A challenge for the direct-drive configuration which has been very much the norm in EVs since they became serious contenders during the past few decades, has been the ability to achieve a smooth launch from a stationary position without sacrificing efficiency at higher speeds such as when driving on the highway.
Regardless of the motive power, whether ICE (internal combustion engines) or electric, a heavy vehicle requires a high level of torque at the drive axle when launching from a standstill, or climbing a grade, and even more torque when launching on a grade.
In an electric vehicle the rpm will essentially be zero at launch and the main need is for torque.
In the opposite situation when cruising at the road’s speed limit, the torque requirement is considerably less, but the rpm is much higher, and the need is for speed.
The inclusion of a transmission such as the Heavy Duty EV from Eaton goes a long way towards addressing these situations. The key purpose of the transmission is to best match the available power to the driving situation.
So, at launch, the need is for lots of torque but at a low speed, and a low gear ratio will accommodate that, and at road speeds, much less torque is required, but more rpm, and the direct 1:1 ratio top gear achieves this.
The Eaton Heavy Duty EV transmission enables a smooth launch on grades of up to 30 per cent, compared with the approximately 10 per cent grade capability of direct drive electric motors.
On grades around three per cent the motor/transmission can hold steady at 95km/h (60mph). With maximum input speeds of 5,000rpm and torque capacities of up to 2,600Nm (1,900 lb/ft) the Eaton Heavy Duty EV transmission can be tailored for a range of electric commercial vehicle applications, motor pairings, and custom shift calibrations.
The standard rating is for up to 43 tonnes GCM, but higher ratings are possible, subject to Eaton application approval. The HD EV casing is light-weight aluminium and the complete transmission weighs 196kg.
Eaton’s Heavy Duty EV transmissions are based on the proven, robust and efficient twin-counter shaft architecture used in the ubiquitous Eaton RoadRanger gearboxes and typical of modern automated manual transmissions such as the Eaton Endurant. Shifting is managed by an integrated electric actuation system and an electronic Transmission Control Unit.
Eaton produces EV specific medium-duty transmissions with four- and six-speeds, and a four-speed for Heavy Duty applications which also incorporates provision for a rear mount PTO.
The reason for a four-speed and not a two, or even an 18-speed, is in line with the ideal rpm range of the electric motor being used.
A typical heavy duty diesel engine might operate best between 1,200rpm and 1,600rpm. Below 1,200rpm, available power will fall, and above 1,600rpm, fuel efficiency will decrease.
Therefore, the working range of a diesel is about 400rpm and to adapt that working range to a commercial vehicle might require 12 or more gears to provide the right balance of torque and speed in all road conditions.
Contrast that to an electric motor which typically can operate between 1,000rpm and 3,000rpm with high efficiency and having 2,000rpm of effective working range means four gears are generally enough to match the wider working range to all road conditions.
A two-speed transmission might limit driveline torque too much, and limiting the operating grade (failure to launch on a steep hill) or it might limit the maximum road speed.
Having two gears spaced a long way apart would create a big gap which will mean a very large step in available torque or speed and that might mean it is not possible to find the right balance between torque and speed for some load and road conditions.
The ratio increments of the four-speed are attuned to match the typical torque curve of the electric motor and the ratios are optimised to deliver maximum gradeability, acceleration and smoothness.
When accelerating, the motor’s torque falls as motor rpm rises and upon an up change, the transmission torque multiplication falls, but the motor rpm is reduced after the gear change, so motor torque rises again.
The reduced torque multiplication of the transmission is almost perfectly matched to the increased torque delivered by the motor, which will make the gear change feel almost seamless to the driver.
The effective amount of torque available drops progressively as the vehicle speed builds which provides a very smooth sensation of acceleration and the upshifts are almost imperceptible.
The transmission’s electronics also control the down shifting strategy to deliver optimum regeneration to recharge the battery and also deliver a better engine braking effect due to the matching of the motor speed with road speed.
Leveraging decades of experience designing and building industry- leading commercial vehicle transmissions, combined with extensive electrical system knowledge, Eaton’s portfolio of purpose-built EV transmissions offers efficient and flexible solutions to improve performance for medium- and heavy-duty electric trucks and buses.
