Car review: BMW i8 hybrid - can a real sports car be eco-friendly?

BMW says its hybrid i8 will heralds a new era of sports cars. It claims to rival a Porsche 911 for performance with the fuel economy of a Toyota Prius. But is it ultimately any good?

Sun, Apr 27, 2014, 22:26

The entire world of sports car makers is pinning its hopes on the i8, and not just BMW. It can run as an electric car, as a fuel-saving plug-in hybrid or as a sports car pumping both electric and three-cylinder, turbocharged petrol power to its wheels. It drapes concept-car-for-the-road skin over a four-seat carbon-fibre chassis and it even introduces laser-fired headlights to the world. It’s supposed to be an eco-friendly Porsche 911 hunter and is priced to match. Can it really do all of that in one car?

There is a lot riding on the i8, and not just for BMW. More than anything else, it’s an attempt to convince the world’s naysayers that even something as selfish as a sports car has a future, without the loss of any of the fun stuff, if the genre steps up to its social responsibilities.

It’s BMW’s supercar; super for its technology even more than its 4.4-second sprint to 100km/h. It takes the i3’s lessons and tacks 911 speed and eye-magnet style onto it. It features laser headlights with high beam clarity at up to 600 metres. It delivers better economy than a Prius, so you can be sexy and fast without feeling guilty.

Its combinations are, by now, well known: a four-seat, carbon-fibre tub atop an aluminium chassis (very much like the i3), plus a five-link rear suspension. A 96kW electric motor up front and a thumper, 170kW version of the MINI Cooper’s 1.5-litre three cylnder down the back. A 7.1kW/hour lithium-ion battery pack stuffed into the transmission tunnel and wire-frying brainpower. Electrical regeneration on the go via the front electric motor and the option of simply plugging it into the wall.

And it’s supposed to do all of that and still deliver a cracking drive that will pull even passionate Porsche drivers out of their 911s and Cayman S models, all while shrinking their running costs with a combined fuel economy number of only 2.1 litres/100km.

It is supposed to be a pure sports car drive at anywhere between a third and a half of the fuel consumption, along with the ability to commute emissions-free for most people. How can that go wrong?

Well, complexity, for one thing. Electronics and software complexity, to be more specific, because the rest of it is surprisingly (in this day and age) conventional.

The engine is a modified version of the MINI’s three-cylinder engine, with more power and more torque. It has variable valve timing and lift, 320Nm of torque (not bad from 1.5 litres) and a big artificial system to help it sound like a sports car. A bit.

BMW chose this because it’s a light engine, but it also chose it because it was one less thing to develop from scratch. Another thing not developed from scratch is the gearbox, because it’s a modified version of the MINI Cooper’s six-speed Aisin automatic. You can almost think of it as a front-drive layout, turned around and plonked in the back, driving the back wheels.

In fact, the petrol powertrain only gets complicated because it needs to turn the back wheels in harmony with what the electric motor is doing with the front ones, plus what the cornering demands are and what traction the tyres can use. Electric motors spin up more or less instantly, but petrol engines need time to pick up their revs. BMW overcomes this by fitting the three-pot with a belt-driven, 11kW electric motor to it to speed up the crankshaft’s acceleration and it also acts as its starter motor when it the engine is chiming in and out during hybrid work.

The electric synchronous motor delivers 96kW of power and, from a standing start, an immediate 250Nm of torque and it delivers it all to the front wheels exclusively. It does that via a two-speed automatic transmission of its own.

That’s the nuts and bolts of what is a very complex system, but there’s a lot more to it than just that.

For starters, there is an electric mode, which runs the car as a pure electric vehicle to allow you to plug it in at home or work, then drive it for 37km (on the NEDC numbers) without paying for fuel. Only in electric mode does the car actually use the two-speed auto’s first gear, because the 96kW motor is moving the 1,485kg car all by itself and needs a bit of help.

Then there’s a Comfort mode, which runs the car as a hybrid, with the petrol motor (or ICE - Internal Combustion Engine - as BMW calls it) chiming in and out as the computers see fit. Sometimes it’s churning more grunt to the rear wheels, sometimes it’s off.

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