Combustion

Combustion is a chemical process in which a substance, called fuel, reacts rapidly with Oxygen and gives off heat.  The source of oxygen is called the oxidizer, which can be a solid, liquid or gas.  The internal combustion engines, found in modern motor vehicles, use air for a source of oxygen, which is a gas.  The air contains 21% oxygen, 78% Nitrogen and 1% trace gases.

O (oxygen) and N (nitrogen)

The Fuel

Gasoline and Diesel Fuel are Hydrogen-Carbon fuels.  The various elements that make these fuels are called hydrocarbons. 

H (hydrogen) and C (carbon)

Ignition

To cause combustion to take place in the internal combustion engine the fuel is mixed with air and ignited.  In a gasoline engine the source of ignition is a spark plug.  In a diesel engine the source is compression.  This results in the rapid expansion of the gas, which causes heat that is the kinetic energy (energy in motion) that drives the piston down and thus propels the vehicle.   During combustion new chemical substances are created from the fuel and the oxidizer.  These substances are called exhaust. 

Re-association

When the oxygen, nitrogen, hydrogen and carbon atoms re-associate they form new substances, which include:

H2O (water), CO2 (carbon dioxide), CO (carbon monoxide), VOCs (volatile organic compounds) and NOx (oxides of nitrogen)

• Water is one of the most abundant elements on earth and is essential for human life.

• Carbon Dioxide is the gas attributed for causing global warming.  Carbon dioxide is one of the trace gases in the atmosphere.  It is uniformly distributed over the earth's surface at a concentration of about 0.033% or 330 ppm. Commercially, CO₂ finds uses as a refrigerant (dry ice is solid CO₂), in beverage carbonation, and in fire extinguishers.  The amount of CO₂ in the atmosphere has increased over the past century.  CO₂ is rising at a rate of about 1 ppm per year.  The rise of CO₂ could cause global climate changes, which are referred to as the greenhouse effect.

• Carbon Monoxide is a gas that is clear, tasteless and odorless.  Heavy concentration and lengthy exposure to CO is fatal to human beings.  CO is caused by the incomplete combustion of fossil fuels. 

• Volatile Organic Compounds are chemicals that evaporate when exposed to air.  VOCs are a contributing factor in air pollution.  VOCs contain hydrocarbons (HC) that are lighter than air.  These hydrocarbons mix with the NOx emissions to create Smog.  Exposure to, or ingestion of, VOCs in high concentrations can cause various health effects, including cancer.

• Oxides of Nitrogen molecules react to sunlight; turn reddish-brown and form ground level ozone or smog.  NOx contain substances that are highly toxic and can cause lung damage or even death.

Summary

For combustion to occur thee things must be present:  fuel, oxygen and a source of heat or ignition.  As a result of combustion, exhausts are created and heat is released.  Heat is the driving force of the internal combustion engine.  Combustion can be controlled or stopped by controlling the amount of fuel available, the amount of oxygen available or the source of heat.