N-heptane Ignition Delay Time Model for Two Stage Combustion Process

Paper #:
  • 2017-24-0071

  • 2017-09-04
Ignition delay time is key to any hydrocarbon combustion process. In that sense, this parameter has to be known accurately, and especially for internal combustion engine applications. Combustion timing is one of the most important factors influencing overall engine performances like power output, combustion efficiency, emissions, in-cylinder peak pressure, etc. In the case of low temperature combustion (LTC) mode (i.e. HCCI mode for example), this parameter is controlled by chemical kinetics and there is no direct control method as in spark ignition engine. For HCCI engine applications and especially Diesel engine, fuels with lower octane ratings such as n-heptane, diesel fuel, dimethyl ether (DME) are preferred. These fuels display a two stage ignition behavior, and therefore it is very difficult to build an accurate ignition delay time model over the wide range of engine operations. On another hand, in view of engine management and tuning applications, only ignition delay time model may be required to be included in a global in-cylinder crank angle model. The objective here is to build a very reduced ignition delay time mechanism which reproduces the same trends as the previous 26-step n-heptane mechanism. In this paper, an ignition delay time model including 7 direct reactions and 13 species coupled with a temperature criterion is described. This mechanism has been obtained from the previous 26-step reduced mechanism, focusing on the low temperature region which is the most important phase during the two stage combustion process. the oxidation of fuel by O addition in a degenerated chain branching has been kept to maintain as close as possible the main pathway of low temperature regime. Starting from the 26-step model, a skeletal scheme including 10 reactions for the low temperature regime has been obtained from a sensitivity analysis. From this latter, only 7 reactions have been retained to describe the first stage. To increase the accuracy of the 7-step model, the rate coefficients of three reactions have been adjusted in the present study. With this model describing only the low temperature phase, no ignition occurs. Consequently, to avoid adding more reaction and more species, a temperature criterion has been chosen, leading to a critical temperature correlation. This phase has been performed thanks to the behavior of this critical temperature which is function of engine external parameters. The complete model works with 7 reactions until the critical temperature is reached, leading to the detection of the ignition delay time value. The resulting ignition delay times obtained with the 7-step model have been compared to those of a calculation using a full kinetics of n–heptane developed by Lawrence Livermore National Laboratory. This comparison has pointed out that the model reproduces the ignition delay time with a good accuracy with differences smaller than 2 CAD. As an example for application, this model can be integrated in differents models (single zone and multi-zone), where the completion of combustion can be described by Wiebe function.
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