An almost forgotten piece of engine technology will help Audi’s next gasoline-powered A4 to slash its fuel consumption to near diesel figures while boosting power and acceleration. While Audi is couching it in different terminology—calling it a “new combustion method,” which it really isn’t—the German premium brand is essentially adapting Miller-cycle breathing technology to its ubiquitous EA888 four-cylinder powerplant.
The new engine was announced at Vienna’s annual Motor Symposium, and the technology will first be applied to a 2.0-liter, turbocharged, direct-injected gas unit making 188 horsepower and 236 lb-ft of torque and installed in the next Audi A4. Due on sale before the end of the year, the all-new A4 will slash the consumption figures of its predecessors — and, critically, undercut its competitors from Mercedes-Benz, BMW, and new class contender Jaguar — with a Euro-cycle combined fuel-economy figure of 47 mpg. By comparison, the BMW 320i, with a similar engine size, has a quoted European-cycle consumption figure of roughly 39 mpg (the more frugal 316i pulls that up by 1 mpg) and the best of the C-class Mercedes-Benz range, the C250, is at 44 mpg.
While refusing to stick the Miller-cycle tag onto its new engine, Ulrich Hackenberg, Audi’s board member for technical development, admitted “at its core, its principle is comparable to the Miller cycle. We are now taking a crucial step further with right-sizing,” he insisted. “Right-sizing 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, with its KZ-JEM 2.3-liter V-6, which ceased production in 2004. Mazda has kept an eye on the technology, though, and applied it recently on the Mazda 2. The system was the brainchild of American engineer Ralph Miller, who earned a patent for it on Christmas Eve, 1957. It works with both gas and diesel fuels and with two- and four-stroke cycles.
For the EA888 engine, Audi has heavily changed the traditional intake period, shortening the duration from around 190 or 200 crankshaft degrees down to just 140 degrees. It also closes the intake valves later than normal, well after the bottom dead center. The expansion portion of the cycle is much longer than the compression phase to extract maximum work from every fuel-air mix.
The engine will capitalize on existing technologies to push the Miller-cycle philosophy even further, making use of the combination of direct 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. 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 in heavy load situations.
This has helped broaden the spread of torque from the 2.0-liter TFSI four, with its 236 lb-ft peaking at 1450 rpm and holding steady until 4400 rpm. The 1984-cc, 309-pound engine also has its exhaust manifold cast integrally with its cylinder head and uses a very low-viscosity oil (0W-20) to minimize windage losses.
“Thanks to this right-sizing approach, the new engine enjoys the consumption benefits of a downsized engine in partial-load operation, while at higher loads it has the advantages of a large-displacement engine,” said Audi head of engine development Stefan Knirsch. “The result is optimal efficiency and performance characteristics across the entire engine-speed range.”