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To reduce heat transfer between hot gas and cavity wall, thin Zirconia (ZrO2) layer (0.5mm) on the cavity surface of a forged steel piston was firstly formed by thermal spray coating aiming higher surface temperature swing precisely synchronized with flame temperature near the wall resulting in the reduction of temperature difference. However, no apparent difference in the heat loss was analyzed. To find out the reason why the heat loss was not so improved, direct observation of flame impingement to the cavity wall was carried out with the top view visualization technique, for which one of the exhaust valves was modified to a sapphire window. Local flame behavior very close to the wall was compared by macrophotography. Numerical analysis by utilizing a three-dimensional simulation was also carried out to investigate the effect of several parameters on the heat transfer coefficient.

This study investigates simultaneous improvement in thermal efficiency and cooling loss in the wider operating condition. To suppress the heat flux of the piston, the piston top and cavity were treated with thin thermal spraying of stainless steel. Thermal diffusivity of stainless steel (X5CrNiMo17-12-2, SUS316) is very low in comparison with the forged steel piston raw material (34CrMoS4, SCM435) to sustain local surface temperature at where spray flame directly interfered. In addition, its surface roughness was very fine finished aiming to reduce the convective heat transfer. The experimental results with the stainless-steel coated piston by utilizing a single cylinder engine showed the significant improvement in both cooling loss and thermal efficiency even in higher load operating conditions with compression ratio of 23.5:1.

Impingement of a spray flame on the periphery of the piston cavity strongly affects heat loss to the wall. The heat release rate history is also closely correlated with the indicated thermal efficiency. For further thermal efficiency improvement, it is thus necessary to understand such phenomena in state of the art diesel engines, by observation of the actual behavior of an impinging spray flame and measurement of the local temperature and flow velocity. A top-view optically accessible engine system, for which flame impingement to the cavity wall can be observed from the top (vertically), was equipped with a high speed digital camera for direct observation. Once the flame impinged on the wall, flame tip temperature decreased roughly 100K, compared to the temperature before impingement.

An experimental study has been made of a single cylinder, direct-injection diesel engine having a re-entrant combustion chamber designed to enhance combustion so as to reduce exhaust emissions. Special emphasis has been placed on controlling the inert gas concentration in the localized fuel-air mixture to lower combustion gas temperatures, thereby reduce exhaust NOx emission. For this specific purpose, an exhaust gas recirculation (EGR) system, which has been widely used in gasoline engines, was applied to the DI diesel engine to control the intake inert gas concentration. In addition, supercharging and increasing fuel injection pressure prevent the deterioration of smoke and unburned hydrocarbons and improve fuel economy, as well.

This paper describes the combustion optimizations of heavy-duty diesel engines for the anticipated future emissions regulations by means of an electronically controlled common rail injection system. Tests were conducted on a turbocharged and aftercooled (TCA) prototype heavy-duty diesel engine. To improve both NOx-fuel consumption and NOx-PM trade-offs, fuel injection characteristics including injection timing, injection pressure, pilot injection quantity, and injection interval on emissions and engine performances were explored. Then intake swirl ratio and combustion chamber geometry were modified to optimize air-fuel mixing and to emphasize the pilot injection effects. Finally, for further NOx reductions, the potentials of the combined use of EGR and pilot injection were experimentally examined. The results showed that the NOx-fuel consumption trade-off is improved by an optimum swirl ratio and combustion chamber geometry as well as by a new pilot concept.

Heat loss reduction could be one of the most promising methods of thermal efficiency improvement for modern diesel engines. However, it is difficult to fully transform the available energy derived from a reduction of in-cylinder heat loss into shaft work, but it is rather more readily converted into higher exhaust heat loss. It may therefore be favorable to increase the effective expansion ratio of the engine, thereby maximizing the brake work, by transforming more of the enthalpy otherwise remaining at exhaust valve opening (EVO) into work. In general, the geometric compression ratio of a piston cylinder arrangement has to increase in order to achieve a higher expansion ratio, which is equal to a higher thermodynamic compression ratio.

The heterogeneous character of diesel engine combustion is well-known. However, in the thermodynamic efficiency calculations, a homogenous combustion is generally assumed. This results in poor accuracy of specific heat ratio estimations. Therefore, this study aims to evaluate how the real diesel engine specific heat ratio behaves by means of a two-zone model calculation. Efficiency improvement from a higher burned zone specific heat ratio was investigated. This was achieved by better air entrainment and a highly dispersed flame in the cylinder. To investigate into the local phenomena, combustion homogeneity was estimated by utilizing the two-zone model where the in-cylinder volume was divided into unburned zones and burned zones. To numerically confirm the effect of a highly dispersed flame on the specific heat ratio, a single-cylinder diesel engine equipped with three injectors (located at the cylinder center as well as at the rim of the piston bowl) was utilized.