Computational Study On One-dimensional Fluid Flow And Design Of Performance Simulation Software For Internal Combustion Engine

In recent twenty years, with the rapid development of computer technology, computationalmathematics and computational fluid mechanics, one-dimensional cycle simulation program is playingan increasingly important role on engine design and performance improvement. Development cycle andcosts of new engine can be reduced greatly. Although commercial software is powerful, new model andalgorithm developed by users are difficult to be integrated into kernel of commercial software becausesource code is secret. Therefore, commercial software can’t support frontier research of internalcombustion engine. It is necessary to develop new model and new program for users. In this thesis,some new models about one-dimensional fluid flow are proposed and the design idea of software isalso presented. New general one-dimensional cycle simulation code with high precision andefficiency is developed.Directed graph in graph theory is used to define the engine thermal fluid network topology. Nodesin directed graph are used to describe all kinds of engine components, and directed edges are used toestablish network topological relationship among components. Using this definition, the engine systemwith arbitrary cylinders, intake and exhaust manifold can be modeled, and engine can also be equippedwith arbitrary series or parallel connected turbochargers. Graph theory is adopted to analyze enginefluid network. Intake and exhaust manifold can be accurately distinguished by depth-first algorithmcombining with features of engine fluid network. Numerical solving order from upstream todownstream can be automatically generated by topological sorting on components in directed graph.These work set a solid base for developing general one-dimensional cycle simulation code.The flow features in intake and exhaust manifold have great influence on overall engineperformance. In order to simulate one-dimensional unsteady flow in intake and exhaust manifoldaccurately, the finite volume method was adopted to divide each pipe into a number of control volumes.High resolution ENO scheme is used to calculate the flux of the internal face of control volumes inpipes. Boundary conditions at the ends of the pipes are obtained by the method of characteristics. Inthis way, the flow conservation can be guaranteed and calculation accuracy is improved. By using the theoretical solution of shock tube, it shows how ENO schemes with different orders affect simulationaccuracy and computational efficiency. The results indicate that simulation accuracy increases as orderof ENO scheme increases. The error between numerical and analytical solutions drops fastest as orderof ENO scheme increases from first-order to second-order. With the improvement of precision, errordrops slower and slower but the computation time grows exponentially. For real application, thesecond-order ENO scheme is recommended because it has high resolution and good computationalefficiency.In order to further improve the computational efficiency of one-dimensional unsteady flow,adaptive local time step marching algorithm is proposed. During calculation, each pipe in engine fluidnetwork has its own time step, which is no longer restricted by time step of other pipe. Inconsistenttime step leads to difficulties of dealing with boundary conditions. By adjusting solving order of pipesdynamically, the above problem is overcome. The practical calculation results indicate that new timestep marching algorithm has no effect on calculation precision and computing time can be reduced by25%-40%.Three way junction model has large impact on simulation precision of pressure wave and massflow in intake and exhaust manifold. Furthermore, overall engine performance can also be affected. Acold wind tunnel experiment with higher air velocity for normal junction in exhaust manifold has beencarried out. The variation law of total pressure loss coefficients is obtained. The measured totalpressure loss coefficients have some deviation from calculated results obtained by Vazsonyi equation,but they have the same trend with flow ratio. The higher the air velocity is, the more obvious thedeviation is. By improving Vazsonyi equation and introducing the distribution function, the correctedpressure loss model considering high air velocity is presented. The calculated pressure before turbineshows that the error of the new pressure loss model has been reduced by4.3%, compared with the oldmodel.On the basis of fluid network defined by directed graph, general framework of cycle simulationprogram is proposed. The efficient data structure is designed to implement the framework. By studyinglogical parallel regions in the serial program, it is the first time to transform the original serial solverinto parallel solver by using parallel algorithm in this article. The modified program is adopted tosimulate one-dimensional fluid flow and the whole engine performance on a multi-core computer. Theresults show that the simulation time with the parallel program can be reduced to one third of the serialprogram.A turbocharged D6114diesel engine test bed was established. The exhaust pressure waves were tested under the entire engine operation range. The simulation code developed by the thesis has beenused to simulate fluid flow in d6114. Under all operating conditions, the measured and calculatedresults were compared and analyzed. The error of simulated average pressure before turbine is under5%.When the engine speed is slow, the error becomes smaller. In one working cycle, the error of flowconservation between the inlet and outlet of exhaust manifold can be controlled under0.053%. Themajor engine performance parameters agree well with measured results.