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Hybrid and electric vehicle testing at Lotus

Major developments in electric, hybrid, and fuel cell vehicle testing are underway at the labs of Lotus Engineering

 

Driven by the increasingly stringent government regulations and energy security concerns, plus higher oil prices, virtually all leading car makers are exploring ways to reduce their vehicles’ carbon dioxide emissions and increase their fuel efficiency. These forces are driving the development of alternative concepts for automotive propulsion as well as alternative fuels. To meet this challenge, the automotive industry is investing large R&D budgets in a variety of new technologies for automotive propulsion.

 

With the recent progress in electric and hybrid vehicle development (EV and HEV), and the expected increase in series hybrids and pure EVs in the marketplace over the next 10 years (see Figure 2), the maximum energy density, currently at 170Wh/kg, is expected to increase to 270Wh/kg in the same timescale. It is therefore imperative that OEMs test and develop motor/inverter efficiency and power management strategies, and that there is an understanding where the inefficiencies are in electric vehicle powertrains, which, in turn, will help OEMs to meet the demands for greater range.

 

Currently there are no drive cycles specifically for EVs and thus the automotive industry tests that have been globally accepted are those of combustion engine test schedules, i.e. NEDC/ FTP/FUDS, etc. Results from these tests will correlate fuel efficiency, reduced CO2 emissions for hybrid vehicles, and range comparisons for pure EVs. It is important that the industry establishes a drive cycle specifically for EVs so that buyers are confident it represents real-world use.

 

Drive system performance is an essential part of the electric vehicle. The drive system is the link between the energy stored in the batteries and the transfer of this energy to the road. A high-efficiency drive system can best utilise the batteries and effectively increase the range of an EV and ultimately, the cost of owning one. Therefore, accurate measurement of drive system efficiency is a primary concern for the EV drive system designer. The most productive route to achieve repeatable and accurate results and reduce development is hardware-in-loop testing. This method negates the use of extensive mule vehicle tests either on the rolling road or test track, plus it can have the added advantage of testing in parallel to vehicle build. Electric vehicles also have a limited on board energy storage system (battery), which depletes very quickly under high load testing. The time required to recharge the high-voltage battery depends on the kWh of the battery pack, but on average via a domestic supply it would be a minimum of five hours. Therefore a facility to emulate the vehicle’s energy storage system during testing becomes advantageous.

 

The hybrid fuel cell taxi project currently underway at Lotus is using such a system. Lotus has designed and commissioned its EV test cell to perform hardware-in-loop testing of the whole hybrid drivetrain. The test bed consists of a Froude Texcel V6 Digital controlled dynamometer connected to a Yokogawa WT3000 power analyzer and other data collection tools.

 

Above: The Lotus Engineering HEV test cell

 

The test bed design incorporates power sharing between the two voltage sources (battery and fuel cell), using high-voltage DC power supply units (PSU). Two 32kW Regatron regulated and fully controllable DC power supplies for EV and HEV subsystem testing  simulate the battery and fuel cell. Both the current and voltage values are programmable, and the units are capable of simulating the discharge characteristics of a battery pack. If used in conjunction with a regenerative module, the units can be used to perform battery cycling tests and function as a battery module for durability testing of other EV and HEV electrical components. The units are based on a modular concept, ensuring systems can be easily expanded as power needs dictate.

The 32kW PSU designated as the fuel cell simulator is an ideal match for the specified vehicle fuel cell, which has a maximum output of 30kW. Although the maximum current output of the PPSU (80A) is less than the fuel cell's 150A, the PSU emulates the fuel cell's optimum efficiency. The 64kW of combined power is enough to develop the control strategy for the high-voltage DC/DC converter (power sharing device under test).

 

For high current/torque demand and peak power testing, the test cell was designed to be able to accept the vehicle 14kWh battery pack alongside the fuel cell DC PSU. The high-voltage distribution box is spilt into two independent supplies to replicate the fuel cell and battery supplies.

 

For the fuel cell taxi project, the objective of the testing program was to develop the software control strategy for the high-voltage DC to DC converter. This involved mapping the voltage and current under different load settings for the converter to manage the power sharing of the two voltage sources (battery and fuel cell). This calibrated model will then be installed into the Lotus Hybrid Controller (LHC), which controls the power management of the hydrogen fuel cell vehicle. Subsequently, the test cell was also used to dynamically test the Lotus Hybrid Controller, simulating vehicle operation encompassing throttle control, power sharing control, voltage and current measuring. A Lotus-designed general purpose module was used to measure the battery and fuel cell voltages and currents. This information was transmitted to the hybrid controller using a PWM output. This multi-purpose module is also used elsewhere on the vehicle, utilised as a safety controller with sensor inputs to detect hydrogen leaks, as well as driver and passenger interface displays.

 

As stated earlier, the test cell was primarily designed to develop the power sharing concept from the dual voltage sources, but it can be easily adapted to supply 64kW of total power by connecting the DC PSUs in parallel. Additional power supplies can be installed to increase the specification of the total power available to 256kW.

 

Currently, to enable peak power testing of the motor, the electric vehicle battery pack is installed into the test cell and used as the primary voltage source. Because a battery can absorb current as well as supply it, this dynamometer can effectively test all four quadrants of motor operation. A 15kW AC motor was coupled to the rear of the dynamometer via a one-way clutch to enable regenerative brake testing.

 

The combination of tools for steady-state and drive-cycle tests offers a complete platform for testing and characterizing electric vehicle drive systems. Data acquisition brings all the data together and data analysis offers insight into the best interpretation of the data to best judge the efficiency of the motor/inverter and drivetrain.

 

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