Research On Heat Transfer Model Of Diesel Engine Cylinder Working Process

The diesel engine cylinder heat transfer process has a great impact on the power, economy and emissions of the engine, and is a necessary part of cylinder working process simulation and component heat load calculation.Since the1920s, the researchers like Nusselt, Eichelberg, Woschni and Hohenberg summarized a series of experimental correlations, empirical or semi-empirical. Some correlations became the important part of cylinder heat transfer models for a long time, and are widely used.However, with the development of PC level and numerical calculation, the diesel engine cylinder multi-dimensional numerical calculation become possible, and some general and special CFD software turn to the simulation of cylinder working process. The2D or3D results of cylinder multi-dimensional numerical calculation provide the spatial and temporal distribution characteristics about the information of cylinder velocity field, temperature field, pressure field, etc, but the traditional cylinder working process cannot provide above information. In order to balance the computing time and computing resources, these CFD software usually use wall function to simulate the cylinder heat transfer process. As all the prerequisite to derive the wall functions is not suitable at the cylinder environment with fuel injection, combustion and piston movement, leading to large deviation about the calculated values between the wall function and the experiment.With the cylinder multi-dimensional numerical simulation software Converge and the cylinder heat transfer experiment data at the cylinder head’s bottom surfece of a single cylinder diesel engine, the traditional Woschni semi-empirical experiment correction is improved spatially from OD to3D. Compared with the traditional wall function method, the modified Woschni correction agrees well with the experiment value. More specifically, the main conclusions of this paper are as follows:(1) The eccentric piston pit has a greater impact on the pressure field, the velocity field and the turbulent kinetic energy field of fluid near the bottom surface of the cylinder head at the combustion TDC. The field parameters contours still distribute around the cylinder axis.(2) Compared with the heat flux experiment data at the measured points which are at the bottom surface of the cylinder head of the single cylinder diesel engine, the heat flux calculated by Han-Reitz temperature wall function is too large. But the modified Woschni model predicts the peak and the peak phase of wall heat flux well.(3) The HTC temporally shows the increased and then decreased trend on the bottom of the cylinder head, the top of the piston and the liner inner wall, and reaches the peak10℃A-20℃A after the combustion TDC. And the HTC spatially is lower at the central region and the edge region, and higher at the middle annular region. As the contact area of liner is different at different time, the HTC is higher at the top and lower at the bottom axially. The HTC at the circumferential directions is not uniform, and may be related to the eccentric piston pit.(4) The diesel engine cylinder global heat transfer characteristics analysis is helpful to understand the macroscopic characteristics of diesel engine. The distribution of time averaged HTC at the bottom of the cylinder head is basically the same as what at the top surface of the piston, lower at the central region and the edge region, higher at the intermediate annular region. The HTC at the bottom surface of cylinder head is the highest, and the HTC at the top of piston is higher than that at the liner. The global HTC lies between the cylinder head and the piston. The value calculated by the original Woschni model is slightly larger than the modified Woschni model. The two values are more or less the same, and indirectly prove the universal characteristics of the original Woschni model.