Development of a Fast Spray Ignition Model

Paper #:
  • 2017-01-0812

  • 2017-03-28
Modern Diesel engines have become ever more complex systems with many degrees of freedom in operation. Simultaneously, with increasing computational power, simulations of engines have become more popular to identify the optimum set up of engine operation parameters achieving the desired point in the emission-efficiency trade off. With the increasing number of engine operation parameter combinations, the number of calculations increase exponentially. Therefore, adequate models for combustion and emissions with limited calculation costs are required. For obvious reasons, the accuracy of the ignition timing is a key point for the consecutive combustion and emission model quality. Furthermore, the combination of turbulence and chemistry during the ignition delay is very challenging to model in a fast way for a wide range of operation conditions. This work focuses on the description of a physics-rich diesel engine spray ignition model, which has the ability of real-time calculation. The spray ignition model uses a simplified 3-Arrhenius approach, based on [1], with conditions of temperature and mixture fraction of the igniting fuel portion. In order to find conceptually the mechanisms for those conditions, 3D computational fluid dynamics simulations were performed using a conditional moment closure (CMC) combustion model. A Lagrangian hybrid tracking method which is capable of capturing spray pattern and fuel trajectory in liquid and vapour phases was contrived which provides information on local equivalence ratio and temperature along the trajectory of the igniting fuel portion of the spray, from injection to ignition. A variation in operating conditions (changing nozzle diameter, injection pressure, ambient temperature and fuel temperature) including reactive and non-reactive calculations provided a broad understanding of the role of mixing and low temperature reactions along the trajectory of the igniting fuel. For the spray ignition model, this trajectory has been conceptually abstracted, using distinct phases identified from the CFD simulations. The equivalence ratio and temperature evolution has been used to develop an ignition integral using a simplified 3-Arrhenius approach which can be parameterized for any fuel exhibiting two-stage ignition behaviour. The spray ignition model contains 5 parameters, which need to be adapted for operation in a Diesel engine. The model has been calibrated on a diesel engine and validated for a broad variation of engine operating conditions (load, boost pressure, start of injection, EGR, intake temperature, intake valve timing, etc.). The accuracy of the modelled, versus the measured ignition delay is very high (r2 = 0.99), and exhibits execution times of one millisecond using Matlab on an Intel Core i7 processor. The dominant effects of the spray ignition process have been identified and successfully transformed into a simplified description, which can therefore be executed in real-time. [1] Blomberg, C., Mitakos, D., Bardi, M., Boulouchos, K. et al., "Extension of the Phenomenological 3-Arrhenius Auto-Ignition Model for Six Surrogate Automotive Fuels," SAE Int. J. Engines 9(3):2016, doi:10.4271/2016-01-0755.
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