In early October 2014, GKN unveiled its Disconnect All-Wheel Drive System. The supplier claims to be the first to market with a system suitable for A-, B- and C-segment vehicle platforms. The technology, which connects and disengages power to the rear axle on demand, can deliver a fuel economy improvement of up to 4% during steady state cruising, says GKN.
AWD differs from 4-wheel drive in that most AWD models are front-wheel drive-based passenger cars. JLR’s Range Rover Evoque was the first vehicle built on a front-wheel drive-based platform to offer a disconnectable all-wheel drive (AWD) system. At the 2014 Paris motor show, Fiat displayed its new 500X, which is offered with AWD with a disconnecting rear axle; the 500X is built alongside the Jeep Renegade at Melfi, and the two B-segment crossovers use common componentry.
And there will be many more, says Rob Rickell, Senior Vice President, Engineering at GKN Driveline. “I would say about half the modern all-wheel drive platforms that we’re working on at the moment will have Disconnect AWD technology – and that’s in the Americas, Europe, China and Asia.”
There are still some front-wheel drive-based AWD vehicles that don’t have disconnect technology, but even though they are lighter and more efficient than older generation versions, their fuel consumption is 3-4% greater than those with disconnect, explains Rickell. “Disconnect gives you a fuel saving on top of the AWD capability, which means you’re getting to a level of carbon emissions, of miles per gallon, which is much more acceptable when compared with a gas-guzzling four-wheel drive vehicle many years ago. It makes all-wheel drive much more acceptable, much more available to a mass market than it used to be.”
In GKN’s system, AWD is always engaged from the outset. “Pulling onto a motorway, pulling out of a corner, or accelerating hard – that’s when you need all-wheel drive,” says Rickell. “Starting off could be the highest torque event. Because you don’t know what’s going to happen, we always engage all-wheel drive at start-up. Then it can disconnect shortly afterwards if you’re driving smoothly, straight ahead, and not under extreme conditions.”
AWD disconnect systems are intelligent, which is part of their widespread appeal. “Whenever torque needs to be distributed over all four wheels, it will connect. And it connects within 300 milliseconds, which is effectively instantly,” says Rickell. “You don’t even notice it – it’s seamless, totally quiet and you don’t feel or hear anything. But if you’re driving down a motorway at constant speed, you only need front-wheel drive. So that’s when you’re saving fuel.”
AWD disconnect-type technology will appear on many new European cars designed in the Americas and Europe for global production, says Rickell. “China’s take-up has been incredible. There are a number of companies we will be supplying in the future where they started with front-wheel drive and all-wheel drive variants, and have now gone to 100% all-wheel drive for the Chinese market. They’re not doing a front-wheel drive any more. They’re doing 100% all-wheel drive. You might think there would be a cheaper variant for China, but it’s not true. The variant for China is the top end one, particularly when we’re talking about European or American designed cars.”
Hybridisation of the undriven axle
In addition to the clear trend towards efficient disconnectable all-wheel drive, there is a notable rise in hybridisation of the undriven axle, says Rickell. GKN Driveline supplies the e-axle to PSA Peugeot Citroen’s HYbrid4 programme, which uses an electric axle on the rear and permanent front-wheel drive, allowing a disconnectable system through on-off control of the rear electric axle, as well as the ability to drive purely with the e-axle as a pure EV for city use. “The first was Peugeot Citroen. There are a number of those coming. I can’t say which they all are but there are a number of manufacturers, particularly in Europe and in America.” This includes some German sports cars. “That’s interesting,” adds Rickell, “because they are traditionally rear-wheel drive cars, so they have the e-axle on the front.”
E-axles will also appear on some forthcoming high volume passenger cars, mainly from European or American OEMs, which will also offer them in China. “I think it’s an ongoing trend. It’s not going to be dominant for the next five years, maybe not even the next ten years, but there will be a trend to increasing hybridisation. And if you’re going to have a hybrid all-wheel drive, then why not do it with an e-axle rather than some other arrangement?” A pure electric axle on the rear or on the front saves on mechanical components, and offers considerable fuel savings. “Disconnect offers 3-4%. When you go to the e-axle you can be talking about a saving of up to 30%.”
Fitment of e-axle hybridisation is partly industry-led but also guided by consumer demand. “If you can offer all-wheel drive as well as hybridisation, you make it more interesting. But it also offers fuel efficiency. People who have driven or owned all-wheel drive cars realise just how nice they are to drive compared to pure front-wheel drive cars. Yes, it’s being offered by the OEMs, but it’s clearly being demanded by the market.”
Just as AWD variants of front-wheel drive models can be mechanical or use e-axle technology, so too can rear-wheel drive models be converted to AWD by using e-axle technology on the front axle. But does this need to be designed in from the platform’s inception, or is it a “drop-in” technology?
“It’s best to be there from the start,” says Rickell. “But yes, the advantage of an e-axle is that you can add it without disrupting the whole of the platform more easily than you can a complete conventional all-wheel drive.”
AWD disconnect, however, must be an integral part of the original design, because of the need for a prop shaft and a power take-up unit. “But that’s the way we develop all those systems. It’s been part of the whole programme as it was being developed right from the beginning.”
Rickell is anticipating significant future growth in all-wheel drive. “It’s been a great success, and continues to be a great success for us. At the moment, we’re investing probably about 40% of my engineering budget, year on year, in all-wheel drive, so we’re talking about £40-50m (US$63-78m) a year on development work on a number of future platforms,” says Rickell, adding that this includes generic product development and specific application work for existing customers. “It’s a major part of our development. The other major part, of course, is hybridisation and electrification, which growing together thanks to e-all-wheel drive systems.”