ElsevierBenajes Calvo, JV.; Novella Rosa, R.; Pastor Enguídanos, JM.; Hernández-López, A.; Hasegawa, M.; Tsuji, N.; Emi, M.... (2016) 6-1, Yokohama (Japan) 10 Jordi Martorell, Marcos Alonso 11Nissan Technical Centre Europe-Spain 12Zona franca sector 080540, Barcelona (Spain) 14Abstract 15 The research in the field of internal combustion engines is currently driven by the needs of decreasing 32The results confirmed the limited benefits, in terms of fuel consumption, around 2%, with constant NOx 33 emission achieved when optimizing the engine hardware, while keeping air management and injection 34 settings. Thus, including air management and injection settings in the optimization is mandatory to 35 significantly decrease the fuel consumption, by around 5%, while keeping the emission limits. 36 37 38

The purpose of this study is to clarify the state of the heat loss in a direct-injection diesel engine. Originally developed thin-film thermocouples (TFTs) are embedded into the combustion chamber walls for accurate measurement of instantaneous surface temperature from which instantaneous heat flux is evaluated through the heat conduction equation. Measured points are arrayed on the cavity bottom, the cavity side wall, the piston top, and the cylinder head. The TFTs are designed and fabricated so that disturbance of the temperature field is minimized when they are embedded into the combustion chamber walls. As a result, it is observed that the behaviour of instantaneous temperature and heat flux depends on the radius of the measured point. Measured points located radially inwards seem to be influenced by the combustion flame considerably more than those located outwards because the flame is presumed to stay in and around the cavity which occupies a region around the central axis of the piston. On the other hand, the heat loss ratio, namely the ratio of lost heat to the heat supplied by the fuel, is larger than in a gasoline engine.

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Thin-film thermocouples were used to measure the instantaneous temperature at 100 points on all the combustion chamber wall surfaces in a naturally aspirated direct-injection diesel engine. Instantaneous heat flux at each measured point was also obtained through heat transfer analysis with the measured instantaneous wall surface temperature applied as a boundary condition. In addition, the instantaneous mass-averaged gas temperature in the combustion chamber was calculated through the equation of state of an ideal gas. As a result, the local and overall heat transfer coefficients were evaluated using the corresponding wall surface temperatures and heat fluxes. The overall heat transfer coefficients thus obtained were compared with those calculated with Eichelberg's and Woschni's empirical equations for five ignition timings and three engine speeds. As a result, it was revealed that an overall average heat transfer coefficient obtained through the authors' experiments has characteristics different from those of the heat transfer coefficients calculated from the empirical equations proposed by Eichelberg and Woschni.