A quartz pressure transducer was flush-installed in the region between the intake-exhaust valves at the side of the spark plug. The transducer allowed performing in-cylinder pressure measurements in real time. An elongated engine piston was used; it was flat and its upper part was transparent since it was made of fused silica UV enhanced (Φ = 57 mm). To avoid window contamination by the lubricating oil, self-lubricant Teflon-Bronze composite piston rings were used in the optical section.
To reduce the initial conditions effects, the engine was preheated by a conditioning unit, and it was maintained in motored condition by an electrical engine until the temperature reached 65°C. After the warm up, the engine worked in fired conditions for 300 consecutive cycles. The engine parameters and pressure measured in the last 200 cycles were considered in the work. Then the engine returned to motored condition for 100 cycles. This phase allowed checking a possible change in thermal and fluid dynamic status of the engine from the beginning of the measurements.
Figure 1 shows the experimental apparatus for the optical investigations and the bottom field of view of the combustion chamber. During the combustion process, the light passed through a quartz window located in the piston, and it was reflected toward the optical detection assembly by a 45°-inclined UV-visible mirror located at the bottom of the engine.