Understanding the characteristics of the various alternative fuels requires the understanding of some important energy concepts.
The different fuels can be compared with their energy content, or more precisely, their specific energy. Specific energy is energy per unit mass: BTU/lb or J/kg. A BTU (British Thermal Unit) is defined to be the amount of energy required to raise one pound of water one Fahrenheit degree. Similarly, a Calorie is the amount of energy required to raise one gram of water on Celsius degree. A Joule is a metric unit derived from the Newton - Metre. Specific energy is also known as the calorific or heating value.
All of the fuels we are considering here are hydrocarbons, that is to say composed of both hydrogen and carbon atoms. When the carbon atoms are completely reacted with oxygen (or burned), their product of combustion is carbon dioxide. When the hydrogen atoms are completely reacted with oxygen, their product of combustion is water. Since the exhaust temperature of any real engine cycle is higher than the boiling temperature of water, the energy available when steam is condensed to liquid water is not available. Therefore, the energy released from burning a hydrocarbon fuel whose products of combustion are at a higher temperature than the boiling point of water is known as the lower heating value (LHV). Similarly, when the products of combustion form liquid water, the amount of energy released is the higher heating value (HHV). The difference between the two is the latent heat of evaporation (or condensation) of water. The latent heat of evaporation is dependent upon the pressure at which the evaporation takes place but the variation will be small at the the range of exhaust pressures considered in internal combustion engines.
When we compare the energy available from various fuels for internal combustion engines, we always use the lower heating value. When do we use the higher heating value? One example is the modern high efficiency furnace, which is an external combustion process. These furnaces are known as condensing furnaces for that reason and they do transfer additional energy to the air that heats your house as the steam condenses to water in the secondary heat exchanger.
This concept is more applicable to spark-ignition engines and describes the fuels' ability to evaporate. Even though gasoline is stored as a liquid, it must become a gas before it can react with air. Gasoline is also not a pure element but is composed of several components.