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To achieve carbon-neutrality, internal combustion engines need to further improve their thermal efficiency to reduce CO2 emissions. To accomplish this, it is necessary to quantify and enhance five factors that control indicated thermal efficiency: compression ratio, specific heat ratio, combustion duration, combustion timing, and heat transfer to wall. In this work, quantitative targets for each factor were defined, which were derived from a simulation that considered the influence of heterogeneity of diesel combustion on thermal efficiency. The simulation utilized a two-zone combustion model. In particular, the targets for the combustion duration, combustion timing and heat transfer to wall were increased significantly compared to those for a conventional engine, in anticipation of an expansion of the load range of premixed charge compression ignition (PCI) combustion to higher loads.

The combustion process, emission formation and the resulting engine performance in a diesel engine are well known to be governed mainly by spray behaviors and the consequent mixture formation quality. One of the most important factors that affect the spray development is the nozzle configuration. Originally, single-hole diesel injector is usually applied in fundamental research to provide insights into the spray characteristics. However, the spray emerging from a realistic multi-hole injector approaches the practical engine operation situation better. Meanwhile, previous research has shown that the reduced nozzle hole diameter is effective for preparing more uniform mixture. In the current paper, a study about the effects of nozzle configuration and hole diameter on the internal flow and spray properties was conducted in conjunction with a series of experimental and computational methods.

Spray characteristics under very small injection amount injected by the hole-type nozzle for a D.I. Diesel engine were investigated using the spray test rig consisting a high-pressure and high-temperature constant volume vessel with optical accesses and a common rail injection system. The Laser Absorption Scattering (LAS) technique was used to visualize the liquid and vapor phase distributions in the evaporating spray. In the very small injection amount condition of the evaporating and free (no wall impingement) spray, the both spray tip penetration and spray angle are larger than those of the non-evaporating free spray. This tendency contradicts the previous observation of the diesel spray with large injection amount and the quasi steady state momentum theory. In the case of the spray impinging on a 2-dimensional piston cavity wall, the spray tip penetration of the evaporating spray is larger than that of the non-evaporating spray.

A new nitrogen oxide (NOx) reduction concept is suggested. A strong vertical vortex generated within the combustion bowl can mix hot burned gas into the cold excess air at the center of the combustion-bowl. This makes the burned gas cool rapidly. Therefore, it is possible to reduce NOx, which would be produced if the burned gas remained hot. In this paper the effect was verified with a 3D-CFD analysis of spray, air, combustion gas, and thermal efficiency as well as experiments on a 4-cylinder 2.0-liter direct injection diesel engine. The results confirmed that the vertical vortex was able to be strengthened with the change of spray characteristics and the combustion bowl shapes. This strengthened vertical vortex was able to reduce NOx by approximately 20% without making smoke and thermal-efficiency worse. Above results proved the effectiveness of this method.