Contrasting Hybrid Electric Bicycles and Electric Bicycle
Michael George & Sam Choi
Abstract — This report examines the difference between Hybrid Electric Bicycles and Electric Bicycles. The differences discussed focuses on the component differences that would be necessary if an Electric Bicycle would be converted to a Hybrid Electric Bicycle. The benefits of Electric and Hybrid bicycles are discussed. Then the benefits and possible methods of implementing a Hybrid Bicycle are briefly summarized.
I. INTRODUCTION
This report will focus the differences between an electric bicycle and a hybrid electric bicycle. It will emphasize what an electric bicycle is and the benefits from using one. Then it will consider the benefits of a Hybrid Electric Bicycle and additional features that are required to transform an electric bike into a hybrid. Then considerations of energy conservation will be looked at and see if that transformation is worthwhile. Ultimately, the question we will try to answer is whether or not regenerative braking is an economically feasible technology to explore on electric bicycles.
II. BACKGROUND
Electric Bicycle (e-bike):
How an e-bike works is by assisting your pedaling. Electric bikes are everyday bicycles with a battery-powered electric motor attached. Although it is capable of pushing you along without any pedaling, electric bikes perform noticeably better augmented by pedaling. The average "couch potato" who normally rides at 10 mph can ride at 15-20 mph using the same effort. He/she’s expected range can vary but distances of 10 miles can be covered with an appropriate battery, with a recharge time of several hours.
Power, when activated by a switch on the handlebar (power-on-demand) or in response to your pedaling (ped-elec), gives you an immediate, nearly silent push. When you release the switch (or stop pedaling), the motor coasts or "freewheels" - like when you stop pedaling a regular bike. Just like a regular bike, e-bike is rounded out with a gear and brake controls as well as the power on demand knob.
Power-on-demand means no pedaling required anytime at any time. Although all electric (or "electric-assist") bikes are designed to work with your pedaling, power-on-demand allows you to ride the bike without pedaling. Most systems offer a variable speed control, although some are simply on. A "ped-elec" won't deliver motor power unless it senses you are pedaling and it's "power output to pedal pressure" ratio is usually adjustable.
When considering an E-bike, battery issue is one of the most talked about isuue. Rechargeable batteries, usually sealed lead-acid, provide power for the electric drive motors. Charging costs less than 5¢ of electricity from common 110V AC wall outlets. Charging times vary widely due to charger output and battery capacity, but you can expect to recharge in less than 8 hours with most stock chargers and if one is not happy with 8 hours of charging, quick chargers are available.
How e-bikes perform depends on many factors. The most important factors are listed below with the most important at the top. You will notice that battery size and system efficiency rank near the bottom. One thing to mention is that the speed you go makes a big difference in how far you go.
1. Terrain (For example, incline of hills)
2. E-bike speed (range at 10 mph is 8 times as far as at 20 mph)
3. Wind conditions (going 10 mph against a 10 mph headwind feels like 20 to the bike)
4. Pulling a trailer (which is like pulling another bicycle)
5. Correct tire inflation (under-inflated tires slow you down)
6. Battery size (measured in volt-amp-hours)
7. Weight of rider and bike frame
8. motor/controller/drive system efficiency
I’ve explained briefly, what people can expect from an E-bike and how it differs from a regular bicycle. Then, how is Hybrid Electric Bike different from an E-bike?
Hybrid Electric Bike
Hybrid bike is similar to an e-bike because they both assist the rider with a second power source. The hybrid engine is a combination of electric motor with a power source, and a means to recharge that power source from energy within the system. (Usually momentum) The major difference between the electric bicycle and the hybrid is that the hybrid employs this recharging to the battery through a regenerative braking system.
To explore level of energy a hybrid electric bicycle can utilize from regenerative braking, proper understanding of the energy usage in a riding situation is necessary. The two largest forces hindering the movement of an in motion bicyclist is air drag and rolling drag. (Air drag becomes a much more significant force to overcome the faster the rider is moving)
Air Drag
Air drag can be calculated from the equation below.
The Ac for racers is between 0.4 to 0.6m2 but in this application users will rarely crouch so the estimate for this calculation will be: Ac = .7m2
The drag coefficient is commonly taken as: Cd = 0.9
The density of air is known to be: Da = 1.226 kg/m3
Velocity is determined by the rider in m/s.
Rolling Drag
Rolling drag can be calculated from the equation below.
The acceleration due to gravity is known as: g = 9.8 m/s2
Different sources give values for Crr but it will be taken as: Crr = 0.007
Mass is determined by the rider in kg.
So the total drag on a bicyclist is the combination of air drag and rolling drag:
(Friction loss within the bicycle system is also a factor but will no be factored in because of its extremely variant nature.)
To find the power needed to operate a bicycle at a certain velocity you use the equation:
Using this equation a rider going 20 MPH with a total combined bicycle and rider weight of 190lbs would have to output 329W to maintain his/her speed. From this equation it can be seen that Pvel grows exponentially with velocity.
