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The tzero Electric Sports Car - Part 1

An electric car prototype like you've never seen before!

By AN Brooks and T B Gage, AC Propulsion Inc.

Click on pics to view larger images


This article was first published in 2001.

In 1996, US company AC Propulsion began development of a high-performance electric sports car, the tzero. Now, tzero prototypes are undergoing in-use testing and safety certification development. In actual tests, the tzero accelerates from 0 to 97 km/h (60 mph) in only 4.1 seconds, yet it is one of the most energy-efficient cars on the road.

The rationale for the tzero is that a car with its unique technology and superior performance can be sold at a sufficient price to cover the high costs of small-volume production.

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The tzero is a purpose-built electric sports car with world-class acceleration and handling performance. It was originally conceived by AC Propulsion in 1996 as a pathway to bringing a small-volume electric vehicle to market with a business case that could support the higher costs of small volume production. As a small company, AC Propulsion cannot subsidize the price of an electric vehicle offering the way the major automakers have. The company had previously made a few EV (electric vehicle) Honda and Saturn conversion vehicles for car manufacturers for evaluation purposes. No real market beyond these few vehicles was apparent, since the high costs of low volume conversion - about US$80,000 - priced such a vehicle outside of the range of any consumer interest. Even though these vehicles had better acceleration and driving feel than their petrol-powered base cars, there was just no market for such a vehicle at a price that would cover the conversion costs.

The concept for the tzero was that a simple purpose-built EV that exhibited performance and driving feel superior to most - if not all - petrol-powered cars could be priced at a level consistent with the costs of low-volume production.

The tzero Prototype

The first tzero was adapted from a kit car known as the Sportech, built by Dave Piontek. The Sportech was originally built around a highly-tuned motorcycle engine with sequential shifting transmission. Its light weight and small size made it a good candidate for electric conversion. AC Propulsion acquired a Sportech, and the rights to the vehicle for EV applications in 1996. In making the first tzero, changes to the Sportech were few - mainly to accommodate the electric drivetrain. That first tzero, was introduced at the Los Angeles Auto Show in January 1997. Performance was very good, with 0 to 97 km/h (60 mph) in 4.9 seconds, with a 160-kilometre range.

The Current tzero

Powertrain

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The current tzero achieves superior acceleration performance and good range with low-cost lead acid batteries. Key attributes that enable this level of performance are a high battery mass fraction of 50%, powerful and lightweight electric drive components, and battery management that includes active thermal management (including battery heating), and pack balancing with module-level charging.





Driveline

The tzero is powered by AC Propulsion's high-performance induction motor (above). For the tzero application it is operated at 37% higher peak current than allowed in our standard system. This is practical since the tzero is relatively light, and periods of peak power are limited to only a few seconds.

Motor Specifications

Motor type AC Induction w/copper rotor bars, 4 poles
Peak torque 246nm (181 ft-lb)
Peak power (at 326V DC input) 177kW (237hp)
Base speed at 330V 5,000 rpm
Maximum speed 12,000 rpm
Peak current 687 A rms
Mass, include plenum and blower 50 kg
Dimensions (less fins, termination, plenum) 213mm dia by 257 mm long
Dimensions of motor incl cooling plenum 305mm dia by 305 mm long
Maximum winding temperature 180 deg C

The motor is forced-air-cooled via fine-pitch fins located around the outer diameter of the motor housing. The cooling blower is run at variable speed, depending on the temperature of the motor. In normal driving, the blower runs very little and the associated energy consumption is inconsequential. The motor stator is constructed in a conventional 3-phase 4-pole configuration with 14-mil laminations. The rotor also has 14-mil laminations and is constructed with copper shorting bars and end rings. A Beryllium Copper ring is added for structural support on each end ring. The copper-based rotor increases both the efficiency and peak power capability of the motor. AC Propulsion developed and patented the design and assembly process for this rotor. Peak output power of the motor as installed in the tzero is about 150kW (200hp). Droop of the battery voltage under high current drain prevents the achieving of the full 177kW output seen on the dynamometer. The peak torque is available in the vehicle; the battery droop only affects the motor speed to which peak torque is maintained.

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Drive power is delivered to the rear wheels through a single speed gearbox with a 9:1 reduction ratio. The gearbox is based on the Honda Civic manual transmission, with all the first-stage gears removed and replaced with a new gear set sized for the increased torque. The final drive gear and differential are stock Honda parts. The gearbox input shaft is connected to the motor through an electrically isolated coupling. (Electrical isolation of the motor is a requirement for the integrated charging system.)

