Perc
Very Odd Looking Vehicular Object
I was just reading a review about a new Subaru. The writer claimed that the "Symmetrical AWD" drivetrain layout hurt fuel economy because all axles are powered at all times. I kinda don't get it, though.
My Opel has a Haldex AWD system which means it's part-time. The rear axle kicks in when front wheel slip occurs. It's the same system as Volkswagen and Volvo among others have been using since around the turn of the century. Off the line, on snow, you can feel the front wheels losing grip for a split second before the Haldex clutch kicks in and sends power to the back... and sends the car away, slightly sideways with all four wheels spinning if you're giving it the beans.
Some people claim this is: 1) inferior because it's a reactive system and 2) widely used because it improves fuel economy, since the car is 2WD most of the time.
The thing is, where does the fuel saving come from? The coupling sits in the back between the prop shaft and rear diff. The gearbox keeps the propshaft spinning even if it isn't connected to anything. The half-shafts are always connected to the wheels, which means the entire rear axle is spinning while the car is moving.
Even Audi is moving away from full-time AWD with their "quattro ultra" which disconnects the rear axle when not needed. The difference here is that the computer brain decides when to disengage the rear, unlike Haldex where the rear is disengaged by default and the brain decides when to engage it.
The car makers are obviously going through great lengths to come up with different fuel-saving techniques for their AWD systems so there has to be something to be gained from disconnecting an axle, but I just don't get it. What am I missing here?
My Opel has a Haldex AWD system which means it's part-time. The rear axle kicks in when front wheel slip occurs. It's the same system as Volkswagen and Volvo among others have been using since around the turn of the century. Off the line, on snow, you can feel the front wheels losing grip for a split second before the Haldex clutch kicks in and sends power to the back... and sends the car away, slightly sideways with all four wheels spinning if you're giving it the beans.
Some people claim this is: 1) inferior because it's a reactive system and 2) widely used because it improves fuel economy, since the car is 2WD most of the time.
The thing is, where does the fuel saving come from? The coupling sits in the back between the prop shaft and rear diff. The gearbox keeps the propshaft spinning even if it isn't connected to anything. The half-shafts are always connected to the wheels, which means the entire rear axle is spinning while the car is moving.
Even Audi is moving away from full-time AWD with their "quattro ultra" which disconnects the rear axle when not needed. The difference here is that the computer brain decides when to disengage the rear, unlike Haldex where the rear is disengaged by default and the brain decides when to engage it.
The car makers are obviously going through great lengths to come up with different fuel-saving techniques for their AWD systems so there has to be something to be gained from disconnecting an axle, but I just don't get it. What am I missing here?