Audi evolves the Miller cycle in its new 2.0-L spark-ignition engine

  • 11-May-2015 03:35 EDT
Audi05-15 new 2.0 l.jpg

To debut on the all-new 2016 A4, Audi's new TFSI engine is the industry's latest step in moving spark-ignition technology closer to diesel.

Audi’s next generation A4 due later this year will be the first model to receive a new 2.0-L engine that takes gasoline spark-ignition technology a step further towards the fuel consumption and torque capability of the diesel.

It does so via technology that is mostly on the inlet side of the cylinder. That is due to Audi’s introduction of a design that company engineers describe as being “comparable to the Miller cycle.” Engines using the Miller thermodynamic cycle have been used by other engine and vehicle manufacturers, notably Mazda. They employ late inlet-valve closing, before the piston reaches BDC, which facilitates the use of a higher compression ratio.

Audi revealed the 140-kW (188-hp) turbocharged, direct fuel injection (TFSI) engine at the 2015 Vienna Motor Symposium, and it illustrates the company’s “rightsizing” rather than downsizing philosophy for its new powertrains.

Prof. Dr. Ulrich Hackenberg, Audi’s Board Member for Technical Development, is adamant that end-user expectations of premium products must continue to enjoy satisfying powertrains.

“Rightsizing involves the optimal interplay of vehicle class, displacement output, torque, and efficiency characteristics under everyday conditions,” he explained.

His remarks were backed in Vienna by those of Dr. Stefan Knirsch, Head of Engine Development, who added: “The new engine enjoys the consumption benefits [less than 5.0L/100 km on the NEDC test cycle] of a downsized engine in partial load operation, while at high speeds it has the advantages of a large displacement engine.”

The new TFSI engine weighs about 140 kg (308 lb), the engineers claim.

Knirsch and his team further developed the Miller cycle, originally patented by American engineer Ralph Miller in 1957, “in crucial ways.” Focusing on the inlet side of the engine, intake timing has been greatly reduced by adopting a 140° crank angle compared to typical 190-200º timing. 

And while Miller-cycle engines traditionally employ a Roots or screw-type positive-displacement supercharger, Audi is using a turbo. The engine gets a higher boost pressure on the inlet side to provide what Audi describes as “optimal cylinder charges” despite shorter intake timing. The engineers did not specify the new engine's compression ratio.

To help achieve Audi’s performance targets, in part-load range there is an additional injection upstream from the intake valve. This yields an efficient mixture formation that is already complemented by the direct injection in the combustion chamber as well as in the intake manifold, according to the engineers.

Also on the inlet side, the Audi Valvelift System ensures a short intake time at part-load and a longer at higher loads.

Full details of the engine have yet to be released, but features include subtle coolant flow control to reduce engine warm-up time. This is also complemented by the exhaust manifold being integrated in the cylinder head. Audi adds that consistent reduction of friction, as well as the use of low friction engine oil (OW-20), also contributes to enhanced engine efficiency.

Maximum torque of the engine is claimed to be 320 N·m (236 lb·ft), available from 1450 rpm to 4400 rpm. The lower end of that speed range is diesel-like, while the upper end helps to ensure the level of driving satisfaction required.

Following its introduction in the new A4, the engine will power other Audi models in various states of tune.

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