Development of a Multibody Systems Model for Investigation of the Effects of Hybrid Electric Vehicle Powertrains on Vehicle Dynamics

With ever increasing numbers of Hybrid Electric Vehicles (HEV’s) being developed, come new challenges in the field of automotive engineering. Whilst there has been considerable work conducted on HEV’s from a powertrain, efficiency, and control systems perspective, very little work has been instigated in the field of how the introduction of such hybrid systems effect passive vehicle dynamics. One of the possible obstacles in the way of such studies is the multitude of powertrain architectures that are present or possible in HEV’s. This obstacle can make investigations very application specific, and leads to inefficiencies in the modelling process. This paper discusses the development of a model constructed in Dymola in order to investigate the effects of hybrid powertrains on ride and handling. The modelling methodology is presented, along with model based testing and validation of component and the full vehicle models. Whilst the development of the model is introduced for a specific study, it is shown that the way in which the model has been developed lends itself easily to use in other fields. It is shown that the modular construction of the model, and the physical, object orientated modelling approach facilitated by Dymola, allow varying numbers and complexities of component models to be utilised within the same basic model. Such an approach means that one base model can be utilised for differing hybrid architectures for ride, handling and drivability studies thus reducing modelling time and complexity.


INTRODUCTION
This paper will present an approach for creating a multibody chassis model to be used for studies relating primarily to vehicle ride (primary) and handling of Hybrid Electric Vehicles (HEV's) [1] [2]. It will be shown that utilising the Dymola modelling packages that such models can be created in a modular fashion, which can be used to facilitate the easy adaption of the model from one hybrid architecture to another. It will also be discussed how the model complexity can be easily added to, or reduced to investigate primary and secondary ride, handling and driveability.

II. MODELLING METHODOLOGY
Utilising the multi-domain, object orientated, physical modelling approach facilitated by Dymola, can allow the inclusion of mechanical, electrical, thermal and control models in one modelling domain, making it ideally suited for modelling HEV's. Further to this the models are physically based on real systems making them inherently easier to understand, and simple models can be aggregated into single components allowing them to be efficiently reused.
The model developed here makes use of the majority of these points. Chassis systems are built and validated at a component level, and then modularly constructed to produce a full vehicle model.
The model presented here was created for use in a study that investigated the ride and handling of a conventionally powered vehicle and a hybrid vehicle based on the same platform. As such specific detail was given to suspension models as well as detailed mass and inertia models of the vehicle body. Engine and drivetrian models are of basic complexity as they were not of critical importance for the specific study. As the mass and inertia properties of HEV's play such a large roll in dictating their ride and handling, masses of components in the conventional vehicle that were to be removed in creating the hybrid vehicle were modelled separately as multibody masses. By modelling in this way it allows the mass model of the vehicle to be quickly and accurately updated when changing from the conventional to the hybrid vehicle. It also allows the mass and inertia properties to be accurately updated for any hybrid architecture, as components are physically added and removed from the model as in reality. Within the specific study that this model was utilised for a number of road models were required. Ranging from circular path models to obtain steady state characteristics to rough road models to investigate ride domain responses. All requirements could be achieved by utilising the 'Road Builder' function that is in the Modelon library. This function allows a large variety of road surfaces to be utilised with vehicle models such as this one. Exampled an uneven and circular road are given in

A. Component Level Validation
Within the Modelon Vehicle Dynamics Library for Dymola there are a number of pre-defined model based test rigs. Such models allow the mounting of suspension linkages, axles and even full vehicles (in the case of 4/7post rigs).
In this instance, corner and axle models were used with such test rigs in order to obtain kinematic data, this allowed comparison between kinematic metrics obtained from the modelling domain and those obtained from real world tests on a kinematics and compliance (K&C) rig. Figure 5 shows the model setup for the front axle. Motions, forces and moments can be applied to the hubs as inputs and corresponding motions, force and moments in all 3 orthogonal axes obtained.
An example of the kinematic results that can be obtained are shown in Figure 7, and metrics used for correlation to test data are shown in Table 2. It can be seen that the two sources of kinematics data agreed very well.
To validate parameters of the chassis model, an inertia rig was modelled as shown in Figure 6. This allowed the vehicle to be mounted with its Cog coaxial to the rotation axis of the rig in all 3 axes, with at torque applied to the rig, the pitch, roll and yaw inertia of the complete vehicle could be obtained. Comparison between this data obtained in the modelling domain and OEM estimates (as shown in Table 1) increases confidence in model accuracy, especially the detailed mass model.

B. Full Model Validation
In order to validate the complete vehicle model in the handling domain a number of steady state and transient manoeuvres were conducted with a test vehicle of the same type as that modelled.
To illustrate the steady state correlation an example of a steering response plot is shown in Figure 8. The same manoeuvre was carried out in the modelling domain by interfacing the vehicle model with a circular road model produced via the road builder function of the Modelon vehicle dynamics library and a lateral closed loop driver model which could perform route following. Longitudinal control was left open loop to allow for the vehicle speed to be gradually increased.
The model data shown in Figure 8 agrees well with the collected test data, both the undertseer gradients (gradient of plot) and limit conditions correlate well between the two sets of data.
To correlate the dynamic responses a number of sinusoidal and ramp-to-step steer manoeuvres were conducted with the same test vehicle. Here to correlate to model data, all model inputs were open loop and the model was simply fed with the speed and steering profiles from the test vehicle. An example of some sinusoidal steer manoeuvre results are shown in Figure 9. It is again seen that the model outputs agree well with the test data.

IV. MODEL USAGE
The specific model constructed within this paper was utilised in a study that compared the ride and handling of a conventionally powered vehicle and its hybrid counter- Alteration of the aforementioned conventional vehicle model to create the hybrid vehicle model was made easy through the object orientated modelling approach. For example to update the mass and inertia properties of the vehicle, a different instance of the mass and inertia sub model was created that housed data for the additional components of the hybrid vehicle. This model was then simply used to replace the conventional vehicle mass and inertia sub model. Likewise, all changes that had to be made to the conventional vehicle model to create the hybrid model, were made at a component level by simply creating a new instance of the conventional sub model. Conventional and hybrid models are then saved in a hierarchical library and can be swapped in and out of any model of the same format illustrated here.

A. Model Extension
The modelling architecture outlined here can easily be extended through approaches discussed in the previous section to suit any variant of hybrid architecture, for ride, handling and drivability studies. Powertrain and drivetrain models can updated/replaced with models that have representations of electric motor drive and compliances, as shown in Figures 10 to 12. Multiple electric motor models could be utilised and through different transmissions models can be implemented in series, parallel and series parallel configurations. Such models could prove vital for use in ride and drivability studies of current and future hybrid vehicles  [3] Mass models of vehicles can be created with stiffness and damping properties between major components and their mounting to the chassis in order to investigate secondary ride effects. Again this could prove invaluable for investigating the ride comfort of current and future hybrid vehicles, or even for deeper investigation of the environment of developing hybrid technologies, such as battery packs.