CNG System Simulation


The design of CNG-powered cars with direct injection requires a time-efficient system simulation. Modern CFD software packages offer the possibility of a very accurate representation of transient processes in such systems, but require very long computing times, which exceed the development time of such systems. The aim of the project was the development of an application-specific component library in one- or two-dimensional computing domain, which offers high accuracy at short computing times.

Benefit Procedure

Fast system design

Building a library of system components
Increasing efficiency and reducing emissions Validation of the components using analytical examples
Better system understanding Validation of the simulation on the test bench



Olivier Reinertz

Scientific Director


+49 241 80 47703



CNG Direct Injection System Simulation

Compressed natural gas (CNG) is a promising alternative to conventional liquid fuels due to several advantages. The improvement of a CNG-powered engine requires a direct injection system. A time-efficient simulation of the system is essential for a successful engine design. In cooperation with the Ford Motor Company such a simulation model was developed and validated at ifas.

The calculation of the direct injection system of a gas engine requires a high temporal resolution, as the time scales of the fuel injection and combustion processes are within a few milliseconds and below. CFD (Computational Fluid Dynamics) software offers one possibility to simulate such systems. The high accuracy of these software packages is connected with an enormous computing time expenditure. For example, a completely transient three-dimensional simulation of one millisecond for a geometric length of only a few centimeters requires a computing time of one week.

The goal of the project was to reduce the computational effort while maintaining an adequate accuracy. This was achieved by the application, improvement and parameterization of one-dimensional gas-dynamic solvers.


Image of a T-Piece

Simulative image of a T-piece on the left and the corresponding test bench component on the right Simulative image of a T-piece (left) and the corresponding test bench component (right)

The figure below shows the pressure curve within a T-piece. Since a T-piece cannot be represented one-dimensionally, a two-dimensional modelling must be used here. Furthermore, the two-dimensional calculation area must be coupled with the one-dimensional calculation area.

  Simulated frequency pattern of a horizontal pipe (left) with vertical branch (right) at position x=250mm Simulated frequency pattern of a horizontal pipe (left) with vertical branch (right) at position x=250mm

Acoustic theory and test bench measurements were used to validate the developed model. The picture below shows the result of a λ/4 resonator for validation.



The project was financed by the Ford Motor Company. Special thanks for the cooperation go to the Ford Research and Innovation Center Aachen.



Kratschun F., Enking J. Murrenhoff H.: „Transient simulation of a pneumatic sharp edged L-shaped pipe“, 2018, 11th International Fluid Power Conference, Aachen

Kratschun F., van Bebber, D., Murrenhoff H. „One Dimensional Transient Pneumatic System Simulation“, 2017, BATH/ASME Symposium on Fluid Power and Motion Control (FPMC2017), 2017, Sarasota, Florida, USA

Kratschun, F; Köhne, J; Schmitz, K ; Kloft, P; Baum, H. „One dimensional unsteady model of a hydropneumatic piston accumulator based on finite volume method", 12th International Fluid Power Conference (12. IFK) : October 12-14, 2020 in Dresden