小型强化汽油机的超级爆震现象

Limits on Downsizing in Spark Ignition Engines due to Pre-ignition

The combination of gasoline direct injection and supercharging technologies allows the substitution of naturally aspirated engines through downsized supercharged engines with comparable performance. However, increasing the mean effective pressure is limited by the occurrence of unwanted pre-ignition phenomena. The following article provides an insight into the pre-ignition phenomenon and its relevant triggering mechanisms. The presented results stem from a research project by Volkswagen AG Group Research, in cooperation with the Institute for Internal

Combustion Engines and Automotive Engineering at Vienna University of Technology.

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Combustion

1 Introduction

The pursuit to meet the increasing ener-gy demand, the objective to maintain individual mobility as well as future statutory provisions regarding CO 2 emis-sions have seen the automotive industry called on to take suitable measures. Fuel consumption has been continuously re-duced through the introduction of new engine technologies. When it comes to reduce fuel consumption in gasoline en-gines, Volkswagen AG relies on a combi-nation of downsizing, supercharging and direct injection. Its latest engine gen-eration shows high mean effective pres-sure values and is characterised by high efficiency and driving enjoyment. In or-der to raise even further potential on the basis of the downsizing strategy, future engines will achieve even higher mean effective pressure values. However, this increase is associated with unwanted combustion phenomena, which repre-sent the focus of this paper.

2 The Pre-ignition Phenomenon

The pre-ignition phenomenon is more likely to occur at low engine speeds and high charging pressures close to full load. Pre-ignition is initiated when auto-igni-tion conditions are reached in the fuel/air mixture before the spark ignition

time. Therefore, unlike the case of knock-ing phenomena, delaying the spark igni-tion time cannot prevent pre-ignitions.The flame propagation during pre-ig-nition takes place suddenly and leads to rapid pressure increases and consider-able pressure fluctuations in the com-bustion chamber. Figure 1 shows cylinder pressure traces for a regular combustion, a knocking combustion and a combus-tion exhibiting pre-ignition. High cylin-der pressures (> 200 bar) during a pre-ig-nition generally result in engine damage and must be avoided at all costs during regular engine operation.

Investigations on the engine test rig have made it possible to ascertain the fol-lowing influencing parameters and mechanisms for triggering pre-ignition:– oil droplets and deposits – …hot spots“ (hot components in the combustion chamber)– r eaction kinetics.

2.1 Oil Droplets and Deposits

One factor that influences the tendency towards pre-ignition concerns oil drop-lets present in the combustion chamber, as those transported into it inadvertently by the crankcase ventilation. These can be ignited during the compression stroke and therefore function as the origin of a pre-ignition. Furthermore, oil droplets can also be present in deposits in the combustion chamber. The latter can

The Authors

Dipl.-Ing.

Jürgen Willand is Head of the Gaso­line Engines Division at Volkswagen AG Group Research in Wolfsburg (Germany).

Dr.-Ing. Marc Daniel is Researcher in the Field of Thermodyna­mics at the Gasoline Engines Division of Volkswagen AG Group Research in Wolfsburg (Germany).

Dr. Emanuela Montefrancesco is Researcher in the Field of CFD / Reaction Kinetics at the Gaso­line Engines Division of Volkswagen AG Group Research in Wolfsburg (Germany).

Prof. Dr.

Bernhard Geringer is Head of the Institute for Internal Combus­tion Engines at the Vienna University of Technology (Austria).

Dr. Peter Hofmann is Lecturer at the Insti­tute for Internal Com­bustion Engines at the Vienna University of Technology (Austria).

Dipl.-Ing.

Markus Kieberger is Research Assistant at the Institute for Internal Combustion Engines at the Vienna University of Technol­ogy (Austria).

Figure 1: Cylinder pressure traces

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