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Behind Audi’s new “rightsized” efficient EA888 Gen.3B family

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Audi’s new “rightsized” 2.0L Gen.3B engine maintains the displacement of the 2.0L Gen.3 and delivers the power of the 1.8L TFSI with the fuel consumption of the 1.4L TFSI. Click to enlarge.

Audi’s new higher-efficiency series-production 2.0 TFSI turbocharged gasoline direct injection engine introduced at the Vienna Motor Symposium last week is the first of the company’s EA888 Gen.3B family. The engine, featuring a new combustion method based on the Miller cycle, will be used for the first time in the next generation of the A4. (Earlier post.) 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), resulting in a smaller effective compression stroke; combustion and expansion proceed normally.

The new Audi engine combines shorter intake events with a high geometric compression ratio; together with other refinements to the basic engine, the new combustion concept enables a reduction in combined cycle CO2 emissions of up to 10-12 g/km compared to its predecessor.

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Fuel consumption maps comparing Gen.3 2.0L and Gen.3B 2.0L engines. In addition to the Gen.3B advantages shown in the lower load range up to 10 bar, consumption in the upper load range could also be significantly reduced. Click to enlarge.

The Gen.3B approach. Audi initially deployed its turbocharged, direct injection TFSI technology in 2004; the second generation followed in 2007, followed by the third generation, currently on the market, in 2011. The new EA 888 Gen.3B features a combustion method with over-expansion (i.e, a Miller-type cycle). Despite the increase in displacement, the fuel consumption of the new 2.0L EA888 Gen.3B engine in the class up to 147 kW (197 hp) compares with current smaller displacement TFSI engines. Audi says that the new 2.0L TFSI is the first representative of direct-injection gasoline engines based on “right-sizing” rather than “down-sizing”.

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Highlights of the Gen2 TFSI were the Audi exhaust valve lift system and a pressure- and volume-flow-controlled oil pump. With Gen.3, Audi introduced a double-flow exhaust manifold integrated in the cylinder head; an electrical cooling water control; an electric wastegate; and the dual injection system (earlier post)—a combination of direct and manifold injection that helps control particulates. Click to enlarge.

The key element driving the improved efficiency is the use of the Audi valvelift system on the intake side. This allows operation at partial load with a short intake valve timing, thus significantly reducing the intake phase.

Prior to the development of the new engine concept, Audi investigated the potential of known and new technologies to reduce fuel consumption. The engineers opted for the modified Miller firing process. As this Audi process differs in some essential design points from the original Miller cycle, Audi internally labelled this process the “B” cycle—the internal label ultimately became part of the official 2.0L EA888 Gen.3B designation.

The basic Gen.3B idea is to combine a shortened compression with a normal—i.e., longer in proportion to the compression—expansion phase to attain significant gains in efficiency.

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The diagram shows a comparison of the piston position at time of intake valve close (Einlass Schließt, ES) for the 2.0L Gen.3 with the older, conventional combustion processes and the 2.0L Gen.3B with the new B-cycle combustion process. In conventional combustion processes, intake camshafts are usually used with opening durations of 190-200 °CA (Kurbelwinkel, KW). In the 2.0L Gen.3B, a camshaft with an opening event of only 140 ° CA is used in partial load operation. The intake valve close is significantly ahead of bottom dead center of the piston. In order to achieve the necessary fresh air charge to represent the same mean pressure as in the conventional combustion process in the combustion chamber at time greatly shortened suction time, the pressure in the suction tube must be increased, resulting in a de-throttling and thus to a reduction in losses. Click to enlarge.

At the end of the compression, both the Gen.3 and Gen.3B engines broadly show similar levels of pressure. As the Gen 3B average pressure level is lower during compression, this results in work process efficiency gains for the new TFSI combustion process.

The Gen.3B combustion process takes place within a smaller combustion chamber volume and thus with a higher pressure level, leading to a further gain in efficiency. However, the second part of the expansion phase and the subsequent exhaust phase show, due to the slightly different mixture masses in the combustion chamber and to a different heat transfer in both combustion processes even smaller differences, with the result being no no significant contribution to the efficiency differentials.

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Distribution of the efficiency gains on the 4 strokes of the Gen 3B. Intake gains are due to de-throttling; compression gains are due to compression taking place at a lower pressure than conventional combustion; combustion gains are due to the smaller chamber volume. Click to enlarge.

The realizable efficiency benefits are directly dependent on the “shortness” of the intake opening event and the selected geometric compression ratio. The shorter the intake opening period and the higher the compression ratio, the greater the theoretical efficiency gains. However, short intake durations and high compression ratios limit high engine loads due to reduced filling and increasingly strong knock limits.

The EA888 Gen.3B resolves this trade-off through the use of the Audi valvelift system on the intake side. The Audi valvelift system makes it possible to realize a very short intake opening period of 140 ° CA in the partial load range, while switching to a larger intake event at higher loads.

The design of the full-load intake contour and the effective compression ratio depend on the defined full load targets. The higher the targets are, the longer the intake contour and the lower the compression ratio.

The Gen.3B engine, as noted above, uses early intake valve closure (EIVC) to manage combustion. In Audi’s preliminary investigations, the engineers also explored using a classic Atkinson approach—i.e., a process with the late intake valve closure (LIVC). They found that the combustion process with EIVC consistently showed better efficiency in partial load operation. The additional use of a turbocharger helps support relatively high engine loads with the short valve opening contour and the lower valve lifts, enabling the good efficiency of the EIVC-based process over a wider load range.

Changes to the 2.0L EA888 engine. Audi made a number of design changes to the EA888 Gen.3 engine to realize the Gen.3B.

  • The Audi valvelift system applied on the intake side.
  • An increase of the compression ratio from ε = 9.6 to ε= 11.7 by decreasing the combustion chamber volume.

    Audi engineers lowered the combustion chamber roof by about 0.9 mm, and moved the intake and exhaust valves—slightly reduced in diameter—axially downward. The positions of the spark plug and high-pressure injection valve and the piston shape was adapted to the new combustion chamber.

  • Maintaining meaningful charge motion is a significant challenge with reduced intake opening times and valve lifts. EIVC extends the time between tumble-generation and combustion, and thus the time for the dissipation of charge movement. Further, the high compression ratio pistons (with reduced recesses) also negatively impact the development and maintenance of tumble.
    To address this, Audi developed a new intake channel that produces a higher basic level of tumble; added maskings in the lower regions of the intake valves; and optimized the piston cavity.
  • Changes to the cylinder head and basic engine optimizations.

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Maintaining charge motion. The visualization on the left shows charge motion in the Gen.3 engine. The visualization in the center shows the resulting charge motion in a Gen.3 engine using Gen.3B timing—about a 2/3 decrease. The visualization on the right show the effect of the changes to the Gen.3B engine in maintaining charge motion. Click to enlarge.

Resources

  • Dr.-Ing. Rainer Wurms, Dr.-Ing. Ralf Budack, Dr.-Ing. Michael Grigo,
    Dr.-Ing. Günther Mendl, Dr.-Ing. Thomas Heiduk, Dr.-Ing. Stefan Knirsch (2015) “Der neue Audi 2.0l mit innovativem Rightsizing—ein weiterer Meilenstein der TFSI-Technologie” Vienna Motor Symposium 2015