In order to reduce fuel consumption in gasoline engines, manufacturers are focusing their efforts on turbocharging and downsizing. However, this option leads to an increase in the knock phenomenon (destructive engine self-ignition). This can be avoided by significantly increasing the dilution ratio using re-circulated burned gases, but such an approach is to the detriment of flame stability during spark ignition, and hence to the detriment of engine stability.
In this context, manufacturers have turned to CFDa and, in particular, LESb in order to understand the causes of – and thereby address – cycle to cycle fluctuations which reduce engine stability. However, existing combustion models capable of describing spark ignition, do not take sufficient account of these fluctuations.
To develop such a model(1), IFPEN took part in the Famacc project, providing an electric circuit model for a standard vehicle coil, capable of accurately reproducing the energy transferred by the spark plug.
Since energy is deposited along the electric arc, this first model was then supplemented by a Lagrangian arc model, which reproduces the shape and evolution of the real arc (figure 1).
electric arc images.
This approach made it possible to correctly simulate the critical ignition energy at rest, without turbulence (figure 2). The first calculations for “standard engine conditions” (with turbulence) also demonstrated good consistency with experience. These first models representing complex ignitions in turbulent conditions will be evaluated for concrete cases and be used to propose ignition strategies designed to reduce knocking. Ultimately, they will also be used to help design the new, more robust ignition systems that will form the basis of more fuel efficient, cleaner engines.
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78 mJ (bottom) of energy leading to ignition success
and failure respectively.
a - Computational Fluid Dynamics.
b - Large-Eddy Simulation.
c - Ignition fundamentals for the spark ignition engine.
(1) O. Colin et al., DNS and LES of spark ignition with an automative coil, soumis au Symposium on Combustion 2018
Scientific contact: olivier.colin@ifpen.fr