III. RESULTS
Now the question comes as to how much energy can be transferred from the moving bike to a battery. This is the necessary component to deem an electric bike “hybrid.” Several assumptions are going to be made when doing this next calculation. In the braking all the kinetic energy will be stored in the battery, negating any losses through internal friction, power conversion and assuming this braking will not engage the manual brakes, or that air drag and rolling friction are not a major factor in the stopping. The reason why such assumptions are made is to establish an upper bound on how much possible energy could be stored in the battery from braking.
The equation for kinetic energy is:
Using the same rider going 20MPH, the kinetic energy would be Uk = 3443 J.
From this point we start to encounter some real problems that begin to indicate the feasibility of this system. If the rider were to slow down in 1 second from 20MPH, then that would be 3443W of energy sent to the battery. This is potentially large amount of power that could be recovered. But problem comes from finding a battery solution that would be capable of absorbing this much power.
IV. CONCLUSION
Since there is a significant amount of energy that can potentially be reabsorbed by the battery, further exploration into how that energy can be stored is warranted. The main issue is getting the energy recovered from the braking into the battery.
Various batteries have different methods and speeds at which they can absorb power. Usually, slow charge rates are used to extend the life of the battery. For this application batteries would have to be charged as fast as possible without damaging the battery. Fast charge rates can be used to charge some kinds of batteries, but the small batteries used in this application cannot handle 3443W. And if this fast charging method is used, the battery must be below 85% of its charge or the fast charging can damage the battery.
Basically charging a battery is a fairly complicated process. Many chargers are designed to limit the current when the battery nears its capacity, which adds more circuitry to the system. With this added complication of charge rates some sort of ultra-capacitor or secondary fast charging energy storage device would have to be used to conserve the energy from braking quickly and slowly charge the battery with that energy. One nice aspect of this solution is that the ultra-capacitor would be charged from previous braking and would be able to supply the motor after the rider wanted to start back up.
There are also a number of alternative methods of storing mechanical/electrical energy required for propelling the hybrid vehicle that have advantages and disadvantages. An alternative energy storage device that can be used is the flywheel. Flywheels, also known as electromechanical batteries, store energy in the form of rotational kinetic energy. The amount of energy stored in the flywheel is calculated as follows:
Thus, an increase in rotational speed is far more beneficial than an increase in the amount of inertia of the flywheel. This fact has steered research towards developing an optimum flywheel shape that allows for the greatest rotational speed possible. The isotropic hyperbolic shape is the most efficient design thus far.
A. Flywheel Energy Storage Using HTS Magnetic Bearings
Recent advances in the development of very low friction bearings and high-strength fiber composite rotor materials has revived interest in flywheel energy storage (FES). These advances enable efficient diurnal storage with high energy densities. A rotating permanent-magnet bearing assembly can be stably levitated above a stator component composed of high critical temperature -Tc superconductor (HTS) elements, without the need for position sensors and the elaborate feedback control systems required for conventional active electromagnetic bearings. Significant advances have been made at Argonne in developing very low friction magnetic bearings based on the unique levitation characteristics of HTS materials. Major accomplishments include an order of magnitude scale-up in HTS magnetic bearing size and demonstration of friction coefficients (?<>
This option is great because a flywheel can receive large amounts of current quickly so would be able to store the energy from the braking immediately. But it is somewhat less feasible within space constraints because of the extra motor and weight required for the flywheel.
There are some obvious questions that still need to be addressed, including: “What if the rider decided not to brake quickly and slowly braked over time?” This complicates the problem because the longer the rider takes to brake, the lost of energy due to drag becomes greater. And if more energy is lost due to drag then less of that energy gets put back into the battery. Incidentally, the rider is unconcerned with this loss due to drag because if energy to drag is not lost now, it will be lost when the rider speeds back up.
Clearly there is this and many more questions that still need to be answered to determine if a hybrid electric bicycle is economically feasible. The issues addressed in the paper have pointed to a potentially significant source of energy that could be reabsorbed by an electric bicycle system and various means to store it. Although all the ramifications and potential has not been fully explored, we believe that the potential of this project warrants further investigation, even only to satisfy academic curiosity.
REFERENCES
[1] http://www.nlectc.org/txtfiles/batteryguide/ba-cont.htm “New Technologies Battery Guide”
[2] http://www.kreuzotter.de/english/espeed.htm#pv “Bicycle Speed and Power Calculator”
[3] http://www.et.anl.gov/sections/te/research/flywheel.html “Flywheel Energy Storage”
[4] http://www.ott.doe.gov/hev/ “Hybrid Electric Vehicles”
[5] http://www.mech.uwa.edu.au/courses/ES407/Storage/1998/flywheel.html “Flywheel”
E Bike and Hybrid Bike (Contrast Diagram)
http://www.iit.edu/~ipro315/Research/braking/ipro2.htm
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