The advent of electric vehicles (EVs) and the steady increase in global acceptance of hybrid electric vehicles (HEVs), are together introducing new dimensions to automotive design and engineering targets. Electric motive power must achieve ever tougher standards of required and expected end-user demands. Among the many salient issues it continues to raise are weight and size of battery packs, performance on inclines, and the crucial concern of achieving realistic and dependable driving range. Safety is also a mainstream subject.
And there is the added matter of convincing buyers of both EVs and HEVs of their through-life durability and reliability. Through all this runs the absolute necessity to provide realistic value for money, both initially and residually.
So, in parallel with the automotive industry’s development of solutions to these challenges, the need to test and prove them is vital.
It is a tough task, as Alastair Wynn, Senior Engineer for Durability at the U.K. Millbrook Proving Ground, experiences daily as he deals with disciplines specific to EVs and HEVs alongside regular internal combustion engine-powered vehicles and the areas where their technologies cross-link: “The common goal of durability testing is to simulate the real world usage of a vehicle in an accelerated manner and to identify reliability, durability, and, particularly for EVs, interface issues so that they can be addressed.”
Test requirements for EVs and HEVs are very different, but the way a test is designed is the same, he explains, saying that it is imperative that all eventualities that will be experienced in service, are covered without conducting unnecessary tests that an OEM or supplier does not need.
Deciding just what is or is not needed is extremely important. So it is essential to establish the criteria of envisaged usage by the end-user of a particular vehicle and to then test accordingly: “For example, out of 100 customers of a C-segment (compact) vehicle, only two may see vertical loads that are considered excessive. As those loads may only be seen twice during three years of data, is it necessary to test to the needs of those two customers? If so, the vehicle could be at risk of being ‘over engineered’ for the 98 people of that 100 who will never see that event!”
When durability testing, some subsystems are plainly more critical than others. These include brakes and steering, but specific testing should be conducted independently of the whole vehicle test, states Wynn and safety systems, including ABS, TCS and stability control should be exercised periodically during the test to ensure functionality.
“Accelerating real-world usage can be challenging. If there are a number of events, these need to be sequenced without compromising test integrity. For example, it may be necessary to conduct a number of pothole impacts and complete a given distance of pavé. Conducting the pothole maneuver after the shock absorbers have risen in temperature on a paved surface could give unrealistic loads, due to reduced shock absorber performance.”
Durability testing must always be aligned with the intended in-service use of the vehicle. This alignment covers areas such as the intended market for the vehicle, what type of vehicle it is, the body style, including car compared with van derivative, van, minibus or chassis. A single vehicle platform could carry many body styles depending on its intended use. All of these factors will determine the validation regime required, and bespoke durability testing programs developed.
Wynn adds: “Areas for consideration when testing HEVs and EVs that could be different to ICEs, include driving style, average trip distance, speed profile, standard and additional features of the vehicle, the target customer, and risk assessment. Whilst many of these appear to be the same as ICE vehicles on first consideration, there are differences which need to be accommodated.”
One is average trip distance, generally short in the case of pure EVs, the great majority of which at present are only practical for limited mileage, urban, stop-start travel. The second involves risk assessment; special attention must be paid to the risk of fire and the potential danger presented to rescue services of EV batteries.
Due to the nature of EVs and HEVs, the most significant differences are in the usage of electrical system performance, battery performance and engine performance.
The latter two will change depending on vehicle usage. Both battery state of charge (SOC), and the systems utilized in an HEV, has a direct impact. Battery performance can also be affected by temperature changes and different charging strategies.
For an HEV, any use of ancillary systems, such as its air conditioning or lighting, affect engine running time and driving style can have a thermal impact on the motors or power systems.
Constant high load running such as long ascents with throttle wide open can put both of the HEV’s motive power systems under maximum loading, as can descents on regeneration systems, states Wynn: “Pure EVs will see similar issues to that of an HEV, except that the usage of ancillaries will have a dramatic effect on the SOC and ultimately the performance and range of the vehicle. High torque demand can put the batteries and motors under a lot of load, generally through lower speed, high load driving. This can impact on the operating temperatures and overall range. Stop-start driving will have an impact on SOC and range, especially if ancillary equipment is being used at the same time.”
Considerable damage can occur during stop-start non-run time: “With the engine not running there is no pressure in the lubrication system, which can lead to damage of some components, particularly bearings, where a condition known as brinelling occurs. Although the engine is not running, the components are still subjected to loads and vibrations as a result of low temperature operation when most wear occurs.
"The engine oil is not the only system normally hot and pressurized on an ICE vehicle; the cooling system operates under these conditions. An HEV may also have systems operating under similar conditions where the temperature profile is critical to avoid low temperature operation or overheating. This may not only apply to engines but also to their systems, notably electric motor cooling.”
An important aspect of durability testing is sub-system integration, with all functioning together as a single unit. Says Wynn: “Prior to this testing, sign off will have been achieved for the components and individual sub-systems. Therefore, it is important that integration of all sub-systems within the vehicle is subjected to the full load spectrum of all input types to ensure continued good connectivity and interface performance.
“The inspection process should include critical inspection of the batteries, battery carrier and wiring, in addition to the normal criteria.”
To conduct a correlated test in an accelerated manner, the less damaging loads giving rise to fatigue damage and wear are significantly reduced. However, the interface between the systems on an EV or HEV may be more susceptible and should be scrutinized more closely, warns Wynn.
While today’s whole vehicle corrosion protection is now generally exemplary, inspections need to include the extra electrical systems that are necessary for EVs and HEVs.
“Common concerns with electrical systems as a result of corrosion include its effect on connector blocks and terminals, resulting in a breakage of the connection, an increase in heat or a voltage drop. Also of concern is capillary action, which can lead to a short circuiting of a system, a breakage in the wire, or a loss of communication and may occur some distance from the connection,” says Wynn.
Generally, corrosion issues with electrical systems become evident during a loss of function or communication failure between control modules. However, when dealing with potentially high voltage systems it is important to identify any concerns as soon as possible, he warns.
Summing up Millbrook’s approach to durability testing, Wynn emphasizes the over-arching importance of methodology and a thoroughly strategic approach to the task.
“The actual specification of the vehicle/propulsion system must be reviewed and the test must include situations which exercise any additional systems, such as regenerative braking, torque vectoring and torque-on-demand. All of these systems have an impact on how the vehicle feels to the driver. Sometimes it may be necessary to engineer the feedback to the driver. Durability testing is a good opportunity to get lots of drivers into the vehicle during its test; their subjective comments can be invaluable to the OEM, supplier, and end user.”