New Audi petrol engine challenges diesel economy

Audi is about to launch a 1.8-litre petrol turbo engine that challenges the best turbodiesels in terms of power outputs, fuel economy and CO2 emissions.

The new 1.8 TFSI engine displaces the same 1798cc as the regular Audi 1.8 TFSI engine but delivers better power and torque figures, consumes less fuel and produces less CO2 – all by significant margins. Audi says it is more efficient than many hybrids currently available.

Comparisons with similar-capacity turbodiesels are encouraged by not just the torque – it makes 320Nm on a flat band extending from 1400rpm to 3700rpm – but also by the revs at which maximum power is produced.

The new engine develops its maximum 125kW at a super-low 3800rpm – actually better than Audi's own 2.0 TDI turbodiesel which develops 103kW at 4200rpm. The 2.0 TDI produces the same 320Nm of torque as the new 1.8 TFSI, but at higher revs and is less sustained throughout the rpm band. The TDI's maximum comes in at 1750rpm and peters out past 2500rpm.

The closest turbodiesel in terms of capacity is Mitsubishi's 1.8-litre Di-D (direct injection diesel) engine used in the ASX compact SUV, where it produces 110kW at 4000rpm and 300Nm at 2000rpm.

The power and torque figures are not the only impressive aspects of the new Audi engine: Installed in the A5 Sportback, it returns a combined fuel economy figure of 5.7L/100km, along with CO2 emissions of 134g/km and a zero to 100km/h acceleration figure of 7.9 seconds.

In the smaller and lighter A3 Sportback, the current 1.8 TFSI engine quotes 6.7L/100km and 158g/km with practically identical acceleration times (7.8 seconds to 100km/h) to those of the A5 equipped with the new engine (7.9 seconds).

Achieving these results has, according to Audi, largely been due to a focus on combustion behaviour, where the engine uses a combination of direct and indirect injection.

Indirect injection to the end of the inlet manifold is used in the "part load" range, while FSI direct injection, operating at 200 bar rather than 150 bar as on the regular 1.8 TFSI engine, is applied from a standing start and at higher loads.

The system works in with Audi's two-stage variable valve lift system on the exhaust valves, as well as adjustable timing on both camshafts.

Further efficiency gains come from the adoption of a clever cooling system that employs two electrically driven, fast-switching rotating cores that control the flow of coolant to the engine, the heat exchanger for the transmission, and the passenger compartment. This achieves the best possible compromise between minimal internal friction and thermodynamic efficiency. Audi claims the new system reduces CO2 emissions by as much as 2.5g/100km.

Audi also says the integration of the exhaust manifold into the cylinder head provides better efficiency because the more controlled temperature avoids the necessity to enrich the fuel mixture when the engine is being driven hard.

A more efficient turbocharger using a new alloy in the turbine wheel and an electric wastegate actuator that quickly and accurately controls boost pressure also helps.

The engine is lighter than the current 1.8 TFSI too, by 2.5kg, while internal friction has also been further reduced.

With all this, the new engine already complies with future Euro 6 emissions standards that don't come into effect until 2015.

The new 1.8 TFSI engine will debut in Audi's A5 Sportback in the first half of 2012 and will join 2.0 TFSI and 3.0 TDI A5 versions which exist in different universes as far as consumption and emissions are concerned.

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