BYGEN

Rated RPM vs. Efficiency

The farther divergence from rated RPM range to show the highest efficient, the lower energy efficiency of electric motor

ICEs use multi-speed transmission to run engine within rated RPM range (around 2,000 rpm) at any speed, which minimize fuel consumption.

Motor is over 90% efficient, while ICE is below 30% efficient. However, torque of motor over rated RPM range drastically drops, which results in radical decrease of energy efficiency. (refer to motor efficiency graph below)

EV to have wider speed range has wider field weakening range. High speed vehicle has greater potential to improve driving efficiency to adopt multi-speed transmission.

Electric motors have played a great roll in all industrial fiends sine invention in 1800s. They are designed to operate within rated output including rated torque and rated RPM to save electric energy and protect motor. That results in no overheating of motor which can be touched by hand.

To run motor within rated RPM range, difference between gear ratios with less than 40% for transmission would be preferred. (see graph below)

Because torque interruption of motor is much stronger than ICE due to strong initial torque, difference of gear ratio should be lower than 40%.

Energy Efficiency of Motor

The farther divergence from rated RPM range to show the highest efficient, the lower energy efficiency of electric motor.

Torque / RPM of 5-speed Auto Transmission

Total Gear Ratio : 400%

Gear		1st gear	2nd gear	3rd gear	4th gear	5th gear
Ratio	In-Wheel	1.00:1	1.46:1	2.07:1	2.87:1	4.00:1
Ratio	Axle	0.50:1	0.70:1	1.00:1	1.43:1	2.00:1

Tiny 5-speed Auto Transmission – Efficiency By Each Speed

● If we have transmission with more than 5-speed for EV, we cam make motor run at the rated RPM range where motor        efficiency is the highest.
   • Driving at the rated output and rated RPM produces no thermal waste.

No need to use field weakening control (No use of expensive inverter)

No use of water cooling system
• Max output, which is double of rated output, is used only for drastic acceleration within less than 10 seconds. (“temporary rating”).

Reasons for radical decrease of EV2W efficiency at higher constant speed

● Driving efficiency at flat road heavily depends on the resistance forces. Total resistance in auto drive with constant speed consists of rolling resistance which is proportional to vehicle’s mass, and air resistance which is proportional to square of velocity. Acceleration (Inertial) resistance and grade resistance does not depend on a level road and at constant speed.(TR=RR+AR, Frr α Mg, Far α V2)

● Efficiency of 2-wheel vehicle decreases faster along with the increase velocity, because air resistance increases proportionally in spite of relatively small rolling resistance for its lighter mass.

With higher air resistance coefficient of 0.7 compared to 0.3 of 4-wheel vehicle, the increase in velocity of motorcycle consequently results in sharp decrease of efficiency. (This is naturally applied to conventional two wheeled vehicles.)

※ In a heavier vehicle, rolling resistance greatly affects to driving range than air resistance. In a passenger car, the rolling resistance is double of air resistance at velocity of 80km/h , which results in low decrease of efficiency.

Drive efficiency of EV4W would not apparently decrease at higher speed range over 100km/h with in-wheel transmission

Why not conventional transmission?

● Large and heavy transmission with hydraulics and liquid clutch is expensive and inefficient, which could not be directly placed in axles.
Heavy weight and bulky size because of hydraulics and fluid clutch make it impossible to directly connect to chassis axle.

● Heavy hydraulics cannot increase battery-to-wheel efficiency.

● Heavy, inefficient, and expensive transmission cannot reduce EV manufacturing cost.

Test Data