2.0 TFSI with 140 kW (190 hp). Click to enlarge.

The Miller cycle uses a higher expansion ratio than compression ratio (i.e., over-expansion) obtained by either early or late closing of the intake valves (EIVC and LIVC, respectively), and results in a smaller effective compression stroke; combustion and expansion proceed normally.

One effect of the Miller cycle is to reduce pumping losses, improving the thermal efficiency of the engine. The Miller cycle can also deliver hefty torque for a given displacement. Yet another effect is that it can be used to mitigate the propensity for knock in highly boosted engines.

Audi engineers have further developed the Miller cycle approach in crucial ways, the company said. Audi attributed the increase in efficiency in the new 2.0L TFSI to the following factors:

• The intake time has been significantly shortened (140° crank angle (CA) rather than 190 to 200° CA).

• Owing to a higher boost pressure on the inlet side, the engine attains optimal cylinder charges despite the shorter intake time.

• Early intake valve closing—well before the bottom dead center is reached—lowers the pressure, allowing a high, efficiency-boosting compression ratio.

• The Audi engine features, as do other engines in the current Audi lineup, a dual injection system that combines direct injection with indirect injection into the intake manifold. (Earlier post.) In the partial load range, an additional injection upstream from the intake valve yields an efficient mixture formation that is already complemented by the direct injection in the intake manifold and in the combustion chamber. This also addresses particulates from gasoline direct injection.

• The Audi Valvelift System (AVS) on the inlet side allows a short intake time at partial load and a longer time at higher loads (full load: 170° CA).

Thanks to this rightsizing 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. The result is optimal efficiency and performance characteristics across the entire engine speed range.

—Dr. Stefan Knirsch, Head of Engine Development at Audi

Beyond the new combustion method, the new engine, with a weight of only about 140 kg (308.6 lb), avails itself of other efficiency technologies. For example, the coolant flow is controlled so as to greatly shorten the engine warm-up time. Contributing to this benefit is also the exhaust manifold, integrated in the cylinder head.

The consistent reduction of friction as well as the use of low-friction engine oil (0W-20) also increases efficiency.

Resources

• Li, Y., Zhao, H., Stansfield, P., and Freeland, P. (2015) “Synergy between Boost and Valve Timings in a Highly Boosted Direct Injection Gasoline Engine Operating with Miller Cycle,” SAE Technical Paper 2015-01-1262 doi: 10.4271/2015-01-1262

• Luisi, S., Doria, V., Stroppiana, A., Millo, F. et al. (2015) “Experimental Investigation on Early and Late Intake Valve Closures for Knock Mitigation through Miller Cycle in a Downsized Turbocharged Engine,” SAE Technical Paper 2015-01-0760, doi: 10.4271/2015-01-0760

• Tie Li, Yi Gao, Jiasheng Wang, Ziqian Chen (2014) “The Miller cycle effects on improvement of fuel economy in a highly boosted, high compression ratio, direct-injection gasoline engine: EIVC vs. LIVC,” Energy Conversion and Management, Volume 79, Pages 59-65 doi: 10.1016/j.enconman.2013.12.022