Variable Valve Actuation Systems for Homogeneous Diesel Combustion: How Interesting are They?
Abstract
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 non negligible drawbacks such as a limited zero NOx 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™ 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 air flow are analysed.
Domains
Physics [physics]
Origin : Publication funded by an institution
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