Do gears make an electric motor bike more efficient?

Question

Electric motor bikes were sold initially on the idea that there would be lesser maintenance and it would remove the requirements of the gears.

Now, a lot of people like having a geared system. Some argue it makes it more efficient during cruise mode or high speeds. I want to understand if this is correct. Are there added benefits to having a gear system on an electric motor bike?

My hypothesis states that it is not dependent on the torque generated and has to do more with the load generated at the motor.

Edit: this is not a cycle. I'm talking about replacing IC engine powered two wheelers. No pedals. Not a cycle. No assisting. Purely run by the motor

Answer 1

Consider this Torque Curve from the mathworks (https://www.mathworks.com/help/physmod/elec/ref/servomotor.html):

_{(source: mathworks.com)}

It's their idea of available torque for a brush-less AC motor. At low speed, you can get the max torque, but as speed increases, available torque decreases in a way that looks logarithmic. Efficiency under this curve will change, but I think the bigger issue will be the loss of torque output at high speeds. If you think you'll use the motor more at low speeds (launch-assist is great on a bicycle), you're probably alright to have a single high-speed pedaling gear for cruising and getting the bike rolling with the motor.

Answer 2

In general cases, a gear system is usually linked with an electric motor for extreme grade requirements like to carry heavy load or to attain very high speeds. The reason we need a gear system when it comes to internal combustion engines is cause they produce useful torque and power in a narrow band of engine speeds and the gear system helps the engine to be within this useful range and hence lets the vehicle attain high speeds. For example, a hero splendor gives more mileage than a honda activa although both of them have almost the same power output. Both have an internal combustion engine but the honda activa lacks a gear system. But when it comes to electric motors and electric vehicles the scenario seems to be completly different as electric motors give useful torque and power through a wide band of engine speeds hence a gear system isn't necessary at all. Coming to the question, I think the efficiency of the two wheeler will reduce if we add a gear system as weight and friction will increase unnecessarily hence more load on the electric motor and inturn reducing efficiency and increasing the complexity of a rather simple two wheeler. Whilst on the other hand for an electric car, a gear system isn't that bad an idea at all as the wide useful range in electric motors is found at lower rpms and this is where gear systems can be of vital importance for higher velocities.

Answer 3

The thing that accelerates your electric vehicle is not torque- it is power, which is the product of torque times RPM. If the RPM is very small (as it is when you are starting off from a standstill) then the power generated by the motor is similarly small and your acceleration will be small.

Now, if you introduce a gearset between the wheel and the motor, you can gear down the motor so it spins faster at a low wheel speed and thereby accelerate faster. Then, as you speed up, you select different gears to match the peak power point of the motor to the wheel speed.

Note also that the efficiency of any electric motor is a function of its rotating speed, and there is a point on that efficiency curve where it is a maximum. Prudent design then requires that you design the motor so when operating at its rated speed, it is also operating at its peak efficiency point.

Since vehicles have to operate at a range of different speeds, optimum design dictates a variable gearset or transmission to both extract peak power from the motor and also achieve peak efficiency.

Answer 4

Motors for electric bikes may be brushed, or they may be brushless DC motors (BLDC motors). The industry default has become brushless motors, because generally they are quieter, smaller, and lighter, and they don't need to serviced.

Basically, what happens is thick copper coils of wire electric power from the battery into the movement that pushes you along. Instead of having one motor powering all the wheels using gears or chains, they build a motor directly into the hub of each wheel—so the motors and wheels are one and the same thing.

Now due to the availability of high torque over wide range of motor speed directly coupling the motor with the motor is not a great idea. Thus, generally the electric motor is always connected to the drive wheel through fixed gear reduction ratio. for this purpose, we generally use a combination of spur gears or helical gears in order to incorporate the advantages of regenerative breaking (charging of battery during breaking or when no throttle is applied / dynamo) which is an essential requirement for all electrical vehicle.

for the other part of your question

From basic physics, power is defined as rate of energy consumption. In electrical engineering load is defined as power but also at particular voltage & frequency. You often find the heaters are rated as 1.2 kW at 250 Volt that means it will consume 1200 watt if supplied from 250 volt, and if the voltage is less than 250 volts then its power won’t be 1200 watt. similarly, motor is rated to give full load output when connected the source of specified voltage and frequency.

Answer 5

Electric motors generate about same torque at almost all rpms. This means the absolute power they deliver declines together with rpm. Fast riding drains the battery much faster than laborious uphill where the engine simply helps less, leaving for the rider to enjoy pedaling.

In cases like steep uphill it may not be enough power to move up at high speed with more rpm, but also not enough power to move at slow speed because there is less power available at low rpm. Hence you may need to shift the gear for moving steep uphill.

Answer 6

As we know people day by day started preferring automatic transmission instead of manual. in case of electric bike we can use CVT for automatic transmission as transmission is required to separate engine and wheel for eg if we directly connect engine to the wheels then we have to face many problems such as speed control, when engine starts, wheel also start moving respectively. therefore we need a transmission or we can say it gear. but normal gear box couldn't work here and in case of electric bike it consumes more power that makes our bike less efficient so we have to use a medium to separate engine a wheel which consumes less power and while we accelerate then it will engage with engine and wheel simultaneously. so basically CVT are best type of transmission system available in market and try to put your engine near wheels for maximum efficiency and less power consumption. for more details you can check the concepts of Tesla model s sports car.