Simulation And Experimental Investigation Of The Combustion And Emission Performance Of A STC Diesel Engine

With the continuous development of the diesel engines with high power density andwide speed range and the increase of the standards on marine diesel emissions, therequirements for the turbocharging systems to match the diesel engines are also increasing.Sequential turbocharging (STC) system is an effective approach to improve the fuel economyand the exhaust performance. To study the improvement of the engine intake and combustioncharacteristics under the STC system at low operating condition, the diesel engine simulationmodel has been established in this thesis. The transient switching experiments of the STCsystem have been described to obtain the optimal switching control strategy and reduceSOOT emission during the turbocharger switch process.Taking TBD234V12diesel engine as the research object and using reverse engineeringtechnology, a diesel engine geometry models were established. Considering the workingprocess of diesel engine with high compression and transient characteristics, the RNGturbulence model was adopted and transient calculation of the PISO algorithm was used. Thethree dimensional combustion models were verified by test, the results show that the modeland method of calculation satisfy the needs of the research.Using the established three-dimensional combustion model on the TBD234V12dieselengine at the switching point,1TC and2TC intake flow law was obtained under comparativestudy. The calculation results showed that1TC and2TC backflow phenomenon happenedwhen the intake valve opens. The curve of the turbulent kinetic energy shows that in thewhole intake process,1TC and2TC turbulent kinetic energy changes from small to big,experiencing two peaks and then decaying.1TC turbulent kinetic energy is greater than2TC,the biggest increase rate is38%. Swirl ratio curve during the1TC and2TC intake processshows the swirl has been produced in the early intake stage, and it becomes bigger with theincrease of valve lift. During the increase, swirl ratio will reach a maximum value atmaximum moment of the valve lift, and then decrease with the decline of the valve liftgradually.1TC swirl ratio is greater than2TC, and the biggest increase rate is29%.On the basis of the law of the inlet flow of sequential turbocharging diesel engine,combustion and emission process under the switching point between1TC and2TC wasstudied by numerical simulation. The calculation and analysis of the cylinder flow field of the1TC and2TC show that1TC combustion chamber produced more evident large-scale swirl than2TC, which was good for fuel atomization, vaporization and mixing with air. Theemission concentration calculation show that1TC SOOT formation was mainly concentratedin the fuel movement area, and2TC SOOT formation was mainly concentrated in thecombustion chamber recess. Comparison with2TC,1TC SOOT generation was decreased by75%and NOx generation decreased by20%.On the basis of the research on the combustion and emission characteristics of aturbocharged diesel engine under steady state, a turbocharged diesel engine switchingtransient test system was designed, and transient test about the cut-in and cut-out process ofcontrol turbocharger under a switching point was conducted. The transient test of thecontrolled turbocharger cut-in process show that if the air valve and the gas valve are openedat the same time or the air valve delay opens too long, it will cause the engine speed tofluctuate too much, and it is not conducive to the stability of transient switching process.Based on the experimental data,0.5seconds is the best for the appropriate switch delay time.The transient test of the controlled turbocharger cut-out shows that if the air valve and the gasvalve close at the same time, the cylinder pressure can rise steadily and the engine speed canstay stable in the process of switching. If the delay time to close the air valve is too long, itwill make the main turbocharger pressure drop over a longer period, and the engine speedfluctuation will be too big, as a result, the air valve closing delay time should not exceed0.5seconds.On the basis of the research on the SOOT and the NOx formation mechanism, transientSOOT emission curve during the cut-in and cut-out process of controlled turbocharger undera switching point was analyzed. The results show that in the controlled turbocharger cut-inprocess, the SOOT emissions is the minimum when switching time delay is0.5seconds; in thecut-out process, SOOT emissions is the minimum when switching time delay is0second. Inorder to analyze the SOOT formation characteristics in the transient switching process, atransient boundary three-dimensional simulation calculation was conducted when theswitching delay times of the controlled turbocharger cut-in process were0seconds,0.5seconds and1.5seconds. The calculation obtained the following results: in-cylinder SOOTconcentration field, temperature field, concentration field of oxygen and SOOT curve.Numerical simulation analysis shows that the SOOT emissions deteriorates because transientturbocharger response delays while switching, instantaneous excess air coefficient decreases, the mixture of the oil and gas is not uniform in the cylinder, the high temperature and hypoxialocal appeared, so the SOOT emissions is relatively increased.Optimal control strategy of the opening and closing time during the cut-in and cut-outprocess of control turbocharger under a switching point was obtained. The air valve shouldopen when switching time delay is0.5seconds. During the controlled turbocharger cut-outprocess, gas valve and air valve should close at the same time. This can make the engine andturbocharger run smoothly, and can effectively reduce the SOOT emissions in the transientswitching process.