The Worldwide harmonised Light vehicles Test Procedures (WLTP) will be introduced on September 1st 2017. These alter the way cars have emissions tests and replace the New European Driving Cycle (NEDC) that has been around for nearly half a century. NEDC is frequently accused of being too easy for car manufacturers to prepare vehicles for, resulting in favourable and unrepresentative outcomes. Who really gets the quoted MPG and corresponding CO2/km from a new vehicle these days?
Better representation
By having a controlled series of actions such as accelerating, standing still and braking, the WLTP aims to give a better representation of real world driving conditions.
The company at the forefront of test rig supply is Hatton Systems, whose customers include many of the world’s leading automotive manufacturers such as Ford, Honda, Nissan and Jaguar Land Rover.
Hatton started to farm out its design function nine years ago to Bennett Engineering Design Solutions, which remains with them to this day.
The principal product that Bennett Engineering works on for Hatton is emissions dynamometers, more commonly known as rolling roads. Director, John Bennett, admits they are, superficially, simple bits of kit. He says: “It’s basically a pair of rollers with a motor between them. The motor swings about some trunnions and you just measure the load, which is effectively the torque. But they are very, very accurate. So, the losses within any of the movements must be minimum. You’re measuring down to a few Newtons on something like an 8000N capacity. Naturally, we must be careful about vibrations and natural frequencies, deflections and all those sorts of things.”
Holding a vehicle in position during the test is not easy – traditionally this is a combination of driver skill (or at least attentiveness) and straps – two at the front and two at the back to stop the vehicle moving forwards.
Bennett says: “It’s a bit like driving with a rubber band between you and the Earth. The operator finds it really hard to keep within the driving constraints, following the trace [drive cycle] to comply with the test. Having a more rigid point was a huge advantage.”
The rigid point, or restraint, was an innovation that initially separated the Hatton solution from competing products. Compared with a solution involving straps and poles, having a rigid restraining system is safer, quicker and more reliable.
Fundamentally, it is just a post with a rigid restraint that attaches to the vehicle and allows vertical movement, up and down. The thing that complicates this seemingly simple solution is that it needs to fit almost any vehicle – and vehicles have their towing points in different places.
Craig Harbron, mechanical design engineer at Bennett Engineering, says: “There’s such a diverse range of vehicles, from very small passenger vehicles such as the Hyundai i10 and i20, all the way up to things like the Ford trucks in the States. There’s a big difference there in offset height, power and load. We’re confident that the latest design will attach to just about anything by using our special adapters. We can accommodate pretty much the full range of connection points and we are one of the first to be able to achieve that with confidence.”
Another challenge is quite literally an obstacle. Under the new legislation a large cooling fan must sit 300mm from the front of the car to account for finer details like the width of a number plate and its fixings. While a cooling fan has always been necessary to simulate airflow of a travelling vehicle, under the WLTP the position makes it impossible to implement any universal strapping for maintaining a car’s position. In short, the fan is where the straps need to be.
“That has kicked all the other restraint systems into touch until somebody comes up with something like our Hatton Systems’ design,” says Bennett. “And there’s only so many ways you can hold a vehicle in place.”
Vertical movement is the only motion allowed during test, as to constrain it would put load on a vehicle and make it appear heavier or lighter. But no lateral or longitudinal movement is allowed, so the car must be sitting on the crown of the rollers.
Human factors
Tests can last several hours so there must be some allowance for human error. If someone becomes unwell during the test, for example, they may lean on the steering wheel, which will apply different forces.
Harbron says: “We’ve got to keep hold of a vehicle so that it doesn’t suddenly come off the dyno. We’ve done a lot of testing simulations, at a huge range of temperatures, to ensure that we will maintain a vehicle regardless.”
Ease of use is also very important in these facilities as manufacturers are keen to be running tests than overseeing laborious changeovers. Again, the Hatton restraint allows changing from one car to the next in about a minute, considerably quicker than having to strap the cars in place.
The consequence is a component of a system that for all its simplicity has changed the test platform to such an extent that it is now being adopted by two of Hatton Systems’ competitors and specified by end users. Bennett sums it up: “The auto manufacturers are now saying they don’t mind which dynamometer is used, but they want the Hatton Systems restraint.”
The Worldwide harmonised Light vehicles Test Procedures (WLTP)
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