MOTOR GASOLINE

Motor gasoline is a mixture of nearly 400 different hydrocarbons consisting of n-paraffins, iso-paraffins, olefins, aromatics and some cycloparaffins. Typical  gasoline  has hydrogen to carbon atom ratio  in the range 1.8 to 1.95  and it may be assigned  a molecular formula, C₈H₁₅. It is liquid at room temperature and boils in the range from about 35 to 215º C. The principal characteristics of gasoline that are important for engine performance and emissions have been given in Table 8.1.  Key fuel properties and their effects on engine performance and emissions are discussed below.

Antiknock Quality

In SI engines, use of a higher compression ratio results in higher thermal efficiency and higher engine power.  With increase in compression ratio however, knocking combustion occurs. Persistent and high intensity combustion knock causes engine overheating, loss in efficiency and may lead to mechanical damage to the engine components particularly under high load operation. To prevent or minimize knocking combustion, gasoline of a high antiknock quality is needed.
Knock resistance of a fuel is measured by octane number (ON), which is determined by comparing knocking combustion characteristics of the sample fuel to that of standard reference fuels in a standardized   CFR (Cooperative Fuel Research, USA) test engine. The two reference fuels that define the octane scale are:

• A fuel with high knock resistance: Isooctane (C₈H₁₈) or 2-2-4 trimethyl pentane given octane number equal to 100, and
• A fuel with low knock resistance: n-heptane (n-C₇H₁₆) given octane number equal to 0.

The blends of these two reference fuels define the intermediate octane number on a linear scale. For example, a blend of 90 % isooctane and 10% n-heptane has 90 ON.
The   ASTM CFR knock test engine used worldwide  is   a single cylinder, variable compression ratio engine The engine compression ratio can be varied while the engine is running between 3 and 30 by moving cylinder and cylinder head assembly up or down with respect to the crankshaft. Thus the engine clearance volume is changed while the swept volume remains constant.  A knock sensor is mounted on the combustion chamber and knock signal is a function of rate of pressure rise during combustion. Two test methods performed for automotive fuels on the standard CFR test engine are:

• Research method (ASTM D-2699) that measures Research Octane Number (RON), and
• Motor method (ASTM D-2700) measuring Motor Octane Number (MON).