With an increasing public perception of the benefits of driving more economically and efficiently, the demands for electric and hybrid vehicles continues to grow. However, despite the obvious advantages, there remain some challenges with these new technologies, such as the driving range, weight and size of battery packs, and performance on inclines.
In order to address these challenges, manufacturers are working closely with Millbrook Proving Ground in the UK to comprehensively test and develop their vehicles to ensure reliability, safety, and that customer expectations are met.
We talk to Alastair Wynn, senior durability engineer at Millbrook, to get an insight into the specific requirements of durability testing for electric and hybrid vehicles, comparisons between new technologies and traditional internal combustion engine (ICE) vehicles, and where testing crosses over.
Test requirements for EVs and hybrid EVs (HEVs) are very different, but the way the test is designed is the same. The common goal of durability testing is to simulate the real world usage of a vehicle in an accelerated manner, and to identify interface issues so that they can be addressed. It is imperative that all eventualities are covered, while not conducting unnecessary tests that the client does not need.
It is not necessary to cater for every customer, as not all are going to see the most severe inputs, but it is important to know the target customer, and test according to their usage. For example, out of 100 customers of a C-segment vehicle (equivalent to a small family car), 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 who will never see that event.
Safety systems – such as the anti-lock braking system (ABS), traction control system (TCS), and stability control – should be exercised periodically during the test to ensure functionality. Some subsystems are more critical than others – for example, brakes and steering – but specific testing should be conducted independently of the whole vehicle test.
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 (cobblestones). Conducting the pothole manoeuvre after the shock absorbers have risen in temperature on a pavé surface could give unrealistic loads, because of reduced shock absorber performance.
‘Real world usage’ considerations
Regardless of propulsion design, 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 style of the vehicle, and the intended use.
Other uses must also be considered, for example a delivery vehicle may see a lot of kerb impacts during its life, have a lot of ignition cycles, and spend time in urban areas, as well as cover a lot of ground on motorways/highways/autobahns.
All of these factors will determine the regime required, and bespoke durability testing programmes will subsequently be developed.
Areas for consideration when testing EVs and HEVs that could be different to ICE vehicles include driving style, average trip distance, speed profile, standard and additional features of the vehicle, the target customer, and risk assessment.
While many of these appear to be the same as ICE vehicles on first reading, there are differences which need to be accommodated. Two of the most common are the average trip distance which, with the current limited range of EVs, means they are primarily urban vehicles and used for short, stop/start motoring; and risk assessment, where special attention has to be paid to the risk of fire and potential danger to rescue services with EV batteries.
Performance and usage implications
The nature of EVs and HEVs means that the most significant differences are in the usage of electrical system performance, battery performance, and engine performance.
Notably the battery performance and engine performance are going to change depending on the usage. Battery state-of-charge (SOC) and the systems utilised in an HEV have a direct impact. Battery performance can also be affected by temperature changes and different charging strategies.
With an HEV, any use of ancillary systems, such as air-conditioning or lighting, will affect how long the engine runs for.
Driving style can have a thermal impact on the motors or power systems. Constant high-load running, long ascents with wide-open throttle applications can put both systems under maximum loading, as can descents on regeneration systems.
Pure EVs will see similar issues to that of an HEV, except the usage of ancillaries will have a dramatic effect on the battery 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 lower-speed, high-load driving. This can impact on the operating temperatures and overall range.
Stop/start driving will have an impact on battery SOC and range, especially if ancillary equipment is being used at the same time.
Issues that can occur with the engine in a hybrid vehicle can be different to those of a normal ICE vehicle. In particular, a lot of time can be spent stationary or at low speeds, and if there is high demand on the power system, the engine could be required. Also, the lack of air flow when stationary can result in cooling issues.
Considerable damage can occur during non-running time, and this need to be considered. With the engine not running there is no pressure in the lubrication system, which can lead to premature wear of some components. The engine is not the only system normally pressurised on an ICE vehicle which may not be on an HEV, and these other systems should be taken into account.
Structural interface considerations
As previously mentioned, durability testing is to ensure the sub-assemblies all function together as a single unit. Prior to this, sign-off will have been achieved for the components and individual sub-assemblies. Therefore, the vehicle should be subjected to moderate to high-level loading of different types to check that the connections and interfaces cope with the anticipated wear and tear.
The inspection process should include critical inspection of the batteries, battery carrier and wiring, in addition to the normal inspection criteria.
There is less need to test for fatigue as a result of more frequent lower loads, as this can have a low impact on the vehicle and be very time-consuming to replicate. The same can be said for wear-out items; to accelerate a test for these loads is difficult due to their frequency in the real world.
Variations between EV, parallel, and series hybrids
- EV versus hybrid: The obvious difference between an EV and a hybrid EV is the range. To a large extent this will dictate the in-service use, and the test will need to reflect this.
- Parallel versus series: In a parallel system the test will need to include the parameters that dictate that the less dominant drive is in demand on occasions.
Corrosion inspections need to include the extra electrical systems that are necessary. Common concerns with electrical systems as a result of corrosion include:
- connector blocks and terminals corroding, resulting in a breakage of the connection or an increase in heat or a drop in the voltage; and
- capillary action, which can lead to a short-circuiting of a system, a break in the wire, or a loss of communication.
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.
The actual specification of the vehicle/propulsion system must be reviewed, and the test must include situations which exercise any additional systems:
- Regenerative braking
- Torque vectoring
- 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 different drivers in the vehicle during its test, and their subjective comments can be invaluable.
Durability testing at work
These are just some examples of test procedures that Millbrook has run for its customers:
- Commercial vehicle with ‘in wheel’ motors
- Commercial delivery van
- Motor assist drive systems for 4WD vehicle
- Small passenger car ‘cradle to grave’
- Wheelchairs designed for ‘off road’ use
- Hand carts and bicycles.
And in conjunction with Cranfield University:
- Battery performance optimisation and refinement.
Millbrook’s engineers have tested a wide variety of vehicle types, but the testing all has one thing in common: the methodology.