Fuels and combustion

Ever since mankind was able to trigger and control them, combustion phenomena have enabled us to ensure our survival (heating, lighting, food) and then to reach industrial civilization. In our modern world, the vigorous combustion of coal, hydrocarbons and other organic products powers domestic and industrial stoves, as well as the various types of engines used for land, river, sea and air transport, and for agriculture.

This article reviews the main characteristics of common fuels, most of which are petroleum-based hydrocarbons. It provides a few notions of thermochemistry and describes the physico-chemical processes involved in combustion.

Readers may wish to refer to the articles "Liquid fuels" and "Combustibles liquides" in the Génie énergétique treatise [1][2] , which explain the requirements of the various energy converters and the means used by oil companies to obtain products that comply with very precise specifi-cations. Here are just a few of the constraints relating to different types of engine.

Gasolines used in spark-ignition engines must have physical properties (density, distillation curve, vapor pressure) that enable them to be sprayed into the air (by injection) before being introduced into the combustion chamber. In addition, the need to ensure the satisfactory operation of automobiles in extremely variable atmospheric conditions (from -20 C to +40 C in France) implies different gasoline volatility specifications for different seasons. Finally, gasoline engines require their fuel to have a high resistance to self-ignition, expressed by its octane number. This characteristic determines certain engine features (compression ratio, ignition advance), which in turn have a direct impact on the car's performance (fuel consumption, specific power). In practice, however, the octane number is limited by the constraints of the refining industry (composition of the various effluents, costs, volume and distribution of demand). The gradual elimination (between 1990 and 2000) of lead alkyl-based additives, which previously made it easy to increase the octane rating of gasolines, has considerably altered the landscape of engine-fuel tuning.

Simpler fuels than petrol can also be used in spark-ignition engines. These are :

• natural gas (CNG; V for vehicle), stored in a gaseous state at a pressure of 200 bar ;

• liquefied petroleum gas (LPG), maintained in a liquid state at a pressure of around 10 bar and delivered in a gaseous state to the engine.

Although not yet widely available, these...