According to Jaguar, the new I-Pace stands for a fully electric vehicle concept without compromise. Within four years, a new drive architecture with the best available technologies has emerged. Jaguar Land Rover’s complete in-house development focusses on intelligent overall vehicle integration of electric drive, high-voltage battery and vehicle cooling. Performance was just as key in development as efficiency maximisation.
THE ELECTRIC DRIVE UNIT
An all-electric Jaguar SUV must not fall behind the high level of its sister models with internal combustion engines in terms of driving dynamics. The weight distribution plays a major role, which is why from the beginning the four-wheel drivetrain via one drive module per axis was determined. It can realise an ideal 50:50 distribution between front and rear axle. Together with the battery in the middle of the underfloor, a low centre of gravity is created, which further positive influences driving behaviour, (1). When choosing the actual drive unit concept, great importance was placed on compactness, low weight and high efficiency. Accordingly, the choice fell on a coaxial design.
The illustrated peak power of 296 kW and the available maximum torque of 696 Nm result from the drive demand to accelerate in 4.8 s from 0 to 100 km/h. The maximum vehicle speed is limited to 200 km/h for efficiency and battery life optimisation. The gear ratio of i = 9.06 is mapped via a planetary gear set. The highly compact integration of e-machine, planetary gear set and differential – the total weight is only 76 kg per unit – means a low installation space that directly serves the customer benefit, (2). This results in a front luggage compartment and a very large vehicle interior for the vehicle class. Another conceptual advantage is the low front and rear overhangs, which provide optimal vehicle proportions.
THE ELECTRIC MOTOR
Due to the high demands on torque density, agility and efficiency, eight-pole, permanently excited synchronous machines are used in the two drive units, (3). The stator has an outer diameter of 204 mm and consists of one-piece silicon iron (SiFe) laminations. The laminated sheets are fixed by welds, which are located on the stator’s periphery. The stator slots provide space for four hairpin conductors and achieve a fill factor of well over 60 %. The aspect ratio of the conductors has been optimised to limit eddy current and proximity losses, improving the response of the electric motor and supporting a sporty driving style. The high thermal capacity allows frequent load calls without loss of power.
The rotor uses several rectangular magnets per pole, which increase the air gap flux density. The single-piece SiFe lamination structure is reinforced through specially designed bridges that are part of the reluctance circuit and contribute to a significant reluctance torque component. This is added to the excitation torque by the precise control of the current phase advance angle and ensures highest acceleration potential, (4).
The integrated electric motor design provides a wide field-weakening region across the revolutions. This is a major enabler for the single-speed transmission. Even at high speeds, the overall efficiency remains high.
The rotor leakage flux paths are designed so that the reversing fields are directed away from the permanent magnets in case of a short circuit of the power electronics. Demagnetisation is effectively prevented. Likewise, an optimisation of the torque ripple has been achieved. A relative movement between the rotor and the shaft is avoided by shrinking the rotor laminations onto the shaft. To protect the torque transmission, two grooves in the rotor lamination and two keyways in the shaft are additionally used via positive locking. The rotor is designed for a maximum speed of 13,000 rpm and an over-speed of 15,600 rpm.
Component tests were successfully performed in the temperature range of -40 to 150 °C. The e-machine efficiency map is equally optimised for homologation and real driving cycles. High electromagnetic efficiencies of up to 97 % in frequent driving ranges are achieved. Furthermore, repeated full-load accelerations without thermal derating are possible.
The heat losses in the constant torque operation was limited to a level sufficient to allow water jacket cooling of the stator – further cooling such as a stator or rotor oil cooling is not required. An optimised shrink fit between the water jacket cooling and the stator ensures complete contact between the electrical machine and the cooling system. Even at ambient temperatures of 45 °C, the e-machine reliably reaches its full performance potential.
The vehicle has one traction inverter on each of the front and rear drive units. The self-contained design allows use in various vehicle models and different powertrain applications. The three-phase architecture consists of a Jaguar Land Rover specific power module, a DC link capacitor, a gate drive Printed Circuit Board (PCB), a control PCB and a low voltage interface PCB. All elements are housed in a liquid-cooled die-cast aluminium housing. In the I-Pace, the traction inverter can provide a peak three-phase current of 550 A for 10 s and operates without phase current limitation in the range of 290 to 450 V at the DC input.
A bespoke power module with silver sintering technology and the latest-generation of silicon Insulated-gate Bipolar Transistors (IGBT) are coupled with an advanced gate drive PCB. Active IGBT gate current control is used to optimise switching properties and increase silicon utilisation resulting in high performance and efficiency.
The traction inverter houses the sensor interface of the electric motor and the control software. The control of the electric machine is achieved by a space vector modulation strategy, which calculates the optimum switching pattern at different speed and torque levels. The highly dynamic torque control capability is used in active powertrain damping.
