The effects of spark timing, unburned gas temperature, and negative valve overlap on the rates of stoichiometric spark assisted compression ignition combustion Laura Manofsky Olesky a,⇑ , Jason B. Martz a , George A. Lavoie a , Jiri Vavra b , Dennis N. Assanis c , Aristotelis Babajimopoulos d a W.E. Lay Automotive Laboratory, University of Michigan, 1231 Beal Avenue, Ann Arbor, MI 48109-2121, United States b Josef Bozek Research Center, Czech Technical University in Prague, Technická 4, CZ-166 07 Praha 6, Czech Republic c 407 Administration Bldg., Chief Academic Office, Stony Brook University, Stony Brook, NY 11794-1401, United States d 151 Light Engineering, Department of Mechanical Engineering, Stony Brook University, Stony Brook, NY 11794-2300, United States highlights " A SACI engine study demonstrated independent control of burn rate and phasing. " Variations in SACI burn rate affected ringing intensity and combustion stability. " Burn rate had little effect on thermal efficiency at constant phasing. " Stratification due to NVO had minimal effect on combustion rate and phasing. article info Article history: Received 16 October 2012 Received in revised form 25 December 2012 Accepted 11 January 2013 Available online 11 February 2013 Keywords: Auto-ignition Flame propagation Homogeneous charge compression ignition Spark ignition Spark assisted compression ignition Low temperature combustion abstract Spark assisted compression ignition (SACI) combustion is a potential means of extending the high load limit of homogeneous charge compression ignition (HCCI) while maintaining high thermal efficiency. In these experiments, an HCCI engine equipped with fully-flexible valve actuation was used to explore the effect of spark assist in controlling peak heat release rates. Fueling rate was held constant at 19 mg/cycle with a net indicated mean effective pressure (IMEP n ) of 6.5 bar. The fraction of flame heat release was varied from 18% to 34% by controlling spark timing and unburned gas temperature (T u ) via changes in internal and external exhaust gas recirculation (EGR). Internal EGR was adjusted by varying the duration of negative valve overlap (NVO). This strategy also allowed mixture composition and equiv- alence ratio (/ = 1.0) to be maintained. It was found that combustion phasing (CA50) could be held con- stant while reducing peak heat release rates by 40% and ringing intensity by 75% with no penalty in thermal efficiency. Heat release analysis showed that the effects of SACI were consistent with changes in estimated laminar flame speed near the time of spark and a relatively constant temperature at the onset of end-gas auto-ignition. For the range of NVO investigated, potential variations in thermal and compo- sitional stratification were found to have a weak effect on burn characteristics, affirming that tempera- ture and spark timing were the primary variables affecting SACI burn rates under the conditions studied. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction While homogeneous charge compression ignition (HCCI) com- bustion has shown the potential to deliver high thermal efficiency and low emissions compared to its spark-ignited counterpart, the implementation of HCCI remains a challenge due to its lack of a di- rect ignition timing control mechanism. Spark assisted compres- sion ignition (SACI) has been shown to provide such a mechanism, helping to control the phasing and stability of a pri- marily auto-igniting charge [1]. Optical engine studies suggest that the spark triggers a turbulent premixed flame propagating within a highly dilute mixture, which then induces auto-ignition of the sur- rounding charge, primarily via compression heating [1–3]. Previ- ous modeling work has shown that laminar flames can be sustained in ultra-dilute mixtures as long as the temperature of the unburned charge is elevated by means of charge pre-heating 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.01.038 ⇑ Corresponding author. Tel.: +1 734 647 1409; fax: +1 734 764 4256. E-mail addresses: manofsky@umich.edu (L.M. Olesky), jmartz@umich.edu (J.B. Martz), glavoie@umich.edu (G.A. Lavoie), jiri.vavra@fs.cvut.cz (J. Vavra), dennis.assanis@stonybrook.edu (D.N. Assanis), aristotelis.babajimopoulos@ stonybrook.edu (A. Babajimopoulos). Applied Energy 105 (2013) 407–417 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy