The ability of a liquid to change to vapor is known as VOLATILITY. All liquids tend to vaporize at atmospheric temperatures, but their rates of vaporization vary. The rate of vaporiza-tion increases as the temperature increases and as the pressure decreases. (Temperature is more important than pressure.) In general, for a given temperature, a highly volatile fuel will vaporize more readily and at a faster rate than a fuel with a lower volatility.

The volatility of fuel affects engine starting, length of warmup period, fuel distribution, and engine performance. (When compared to diesel fuel (F-76), gasoline is much more volatile.) High volatility, however, can also result in fuel dilution of the lube oil in the crankcase. The ways in which volatility can affect engine operation are discussed in the sections that follow.

INJECTION, IGNITION, AND COMBUSTION

The self-ignition point of a fuel is a function of temperature, pressure, and time. In a properly operating diesel engine, the intake air is compressed to a high pressure (increases the temperature), and the injection of fuel starts a few degrees before the piston reaches TDC. The fuel is ignited by the heat of compression shortly after fuel injection starts and combustion continues throughout the injection period. Combustion in a diesel engine is much slower than it is in a gasoline engine, and the rate of pressure rise is relatively small.

Immediately after injection, the atomized fuel partially evaporates with a resultant chilling of the air in the immediate vicinity of each fuel particle. However, the extreme heat of compression rapidly heats and vaporizes the fuel droplets to the self-ignition point and combustion begins. The fuel particles burn as they mix with the air. The smaller particles burn rapidly, but the larger particles take more time to ignite because heat must be transferred into them to bring them to the self-ignition point.

There is always some delay between the time fuel is injected and the time it reaches the self-ignition point. This delay is commonly referred to as IGNITION DELAY or lag. The duration of the ignition delay is dependent upon the characteristics of the fuel, the temperature and pressure of the compressed air in the combustion space, the average size of the fuel particles, and the amount of turbulence present in the space. As combustion progresses, the temperature and pressure within the space rise rapidly; therefore, the ignition delay of fuel particles injected later in the combustion process is less than in those in-jected earlier. In a diesel engine, the delay period between the start of injection and the start of self-ignition is sometimes referred to as the first phase of combustion. The second phase of combustion is ignition of the fuel injected during the first phase and the rapid spread of the flame through the combustion space, as injection continues. The resulting increases in temperature and pressure reduce the ignition lag for the fuel particles entering the combustion space during the re-mainder of the injection period.

Remember, only a portion of the fuel has been injected during the first and second phases. As the remainder of the fuel is injected, the third or final phase of combustion takes place. The increase in temperature and pressure during the second phase is sufficient to cause most of the remaining fuel particles to ignite with practically no delay in the third phase as they come from the injection equipment. The rapid burning during the final phase of combustion causes an additional, rapid increase in pressure.

The knock that occurs during the normal operation of a diesel engine should not be confused with detonation. Generally, DETO-NATION in a diesel engine is caused by a simultaneous combustion of all particles of the fuel spray in the cylinder. COMBUSTION (DIESEL) KNOCK in a diesel engine is directly related to the amount of ignition delay and will take place at the end of the second phase. Diesel knock occurs from the rapid burning of large amounts of fuel (gathered in the cylinder before combustion begins). Whether combustion is normal or whether detonation occurs is de-termined by the amount of fuel that is ignited instantaneously. The greater the amount of fuel that ignites at one time, the greater the pressure rise and the more severe the knock.

Detonation in a diesel engine is generally caused by too much delay in ignition. The greater the delay, the greater the amount of fuel that accumulates in the cylinder before ignition. When the ignition point of the excess fuel is reached, all of this fuel ignites simultaneously, causing extremely high pressures in the cylinder and an undesirable knock. Thus, detonation in a diesel generally occurs at what is normally considered to be the start of the second phase of combustion. Detonation in a diesel may occur when the engine is not warmed up sufficiently or when fuel injection equipment is not operating properly. These conditions may allow excessive fuel to accumulate in the cylinder.

Even though diesel fuel must have the ability to resist detonation, it must ignite spontaneously at the proper time under the pressure and temperature conditions existing in the cylinder. The ease with which a diesel fuel ignites and the manner in which it burns determines the ignition quality of the fuel. The ignition quality of a diesel fuel is determined by its CETANE RATING, or CETANE NUMBER. In fact, the cetane rating of a diesel fuel is identified by its cetane number. The higher the cetane number, the less lag there is between the time the fuel enters the cylinder and the time it begins to burn.

The cetane number of a diesel fuel is derived from a comparison test. The cetane number of diesel fuel is the numerical result of an engine test designed to evaluate fuel ignition delay. To establish the cetane number scale, two reference fuels are used, cetane and heptamethylnonane. Cetane has an excellent ignition quality (100), and heptamethylnonane has a very poor ignition quality (15). The cetane rating of a fuel in which the ignition quality is unknown can be determined by a comparison of the performance of the fuel with that of a reference fuel. The cetane number represents the percentage of pure cetane in a reference fuel that will just match the ignition quality of the fuel being tested. A higher cetane number means a quicker burning of the fuel, a condition that tends to result in easier engine starting, particularly in cold weather.