The biggest challenge for Jaguar Land Rover’s high-voltage battery development team was to portray the brand’s signature of performance. This specifically means the ability to retrieve a high peak power repeatedly and to offer a relevant maximum speed. In addition, a large vehicle range was to be ensured with a capacity of 90 kWh. To ensure this, cell chemistry, design methodology and the materials used are state of the art. The battery is designed to get the most out of the available space. The total package weighs 606 kg and is mounted under the vehicle floor. This favours a low centre of gravity. The mass of the battery pack reduces vehicle noise. Heavy vehicle carpets in the interior could be replaced by lighter ones, reducing the overall vehicle weight. The flat bottom of the battery supports aerodynamics; the low drag coefficient of cd = 0.29 in turn has a positive effect on the range.
The complex technology of the battery system requires a cross-functional engineering approach from the fields of electrical engineering, mechanics and chemistry. It consists of the following components, (5):
:: Frame with six cooling plates;
:: 36 modules with 12 cells each;
:: 432 battery cells;
:: Battery Electric Module (BEM);
:: Battery Electric Control Module (BECM);
:: Six Cell Monitoring Circuits (CSC);
:: Battery Management System (BMS);
:: High-voltage protection (HVIL);
:: Harness and cover.
When deciding on the right battery cell, the choice fell on a 58-Ah pouch cell. The deciding factors were the positive properties in terms of performance, energy density and life cycle. The battery pack is designed as a 108S4P configuration. There are 108 cells connected in series and four in parallel. The battery consists of 36 modules containing 12 cells per module, and a total of 432 cells. The rated voltage of the battery is 389 V and has a capacity of 90.2 kWh. The total energy density is 149 Wh/kg. The battery pack can deliver 358 kW of instantaneous power for up to 10 s and has a continuous output of 110 kW. The high-energy pouch cells use the latest NMC (Nickel Manganese Cobalt) technology and provide high volumetric and gravimetric density of 541 Wh/l and 257 Wh/kg.
The pouch cell consists of individual layers of anode and cathode material with an intermediate plastic separator. Although the cell construction is designed for high energy content, the pouch cell has a low internal resistance. The discharge rate is over 6C. Charging from 0 to 80 % State of Charge (SoC) is achieved with DC at 100 kW in just 40 min. The cells are designed for operation at extreme temperatures of -40 to +60 °C. Performance is reduced at the lower and upper temperature limits to prevent cell damage. Within a module, the cells are welded together at the tabs. Held by plastic caps, they are enclosed in a thin extruded aluminium housing to keep the weight low. The module contains two thermocouples for battery temperature monitoring. Each module is connected to the battery controller through the integrated wiring harness, which monitors the individual module voltage and temperature.
The lifetime goal of ten years is achieved, inter alia, by limiting the SoC. The usable range is 0 to 96 % and is monitored by the battery control module. The I-Pace battery has been subjected to rigorous testing to ensure maximum reliability and safety throughout the vehicle’s service life. The battery was subjected to temperature shocks between -40 and +85 °C. Between +25 and +80 °C, the absence of corrosion was detected with high humidity of 70 to 95 %. All legal crash tests were successfully completed, including drop tests from 4.9 m height on concrete floor.
Jaguar’s experience in the use of aluminium in the field of bodywork benefits the weight-optimised battery frame, consisting of extruded aluminium side rail profiles, which are welded together. Carrier elements on the underside of the frame are used to attach the battery modules. In addition, an aluminium bash plate is riveted and bolted at the bottom. Overall, an 8 mm thick aluminium protection is installed between the underside of the vehicle and the battery cells. The volume that encloses the frame construction is 457 l. Despite this size, a relatively flat battery has been implemented, which allows sufficient ground clearance. The battery is mounted under the vehicle body and increases the body stiffness by 50 %.
The I-Pace battery has a cooling plate water cooling system integrated into the frame. Six separate cooling plates are mounted together with the modules on the base plate. The coolant is pumped from the left front of the frame through the cooling plates and returns from the front right into the vehicle cooling system.
THE COOLING SYSTEM
The efficiency of the cooling system plays a key role in maximising the range of the vehicle. The cooling system, (6), consists of three cooling circuits: the drive cooling circuit, the battery cooling circuit and the interior cooling circuit. The drive cooling circuit is responsible for the cooling of the power electronics, the electric drive units, the on-board charger and the DC/DC converter. It operates in three different states: self-heating, low-temperature cooling (NT cooling) or heat pump. These operating modes are listed in (7).
Immediately after the vehicle starts, the drive cooling circuit operates in self-heating. In this mode, the NT cooler is bypassed and the coolant is routed through an inactive chiller. Without an active cooling source in the circuit, the electric drive modules can heat in a short time to optimum operating temperature with the best efficiency.
