From System Simulation to CFD: 1D–3D Coupling with AVL CRUISE™ M and AVL FIRE™ M

Published on August 04, 2026 · 7 min read

Perhaps you are familiar with Edwin A. Abbott’s novella Flatland (Flatland: A Romance of Many Dimensions; Abbott, Edwin A.; 1884). In addition to satirizing the class and gender hierarchies of Victorian society the author examines how knowledge is constrained by the number of spatial dimensions of its inhabitants’ perceptual reality. It turns out that some phenomena cannot be explained within a lower dimensional (sub-)space. This might also be the case for the usually applied degree of spatial discretization in system simulation and, unlike the citizens of Flatland, this is not approached with ignorance. In AVL CRUISE™ M this topic is addressed at many places by a local spatial refinement and in addition by introducing a new dedicated CRUISE M – AVL FIRE™ M gas flow coupling solution. A specific gas path part of interest is modelled within FIRE M in a highly predictive manner surrounded by a CRUISE M plant model.

AVL CRUISE™ M – AVL FIRE™ M Gas Flow Co-Simulation

Summary

AVL CRUISE™ M – AVL FIRE™ M Gas Flow Co-Simulation

A gas path in CRUISE M is modelled either in 0D (HIL/SIL Real Time applications) or 1D (higher predictivity and consequently computational effort). The level of predictivity regarding geometrical modifications may be enhanced by modelling selected parts of the gas path with 3D CFD, like in the following list of internal combustion engine application cases

Intake manifold EGR feed setup for predicting cylinder individual EGR content
Intake manifold for naturally aspirated engines for predicting cylinder individual volumetric efficiency (fresh charge filling)
Exhaust system with high gas exchange pulsations improving cylinder scavenging by taking advantage of surge phenomena

The first listed application example is chosen to show the CRUISE M - FIRE M gas flow co-simulation by analyzing the cylinder individual EGR content of a charged 4-cylinder compression ignited engine (Figure 1).

Figure 1: Charged 4-cylinder Compression Ignited Engine Model

The co-simulation is managed according to the FMI standard utilizing the FIRE M FMU Export App. The resulting FIRE M FMU instance is imported into the dedicated CRUISE M Gas flow FMU component, which also serves as container for the 1D representation (surrogate) of the FIRE M gas path part separated by Gas flow Interface components.

Dependent on the size of the gas path part modelled in FIRE M the Real Time Factor (RTF = (wall-clock time)/(simulation model time) ) can be up to four magnitudes larger than the one of CRUISE M. To minimize the simulation time spent in FIRE M the necessary convergence period (run-up) to get a CRUISE M engine cyclic steady state operation is performed without the FIRE M part, requiring the mentioned CRUISE M 1D surrogate of the FIRE M gas path part.

The CRUISE M - FIRE M gas flow co-simulation is therefore separated into 3 operation mode periods (Figure 2):

FMU Co-simulation pausing phase (CRUISE M surrogate subsystem in operation, FIRE M gas path simulation disabled) for reaching a CRUISE M (cyclic) steady state operation
FIRE M 3D response suppressed 1D-3D FMU co-simulation (CRUISE M surrogate subsystem and FIRE M FMU in order, but provided flow state boundaries of FIRE M are not recognized in CRUISE M to avoid a potentially significant disturbance of its (cyclic) steady state operation as the FIRE M gas is being accelerated
Full 1D-3D FMU Co-simulation (CRUISE M surrogate subsystem simulation disabled)

The Gas flow Interface components steer the gas flow state communication (in a 3/2-way directional control valve manner) across the different operation mode periods.

Figure 2: Co-simulation Operation Mode Period Control

This method allows to reduce the overall simulation time significantly. In the shown example just five of the forty-five engine cycles are performed with an active FIRE M simulation enabled.

According to the consistent gas species treatment in the CRUISE M and FIRE M gas flow domains the spatial cyclic flow pattern for the combustion product fraction in the 3D discretized FIRE M intake manifold can be properly recognized by the intake channels of the individual CRUISE M combustion chambers. At the investigated full-load operating point of 3500 rpm, the EGR fraction varies significantly across the combustion chambers, ranging from 6% to 29% (Figure 4). This large spread indicates an unsuitable EGR feed location, positioned opposite the fresh air intake. The resulting variation in the combustion excess air ratio (±25% around the mean), caused by equal fuel injection into the different combustion chambers, leads to a significant increase in NO_xemissions.

As shown in Figure 3 capturing the variation of combustion chamber residual gas fraction is beyond the modelling depth of the CRUISE M surrogate subsystem. For the intake manifold represented by a 0D plenum component, a perfect mixing approach is applied for balancing the gas species. This leads to the expected equal residual gas distribution in the first two operation mode periods of the simulation.

Figure 3: AVL CRUISE™ M Combustion Chamber Residual Gas Fraction Spread

Figure 4: AVL FIRE™ M Intake Manifold EGR Flow Tracing

Summary

In terms of user experience, embedding this co‑simulation solution within the common AVL Simulation Desktop (SDT) GUI is highly advantageous, opening a convenient way for combining gas flow simulations of different degrees of spatial discretization. From a practicality standpoint, it is essential to minimize the duration during which the FIRE M simulation is active.