Electric Vehicle Conversion Transmission Analysis and Selection

Electric vehicle transmission selection

Overview of all transmissions for EV conversions

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 Do Electric Cars Have Transmissions?

One of the most asked question when talking about an electric car conversion, especially when the topic of diy ev drivetrain is brought up.  In short, an electric car transmission is required to increase the torque at the wheel.  This applies most to an EV car whether it is through an electric conversion professional shop or if you chose to make your own car.

Transmission Purpose

Simply defined, a transmission transfers power from the engine to the wheels through the driveshaft and differential.  This is obtained by using gears to adjust the gear ratios in each desired gear selection through the transmission.  A vehicle will have a either manual or automatic transmission, which includes continuous variable transmissions since the driver does not have to physically change the gear selection while driving.  After the transmission there is one last gear ratio in the differential, which is independent of the transmission gear selection.  The overall gear ratio that affects the wheels is a combination of the selected gear ratio multiplied by the differential, or final gear ratio.

The number of selectable gears in any transmission will depend on the original equipment manufacturer (OEM) for that vehicle model year.  Typically the latest vehicles will have a 5 or 6 speed transmission no matter if they are automatic or manual transmissions.

Engine / Motor Speed and Torque

An internal combustion engine produces a decent amount of power from the ignition of fuel but its greatest asset is the ability to generate high revolutions per minute (RPM).  The gearing of the transmission and differential together increases the torque transferred to the wheel by reducing the rotation speed and increasing the torque.  This is the nature of gear ratios, to produce more torque the rotation speed will decrease.

You may have heard that an electric motor produces more torque at a lower rotation speed then an internal combustion engine (ICE).  Even the torque produced by an electric motor needs to be increased with gears in some form of a transmission or, at minimum, a fixed gear ratio like a differential.

Whether it is an internal combustion engine or an electric motor driving the vehicle, the selection of gear ratios can change the vehicle characteristics drastically.  Typically ads for new vehicle will include engine output like 250 horsepower and 200 foot-pounds of torque each at a certain RPM.  The issue is that the engine specifications only is not all the story, it is just the easiest to understand and compare from all the information on the brochures.  Reading transmission gear ratios are not the easiest to digest instantly on the sales floor.  This is one of the reasons why you should always take a car for a test drive, whether you are looking for fast acceleration, fuel efficiency, towing, or any variation of the three.  For the biggest difference, test drive a truck like a Chevy Silverado 1500 and a Ford Mustang.  They both have comparable engine output horsepower and torque but behave differently, and this not just from the added vehicle weight of the truck.  The difference is in the transmission gear ratios and axle ratio, which changes the overall gear ratio generated at the wheels.

Vehicle Gearing Example

In order to properly identify how the vehicle will operate after converting to an electric motor, we first need to determine how the vehicle operates before the conversion.  To get all the possible information each available transmission will be included in the baseline analysis.  First all the variables need to be defined before diving into the analysis for each transmission.

For this example of a vehicle transmission and differential gear ratios I am using a 2014 Toyota Corolla model with L and LE badges which is offered with either a manual, automatic, or continuous variable transmission.  The identified specifications for this vehicle include engine displacement, horsepower, torque, transmission specific gear ratios, differential ratio and curb weights.

L / LE Models

  • Engine Displacement 1.8 Litre
  • Horsepower 132 hp @6000 rpm
  • Torque 128 lb-ft @4400 rpm

6-Speed Manual Gear Ratios:

  • 1st 3.538 : 1
  • 2nd 1.913 : 1
  • 3rd 1.310 : 1
  • 4th 0.971 : 1
  • 5th 0.714 : 1
  • 6th 0.619 : 1
  • Differential Ratio 4.214 : 1
  • Curb Weight 2,800 lbs.

4-Speed Automatic Gear Ratios:

  • 1st 2.847 : 1
  • 2nd 1.552 : 1
  • 3rd 1.000 : 1
  • 4th 0.700 : 1
  • Differential Ratio 4.130 : 1
  • Curb Weight 2,820 lbs.

CVTi-S

  • 2.48 : 1 to 0.396 : 1
  • Differential Ratio 4.761 : 1
  • Curb Weight 2,855 lbs.
2014 Toyota Corolla Product Specifications

Source PDF: Toyota Corolla Specifications

 

Vehicle Tire Size:

For the vehicle selected for this gearing example, 2014 Toyota Corolla models L / LE, the tire sizes are shown below.  The reason to include the tire specifications is to determine the overall rolling circumference which would be required to find the vehicle speed at any given gear and engine speed.

P205/55 R16

  • Sidewall Aspect Ratio 55% of section width, (205mm / 25.4) x 55% = 4.44”
  • Tire Diameter, 16” + (2 x 4.99”) = 24.88”
  • Tire Circumference, pi x diameter = 3.14159 x D = 78.15”
Tire Size Guide

(www.tirerack.com)

(www.tyresizecalculator.com)

Corrected Toyota Corolla HP and Torque:

I have extrapolated the torque value at 3000 revolutions per minute using the engine horsepower and torque curve. The purpose of using a lower engine speed is to provide a realistic example of a highway driving scenario.

