13th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 26-29 June, 2006 - 1 - In-Situ Fuel Concentration Measurement Using an IR Spark Plug Sensor by Laser Infrared Absorption Method - Application to a Rotary Engine- Nobuyuki Kawahara 1 , Eiji Tomita 2 , Kenta Hayashi 3 , Michihiko Tabata 4 , Kouhei Iwai 4 , and Ryoji Kagawa 4 1: Department of Mechanical Engineering, Okayama University, Okayama, JAPAN, kawahara@mech.okayama-u.ac.jp 2: Department of Mechanical Engineering, Okayama University, Okayama, JAPAN, tomita@mech.okayama-u.ac.jp 3: Department of Mechanical Engineering, Okayama University 4: Mazda Motor Corp., Hiroshima, JAPAN Abstract Cycle-resolved measurements of the fuel concentration near a spark plug in both a commercial spark-ignition piston engine and a commercial rotary engine were performed. An in situ laser infrared (IR) absorption method was developed using a spark plug sensor and a 3.392-μm He–Ne laser as the light source. This wavelength coincided with the absorption line of hydrocarbons. The newly developed IR spark plug sensor had a higher signal-to-noise ratio than its previous version due to the optimization of its quartz lens and two optical fibers. The new sensor provided quantitative cycle-resolved fuel concentration measurements around the spark plug with a high temporal resolution. At lean preset air/fuel (A/F) ratios, fuel was mixed with the surrounding air gradually near the spark plug. Strong mixture inhomogeneities were measured during the compression stroke; the magnitude of these inhomogeneities decreased throughout the compression stroke. There was a strong correlation between the fuel concentration measured with the spark plug sensor and the combustion characteristics during the initial combustion period, which occurred faster when conditions were slightly richer than stoichiometric near the spark plug. The indicated mean effective pressure (IMEP) was slightly related to the A/F ratio near the spark plug. It was possible to measure the cycle-resolved A/F ratio near the spark plug and investigate its cycle-to-cycle fluctuations to achieve stable operation using the newly developed spark plug sensor. 1. Introduction Requirements of spark-ignition (SI) engine are lower fuel consumption and reduction of pollutants. The Wankel rotary engine is an alternative to a reciprocating piston engine (Heywood 1988, Stone 1999, van Basshuysen and Schäfer 2004). The advantages of this rotary engine are its compactness, higher engine speed (resulting in more power), inherent balance, and smoothness. The disadvantages are its sealing and leakage problems, lower efficiency, and higher unburned hydrocarbon emissions resulting from the flattened combustion chamber shape. Since the 1960s, several studies have attempted to improve the fuel efficiency and exhaust emissions of the Wankel rotary engine (Kanbara, et al. 2003, Jones et al. 1971, Shimamura et al. 1981, Muroki and Fujimoto 1987a, Muroki 1987b, Shimizu, et al. 1995). Recently, a new side exhaust port rotary engine has been developed (Tadasu et al. 2003, Masaki et al. 2004) that reaches higher speeds through the use of lightweight rotating parts, optimizes the intake air according to the engine speed, and uses three fuel injectors per rotor to generate high torques at both low and high engine speeds up to 9,000 rpm. The fuel injectors and spark plugs have also been upgraded to improve the fuel economy of the engine. The flow field in the combustion chamber and the fuel concentration around the spark plugs should be optimized in commercial rotary engines to improve their fuel economy. Mixture composition of the unburned charge fundamentally influences the combustion process in a spark ignition engine. The fuel concentration around the spark plug together with the fluid motion strongly influences the combustion initiation duration. This causes cycle-to-cycle variations, which can become large in spark-ignition engines.