It's not the sort of noise you expect to hear from a mid-engined sports car.In fact, it’s not the noise you’d expect to hear from a car in good mechanical health, either. The little 1.5-litre four-cylinder turbodiesel stuffed behind the seats appears to have a severe case of cylinder knock – a result of the experimental combustion process, which will be refined for production. And when you don’t hear the sound of metal momentarily tortured by exploding gobbets of fuel, it sounds like a diesel.Which it is, kind of. In fact, this advanced engine is an early step towards Volkswagen’s goal of producing a combined combustion system (CCS) motor, whose combustion habits combine the best features of a diesel – low CO2 emissions, loads of torque – with the best of a petrol, namely low NOx and hydrocarbon emissions, and a willingness to rev. The EcoRacer’s experimental engine is not actually a combined-cycle motor, but the real thing could arrive by 2012.But we can try this intermediate engine, and the rather unusual Volkswagen it sits in, right now, in the pouring rain at the Pau race circuit in the south of France. The EcoRacer has never been run in the wet before, the dribbles that seep past the roof seals are proof of that.This is quite a dinky little sports car. It looks low and short – just shorter than a Lotus Elise, though a little wider and taller – and it’s striking for the length of the roof and the narrowness of the tyres. They’re thin to minimise rolling resistance, but also promise more feelsome moments at the wheel.VW and Audi have offered ultra-economical cars before, at least on the Continent, including the 3L (meaning 3.0 litres of fuel per 100km, which equates to around 94mpg) Lupo and A2. These ingeniously lightweight, ultra-efficient machines certainly delivered economy, but they came with off-puttingly high prices.So VW boss Bernd Pischetsrieder asked the R&D department to come up with a low-consumption car that was fun to drive. The EcoRacer is the result and provides ‘driving pleasure without regret’. Well, it does to the lucky bunch of journalists who’ll get to drive it today. Trouble is that, unlike the 3L Lupo, there is only one EcoRacer and it doesn’t look like any more will be appearing any time soon.The issue, they say, is whether this sophisticated machine could be sold in modest volumes at a price high enough to cover the cost of making it – which would be a lot higher than that of a Mazda MX-5. That’s because the EcoRacer has a carbonfibre tub and panelling in the quest to pare weight, reduce fuel consumption and lend it the dynamic character of the Elise, which has been a benchmark for this project. Admittedly the Lotus has an aluminium chassis, glassfibre panels and a petrol engine, but it’s not hard to see major similarities.You’ll see even stronger parallels between this car and the Opel Eco-Speedster prototype from 2003. This VW is a carbon copy of the Opel, which was built on an Opel Speedster chassis (aka Vauxhall VX220 and so Elise-based) and propelled by a 1.3-litre diesel.This EcoRacer was conceived with several objects in mind. It had to be light-weight and aerodynamic, it had to handle, it needed to go hard and use minimal fuel. And VW wanted attractive, versatile bodywork. You can be the judge of that as easily as us, but your reporter’s view is that it looks cute without being beautiful.It also provides no fewer than four roofing arrangements. But when it’s raining there’s really only one, which is fully enclosed and with the tunnel-like extended rear roof in place to improve aerodynamics. Lift away the extra roofing and you can install a luggage rack on the engine cover, while the targa top can be stowed behind the seats in any configuration.The targa roof is ingenious. The central backbone section provides hinge mounts for the larger outer sections of roof which swing upwards when the doors are opened, making it far easier to get into the closed version of this VW than it is to get into a roofed Elise.The final option involves removing the windscreen – not as hard as it sounds when the frame is made from lightweight carbonfibre – and replacing it with a much smaller screen, creating a speedster.The EcoRacer is built around a very impressive-looking carbon tub from which all the exterior body panels are hung. A full carbonfibre tub is expensive, but contributed to the EcoRacer’s 916kg overall weight. And VW believes that the cost of making carbonfibre components may eventually drop low enough for it to become viable for specialist cars like this. Not that the EcoRacer’s weight is as impressive as the 838kg of a 1.8 Elise, but the VW carries superfluous weight in the form of an over-specified DSG dual-clutch transmission – a lighter version for smaller VW Group cars is on the way – and there is doubtless scope for further weight-paring from this relatively undeveloped prototype.The rear suspension is a multi-link set-up borrowed from the Golf, part-lightened to suit. The double-wishbone front suspension has been custom made for the EcoRacer, while the steering system, complete with controversial electric assistance, is also from the Golf. The brakes are stock VW, too.Some key figures, then. The EcoRacer will knock off the 0-62mph sprint in 6.3sec, run to 143mph and turn in 83mpg, which is pretty spectacular. For comparison, the entry-level (but now defunct) K-series-powered Elise 1.8 produced 118bhp, turned in a 5.8sec sprint to 60mph, a top speed of 124mph and fuel consumption over double the VW’s at 38.2mpg.Get yourself snug inside – so much easier with those swing-up roof sections – and you begin to see how the EcoRacer achieves 83mpg. Leave the DSG transmission in ‘D’ and it will change gears in the most unsporting way imaginable, short-shifting at very low revs into the highest ratio feasible to produce grumbling diesel sounds. Still, just think of the money you’re saving. Stick the lever in sport and the engine, suddenly freed from pulling improbably tall gears, feels more like it actually has the 184lb ft claimed between 1900-3750rpm.Now you’ve got the power to enjoy the EcoRacer’s chassis, which on this twisty, undulating track is hugely entertaining. Just as you’d hope, the VW feels wonderfully light and deft, darting into bends with the easy agility for which the Elise is famed. It feels stable despite this lightness and the fact that most of its weight lurks behind you – 61 per cent, the same as the Elise – and it doesn’t take long for the cornering forces to build.Eventually the EcoRacer will spin if you push it too hard, but at the prototype-preserving speeds we managed it felt well-mannered and great fun. The steering is a major contributor: it’s quick, direct and feels unassisted despite the fact that it isn’t.You also discover that the VW’s low rolling resistance and drag enables you to give it a burst of power and then coast, because it takes a long time to slow down. It allows for an intriguingly different driving style that gives you something interesting to think about besides spearing the next bend. Still, a good job the brakes feel firm underfoot, given all that scope for coasting.All of which is cause for optimism about the eco-cars that are coming our way. VW probably won’t make the EcoRacer, but its existence suggests that we don’t need to get depressed about driving future cars that are super-economical.In fact, VW’s own Golf Twincharger (supercharged and turbocharged) engine is impressive proof of that, and the production version of the EcoRacer’s torquey little unit – something we definitely will be seeing – should only ram that fact home.