Horsepower

  • 64 hp @3000 RPM
  • 48 kW @3000 RPM

Torque

  • 117 ft-lbs @3000 RPM
  • 158 Nm @3000 RPM

 

2008 Toyota Corolla Engine Horsepower & Torque Curve

Based on 1.8 1ZZ-FE Engine Specifications

Toyota Altis/Corolla Torque Chart

Image Source: Paultan.org

 

There will be two speeds tested to take into account city and highway driving. For the slower city speed, 50 kph or 30 mph will be used, and for the faster multi-lane highway speed 105 kph or 65 mph will be used. These speed limits listed are common in most North American cities and highways.

 

Toyota Corolla Gasoline Engine Wheel Torque and Engine Speed Baseline

Tire Circumference = 78.15” per tire revolution

Starting with vehicle speed in the city 50 kph or 30 mph.

  • Required Tire Rotation = (50000 metres / 1 hour) x (1 hour / 60 minutes) x (39.3701 inches / 1 metre) x (1 revolution / 78.15 inch travelled) = 420 revolution per minute
  • Manual Transmission 2nd Gear Reduction = (1.913 : 1) x (4.214 : 1) = 8.061 : 1
  • Engine Rotation Speed = 420 RPM x 8.061 : 1 = 3385 RPM
  • Effective Torque at Tires = 117 ft-lbs @ 3385 rpm x 8.061 : 1 = 943 ft-lbs

 

  • Automatic Transmission 2nd Gear Reduction = (1.552 : 1) x (4.130 : 1) = 6.410 : 1
  • Engine Rotation Speed = 420 RPM x 6.410 : 1 = 2692 RPM
  • Effective Torque at Tires = 117 ft-lbs @ 2692 rpm x 6.410 : 1 = 750 ft-lbs

 

Now with vehicle speed on the highway 105 kph or 65 mph.

  • Required Tire Rotation = (105000 metres / 1 hour) x (1 hour / 60 minutes) x (39.3701 inches / 1 metre) x (1 revolution / 78.15 inch travelled) =  882 revolution per minute
  • Manual Transmission 6th Gear Reduction = (0.971 : 1) x (4.214 : 1) = 4.091 : 1
  • Engine Rotation Speed = 882 RPM x 4.091 : 1 = 3609 RPM
  • Effective Torque at Tires = 117 ft-lbs @ 3609 rpm x 4.091 : 1 = 479 ft-lbs

 

  • Automatic Transmission 4th Gear Reduction = (0.700 : 1) x (4.130 : 1) = 2.891 : 1
  • Engine Rotation Speed = 882 RPM x 2.891 : 1 = 2549 RPM
  • Effective Torque at Tires = 117 ft-lbs @ 2549 rpm x 2.891 : 1 = 338 ft-lbs

 

Transmission Analysis for Electric Vehicle Conversion

The purpose of this analysis is to determine which transmission type would be ideal for your electric vehicle conversion.  Each transmission type will be evaluated below to determine the equivalent motor speed based on the variables indicated above in the baseline information.  From the motor speed results, the AC50 electric motor torque can be estimated from the motor torque chart.  The torque of the wheel at the evaluated vehicle speed can be determined and measured against the original vehicle configuration.

For each scenario the transmission gear for city will be assumed to be 2nd gear and the gear for highway will be the highest gear, which is set to the highest ratio for the CVT.

 

AC50 Electric Motor Torque Curve

Source: HPEVS (High Performance Electric Vehicle Systems)

Source PDF: AC-50 72 Volt

 

5 Types of Transmissions for Electric Vehicle Conversions

1. Direct Drive

Direct Drive Overview

As the name implies, direct drive does not have a power transfer component.  Direct drive setup is connected directly to the driveshaft.  There is still the differential that adjusts the gear ratio of the motor.

The analysis below assumes that the vehicle was a manual transmission and will use that differential ratio.

Effect of EV motor AC-50 at 72 Volts

Vehicle speed in the city 50 kph or 30 mph.
  • Required Tire Rotation = 420 revolution per minute
  • Differential Gear Reduction = 4.214 : 1
  • Motor Rotation Speed = 420 RPM x 4.214 : 1 = 1770 RPM
  • Estimated Effective Torque at Tires = 92 ft-lbs @ 1770 rpm x 4.214 : 1 = 388 ft-lbs

 

Vehicle speed on the highway 105 kph or 65 mph.
  • Required Tire Rotation = 882 revolution per minute
  • Differential Gear Reduction = 4.214 : 1
  • Motor Rotation Speed = 882 RPM x 4.214 : 1 = 3717 RPM
  • Estimated Effective Torque at Tires = 70 ft-lbs @ 3717 rpm x 4.214 : 1 = 295 ft-lbs

2. Transfer Case

Transfer Case Overview

The transfer case is a form of transmission in that it modifies the speed to torque ratio from the motor, similar to the differential.  The transfer case has only one gear ratio though compared to a transmission, making it a compromise between direct drive and a transmission.

The analysis below assumes that the vehicle was a manual transmission and will use the appropriate differential ratio.  Also, for the purpose of analysis the transfer case gear ratio of 1.5 : 1 is used.

