Effects of ethanol–unleaded gasoline blends on cyclic variability and emissions in an SI engine M.A. Ceviz * , F. Yu ¨ ksel Department of Mechanical Engineering, Faculty of Engineering, University of Atatu ¨ rk, Erzurum 25240, Turkey Received 5 July 2003; accepted 26 July 2004 Available online 21 September 2004 Abstract One important design goal for spark-ignited engines is to minimize cyclic variability. A small amount of cyclic variability (slow burns) can produce undesirable engine vibrations. On the other hand, a larger amount of cyclic variability (incomplete burns) leads to an increase in hydrocarbon consumption and emis- sions. This paper investigates the effects of using ethanol–unleaded gasoline blends on cyclic variability and emissions in a spark-ignited engine. Results of this study showed that using ethanol–unleaded gasoline blends as a fuel decreased the coefficient of variation in indicated mean effective pressure, and CO and HC emission concentrations, while increased CO 2 concentration up to 10 vol.% ethanol in fuel blend. On the other hand, after this level of blend a reverse effect was observed on the parameters aforementioned. The 10 vol.% ethanol in fuel blend gave the best results. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Cyclic variations; SI engines; Exhaust emissions 1. Introduction From the measurements of the pressure–time history of consecutive cycles in the combustion chamber in an SI engine, it can be easily seen that variations from one cycle to another exist. 1359-4311/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.07.019 * Corresponding author. Fax: +90 442 235 4493. E-mail address: aceviz@atauni.edu.tr (M.A. Ceviz). www.elsevier.com/locate/apthermeng Applied Thermal Engineering 25 (2005) 917–925