Test Data

Seoul National University of Science & Technology (Eco Automotive Technology Institute)

Test Data
RPM vs. Efficiency
Main Drivers for Increase in Efficiency
Single Speed EV Efficiency Analysis
EV Cost Reduction for Auto Makers
RPM vs. Efficiency
Energy efficiency decreases significantly as the motor is not run within the range of rated RPM
Motor has replaced internal combustion engine (ICE) in overall industry, for its energy efficiency is over 90%, which is one fourth of that of ICE. Average efficiency of ICE is below 30%.

Especially, the efficiency drastically drops when the motor is run in excess of RPM range over rated RPM, which is not recommended due to possible serious damage and over-heating of motor.

※ Multi-speed transmission of ICE automotive also keeps engine to operate within rated RPM range (around 2,000 RPM for a medium-sized car), adjusted to speed and road condition, which enables to reduce fuel consumption and to drive smoothly.

※ As seen in Fig. 2, 5-speed Transmission , shifting ratio by gear of which is under 40%, will make motor run within rated RPM at any load condition. This contributes to the most efficient and smooth driving to electric vehicles
Efficiency Graph of PMDC Motor
As showed in above graph, motor efficiency radically decreases as further out of range of rated RPM, which is worse than ICE’s.
Torque-RPM Graph with Gears
5-speed Auto Transmission with total gear ratio of 352% is under development
Why does EV2W efficiency decrease for higher constant speed driving?
● Driving efficiency heavily depends on the resistance forces. Total resistance in auto drive with constant speed consists of rolling resistance, which is proportional to vehicle 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 of its lighter mass.
With higher air resistance coefficient of 0.2 compared to 0.3 of 4-wheel vehicle, the increase in velocity of motorcycle sensitively drops efficiency. In a passenger car, the rolling resistance is double of air resistance until velocity of 80km/h , which results in low decrease of efficiency.

Drive efficiency of passenger car with higher speed does not apparently decrease with in-wheel transmission
How and why can EVs move without transmission?
● Because initial (stall) torque of motor is enough to move vehicle without transmission.
At the constant torque range , torque is the highest. Torque is proportional to current, which means strong current is supplied to motor at starting point of motor.

● In higher speed range, relatively strong current is supplied to keep enough torque to move.

● The development in AC and DC drives is mainly because of development in power electronics, modern day algorithms (like scalar control, field oriented (FO) control, Direct Torque Control (DTC), V/f control, and vector control) and fast controllers to implement them. Now it is possible to extend and modify performance characteristics of both the AC and DC motors.

● Gear shifting of multi-speed transmission at each speed could provide, not electrically, but mechanically, enough torque to move smoothly with relatively weak current, which results in higher efficiency than single speed.
The Faster, the More Efficient