Ford Motor Co. has developed a prototype electric small car, the e-Ka. Based on Ford's Ka production car, it is powered by a new generation of lithium ion batteries. Ford claims to be the first manufacturer to produce an electric vehicle using this type of battery propulsion, which the company says has the performance of a gasoline-fueled car and a range of 150 km (93 mi). The e-Ka was developed at Ford's Research Center in Aachen to demonstrate the capabilities of new "dedicated soft lithium ion" watercooled battery technology.
The lithium ion battery pack is approximately 70% lighter than an equivalent pack using conventional lead acid technology. The result is that the e-Ka can accelerate from 0-100 km/h (0-62 mph) in 12.7 s and 0-50 km/h (0-31 mph) in 3.9 s, with a maximum speed of 130 km/h (81 mph). If the vehicle is maintained at a constant speed of 80 km/h (50 mph) it could travel more than 200 km (124 mi) before needing a recharge.
A particular advantage of lithium ion battery technology is its tolerance to high electric current when recharging. Unlike other battery systems, lithium ion packs are immune to the so-called "memory effect," retaining full charging capacity regardless of usage. Featuring an onboard recharger, the e-Ka can be fully charged in just six hours.
The e-Ka battery pack consists of a total of 180 cells divided into three separate battery "troughs." One trough is located in the engine compartment, the other two directly behind and in front of the rear axle. The entire bundle of lithium ion batteries have a combined mass of 280 kg (617 lb), which is 70% less than that of normal lead-acid battery packs. The battery troughs are divided into modules that each contain six lithium ion cells. An onboard battery-management system (BMS) constantly monitors the modules both during operation and while they are being recharged. Data concerning cell voltage, battery charge status, and the operating temperature of the battery module are passed through an onboard communications network to the e-Ka's central control unit, called the Vehicle System Controller (VSC). The VSC ensures optimum operation of the battery charger, the battery itself, and the electric motor.
The flow of electricity from the battery unit is automatically shut off if the system is turned off or if sensors detect a crash. There are independent coolant systems: one for the drive system consisting of electric engine, transmission, and differential, and the other for the lithium ion battery pack. The three battery troughs in the system are completely sealed.
Ford engineers equipped the vehicle with electric power assisted steering (EPAS), which uses sensors to register torque as well as steering wheel position. This data are continually conveyed to an electronic module, which controls the precise amount of electrically powered assistance required. The advantages of the EPAS system compared to conventional hydraulic power steering systems include compact construction, reduced weight, and environmental benefits. The e-Ka is also equipped with an electric brake booster and ABS.
The use of a composite material called Hylite, which consists of two aluminum sheets sandwiching a layer of polypropylene, contribute to a mass reduction of 5.5 kg (12.1 lb) in the roof and 5.3 kg (11.7 lb) in the hood. Hylite is up to 65% lighter than aluminum or steel of comparable stiffness. Aluminum was used for the rear axle, wheels, and brakes to provide another 35 kg (77 lb) mass reduction. The car is also equipped with low rolling resistance tires to maximize the vehicle's range.
The lithium ion battery in the e-Ka has a power density of 126 W•h/kg and a high-energy capacity for its volume, 197 W•h/L. In practice, this allows for more energy charging capacity without an increase in volume or mass. Other advantages of the lithium ion battery system are longevity, cycle consistency, and temperature stability.
Jean L. Broge
AEI July 2000
