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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.

The emissions reduction potential of neat GTL (Gas to Liquids: Fischer-Tropsch synthetic gas-oil derived from natural gas) fuels has been preliminarily evaluated by three different latest-generation diesel engines with different displacements. In addition, differences in combustion phenomena between the GTL fuels and baseline diesel fuel have been observed by means of a single cylinder engine with optical access. From these findings, one of the engines has been modified to improve both exhaust emissions and fuel consumption simultaneously, assuming the use of neat GTL fuels. The conversion efficiency of the NOx (oxides of nitrogen) reduction catalyst has also been improved.

A study has been made of an automotive direct-injection diesel engine in order to identify the effects of the combustion chamber geometry on combustion, with special emphasis focused on a re-entrant combustion chamber. Conventional combustion chambers and a re-entrant one were compared in terms of the combustion process, engine performance and NOx and smoke emissions. Heat transfer calculations and heat release analyses show that the re-entrant chamber tends to reduce ignition lag due to the higher temperatures of the wall on which injected fuel impinges. Analyses of turbulent flow characteristics in each chamber indicate that the re-entrant chamber enhances combustion because of the higher in-cylinder velocity accompanied by increased turbulence. Further, analyses of in-cylinder gas samples show lower soot levels in the re-entrant chamber. As a result, a good compromise can be achieved between fuel economy and exhaust emissions by retarding the fuel injection timing.

In heavy duty diesel engines, the waste heat recovery has attracted much attention as one of the technologies to improve fuel economy further. In this study, the available energy of the waste heat from a high boosted 6-cylinder heavy duty diesel engine which is equipped with a high pressure loop EGR system (HPL-EGR system) and low pressure loop EGR system (LPL-EGR system) was evaluated based on the second law of thermodynamics. The maximum potential of the waste heat recovery for improvement in brake thermal efficiency and the effect of the Rankine combined cycle on fuel economy were estimated for each single-stage turbocharging system (single-stage system) and 2-stage turbocharging system (2-stage system).

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.

Improvement in heat loss could be an important factor to increase the brake thermal efficiency (BTE) of an internal combustion engine; however, the heat energy saved isn’t all converted to brake work. Theoretically, to increase the conversion efficiency of heat energy into indicated work, the compression (or expansion) ratio and specific heat ratio (γ) are important. Nevertheless, γ has not been well-studied thus far, since it can’t be easily controlled. This study utilized a two-zone model to calculate the time-resolved γ and local excess air ratio of the burned gas (λb), which varied with the heat release rate. The two-zone combustion model, in which the cylinder volume is simply separated into burned and unburned zones to simulate the overall diesel combustion phenomena, was developed to investigate the current status of heterogeneous (diesel) combustion compared to ideal homogeneous combustion.

To overcome the trade-offs of thermal efficiency with energy loss and exhaust emissions typical of conventional diesel engines, a new diffusion-combustion-based concept with multiple fuel injectors has been developed. This engine employs neither low temperature combustion nor homogeneous charge compression ignition combustion. One injector was mounted vertically at the cylinder center like in a conventional direct injection diesel engine, and two additional injectors were slant-mounted at the piston cavity circumference. The sprays from the side injectors were directed along the swirl direction to prevent both spray interference and spray impingement on the cavity wall, while improving air utilization near the center of the cavity.

Cooling loss reduction is essential to enable further increases in thermal efficiency of reciprocating internal combustion engines. Many in-cylinder cooling loss reduction studies have been carried out by applying various thermal barrier coatings to the piston and/or other in-cylinder surfaces, taking advantage of the lower thermal effusivity of ceramic materials. However, the end result was mostly minimal or in some cases, negative. In our previous study, significant cooling loss reduction was experimentally confirmed by utilizing a mirror-like polished stainless-steel thermal sprayed surface (HVOF: high velocity oxy-fuel) on a forged steel piston. This study firstly investigated an alternative insulating layer material to stainless-steel, along with effects of its thickness on heat transfer by a one-dimensional unsteady numerical model. Results showed that lower thermal effusivity doesn’t always reduce heat transfer, but increases nonuniformity of surface temperature.

A study has been made of an automotive direct injection diesel engine designed to reduce exhaust emissions, particularly NOx and particulates, without performance deterioration. Special emphasis has been placed on air-fuel mixing conditions controlled by the fuel injection rate, the intake swirl ratio, and the intake boost pressure. By means of increasing the injection rate, ignition delay can be shortened enough to improve particulate emissions at retarded injection timings. Enhancing the intake swirl velocity contributes to the reduction of soot emission in spite of the deterioration of NOx emission. Supercharging can favorably enhance diffusion combustion resulting in improved fuel economy for retarded injection timings and reduced emissions. As a result, a good compromise can be achieved between fuel economy and exhaust emissions by increasing the injection rate along with retarding the injection timing. Supercharging was found to be more favorable than swirl enhancement.

This study focused on the technology integration to aim beyond 60% indicated thermal efficiency (ITE) with a single-cylinder heavy-duty diesel engine as an alternative to achieve 55% brake thermal efficiency (BTE) with multiple-cylinder engines. Technology assessment was initially carried out by means of a simple chart of showing ITE and exhaust heat loss as functions of cooling loss and heat conversion efficiency into indicated work. The proposed compression ratio (28:1), excess air ratio and new ideal thermodynamic cycle were then determined by a simple cycle calculation. Except for peak cylinder pressure constraint for each engine, the technical barriers for further ITE improvement are mainly laid in cooling loss reduction, fuel-air mixture formation improvement, and heat release rate optimization under very high temperature and density conditions with very high compression ratio (smaller cavity volume).