Power Electronics Unit

The power electronics unit (PEU) contains the traction inverter, charger components, and 12V auxiliary power supply in one enclosure. The system is air-cooled with a speed-controlled squirrel-cage blower. Cooling air is forced past fine-pitch cooling fins on the bottom of heat sink plates. Outside air does not flow through the electronics. Air inside the electronics enclosure is circulated internally with a small fan. An air-to-air heat exchanger removes heat from this closed volume.

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The traction inverter power stage is based on three bipolar IGBT-based 'smart poles' switches. Unlike most other traction inverters, the switches are made up of paralleled discrete IGBTs in TO220 packages. This approach allows the heat load to be spread over a larger area, making air-cooling possible. Additionally, the cost per kVA for discrete IGBTs is currently about half that for packaged IGBT modules. Control of the phase current is through discrete analog circuits, with a lookup table for optimum slip speed. Patented techniques are employed for high-speed stability and for maximizing the motor output in the voltage-limited (high speed) portion of the operating region. With the tzero fixed-ratio gearing, 8000 rpm corresponds to 100 km/h.

Charger

The tzero's Reductive? 1 charger uses the motor and power switches from the drive system to serve as the power elements of the charger. The Reductive charger is conductively coupled to the power grid, and operates with standard J1772 conductive wall boxes and can be directly plugged in to 110- to 240-V outlets. Since the Reductive charger is based on a powerful drive system, it can charge at higher power levels than standard chargers. The Reductive charger in the tzero can charge at up to 20kW compared to 4-7kW for conventional chargers and, with minor modifications, can be configured to charge at 40kW.

In the tzero, a 60% charge can be completed in as little as 30 minutes.

Power Electronics Specifications

Voltage 336V nominal 240 min, 450 max
Motor Current 687 A rms max tzero only 500 A rms (other applications)
Max input power 206 kW (tzero only)
Dimensions 760 mm x 313 mm by 186mm 29.9 in x 12.3 in x 7.3 in
Weight 30 kg (includes cooling blower)
Charger 20 kW max
Auxiliary supply 100 A at 13.5 V

Battery Pack

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The battery pack consists of 28 series-connected 12V lead acid batteries. Two rows of 6 batteries are packaged along each side of the vehicle between the front and rear wheels, and the remaining 4 batteries are packaged just ahead of the motor. There are no separate or removable battery 'packs'. For packaging volume efficiency and light weight, each battery is mounted individually to the vehicle chassis. The side mounted batteries are supported on light-weight sheet-steel mounts welded to the frame, and secured with tension rods and aluminium right angle brackets. The rear batteries mounted to the chassis bottom with shaped delrin blocks and tension rods. Battery interconnects are custom-made with braided copper. The battery pack fuse is in the electrical centre of the pack.

The battery modules are a deep-cycle spiral-wound recombinant lead acid type made by Optima Batteries, commonly known as the "yellow-top". When new, these batteries deliver 44 Ah on typical driving cycles. In the tzero application, these batteries deliver up to 600A discharge current for brief periods. Variants of these batteries are also marketed as premium automotive starting, lighting, and ignition (SLI) batteries. A deep cycle yellow top battery that has reached the end of its service life as an EV battery will still function quite satisfactorily as an SLI battery. AC Propulsion has implemented a very successful secondary market for spent yellow top batteries. They are sold for SLI application at US$25 each - about 25% of the original cost - with a two-year warranty. Several of these are still going strong after 5 years of SLI service.

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The tzero is fitted with AC Propulsion's BatOpt battery management system. This system consists of a BatOpt module mounted to each battery and a central controller called the BatOpt computer. The BatOpt modules are interconnected on a 4-wire bus; two wires for digital communications, and two for connection to the full voltage of the battery pack. The functions of the BatOpt module are:

  • Voltage measurement - The voltage is sensed across the battery terminals and communicated to the control computer.
  • Temperature measurement - A thermistor senses the temperature on the side of the battery. The measured valued is communicated to the control computer.
  • Balancing and weak module support - Each BatOpt module contains a 5-amp switching power supply powered by the full pack voltage. This supply can be switched on and off by the BatOpt computer for purposes of balancing the pack or bringing up a weak module. This function is available while driving as well as while charging.
  • Battery Heating - The BatOpt module can control a 3-amp heater load to enable each battery to be individually heated to the optimum temperature. The heaters are individually controlled by the BatOpt computer. In the tzero, the battery temperature is usually set to 40 deg C. There is a thin-film heater blanket installed in each of the two central cavities in the battery. The heaters are powered with 12V directly from the battery being heated. In order to preclude charge imbalance due to varying levels of heating in different modules, the 5-amp power supply in each BatOpt unit is cycled on and off to provide an average of 3 amps, so that the heating energy for individual modules is effectively shared uniformly across the whole pack.

