Study On Ultra-Low Emissions Based On Multi-Injection Strategy Coupling With EGR Technology

With high thermal efficiency, good economy and reliability, and wide power ranges, diesel engine is widely used in power machinery; with viechel population increased year by year, controling diesel engine emissions has become a wide concern all over the world.A necessary technique meeting Euro-â…¢and further higher emission control regulations is an adoption of Electronic Fuel Injection Technology, which promotes the air and fuel mixture and optimizes the combustion process by increasing the injection pressure. Nowadays mostly turbocharged HD diesel engines are equipped with high-pressure common-rail fuel injection system, which can meet the requirement of Euroâ…¢emissions regulations after the conditions calibration test. With a view to further strict diesel engine emission regulations, In addition to combining machine outside the decontamination measures to meet emissions regulations requirements, The current study focused on the adoption of mixed-time control (quasi-homogenous mixture formation) and chemical process control (combustion reaction control) to achieve optimization of the combustion path, which in essence are coupled with high-pressure common-rail injection and EGR technology, strengthen the combustion gas mixture of high discrete distribution based on the active control between energy mix and mixed atmosphere under different periods, and migrate to the low temperature and qualitative distribution to avoid the generation regions of NOx and PM in order to achieve ultra-low emissions.Because most of time diesel engine operated in start, acceleration and deceleration transient conditions on the road, the emission concentrations are very different with ones in steady conditions, and especially obviously appeared on the diesel engine equipped with turbochargers, smoke emission is particularly prominent. With the more stringent emission regulations and the increasing prominence of environmental issues, it has a very important practical significance to study the influent law on combustion and emissions in transient conditions for deeply understanding the combustion characteristics and improving the emissions. However, due to the complex transient conditions and expensive testing bench of transient process, the research on the engine performances and emission under transient operating conditions is still at the preliminary stage.On the one hand to explore how fast to form the air-fuel mixture and how to organize combustion control to achieve the purpose of energy saving and low emission under rich homogeneous mixture conditions, on the one hand to solve the problems of combustion deterioration and obviously soot emissions generated in the transient process on a turbocharged diesel engine, a large number of tests and simulations were conducted on a CA6DL2-35E3 high-pressure common-rail turbocharged and intercooled heavy-duty diesel engine. Main contents and conclusions are summarized as follows:1.A test bench was set up. It includes a measurement system, EGR control system and fuel injection control system.1) A real-time parameter measurement system was setup by a Milliseconds A/D data acquisition system with high-response sensors, which realized the real-time measurement and recording (10 ~ 100ms) of engine performances and emissions;A single-cycle dynamometer measurement was achieved by the angle calibrator and IndiPar Combustion Analyzer2) Exterior low-press-loop EGR system was applied. Through hardware and software designs of the electronic throttle and electronically controlled EGR system, it realized full-condition ranges of throttle adjustment and full-degree ranges of EGR valve adjustment within 3% accuracy error.3) Injection modes (pilot injection (PII), main injection (MII) and post injection (POI)), injection timings (PII, MII, POI) and injection pressures were independently controlled based on the common rail injection calibration system.2.Aimed at energy conservation and ultra-low emissions, coupling EGR and flexible injection control by means of ETAS calibration and CFD calculation system was conducted to explore the possibility of achieving low temperature combustion(LTC) mode with EGR in the ESC-B speed(1650r/min) condition.1) Boosting injection pressure can improve all performance except NOx emission, combined with delay of injection timing, a trade-off point could be determined, which can totally improve the engine performances.2) EGR has a significant effect on promoting fuel-air mixture under low load conditions and strengthening the combustion speed control under high load conditions, which demonstrates that EGR is an effective measurement to realize the low temperature combustion mode.3) After post injection (POI) is added, decreasing the time interval between main and post injection under the promise of sufficient oxygen in the spray direction, and selecting the proper quality of post injection to reinforce the disturbance of air-fuel mixture at the late injection, which can realize the control of air-fuel mixture under the whole combustion progress and the performance optimization. Therefore, collaboratively optimizing combustion process with in-cylinder flow characteristic, chemo-atmosphere and thermo-atmosphere based on multi-injection could be considered as a time scale combustion.4) Collaboratively controlling in-cylinder flow status and chemo-atmosphere migration with three-stage injection realized the applicable conditions of time scale combustion in full speed up to 60% load rate. According to the EGR dual nature of extending premixed time and restraining combustion rate and the characteristic that post injection provides soot allowance under EGR strategy, EGR intervention realized LTC mode with low combustion noise.5) It is difficult to realize the decrease of PM and NOx emissions at the same time by enhancing injection pressure and post injection for better air-fuel mixture or controlling the combustion process by EGR. It is feasible that Coupling with EGR, the time scale combustion based on controlling in-cylinder flow status and chemo-atmosphere migration with three-stage injection collaboratively can optimize the combustion path to achieve ultra-low emissions.3.Transient contol and measurement systems were setup. According to EGR measurement ways about poor response of CO2 tracer method and poor accuracy measurement of temperature method, Coriolis flowmeter measurement system was used as the EGR measurement system, which realizes the good linearity, stability, accuracy and response characteristics of real-time transient EGR measurement.4. To solve the combustion deterioration and obviously soot emissions generated in the transient process on a turbocharged diesel engine, study on basic prototype engine performances and emissions in EGR step response and CTIS process was conducted.1) The actual EGR flow has a linear relationship with EGR-flow?s analog output within 5% maximum error; EGR rate step response period is about 0.7s in 50% load rate conditions of different speeds as the same with EGR system so that it can be used for real-time measurement of EGR rate; The response periods of Basic prototype engine performances are as the same as EGR rate response period; because of delayed response of combustion boundary conditions, the response periods of combustion parameters and emissions are much longer than EGR rate one.2) After optimizing the EGR rate in ESC-B speed conditions, CTIS processes test (from 10% to 90% load rate) in different transient times were conducted. Due to the effects of the delayed response of turbocharger and overshooted EGR, soot emission seriously deteriorated; Especially in the condition of large transient rate (5s transient time), EGR overshooted obviously and caused smoke opacity was increased by 50%; transient rate had less effect on NOx emissions.3) Under ESC-B speed conditions, adopting low temperature combustion mode which is coupling with EGR and multi-injection in moderate load conditions (from 10% to 60% load rate), and EGR optimization in heavy load conditions (from 60% to 90% load rate) realized the optimization of engine performances and emissions; CTIS process test was conducted by mode convention in 5s transient time. Because the datas of EGR rate is similar to the barely optimized EGR ones in stedy conditions, overshooted EGR has not been alleviated; compared with the emissions between mode convention optimization and barely EGR optimization, soot emission has not deceased, and NOx emission is significantly reduced with a 15% drop.4) A transient response optimization strategy was brought forward by adapting delayed intake air to improve air-fuel ratio based on the fuel injection non-linear correction. The transient condition test with 5s transient time and EGR optimization of steady conditions was conducted. The results showed that due to lack of air-fuel Mixing time in heavy load rate conditions it caused a serious deterioration in soot emission, and smoke opacity is up to 90%; Considering the quality ratio of air and fuel mixture and the mixture time distribution during different stages of transient conditions, EGR overshoot was effectively controlled and soot emission was Significantly decreased with a 20% drop compared with barely EGR optimization.5) Closing the EGR valve can successfully reduce soot emission in the transient process. Selecting the EGR closure time relatively delayed 2.5s with transient Start moment can achieve smoke opacity down to 15%under the conditions of 5s transient time and the EGR valve corresponding to less than 10% smoke opacity in 90% load rate.