Ignition delay

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. 

Peroxy and hydroperoxy radicals play important roles ia the knock process. A number of good reviews have discussed the details of the chemical mechanisms (16). Ignition delay (tau) has also been used for description of the chemical tendency to knock (17). The chemical factors affecting knock are... 

The cetane engine is a variable compression single cylinder engine very much like the octane engine. The engine is mn at 900 rpm and injection is timed to start at 13° before top dead center (BTDC). The compression ratio is adjusted so that the test fuel starts to ignite at exacdy top dead center (TDC), for an ignition delay of 13° or 2.4 ms. Reference fuels are chosen which bracket the sample and the cetane number of the sample is estimated by interpolation between the two reference fuels. 

The electrical energy is rapidly transformed into thermal energy, and because the temperature of the ionized gas is generally above 300 K, the ignition delay time is short compared with the spark duration, . Ignition only takes place if the electrical energy exceeds the critical value, and if this energy is... 

For Hquid fuels, ignition delay times are of the order 50 ]ls at 700 K and 10 ]ls at 800 K. At low temperatures most of the ignition delay is the result of slow, free-radical reactions, and a distinction between the initiation and explosion periods within the ignition delay time can be made. With increasing ignition temperature for a given mixture, these times become comparable and at temperatures as high as 1500 K, both times may be of the order of lO " s. Consequently, the reaction zone in the flame of a mixture is observed to be one continuous event (12—14). 

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 ... 

If environmental and atmospheric conditions are such that vapor cloud dispersion can be expected to be very slow, the possibility of unconfined vapor cloud detonation should be considered if, in addition, a long ignition delay is likely. In that case, the full quantity of fuel mixed within detonable limits should be assumed for a fuel-air charge whose initial strength is maximum 10. 

A flammable vapor flows tlu ough a 2-inch-insulated pipe at a flow rate of 4.5 acfm. A lagging fire started and heated a 4-foot lengtli of the pipe to 150 °F, vvliich is above tlie ignition temperature of tlie vapor. The ignition delay time of tlie vapor is e.xpressed by... 

The residence time of tlie vapor in the pipe section is less than tlie ignition delay time. Therefore, ignition will not take place. 

Quieter operation (no ignition delay or diesel knock lower peak cylinder pressures and temperatures). 

Quicker acceleration (no ignition delay) at lower engine speeds. 

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. 

Lu, Vyn, Sandus and Slagg (Ref 17) conducted ignition delay time and initiation studies on solid fuel powder-air mixts in an attempt to determine the feasibility of solid-air detonations. The materials investigated included Al, Mg, Mg-Al alloy, C and PETN. Ignition delay time was used as a method of screening the candidate fuels for further work in initiation studies which determined detonation wave speed, detonation pressure, detonation limits, initiation requirements, and the effect of particle size and confinement. The testing showed the importance of large surface area per unit mass, since the most... 

The pyrotechnic literature does not contain a critical evaluation of the ignition response time. of primary initiators in terms of their compn, temp tolerance and shock sensitivity. In general, primary expls such as Pb Azide or styphnate are selected whenever a brief (microsecond) response is desired, while, for instance, Pb thiocyanate-chlorate mixts are selected when high temps and high radiation environments are encountered, and presumably a longer ignition delay is the price which is paid for the extra margin of safety... 

In devices which can tolerate moderate ignition delay, further development of non-primary initiators (Refs 31, 77, 97 112) will extend the storability, the manufacturing safety, the electromagnetic Held and spark sensitivity and the high temp compatibility of pyrotechnic devices... 

Altman and Nichols (A4) were the first to test the constant ignition-temperature approach experimentally. These investigators ignited samples of double-base propellants with electrically heated wires located on the propellant surface. The ignition delay was measured from the time of application... 

Inspection of the numerical solutions of the equations shows that, with the exception of Es= 0 kcal/mole, the rate of surface temperature increase with time is very large once the surface temperature reaches approximately 420°K—on the order of 108°K/sec. Because typical autoignition temperatures are of the order of 625°K for composite propellants, the particular value of the ignition temperature does not affect the computed numerical value of the ignition-delay time. 

Parametric studies showed that mass diffusion in the gas phase could be neglected under most conditions. The calculations also show that the selection of the hypergolic combination (i.e., the gaseous oxidizer and the propellant system) fixes all of the parameters except the initial temperature and the oxidizer concentration. A general solution of the model shows that the ignition-delay time is approximately rated to the gaseous oxidizer concentration by the relation... 

Equation (16) suggests that for a given propellant the ignition delay is a unique function of flux and pressure. 

---