Variable valve actuation systems for homogeneous Diesel combustion: How interesting are they?

With a high thermal efficiency and with low CO2 (carbon dioxide) emissions, Diesel engines would become leader of tomorrow's transport market. However, the evolution of the regulatory constraints leads to a drastic reduction of their NOx (nitrogen oxide) and particulate emissions. Another interesting competitor to the NOx after-treatment systems is the use of new, combustion concepts to reduce the raw emissions: homogeneous combustion (Homogeneous Charge Compression Ignition - HCCI- or Highly Premixed Combustion - HPC). Nonetheless, such concepts present lion negligible drawbacks such as a limited Zero NO-v operating range, huge external gas recirculation (EGR) needs which forces a complete redesign of the air path circuit maintaining a high level of full load performances, and above all excessive HC (unburned hydrocarbon) and CO (carbon monoxide) emissions at low load. The emerging Variable Valve Actuation technologies (VVA) could totally change the deal of the new combustion processes especially through the internal EGR possibilities and the effective compression ratio reduction. Through the well known dual mode HCCI combustion concept NADI(TM) developed by IFP, this paper aims at evaluating the improvement potential of this kind of combustion concept with the use of several intake and exhaust VVA configurations and makes a comparison between these approaches. The results come from a single cylinder engine equipped with a fully variable intake and exhaust VVA (camless). After a brief description of the VVA system, and the possibilities offered, two aspects of the potential are studied: - the different ways to obtain internal EGR and its main interests with the high potential to reduce HC and CO emissions at low load in homogeneous combustion (HCCI or HPC): 70% reduction on the HC emissions, 40% on CO emissions keeping same NOx emissions level; - the effective compression ratio reduction with a view to increase the maximum load with very low NOx emissions. First, the ways to reduce the effective compression ratio are presented and then, the reduced possibilities given in this field due to decrease of airflow are analysed.