Overcoming EV Drivetrain Testing Challenges

March 19, 2024

Randal Beattie

Transportation markets have seen a tremendous surge of interest in hybrid and electric vehicle technology. To gain the promised efficiency benefits and green profile of these vehicles, it is important to conduct driveline and component testing during design and manufacturing that is specially adapted to hybrids and EVs.

Hybrid and electric drivetrains have several features that make testing them very different from the standard testing conducted on internal combustion (IC) only systems. Hybrid and electric systems use regenerative braking, which typically requires the addition of AC inverter technology and often more complex transmissions.

These vehicles also typically have several module control units (MCUs), essentially small onboard computers, that control the functions of major subsystems such as the engine, transmission and charging system. To properly test these components, the test system needs to be able to communicate with one or more of these units via high-speed in-vehicle networks. The technology exists to ensure proper testing and realization of the energy-efficiency benefits promised by hybrids and EVs.

Types of hybrid/EV driveline testing

Hybrid or electric driveline testing is conducted at several stages during the development of a vehicle, and each has an important role to play.

Engineering testing – Accurate measurements are critical so design engineers can extract every bit of efficiency from their designs. Most vehicles use three-phase AC motors driven by inverter technology, so sophisticated power analyzers are needed to properly measure three-phase AC power with a large amount of harmonic content. These test systems tend to be rather complex and are usually the most sophisticated, with many elements to be tested and coordinated.

In-process and end-of-line testing – Manufacturing end-of-line testing is usually performed to verify that no defects were introduced in the manufacturing process and that the components will perform to specifications. Typical tests include operational validation, quick performance testing, as well as rigorous testing to validate that high-voltage electrical systems are properly isolated and are therefore safe to use in vehicles.

In-process testing also may be conducted to test partial assemblies along the production line. This improves manufacturing efficiency and significantly reduces the chance that faulty components will find their way into the finished product.

Quality control testing – Quality control (QC) testing is usually done on a percentage of the components to verify that they perform over the specified range and are relatively free of defects. For example, a forklift company may conduct QC testing on a shipment of imported electric motors. The company would use QC testing to verify that the shipment coming from its supplier performs as specified and will not experience high failure rates in the field. This type of test system is typically less complex because it does not have to measure as many items, nor to the degree of accuracy, as those tested in engineering systems.

Electrical system testing

Traditional ICE testing typically measures speed, torque and a few temperatures, pressures and flows. Very precise control of speed and torque is typically not required in testing IC engines, so dynamometers used for standard combustion-engine testing (for example, water brake and eddy current) were never designed to handle the types of precision required by hybrid or electric powertrains. Nor can they test the regenerative (motoring) modes of operation.

Hybrid/EV test systems must provide all the functionality of traditional systems, with the added ability to test high-power regenerative electrical drives, high-voltage battery and charging systems and communicate with any number of smart control modules.

For many larger hybrid/electric drivetrains, there is a strong trend toward using higher voltage, higher efficiency drive systems. Going from the traditional 12/24-volt DC electric system to one using 240 volts AC will typically require one-eighth or less of the current to deliver the same power. Not only is this more efficient, but it also requires much smaller/lighter wiring and smaller components to transfer the energy. Many current designs operate at 800 volts or more, making the vehicles even more efficient.

Hybrid and electric vehicles use four-quadrant motor/inverter technology, meaning that the electric motor can control velocity or torque in either direction – the motor can accelerate, run and decelerate forward or backward. A four-quadrant motoring dynamometer with the ability to drive or load in either direction is essential to simulate/test all modes of operation in a hybrid or EV. A standard dynamometer is not capable of testing the system during braking when it is in regenerative mode.

Creation of high-efficiency, AC-powered systems typically involves the use of three-phase, inverter-based technology to precisely control the electric motor(s) in the system. These systems tend to be very efficient but also generate a great deal of harmonic distortion in the power output. So, in addition to the motoring dynamometer, a hybrid/EV test system typically includes a three-phase power analyzer. This unit must be specifically designed to measure high-power electrical values with a great deal of harmonic distortion present.

To meet the need for a system that can fully test hybrid and EV drive systems, SAKOR developed HybriDyne, a comprehensive test system for determining the performance, efficiency and durability of all aspects of hybrid drivetrain systems, including electrical assist (parallel hybrid), diesel electric (serial hybrid) and full-electric vehicle systems.

HybriDyne integrates components of SAKOR’s DynoLab powertrain and electric motor data acquisition and control systems. Coupled with one or more of its AccuDyne AC Motoring Dynamometers, and one or more precision power analyzers, the modular HybriDyne can test individual mechanical and/or electrical components, integrated sub-assemblies and complete drivetrains with a single system.

High-voltage battery simulation and testing

A critical element of hybrid or electric vehicles is the high-voltage battery and charging system. To accurately test a high-voltage hybrid or electric drivetrain, the test system must provide precise, repeatable high-voltage DC power. Since battery performance changes over time depending upon their charge state, ambient conditions and age, they are typically not acceptable for powering the DC components of a hybrid/EV test system. A standard off-the-shelf power supply will not work, because it cannot absorb power from the regenerative system and may be damaged or destroyed.

SAKOR developed a Solid-State Battery Simulator/Test System specifically to test high-voltage hybrid vehicle batteries and simulate these batteries in an electric drivetrain environment. At the heart of the system lies a high-efficiency, line-regenerative DC power source. During regenerative modes, absorbed power is regenerated back to the AC mains instead of being dissipated as waste heat, which is common practice among previous-generation testing systems. This method provides much greater power efficiency and measurably reduces overall operating costs.

Coupled with DynoLab, the Solid-State Battery Simulator/Tester accurately simulates the response of the high-voltage battery in real-world conditions. However, since it is not subject to a variable charge state, it provides repeatable results. This same unit, when operated as a battery tester, subjects the battery to the same charge/discharge profile as it would encounter in an actual vehicle on an actual road course.

One of the advantages of using the AC dynamometer with a regenerative DC power source is that when the two are coupled together, the power absorbed by one unit can be re-circulated back to the other unit within the test system. This greatly reduces the power drawn from the AC mains – by as much as 85% to 90% – and therefore significantly reduces total cost of operation. Low maintenance requirements also contribute to lowering operating costs.

Communication with control modules

Communication with individual MCUs is another feature that must be built into testing systems for hybrid or electric vehicles. In the past, the engine was primarily controlled using the throttle and ignition. Now, engines have an engine control unit (ECU), the vehicle will likely have a separate MCU that controls the electric drive and may have separate units for controlling the transmission and/or charging systems.

These units typically communicate commands and/or data between themselves via high-speed vehicle networks, such as CAN, LIN, FlexRay, etc. To properly test this complex drivetrain configuration, the test system must be able to communicate with these control units simultaneously and efficiently.

There is great excitement in the automotive, heavy equipment, military and aerospace industries over the promise of improved environmental performance of hybrid and electric vehicles. To achieve that promise, driveline testing programs must be adopted that meet the needs of these technologies.

Randal Beattie, President at SAKOR Technologies Inc., wrote this article and submitted it to SAE Media.

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