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Carsales Staff8 May 2015
NEWS

New Audi A4 to go Miller-cycle

Back-to-the-future tech makes the new Audi A4's petrol engine almost as efficient as its diesels

An almost forgotten piece of engine technology will help Audi’s next-generation A4 to slash its CO2 emissions to near-diesel levels while boosting power and acceleration.

While Audi is caching it in a different term, the German premium brand is essentially grafting Miller-cycle breathing technology on to its ubiquitous EA888 four-cylinder petrol powerplant.

Announced at the annual Vienna Motor Symposium today, the technology will see Audi deliver a 2.0-litre turbocharged direct-injection petrol engine with 140kW of power and 320Nm of torque in its earliest iteration.

Due on sale in Europe before the end of the year – and in Australia next year — the all-new A4 will slash the consumption figures of its predecessors and, critically, it’s competitors from Mercedes-Benz, BMW and new class contender Jaguar, with an NEDC combined figure of less than 5.0L/100km.

While it is refusing to stick the Miller cycle tag onto its new engine, Professor Doctor Ulrich Hackenberg, Audi’s board member for technical development, admitted: “at its core, its principal is comparable to the Miller cycle.”

“We are now taking a crucial step further with right-sizing,” said Prof Hackenberg.

“Right-sizing thus involves the optimal interplay of vehicle class, displacement, output, torque and efficiency characteristics under everyday conditions.”

The Miller-cycle system was most famously used in production by Mazda’s Millenia/Xedos 9/Eunos 800, with its KZ-JEM 2.3-litre V6, which ceased production in 2004. Mazda has kept an eye on the technology, though, and used it recently on the Mazda2.

The system was the brainchild of American engineer, Ralph Miller, who filed a patent for it on Christmas Eve, 1957. It works with either petrol or diesel fuels and with either two- or four-stroke motors.

Typically, a Miller-cycle engine leaves the intake valve open on the compression phase longer than a normal engine. It operates in a similar fashion to the Atkinson-cycle motor, but uses forced induction (usually, but not always, a supercharger) to make up for the loss in engine compression.

The turbocharger helps the Miller-cycle engine avoid power losses, even as it slashes fuel consumption by getting more energy out of each cylinder’s combustion process.

For the EA888 engine, Audi has heavily changed the traditional intake period, shortening the crank angle from around 190 or 200 degrees down to just 140 degrees.

It also closes the intake valves earlier than normal, before the bottom dead centre of the crankshaft’s rotation is reached.

Audi says this lowers the medium cylinder pressure, allowing for a high compression ratio, which usually translates to higher energy levels from each combustion cycle.

The engine will capitalise on existing technologies to push the Miller-cycle philosophy even further, making use of the combination of direct (in the combustion chamber) and indirect (in the intake manifold) fuel-injection and variable valve timing and lift already fitted to the EA888.

When the engine runs at part throttle or low loads, Audi says it will deliver an extra fuel-injection burst from its indirect fuel-injection system before the air-fuel mixture even reaches the combustion chamber. It will then flesh this out with its existing systems, which sounds like Audi is effectively delivering a third fuel-injection system.

It also uses its existing variable valve lift to give the engine a short intake time on part throttle and up to 170 degrees of intake timing on full throttle or heavy load situations.

This has helped broaden the spread of torque from the 2.0-litre TFSI four, with its 320Nm peaking at 1450rpm and staying there until 4400rpm.

The 1984cc, 140kg engine also has its exhaust manifold integrated into its cylinder-head and uses a very low friction oil (0W-20) to minimise internal friction.

“Thanks to this right-sizing approach, the new engine enjoys the consumption benefits of a downsizing engine in partial load operation, while at higher loads it has the advantages of a large-displacement engine,” said Audi’s head of engine development, Dr Stefan Knirsch.

“The result is optimal efficiency and performance characteristics across the entire engine speed range."

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Written byCarsales Staff
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