Fiber Optic Sensor for Crank Angle Resolved Measurements of Burned Gas Residual Fraction in the Cylinder of an SI Engine

A fiber optic infrared spectroscopic sensor was developed to measure the crank angle resolved residual fraction of burned gas retained in the cylinder of a four-stroke SI engine. The sensor detected the attenuation of infrared radiation in the 4.3 μm infrared vibrational-rotational absorption band of CO₂. The residual fraction remaining in the cylinder is proportional to the CO₂ concentration. The sensor was tested in a single-cylinder CFR spark ignition engine fired on propane at a speed of 700 rpm.

The sensor was located in one of two spark plug holes of the CFR engine. A pressure-transducer-type spark plug was used to record the cylinder pressure and initiate the spark. The temporal resolution of the measurements was 540 μs (equivalent to 2.3 crank angle degrees) and the spatial resolution was 6 mm. Measurements were made during the intake and compression stroke for several intake manifold pressures. The compression ratio of the engine was varied from 6.3 to 9.5. As expected, the residual mass fraction was found to fall rapidly during the start of the intake stroke. Fluctuations in the local burned gas concentration were a maximum near BDC of the intake stroke at which time the local burned gas concentration also reached a minimum. Cycle-to-cycle variations in the residual gas fraction were evident. The residual gas fraction increased as load was reduced and as the compression ratio was reduced.