The battery pack is cooled with ambient air ducted in from a variable-speed squirrel cage blower in the nose section of the car. A close-fitting cover is fitted over the side-mounted batteries to assist in directing airflow over them. The space between this cover and the body side pod forms part of the air ducting that delivers air to the pack. The air enters the pack at the centre of each side battery pack. The flow path splits, with some air moving forward and out at the front wheel well and some moving rearward. The rearward-moving air passes over the rear side batteries and the 4 batteries in front of the motor before exiting through the rear wheel wells.

The PEU houses a 100A auxiliary power supply to provide power to for the vehicle's electrical loads. A small 12V 7-Ah battery is also incorporated to provide peaking for short loads above 100 Amps and to provide unswitched accessory power when the vehicle is turned off.

Powertrain Control

Control of torque delivery of the powertrain is primarily through the accelerator pedal, subject to several limit conditions for parameters such as motor temperature, PEU temperature, upper and lower battery voltages, and to limit wheel slip.

Traction Control

With rear-wheel drive, high power, and short wheelbase, the tzero could be expected to be prone to wheel spin and/or loss of traction of the drive wheels, resulting in a vehicle spin. To reduce this possibility, the drive system incorporates a traction control system that prevents the drive wheels from spinning or slipping under acceleration or regenerative braking. The system operates in forward and reverse direction, and for acceleration torque and regeneration (ie electrical braking) torque.

The system works by limiting the slip of the rear (driven) wheels with respect to the front (un-driven) wheels. The speed of each front wheel is sensed, and the average of these is used in the traction control system. The system allows for different slip limits in motoring and regeneration. The slip fraction has been empirically determined to provide optimum motoring traction and safe regeneration.

The motoring slip limits are set higher than the regeneration limits. This approach allows the traction control to be set up such that there is enough wheel slip to achieve maximum acceleration, but with slightly diminished directional stability. Drivers are generally familiar with the potential for loss of traction while accelerating and its effects on vehicle stability. The natural instinct is to back off on the accelerator if the car starts to get 'sideways'.

But because there is not a comparable driver instinct for potential loss of traction while slowing with strong regenerative braking, regeneration traction is set up with more margin - there should be no possibility for regeneration to upset the stability of the vehicle. The overall effectiveness of the traction control system was demonstrated recently in a staged drag race in the rain against an all-wheel-drive Porsche Carrera 4. The tzero handily out-accelerated the Porsche without a hint of wheel spin, and at the end of the run, transitioned to full regenerative braking at 145 km/h on the rain-slick runway without any loss of traction or directional instability.

Regenerative Braking

Strong regenerative braking is provided and is crucial to the high overall energy efficiency of the tzero. Driver control of the level of regeneration is through the first quarter of the accelerator pedal travel, with a slider control on the instrument panel to set the maximum regeneration level when the pedal is all the way up. The feel is like that of engine braking, but with a continuously-variable gear ratio to set the level. No regeneration is mixed in with the service brakes. In this way, the driver always knows which type of braking is being used, and quickly adopts a driving style that almost completely eliminates the use of friction brakes.

At its maximum level, the regenerative braking will slow the tzero at about 0.3 G, adequate for virtually all normal driving. The 'zero torque' point of the accelerator pedal travel varies with vehicle speed, such that at zero speed there is no regeneration commanded, and hence no 'dead zone' in the pedal travel when starting up from rest.

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Under very hard cornering, any significant levels of motoring or regenerative braking can potentially exceed the traction margins in the rear tyres, resulting in a spin. Traction control alone is not enough to eliminate this occurrence. Driver experience and instinct usually result in judicious application of power when cornering. In regeneration, the goal is to preclude any possibility of having regenerative braking induce a spin while cornering. This is accomplished by reducing the maximum level of regeneration allowed as the cornering load increases. Regeneration tapering starts at 0.3G and is down to zero at and above 0.5-G cornering level - if regeneration alone is applied. If the service brakes are also applied, then regeneration is allowed in proportion to brake pedal pressure, since the front/rear balance of the service brakes is somewhat front-biased to accommodate the regenerative braking that is usually present when the service brakes are applied. If regeneration were not enabled in this fashion, application of the friction brakes while cornering leads to an undesirable level of understeer.

Next week: the chassis, tyres - and performance!

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