**1. Introduction**

Automotive engines are designed to convert chemical energy stored in the fuel to mechanical energy through combustion. Thus, the working fluids in the engine are the fuel and air with which it is mixed. The combustion process can be viewed to convert chemical energy of the reactants to thermal energy of the products. The thermal energy translates into higher temperature and pressure in the combustion chamber which in turn is converted to work by the engine.

There are two main types of automotive engines which are classified according to how the ignition in the combustion chamber is induced. These are the spark ignition (SI) engine and the compression ignition (CI) engine which determine which fuel is fit for purpose.

It is worth noticing that the combustion of both types of engines are governed by the laws of thermodynamics. The first law states that internal chemical energy in the reactants is converted in heat added to the system and work done by the system. The second law precludes the possibility of a perfect engine where all the internal chemical energy is converted to work as some heat must be exhausted to the environment. Thus, engines are designed to extract the maximum amount of work and minimizes the heat release which improves efficiency.

In this chapter the two types of engines will be discussed which leads to the desired combustion characteristics of the fuels. However, combustion is not the only aspect to be tested for automotive fuels. In fact, the quality assessment performed on the fuels to ensure that, in addition to combustion, other aspects

of the fuel are properly measured such as energy density, stability, fluidity, corrosion, contaminants, handling, wear, composition and environmental aspects will be illustrated. Impact of additives and renewable components will also be discussed.
