Ultra-light City Car

© Maciej Kaniewski – All rights reserved

(This paper was written as an assignment for the Electric Vehicles Course offered by TU Delft. No liability shall be taken for the correctness of the data or facts quoted.)

This vehicle is designed for the suburban population in Warsaw, Poland. Young couples often move into compounds in sprawling suburbs, taking advantage of lower real estate prices, but face a limited public transport infrastructure. Though cycling is feasible in the summer months, cycle paths are scarce, and it is not realistic during the colder seasons. Consequently, young people usually drive cars to work. Since jobs are located in the central districts, commuters face annoying traffic jams.  As a standard, both spouses work, and compared to most European countries, they work very long hours. Driving kids to school, often different schools for each kid, as well as taking them to after school activities, is thus an additional challenge, so usually both parents share this task. Suburban shopping usually takes place in shopping malls away from home, again requiring some form of transport. The only practical solution for most suburban couples is two cars, increasing traffic congestion and pollution.

As an alternative to public transport, I propose a miniature electric one-seater city car, capable of taking two small children plus some luggage in the back.

The car could be sold or leased, or a car-sharing scheme could by set up by compound administrations, similar to bicycle-sharing which is already popular. Elderly people with purely urban travel needs, e.g. city gardens, could be another target group for this car.

Car type

Warsaw and its surroundings are essentially flatland. In Warsaw itself, the only elevation is the 15 meter river bank.  The metropolitan area houses about 2.7 million people, with an additional 0.5 daily commuters. The average distance to be covered daily by suburban commuters is 30-50 kilometers. This means that the city car would not need a powerful motor, high speeds or a great range. The focus should be on driving comfort and fun, safety and affordability.

Based on these considerations, the car will be a three-wheeler:  driving wheel in front and two wheels at the back for better aerodynamics. There will be one full-size seat in the front and a smaller, dual foldable child seat in the back. The car will have a single big door accessing the driver’s seat and the child seat plus any small luggage in the rear.

With dimensions of ca. 2500 mm (length) x 1200 mm (width) x 1600 mm (height), two city cars will fit side-by-side into a standard-size parking slot or a driving lane. The net weight of the car will not exceed 300 kg. The car will have a maximum speed of 90 km/h, for smooth city traffic and driving on high-speed thoroughfares within the metropolitan area. It will have a maximum range of ca. 100 km, enough for daily commuting.

Car driveline

The car will be a battery electric vehicle (BEV).  Other options (PHEV, HEV, FCEV) are not considered, since they would involve unnecessary drivetrain complexity,  increasing the car size, weight and ultimately price.

Car motor

A one-phase 6 kW induction motor will power the front wheel. A permanent magnet motor is not considered because of the requirements for a position sensor and starter, and a switched reluctance motor because of its higher price.

Power conversion

The car will have a combined bi-directional (buck/boost) DC/DC converter providing energy from the battery and capable of charging the battery with energy from recuperative braking. The current from the battery, supplied to the high-voltage bus operating at 100V, will pass via a DC/AC converter to power the induction motor with a maximum current of 60 A. A uni-directional converter to 12 V will be used to power car equipment.

The car (including driver and two children) will have a kinetic energy of ca. 125 kJ (34,72 Wh) at 90 km/h.

Energy storage (battery)

Assuming an average consumption of 4-5 kWh/100km[1] and 80% discharge, a 10 kWh Li-ion battery will more than suffice for the planned 100 km range. A 200 Wh/kg energy density derived from the Ragone plot implies a weight of only 50 kg, excluding the battery management system.

The battery unit will be located at the rear bottom. The chassis will be entirely carbon fiber, so the center of gravity will lie low enough for the driver seat to be located higher for comfort, visibility, and additional storage under the seat.

Charging

The car will use a built-in AC charger (230 V 1-phase and 400 V 3-phase charging) with a type 2 and conventional mains connector.  Inductive charging is currently not considered owing to its high cost. Direct high-current charging is not necessary (no long transits).

The cars will be equipped with a special DC connector to other cars, allowing parallel charging of several cars from a single charging outlet. (The battery management system will be capable of handling multiple batteries as one unit). In the absence of an outside energy source, one car’s battery will also be chargeable from another car’s battery.

While driving, it will be possible to charge the car mechanically by pedaling, as in cycling. As this charging method will generate not more than 75 W, it will not replace mains charging. Rather, it will offer an opportunity for healthy movement while contributing to replenishing the battery. The pedals will not be part of the drivetrain, but the speed of the car will be electronically adjusted to the pedaling effort to create a realistic driving experience.

Future developments

The city car will offer plenty of opportunities to include future technology development:

  • The roof may be fitted with solar panels for additional charging. Owing to the car’s small footprint, solar cells with a higher efficiency than today would have to be used.
  • Higher battery energy density could lead to increased range at the same weight.
  • A convoy of connected cars could be driven by just one driver, or autonomously in the future.
  • Using vehicle-to-grid technologies, the car could be an additional energy storage for the home.

[1] Assumption: power consumption is 25% of a BMW i3, source: https://en.wikipedia.org/wiki/Electric_car_energy_efficiency