Auto-ignition

The motor fuel under test is compared to two pure hydrocarbons chosen as references. The first is 2, 2, 4-trimethylpentane or isooctane which is very resistant to auto-ignition and to which is arbitrarily given the number 100 /... 

The lead alkyls inhibit auto-ignition which, by intermediary of lead oxide, PbO, increases the resistance to knocking. 

The diesel engine takes in and compresses the air. The fuel is injected into the cylinder in atomized form at the end of the compression stroke and is vaporized in the air. Ignition begins by auto-ignition in one or several zones in the combustion chamber where the conditions of temperature, pressure and concentration combine to enable combustion to start. 

For diesel engines, the fuel must have a chemical structure that favors auto-ignition. This quality is expressed by the cetane number. 

Another characteristic used for some time to measure the propensity of a diesel fuel for auto-ignition, is the Diesel Index (Dl). This is defined by the relation ... 

The additives for improving the cetane number, called pro-cetane, are particularly unstable oxidants, the decomposition of which generates free radicals and favors auto-ignition. Two families of organic compounds have been tested the peroxides and the nitrates. The latter are practically the only ones being used, because of a better compromise between cost-effectiveness and ease of utilization. The most common are the alkyl nitrates, more specifically the 2-ethyl-hexyl nitrate. Figure 5.12 gives an example of the... 

Hydrotreating the LCO increases its cetane number to around 40 (Table 5.16), but this technique needs large amounts of hydrogen for rather mediocre results, the aromatics being converted into naphthenes which are still not easily auto-ignited. That is why LCO is sent to the domestic heating oil pool. 

Outside of their very high resistance to auto-ignition, the aviation gasolines are characterized by the following specifications vapor pressure between 385 and 490 mbar at 37.8°C, a distillation range (end point less than 170°C), freezing point (-60°C) and sulfur content of less than 500 ppm. 

The auto-ignition delay of a heavy fuel measured in the engine increases linearly with the CCAl it is therefore desirable that the latter value be as low as possible. 

Unlike spark-induced combustion engines requiring fuel that resists autoignition, diesel engines require motor fuels, for vhich the reference compound is cetane, that are capable of auto-igniting easily. Additives improving the cetane number will promote the oxidation of paraffins. The only compound used is ethyl-2-hexyl nitrate. 

The octane number is a measure of a fuel s ability to resist auto-ignition during the compression phase prior to ignition. 

Alcohol Open cup Closed cup Lower Upper Auto ignition temperature, °C... 

Health and Safety Factors. Sulfur monochloride is highly toxic and irritating by inhalation, and is corrosive to skin and eyes (156). The OSHA permissible exposure limit is 1 ppm (6 mg/m ). Pulmonary edema may result from inhalation. Because its vapor cannot be tolerated even at low concentrations, its presence serves as a warning factor. Sulfur monochloride is not highly flammable, having flash poiats of 118°C (closed-cup) and 130°C (open-cup) and an auto-ignition temperature of 234°C. 

Autoignition Temperature (AIT) The auto ignition temperature of a substanee, whether solid, liquid, or gaseous, is the minimum temperature required to initiate or eause self-sustained eombustion, in air, with no other souree of ignition. 

DEE eombustors have pre-mix modules on the head of the eombustor to mix the fuel uniformly with air. To avoid auto-ignition, the residenee time of the fuel in the premix tube must be less than the auto-ignition delay time of the fuel. If auto-ignition does oeeur in the pre-mix module then it is probable that the resulting damage will require repair and/or replaeement of parts before the engine is run again at full load. 

If auto-ignitions occur, then the design does not have sufficient safety margin between the auto-ignition delay time for the fuel and the residence time of the fuel in the pre-mix duct. Auto-ignition delay times for fuels do exist, but a literature search will reveal that there is considerable variability for a given fuel. Reasons for auto-ignition could be classified as follows ... 

Flammable limits, flash point, auto-ignition temperature etc. 

Auto-Ignition Temperature — the temperature at whieh ignition oeeurs without an ignition souree and the material continues to burn without further heat input. 

After the ceramic heat transfer beds have reached an operating temperature of 1500 F the unit is ready for the process airstream. As the process airstream enters the ceramic heat transfer beds, the heated ceramic media preheats the process airstream to its oxidation temperature. Oxidation of the airstream occurs when the auto-ignition of the hydrocarbon is reached. At this point the heat released by the oxidation of the process hydrocarbons is partially absorbed by the inlet ceramic heat transfer bed. The heated air passes through the retention chamber and the heat is absorbed by the outlet ceramic heat transfer bed. 

Certain heating fluids are not compatible with some insulating materials, and auto-ignition may occur. Caution should be exercised in sek ciing materials. 

A pump was being dismantled for repair. When the cover was removed, hot oil, above its auto-ignition temperature, came out and caught fire. Three men were killed, and the plant was destroyed. Examination of the wreckage after the fire showed that the pump suction valve v/as open and the drain valve shut [3]. 

Unexpected concentration of oxygen can occur when compressed air is dried or purified by pas.sing it over certain types of molecular sieves. Nitrogen is absorbed preferentially after regeneration, and the air first produced may be rich in oxygen. This can widen flammability limits and lower auto-ignition temperatures. At least one explosion has occurred as a result. If possible, use Type 3A molecular sieves [21]. 

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