August 4, 2017

Phenomenological models and engine cycle simulations


The ability to accurately predict combustion behavior and emission formation at an early stage of new engine development is becoming increasingly important. Therefore, CFS has developed phenomenological models for the analysis of combustion behavior and emission formation for forecasting purposes. These models can handle variations in boundary conditions and are highly suitable to study different operating conditions and engine designs. In many cases many unknown parameters exist especially at the beginning of the development or at a new design. The optimization tool can be used simultaneously for automatic optimization of engine performance, thus providing an effective range of tools.

Advanced flame propagation mode CFS.AFPM

A newly developed extended flame propagation model cfs.afpm is used for premixed combustion in the cycle simulation tools. The model has been successfully tested for gas and gasoline engines.

The capability for accurate prediction of combustion behavior and The turbulent flame propagation is described in a phenomenological manner, including sub-models for laminar and turbulent flame velocity taking into account in-cylinder pressure, temperature, amount of exhaust gases, equilibrium distribution and turbulence. The turbulence model takes into account the turbulence generated at intake, injection, pinch flow, and the change in the strength of the turbulence due to pressure differences and losses. The geometric properties of the combustion chamber are described by a flame front area function, which is a fingerprint of the combustion chamber geometry.

The phenomenological model can be used in two ways: On the one hand, in "backward" applications, the interaction of flame and wall can be studied; on the other hand, in "forward" applications, the combustion rate and thus cylinder pressure, temperature, efficiency and many other relevant quantities can be predicted.

Reverse application: Based on the measured pressure patterns, the flame-wall interaction can be estimated. The following figure shows the experimentally determined combustion rates of two different combustor designs and the corresponding calculated flame front areas. It can be seen that the curves overlap by CA=13.4°. This means that the flame propagation is limited to the engine with the higher compression ratio (which is achieved by a filled piston cavity).

The point was further investigated and it was observed that the flame front touches the piston surface. The example shows the "backward" calculation. The following images show the spherical flame in the CAD drawing of the combustion chamber. The flame radius was determined based on the calculated flame front area.

The following graph shows the comparison of the experimentally determined and calculated crank position at 50% combustion. The example corresponds to the mentioned "forward" application. The calculations were performed with a cycle simulation code. Good agreement was observed for the operating conditions studied.

Diesel combustion and emission models CFS.DCEM

The phenomenological diesel combustion model first describes the mixture formation where submodels for spray formation, evaporation and air-fuel mixture are available. Ignition timing delay accounts for physical and chemical ignition delay. Fuel is distributed between premix and diffusive combustion. Premix combustion is modeled by flame spread, while diffusive combustion is modeled by controlled mixture combustion. The two domains, premix and diffusive combustion are finally superimposed to obtain the final combustion rate.

Thus, combustion and flow solutions GmbH uses such models in the design phase to test new concepts. The following example shows the change of the injection timing for a diesel engine.

The soot model includes a formation term and an oxidation term in different lambda regions. During diffusive combustion, a 3D lookup table based on Akihama (SAE 2001-01-0655) is used to account for soot formation, while soot oxidation is calculated based on soot mass, oxygen concentration, and characteristic inverse mixing time.

The NO model uses an extended Zeldovich mechanism where virtual combustion zones take into account the different air-fuel rates and combustion progress.

Both emission formation models are under continuous development and a renewed version will be presented soon.

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