Following the successes of progressively more complex hybrid vehicle projects, including the hydrogen fuel cell taxi testing and development program. Lotus Engineering’s EV test cells satisfied a high demand for the development of the Lotus Evora 414E REEVolution prototype.
Complete testing facility for hybrid systems
Lotus EV Test cells played a key part in developing the complex hybrid drive capability, including pairing twin motors to corresponding twin inverters on a common drive to power one of the vehicles rear wheels. This complexity is magnified by a mirrored, synchronised system driving the second rear wheel.
Range extender engine validation
Whilst electrically less complex, the range extender relied on successful development in the high voltage test cell. Using a battery simulator and dynamometer the program coupled the engine with an adaptable control strategy to a generator with a previously unpaired inverter. The test cell answered many questions regarding operating strategy, from mechanical variables associated with the combustion engine (including fueling and timing) to managing the electrical energy generated across the operating range of the engine.
Integrating the high voltage, high power systems required extensive testing and debugging and the test facility revealed essential changes to hardware and software. This allowed part suppliers to modify components before they became critical to the success of the project.
A success factor of the off-line testing was that in additional to traditional test cell capabilities the HEV test facility at Lotus Engineering included a high powered battery simulator. Supplied by AVL, the unit is capable of managing 200 kW of power flow up to 600 V, sinking or sourcing
As a programmable bi-directional power supply the simulator emulates automotive energy storage systems, including super capacitors, fuel cells and for this project, the vehicle’s high performance battery. Simulating the vehicle battery permitted any operating point to be available for any length of time – there was no down-time due to battery charging cycles. Additionally, the consistency and repeatability of test results was essential and the simulator offered increased safety for prototype development.
Evolving a working system
An objective of the testing program was to develop a control strategy and characterise the electric drive-train across its entire operating range. Early on in the project test data revealed many technical challenges to overcome. For example, preliminary testing of the motor and inverter pairing produced unexpected results where approaching peak performance conditions, the inverter failed on over-voltage and over-current faults before reaching operating speed. The power/torque curve below (Fig. 3) highlights the shortfalls of the first run single stage testing.
Findings from each test were fed back to the inverter supplier and several revisions of firmware were released and retested, until the traction motor could deliver the speed and torque required to enable the vehicle to achieve its performance targets.
The electrical drive-train drives the rear wheels independently using dual stage motors. In order for the vehicle to pass stringent safety case criteria it is imperative that the amount of torque being produced at each wheel is accurately predicted and controlled such that cross axle difference does not lead to instability
The system is controlled via CAN using an interface developed by Lotus and Rinehart that includes monitoring and safety functions. Early Test cell validation revealed that whilst the Lotus vehicle dynamics controller requested a specific torque from each stage, the actual torque produced did not match the demanded value. Typically a look up table embedded in the inverter is sufficient to regulate the torque output but for this high performance vehicle with independently driven wheels a standard open loop control system was insufficient. In order to satisfy the safety case criteria, Lotus was able to use the test facility to derive an innovative adaptive mathematical function to correct the torque errors in the drive-train, effectively creating a dynamic motor map in the Lotus controller.
The EV test cell proved instrumental in developing the control algorithm by replicating a dual stage motor with two inverters, i.e. a single axle of the 414E vehicle installation. Over two hundred measurement points were recorded throughout the motor’s torque and speed operating range, yielding an equation that takes into account measured motor variables and accurately predicts the torque produced for a given torque demand. Fig. 6 shows the predicted torque based on the equation against the torque measured in the test cell.
Testing showed that an accuracy to +/- 5 Nm for over 90% of the test points, with more than 98% within +/- 8 Nm. Across the whole range of the data the absolute error in Nm can be seen in the graph (Fig. 7). The vast majority is with +/- 4 Nm, with a few regions slightly outside of that. All the results outside the +/- 4 Nm region occur at higher speeds with higher demands, so the net percentage error is relatively low. The equation has been implemented on the 414E REEVolution vehicle and has enabled the vehicle to successfully pass the safety case criteria.
Complete series hybrid testing
With two test cells configured back to back, the driveline from the range extender to traction motor was validated in a controlled environment. As illustrated in Fig. 8 below, both stages of the traction motor were connected to two 100 kW inverters and the range extender system in cell 38 is electrically coupled to the traction motor load in cell 39. Both cells are linked to the battery simulator.
In parallel with the test cell development activities, a mathematical model of the vehicle and hybrid drivetrain was constructed. This allowed a virtual development programme to be conducted for proving the functionality of the drivetrain. Additionally, simulation work with the virtual car made it possible for Lotus Engineering to optimise the vehicles’ energy management strategy and compare it to previously simulated software models. The output of the simulation work could be directly copied over to the vehicle controller hardware which meant significantly reduced vehicle development time with improved overall vehicle safety.
The vehicle is capable of running multiple NEDC cycles on pure electric from a battery pack that is fully charged (100% SoC). The vehicle has an option during this charge depleting mode such that the range extender can be used to boost acceleration performance upon driver request. After the energy stored in the battery is considered depleted by the vehicle controller the vehicle is switched to charge sustaining mode. The energy management strategy manages power flow from the battery and APU to maintain the battery SoC within a predefined band. To optimise emissions performance and running costs the battery is only permitted to return to its maximum charge level via external charging from the power grid. Fig. 9 illustrates this drive strategy.
As part of the strategy to reduce rapid depletion of the battery during high energy consumption events there are two additional triggers to switch on the range extender irrespective of SoC. The triggers are based on consumption and rof change of power consumption, if definable values for either are exceeded the engine is used to generate the rate of power being consumed (subject to max engine power). When the range extender is activated by this part of the strategy, the SoC based power demand correction is disabled and the power demand is purely load following i.e. matching as close as possible the driver demands.
Using the test cells to support design and build of the 414E allowed the team to identify problems and develop solutions as early as possible in the program. In addition to the areas previously mentioned, looking further into the project would reveal how the test cells were used to also tackle issues with component performance, electromagnetic interference, CAN error frames, software development, engine tuning, thermal management and almost as critically, how Lotus Engineering benefited from the experience and intellectual property generated from creating solutions.
Looking to the future and based on the success of the current HEV test facility, Lotus Engineering is planning to grow its capability with a battery simulator dedicated to supply hybrid and electric vehicles on a rolling road.
Using the knowledge gained and the facilities invested in the programme, the benefits of reduced programme delivery time and cost have been realised and these can be carried forward to future complex hybrid and electric vehicle programmes internally and for third party clients.
Writer: Gary Spinks