Browse Publications Technical Papers 2010-01-0844
2010-04-12

Quantifying Benefits of Dual Cam Phasers, Lean Mixture and EGR on the Operating Range and Fuel Economy of a PFI NVO CAI Engine 2010-01-0844

Among the existing concepts that help to improve the efficiency of spark-ignition engines at part load, Controlled Auto-Ignition™ (CAI™) is an effective way to lower both fuel consumption and pollutant emissions. This combustion concept is based on the auto-ignition of an air-fuel-mixture highly diluted with hot burnt gases to achieve high indicated efficiency and low pollutant emissions through low temperature combustion. To minimize the costs of conversion of a standard spark-ignition engine into a CAI engine, the present study is restricted to a Port Fuel Injection engine with a cam-profile switching system and a cam phaser on both intake and exhaust sides.
In a 4-stroke engine, a large amount of burnt gases can be trapped in the cylinder via early closure of the exhaust valves. This so-called Negative Valve Overlap (NVO) strategy has a key parameter to control the amount of trapped burnt gases and consequently the combustion: the exhaust valve-lift profile. The present study shows that intake valve-lift profile, air-fuel ratio and EGR are also relevant parameters to control CAI combustion mode.
First, a reference CAI operating area with constant valve-lift timings and stoichiometric mixture is presented. Specific intake and exhaust valve-lift profiles with reduced maximum valve-lift and opening duration are implemented. The gains associated with dual cam phasers and lean burn mixture on both the CAI operating area and fuel consumption are then detailed. Lastly, the positive impact of EGR on the extension of the CAI operating area towards higher loads and on the improvement of efficiency is quantified.
To complete the test bench data, the operating points are analyzed with system simulation to investigate the mean in-cylinder conditions (species and temperature) just before auto-ignition and to understand the consequences of the different strategies. An energy balance on a standard spark-ignition operating point and on various CAI operating points is also presented to point out the origins of the fuel economy.

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