Ignition delay period

This is the temperature at which the liquid fuel will vaporize when injected into the combustion chamber therefore it becomes an important factor in the ignition-delay period of the fuel. Lower-boiling-range fuels such as No. 1-D are more volatile, while No. 2-D has a lower volatility, therefore requiring higher temperature to vaporize or boil. Due to these factors. 

Ignition delay can be defined as the time period between the injection of fuel into the combustion chamber and ignition of the fuel. A long ignition delay period is characteristic of low-cetane-number fuels a shorter period is a characteristic of higher-cetane-number fuels. 

Ignition delay between injection of fuel and autoignition can be controlled by engine design, fuel and air temperature, fuel atomization, and fuel composition. The ignition delay period is typically shorter than the fuel injection period. The effect of temperature on ignition delay is shown in FIGURE 4-10. 

The presence of low molecular weight, low flash point compounds in diesel fuel could lead to a shortening of the fuel ignition delay period. In a diesel engine, this could cause rough running due to early combustion of the low flash point compounds. 

The cetane number is an important property of diesel fuel. In the majority of diesel engines, the ignition delay period is shorter than the duration of injection. Under such circumstances, the total combustion period can be considered to be divided into the following four stages (1) ignition delay, (2) rapid pressure rise, (3) constant pressure or controlled pressure rise, and (4) burning on the expansion stroke. 

The shorter the ignition delay period, the higher the cetane number of the fuel and the smaller the amount of fuel in the combustion chamber when the fuel ignites. Consequently, high-cetane-number fuels generally cause lower rates of pressure rise and lower peak pressures, both of which tend to lessen combustion noise and to permit improved control of combustion, resulting in increased engine efficiency and power output. 

Ethylhexyl nitrate (EHN) is the most widely used cetane number improver. It is also sometimes called, octyl nitrate . EHN is thermally unstable and decomposes rapidly at the high temperatures in the combustion chamber. The products of decomposition help initiate fuel combustion and, thus, shorten the ignition delay period as compared with that of the fuel without the additive. 

A detailed study of the slow reaction of methane with air during the ignition delay period in a static stainless steel reactor [54] showed not only quite natural increases in the methane conversion and in the final temperature of the reaction products with the oxygen concentration, but also an elongation of the ignition delay time (Fig. 3.42). This means that, in the induction period, increasing the concentration of air in the methane—air mixture from 10 to 40% (increase of [O2]o from 1.9 to 8.8% relative to the sum [CH4]o + [O2]o) clearly reduces the rate of the CH4 + O2 reaction. 

Spray combustion phenomena, flame temperature, and soot distribution Summarizing the results of these sections, as shown in Fig. 23.14, it can be noted that the visible combustion course in a swirl chamber occurs without any starting aids. The visible inflammation appears above the fuel jet. From there the flame engulfs the whole swirl chamber very quickly. This process needs some delay time. The comparison of the observed luminous spray combustion between reference diesel and 10% CPO diesel is shown in Table 23.6. It was found that 10% CPO diesel has a longer ignition delay period than diesel. The combustion for both fuels tends to start faster with increasing... 

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