Audi pushes ef­fi­ciency bound­aries

Motor Equipment News - - OEM - READER RE­PLY 01506014A

Audi has pre­sented what it terms the most ef­fi­cient two-litre gaso­line en­gine in its class, the new 2.0 TFSI. Audi will be us­ing the en­gine for the first time in the next gen­er­a­tion of the A4.

The four-cylin­der en­gine pro­duces 140kW) of power and 320Nm of torque from a dis­place­ment of 1,984 cc. Be­cause the high torque is ap­plied in a wide speed range of 1,450 to 4,400 rpm, and boasts ground-break­ing low fuel con­sump­tion lev­els at less than 5L/100 km.

Tech­nol­ogy in de­tail

The break­through of the 2.0 TFSI lies in a new com­bus­tion method. At its core its prin­ci­ple is com­pa­ra­ble to the Miller cy­cle. Audi en­gi­neers have fur­ther de­vel­oped that method in cru­cial ways, how­ever. The in­crease in ef­fi­ciency is based on the fol­low­ing fac­tors:

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

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

The in­take valve also closes ear­lier – well be­fore the bot­tom dead cen­tre is reached. This low­ers the medium pres­sure, al­low­ing a high, ef­fi­ciency-- boost­ing com­pres­sion ra­tio.

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.

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

In the Miller cy­cle, the in­take valve is left open longer than it would be in an Otto cy­cle en­gine. In ef­fect, the com­pres­sion stroke is two dis­crete cy­cles: the ini­tial por­tion when the in­take valve is open and fi­nal por­tion when the in­take valve is closed. This two-stage in­take stroke cre­ates the so-called “fifth” stroke that the Miller cy­cle in­tro­duces.

As the pis­ton ini­tially moves up­wards in what is tra­di­tion­ally the com­pres­sion stroke, the charge is par­tially ex­pelled back out through the still-open in­take valve. Typ­i­cally this loss of charge air would re­sult in a loss of power, but in the Miller cy­cle, this is usu­ally com­pen­sated for by thee use of a; how­ever Audi has man­aged to make it work with tur­bocharg­ing.

In the Miller cy­cle en­gine, the pis­ton be­gins to com­press the fuel-air mix­ture only af­ter the in­take valve closes; and the in­take valve closes af­ter the pis­ton has trav­elled a cer­tain dis­tance above its bot­tom-most po­si­tion: at around 20 per­cent to 30 per­cent of the to­tal pis­ton travel of this up­ward stroke. So in the Miller cy­cle en­gine, the pis­ton ac­tu­ally com­presses the fuel-air mix­ture only dur­ing the lat­ter 70 per­cent to 80 per­cent of the com­pres­sion stroke.

Dur­ing the ini­tial part of the com­pres­sion stroke, the pis­ton pushes part of the fuel-air mix­ture through the still-open in­take valve, and back into the in­take man­i­fold.

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