Effect of EV motor AC-50 at 72 Volts

Vehicle speed in the city 50 kph or 30 mph.
  • Required Tire Rotation = 420 revolution per minute
  • Transfer Case Gear Reduction = (1.5 : 1) x (4.214 : 1) = 6.321: 1
  • Motor Rotation Speed = 420 RPM x 6.321 : 1 = 2655 RPM
  • Estimated Effective Torque at Tires = 92 ft-lbs @ 2655 rpm x 6.321 : 1 = 582 ft-lbs

 

Vehicle speed on the highway 105 kph or 65 mph.
  • Required Tire Rotation = 882 revolution per minute
  • Transfer Case Gear Reduction = (1.5 : 1) x (4.214 : 1) = 6.321 : 1
  • Motor Rotation Speed = 882 RPM x 6.321 : 1 = 5575 RPM
  • Estimated Effective Torque at Tires = 30 ft-lbs @ 5575 rpm x 6.321 : 1 = 190 ft-lbs

 

3. Manual Transmission

Manual Transmission Overview

For the manual transmission this retains the original transmission, clutch and clutch pedal.  This would need an adapter plate to bridge the connection between an electric motor and bell housing of the transmission.

Effect of EV motor AC-50 at 72 Volts

Vehicle speed in the city 50 kph or 30 mph.
  • Required Tire Rotation = 420 revolution per minute
  • Manual Transmission 2nd Gear Reduction = (1.913 : 1) x (4.214 : 1) = 8.061 : 1
  • Motor Rotation Speed = 420 RPM x 8.061 : 1= 3386 RPM
  • Estimated Effective Torque at Tires = 85 ft-lbs @ 3386 rpm x 8.061 : 1 = 685 ft-lbs

 

Vehicle speed on the highway 105 kph or 65 mph.
  • Required Tire Rotation = 882 revolution per minute
  • Manual Transmission 6th Gear Reduction = (0.971 : 1) x (4.214 : 1) = 4.091 : 1
  • Motor Rotation Speed = 882 RPM x 4.091 : 1 = 3608 RPM
  • Estimated Effective Torque at Tires = 73 ft-lbs @ 3608 rpm x 4.091 : 1 = 299 ft-lbs

4. Automatic Transmission

Automatic Transmission Overview

For the manual transmission this retains the original transmission, and torque converter.  Just like the manual transmission, an adapter plate is required to connect an electric motor and bell housing of the transmission.

Effect of EV motor AC-50 at 72 Volts

Vehicle speed in the city 50 kph or 30 mph.
  • Required Tire Rotation = 420 revolution per minute
  • Automatic Transmission 2nd Gear Reduction = (1.552 : 1) x (4.130 : 1) = 6.410 : 1
  • Motor Rotation Speed = 420 RPM x 6.410 : 1 = 2692 RPM
  • Estimated Effective Torque at Tires = 92 ft-lbs @ 2692 rpm x 6.410 : 1 = 590 ft-lbs

 

Vehicle speed on the highway 105 kph or 65 mph.
  • Required Tire Rotation = 882 revolution per minute
  • Automatic Transmission 4th Gear Reduction = (0.700 : 1) x (4.130 : 1) = 2.891 : 1
  • Motor Rotation Speed = 882 RPM x 2.891 : 1 = 2550 RPM
  • Estimated Effective Torque at Tires = 92 ft-lbs @ 2550 rpm x 2.891 : 1 = 266 ft-lbs

 

5. Continuous Variable Transmission (CVT)

CVT Overview

For the continuous variable transmission this retains the original transmission.  An adapter plate will be required to connect an electric motor to the transmission.  As stated earlier, for this analysis the gear ratios used in the CVT will be the same as the automatic transmission for city speeds and use the highest available gear ratio for highway speeds.  The exact ratio used while driving a CVT will depend on the design characteristics from the original equipment manufacturer.

Effect of EV motor AC-50 at 72 Volts

Vehicle speed in the city 50 kph or 30 mph.
  • Required Tire Rotation = 420 revolution per minute
  • CVT Equivalent 2nd Gear Reduction = (1.552 : 1) x (4.130 : 1) = 6.410 : 1
  • Motor Rotation Speed = 420 RPM x 6.410 : 1 = 2692 RPM
  • Estimated Effective Torque at Tires = 92 ft-lbs @ 2692 rpm x 6.410 : 1 = 590 ft-lbs

 

Vehicle speed on the highway 105 kph or 65 mph.
  • Required Tire Rotation = 882 revolution per minute
  • CVT High Gear Reduction = (0.396 : 1) x (4.130 : 1) = 1.635 : 1
  • Motor Rotation Speed = 882 RPM x 1.635 : 1 = 1442 RPM
  • Estimated Effective Torque at Tires = 93 ft-lbs @ 1442 rpm x 1.635 : 1 =  152 ft-lbs

 

 

I hope this has provided more information on the available transmissions for electric vehicle conversions.  Let me know in the comments section if there is more information that I might have missed.  Also, if there is another vehicle scenario that you would like analyzed, leave the request in the comments section or send me an email through the contact page.

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