q Fuels q Fuel Systems
LEKQ7256 (Supersedes LEKQ2461)
Fuel Characteristics Hydrocarbons Standard Condition of a Gas Heat Value Methane Number Air Required for Combustion Common Fuels Natural Gas Sour Gas Propane Propane-Butane Mixtures Propane-Air Propane Fuel Consumption Calculations Digester GasSanitary Landfill Gas Manufactured Gases Constituents of Gas by Volume - Percent Producer Gas Illuminating Gas Coke-Oven Gas Blast Furnace Gas Wood Gas Cleaning
Fuel Effects on Engine Performance Heat Value of the Air-Fuel Mixture Turbocharged Engines Methane Number Program Calculations Fuel Consumption Detonation Methane Number Compression Ratio Ignition Timing Load Inlet Air TemperatureAir-Fuel Ratio Emissions Variations in Heating Value Fuel Temperature Recommendations Fuel Requirements Heating Value
Most of the fuels used in internal combustion engines today, whether liquid or gaseous, are composed primarily of hydrocarbons (hydrogen and carbon); their source is generally petroleum. Natural gas is the most popular and widely used of the petroleum gases. Digester gas (alsoa hydrocarbon) and some manufactured gases (from coal), which contain hydrocarbons, are also used in engines with varying degrees of success. Digester gas is the most practical of the manufactured or by-product group. Each commercial fuel gas is a mixture of gases, some combustible and some inert. The different mixtures have extremely wide variations in composition. Consequently, it is necessaryto closely examine the characteristics and behavior of an individual gas.
As the number of atoms increases, the molecular weight of the molecule increases and the hydrocarbons are said to become heavier. Their physical characteristics change with each change in molecular structure. Only the first four of the Paraffin series are considered gases at standard conditions of 101.31 kPa (14.696 psia)and 15.55°C (60°F). Several of the others can be easily converted to gas by applying a small amount of heat.
Standard Condition of a Gas
It is important to note that when standard conditions are referenced, it means 101.31 kPa (14.696 psia) and 15.55°C (60°F). When a gaseous fuel flow is stated in SCF, it means standard cubic feet (or standard cubic meters - SCM) and is referenced to a gas atstandard conditions. In some places, Europe for example, gas is referenced to 101.31 kPa (14.696 psia) and 0°C (32°F). When gases are referenced to 0°C (32°F), the units are called normal cubic meters (NM3) or normal cubic feet (NF3).
Hydrocarbons are grouped into three classifications according to their molecular structure. • Paraffins - CnH2n+2 • Napthenes- CnH2n • Aromatics - CnH2n-6 Most of the important fuel gases used in engines today are of the Paraffin series. This includes both natural gas and digester gas. This series starts with methane (CH4); each succeeding member of the series has one more carbon (C) atom and the corresponding number of hydrogen (H) atoms, etc. The normal Paraffin hydrocarbons are said to have straight chain molecularstructures, having one bond between each atom. The first four of the Paraffin series would have structures as follows:
H H-C-H H Methane CH4 HH H-C-C-H HH Ethane C2H6 HHH H-C-C-C-H HHH Propane C3H8 HHHH H-C-C-C-C-H HHHH Butane C4H10
Heat value is defined as the amount of energy (heat) released during the combustion of a fuel with the correct amount of oxygen (air). It is determinedwith a device called a calorimeter. A known quantity of fuel and oxygen are combined in a calorimeter and burned. Heat is generated and, the water produced (from the combustion of fuels containing Hydrogen; either Cx Hy or H2) is condensed. The heat measured by the calorimeter is called the high heat value of the fuel (also referred to as the gross heat value). It is important to understand the...