eBici - An Electric Bicycle with Human Assist

The Concept - A New Approach

All "electric" bicycles available on the market today are electric assist bicycles. This means that a regular bicycle design, optimised for human power, has been modified to include an electric motor add-on to assist the human, mainly for going up hills. There are various methods of adding the electric power to the wheels, either by a friction drive, a hub motor built into the wheel, through the regular drive chain, and so on.

All of these approaches suffer from a basic incompatibility of matching between the human engine and electric motor, typically resulting in an inefficient system requiring a large, heavy battery, making pedalling without the electric assist more difficult. The concept of advantageously combining different types of engines in a single "hybrid" vehicle has been used for many years in the Diesel-electric locomotive, and more recently in the modern hybrid motorcar. The approach that we have been using to develop the eBici bike is to treat it as a Parallel Hybrid machine, designed from the ground up to merge the human engine and electric motor in an optimal manner, resulting in an efficient and light machine. The Parallel Hybrid system has unique advantages in merging different types of engines in vehicles, and is currently under development at eCycle with their unique hybrid motorcycle. In this vehicle the electric motor is used in parallel with an internal combustion (IC) engine, however each has its own specific complementary functions. The electric motor is used mainly for acceleration (in particular from a complete stop) and for regenerative braking, in that the braking energy is used to recharge the battery. The IC engine can then be a much lower power device since it is used mainly for the constant power cruising, in addition to keeping the batteries charged. The eBici bike uses the same approach, with the human motor taking on the task of the IC engine - that of maintaining a constant cruising speed, trickle charging the batteries whenever convenient with "extra" human power, and assisting the electric motor on an uphill grade.

The development of the eBici electric bike is the combined effort of eCycle and Izzi Urieli of eBici. It was decided that the design should concentrate on optimising the combined requirements of efficiency, comfort, safety, maneuverability, simplicity, affordability, ease of manufacture and assembly, portability (lightweight and take-apart), and a clean, oil-free system (no chain).

The central element of the design is the extremely efficient series of brushless electric motors and controllers developed by eCycle. These were developed for high speed 5 kW applications, but can be used to advantage at the low voltage, low power and low speed required by the eBici bike. The comfort and maneuverability is obtained by using the short wheel base recumbent configuration, which was extensively evaluated and developed by Izzi Urieli with the Grasshopper series of bicycles. The simplicity is obtained through a number of unique features. The frame structure has no welds, and the entire frame is simply bolted together - the complete front section assembly can be easily removed by removing 4 bolts. The handlebar stem is connected to the steer tube by a unique clamping mechanism, allowing a convenient handlebar height adjustment. Both wheels are identical and can be interchanged.
 The crankset drives the electric motor shaft through a belt drive and roller clutch, and on the opposite side the motor shaft drives the rear wheel through a second belt. Both belts are Gates Poly Chain GT2 toothed belts, and require no maintenance. The only gear available to the rider is the two speed Swiss made Speed-drive with a planetary gear mechanism built into the crankset. Changing gear between low and high is done by kicking a button on either side of the crank axle with ones heel. Notice that the crankset can be mounted in either of two positions - for shorter riders (above left) or in the front crankstay hole for longer riders (above right). In this position the belt would be replaced by one of larger pitch length. Thus with a seat position adjustment of around six inches we cover a wide range of rider heights and still maintain near optimal weight distribution. The only controls on the handlebars are the twist throttle on the right allowing acceleration (clockwise twist) or regenerative braking (counterclockwise twist), and the front wheel brake lever on the left.

Power Required and the Human Engine

The nomograph shows the power required to drive the eBici bike at a constant velocity with various grades of slope (lower portion), and the long term capability of the human engine to provide power before becoming exhausted (upper portion). The human power curves were presented by Douglas Malewicki at the 1983 IHPV Scientific Symposium. To use the curves we first choose a baseline operating point. For example, the power required to propel the bike at 15mph on a zero slope is seen to be 100 watts. Following up the 100W line we find that a 'healthy human' can provide that power for about 5 hours before becoming exhausted. However the same human providing all of the 300W required to ride up a 6% grade at 10 mph would become exhausted within a few minutes. In this case we would let the electric motor provide 200W, while the human would assist with the extra 100W. On the other hand, while relaxing at 10 mph on a zero slope (40W), or while riding downhill, the rider could use excess human power to trickle charge the battery by any convenient amount using reverse throttle control (counterclockwise twist).  
 The eCycle MG62 motor/generator and controller have been designed to cover the entire range of speeds of the eBici bike from standstill to 20 mph. With a pedalling cadence range of 60 to 100 revolutions per minute the human rider will be able to assist the motor over the range of 7 to 12 mph (low gear) and 12 to 19 mph (high gear), as indicated on the graph above. The initial acceleration from standstill to 7 mph will be done exclusively with the electric motor (about 3 seconds at full throttle on zero slope).