the cylinder and other components.
• A cylinder head sealing the top of the cylinder.
• A crankcase enclosing the crankshaft.

Piston Displacement

Piston displacement is determined by:

• Top Dead Center (TDC): The piston's highest position in the cylinder.
• Bottom Dead Center (BDC): The piston's lowest position.
• Stroke: The distance the piston travels between TDC and BDC.
• Unit Cylinder Volume: Calculated as (π * D² / 4) * stroke, where D is the cylinder bore.
• Total Displacement: Unit cylinder volume multiplied by the number of cylinders.

The compression ratio is the ratio of the cylinder volume at BDC to the volume at TDC (combustion chamber volume). It affects the pressure and temperature at the end of the compression stroke and is calculated as: (Unit cylinder volume + combustion chamber volume) / combustion chamber volume.