When it is necessary to cool the drive cooling circuit at ambient temperatures in excess of 20 °C, the NT cooling is used, which is similar to a conventional engine cooling circuit. For this purpose, a proportional valve directs the coolant, bypassing the chiller to the NT cooler. At ambient temperatures below 20 °C, the drive cooling circuit may switch to heat pump mode if there is a demand for heating from the passenger compartment. To do this, the proportional valve diverts the coolant to the chiller. The heat is transferred from the drive unit into the refrigerant. The battery cooling circuit cools or heats the high-voltage battery.
The intended operating temperature range for the battery cells is between 15 and 30 °C. The battery cooling circuit operates in five operating states: “off,” cell temperature compensation, active heating, low temperature cooling, and cooling, (7).
The battery cooling circuit will remain off when the cell temperatures are within the optimal operating range. In this case, the pump is switched off and there is no cooling or heating load. Since this is the most efficient operating state, ideally it is most commonly used. If an uneven distribution of cell temperatures across the battery unit is detected, it can be compensated.
In this mode, a solenoid valve sends coolant back to the battery via an inactive coolant heat exchanger and chiller. Since there is no active cooling or heating source, a homogeneous temperature distribution results.
To heat the battery, active heating is used. A solenoid valve leads coolant to the heat exchanger. In the interior cooling circuit, the high-voltage heater or the indirect condenser heats up the coolant. The heat enters the battery circuit via the heat exchanger. Cooling of the battery at ambient temperatures lower than the battery cell temperature is carried out passively via the NT circuit. Active cooling starts at outside temperatures above the battery cell temperature. In this case, the battery coolant circuit is connected via a chiller with the refrigerant circuit. Active cooling requires that the refrigerant circuit for the passenger compartment is in operation. The high-voltage compressor used here consumes up to 5 kW of power. Due to the negative effect on the vehicle range, the control is optimised so that active cooling is used as seldom as possible.
The interior cooling circuit is responsible for the heating of the passenger compartment and the battery cooling circuit. The passenger compartment is heated with a conventional radiator. Since an internal combustion engine is not available as a heat source, alternatively a high-voltage coolant heater or an indirect condenser is used. The high-voltage coolant heater can take up to 7 kW of power, which, just like active cooling, reduces the vehicle range. For this reason, the use of the indirect condenser for heating the refrigerant circuit is preferred.
If the customer wishes to heat the passenger compartment, heat pump operation becomes active. Heat is recovered from the drive unit and the electrical components in the drive cooling circuit and transferred to the refrigerant via the chiller. By heat transfer, the refrigerant changes from the liquid state to a gaseous one, resulting in an enthalpy increase. The high-voltage compressor continues to increase this enthalpy increase many times over.
The connected indirect condenser ensures a second phase change and transfers the heat to the interior coolant circuit, which finally allows the radiator to heat the passenger compartment. A proportional valve diverts some of the coolant from the chiller to the NT cooler if there is less heating demand from the passenger compartment. This increases the scope of heat pump operation and reduces the need for the high-voltage coolant heater.
Using the heat pump is an extremely efficient way of warming up the passenger compartment. Compared to a concept that only uses high-voltage coolant heaters, the range increases by up to 80 km. The three-stage cooling module architecture is determined by the different operating temperature levels of the cooling circuits.
Since the battery circuit has the lowest operating temperature level of approximately 20 °C, the low-temperature cooler is furthest forward. The drive module cooling circuit runs at a higher temperature level than that for the battery and is therefore in the second row. The refrigerant circuit has the highest average operating temperature, which is why the condenser, unlike in a vehicle with an internal combustion engine on the back of the cooling module, is arranged in the third row. The fan on the back of the cooling module is dimensioned with 850 W and is switched on as required.
The cooling module has an air duct and active vanes on the front and an air discharge duct on the back. With the help of the active vanes, the aerodynamics and operating modes of the cooling circuit fan can be optimally supported, (8).
DIPL.-ING. STEFAN FUCHSS is Chief Engineer Electric Drive Units and Driveline at Jaguar Land Rover in Whitley (UK).
DR ALEX MICHAELIDES is Senior Manager E-Machines at Jaguar Land Rover in Gaydon (UK).
OLIVER STOCKS, CENG is Senior Engineer Cooling Systems at Jaguar Land Rover in Whitley (UK).
RICHARD DEVENPORT, BSC HONS is Manager I-Pace Energy Storage at Jaguar Land Rover in Gaydon (UK).
The author team thanks the team members of Jaguar Land Rover Engineering and Public Relations for their energetic support. In particular, Mike Byrne, Tobias Burgstaller, Harry Mustoe, Imogen Pierce, Simon Bickerstaffe and Andrea Leitner-Garnell should thankfully be mentioned.