Single cylinder optical engines are used for internal combustion (IC) engine research as they allow for the application of qualitative and quantitative non-intrusive, diagnostic techniques to study in-cylinder flow, mixing, combustion and emissions phenomena. Such experimental data is not only important for the validation of computational models but can also provide a detailed insight into the physical processes occurring in-cylinder which is useful for the further development of new combustion strategies such as gasoline homogeneous charge compression ignition (HCCI) and Diesel low temperature combustion (LTC). In this context, it is therefore important to ensure that the performance of optical engines is comparable to standard all-metal engines. A comparison of optical and all-metal engine combustion and emissions performance was performed within the present study. The objective was to investigate the principal differences between optical and all-metal engines and how these differences ultimately affect mixing, combustion and emissions formation processes. Experimental results reveal the significant impact of differences in heat transfer characteristics between optical and standard engine piston bowls on combustion phasing and engine-out emissions. Quantitative measurements of piston wall temperatures using a laser induced phosphorescence technique were performed which allowed the subsequent definition of appropriate engine operating strategies so as to compensate for differences in heat transfer properties. Furthermore, differences in combustion chamber geometry were also studied. Geometrical differences can arise as a result of dynamic (compressive/tensile) and thermal loading of the extended piston-liner assembly on the optical engine, potentially leading to changes in the effective compression ratio. In addition, intake charge dilution in optical engines is often achieved via the use of simulated exhaust gas recirculation (EGR). A comparison has been made between simulated EGR (using pure nitrogen) with real EGR under Diesel LTC conditions. Finally, 'pure', single component fuels are often employed in optical Diesel engines due to laser diagnostic constraints. However these fuels generally differ from standard Diesel fuel in terms of cetane number and fuel volatility which can significantly influence the combustion and emissions characteristics in optical engines. These aspects are discussed within the present study. An improved understanding of the differences between optical and all-metal engines has allowed us to develop appropriate strategies to compensate for these differences on the optical engine. It is shown here that combustion phasing (and engine-out emissions) matching between optical and all-metal engines can be achieved even for advanced LTC Diesel combustion strategies. The ability to ensure fully representative combustion and emissions behaviour of optical engines ultimately increases the value of optical engine data, highlighting the importance of using such engines as research tools for the further development of innovative, low emission combustion concepts.
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