Dynamic Characteristic Of High Speed Direct Injection Diesel Engine Air-fuel Mixture Formation And Its Effect On Combustion

As the internal combustion engine provides people more convenience it also brings about serious energy crisis and environment pollution. At the circumstance of whole word advocate low carbon economy trend, the energy crisis and environment protection becomes the hot topic. Current diesel engine already has high thermal efficiency, high power, high torque, low specific fuel consumption, low exhaust emission and low noise et al. excellent properties. Compare with identical capacity gasoline engine the diesel can save 15%~ 30% fuels, decrease 35% CO2 emission and exhaust little CO and THC. And so, the green house gas decreased at least 45% totally. For the diesel engine has aforementioned advantage, more and more people willing to accept diesel engine. Along with the enhancement of environmental concern and comfort the entire researcher should make further effort to reduce NOX emission, smoke emission, noise and brake specific fuel consumption (BSFC).In recent years, Homogenous Charge Compression Ignition (HCCI) and Premixed Charge Compression Ignition (PCCI) were been given much attention for its high thermal efficiency and low emission. Meanwhile, both of the new combustion modules have the problem of narrow load rang and hard to control. Whether HCCI, PCCI or improved traditional combustion mode, the final purpose is high efficiency and low emission combustion. The crux of achieve high efficiency low emission is complete combustion and control the maximum temperature and heat release rate(HRR) and, moreover, control the in-cylinder equivalence ratio distribution to avoid particulate matter(PM) and Prompt NO production. Therefore, this dissertation proposed the diesel engine combustion process active control method base on the control of target heat release rate and combine with the high pressure common rail (HPCR) injection system matches in-cylinder airflow characteristic to control the HRR and equivalence ratio distribution, and finally achieve high efficiency and low emission combustion process.Because of the weak volatility of diesel oil, it must be injected into combustion chamber under high pressure to mix with the air and form combustive air-fuel mixture. The air-fuel mixture can auto ignition under certain high pressure for the low auto ignition temperature of diesel oil. The key factors affect the diesel combustion process are injection system, airflow inside the combustion chamber and the optimization of matching between both. HPRC is a kind of ideal new type time-pressure electronic control injection system for vehicle diesel. The control of pump, pipe and injector of HPCR injection system are independent for each other, therefore the control degree of freedom greatly increased and can achieve injection rate flexibility control. In order to choose a suitable HPCR for a diesel engine to obtain ideal injection rate, this dissertation firstly investigated the effect of injection system parameters on injection rate and engine performance. The results as following: 1) anchor spring stiffness coefficient have little influence on injection rate; anchor spring preload have great influence on injection rate during needle opening process but little influence on closing process; the larger the electromagnetic force the needle open sooner, but the electromagnetic force nearly not influence the closing process. 2) If the control volume of injector is big, the needle may open slowly in the initial stage of the opening, accordingly resulting very slow initial injection rate and poor spray quality. If the control volume is small, the pressure fluctuation may cause the needle open twice in closing process, and so result in unexpected injection. The bigger the inlet orifice the pressure in control volume rise soon and the needle close more quickly. The bigger the outlet orifice the pressure in control volume drop soon and the needle open more quickly. The flow area difference between inlet and outlet orifice is the key factor that affects the injection rate.3) Needle mass mainly affect the injection rate during opening and closing process, but nearly have no influence on injection rate in injection duration. High injection pressure and small injector hole are efficiency way to improve the spray quality, and the injector hole diameter have greater influence on spray quality than injection pressure.The airflow characteristic directly influences the air-fuel mixture distribution in combustion chamber. The airflow mainly affected by the structure of the chamber after intake valve closing. High speed direct injection diesel engine requires fast mixture formation. So that, it should take advantage of the airflow intensity and its maintenance of reentrant combustion chamber to find out the way to promote high speed diesel engine air-fuel mixture formation. To further exploit the advantages of reentrant combustion chamber, this dissertation investigated the effect of structure parameters on airflow characteristic and put forward the evaluating indicator for airflow characteristic. The results as following: 1) smaller throat chamber have better squish flow intensity and maintenance, but have no influence on swirl flow. 2) Deeper chamber have better airflow intensity and maintenance, include squish flow and swirl flow. 3) The central boss height nearly has no influence on airflow characteristic, but it can improve dead area.Good airflow characteristic and injection characteristic just the two of prerequisite for high efficiency low emission combustion. What is more important is to optimize the matching between the airflow characteristic and injection parameters. If the airflow not matches the injection suitably, the spray might interpose each other or impact on unexpected position of the chamber, and so have a bad effect on the air-fuel equivalence ratio distribution, temperature filed distribution, finally aggravate the engine economy and emission. As mentioned above, optimization of the matching between the airflow characteristic and injection parameters is the real way to achieve high efficiency low emission combustion for HPCR direct injection diesel engine. As the engine matches different specification injector, the test results show that as the normalized flow rate difference 100cm3/min, the external characteristics BSFC maximum difference up to 10%, smoke emission maximum difference up to 10%. As the engine matches same specification injector but different install position, the test results show that as the injector gasket height changes 1mm, the external characteristics BSFC maximum difference up to 10%, smoke emission maximum difference up to 50%, exhaust temperature maximum difference up to 15%.For a specific engine, every operating points corresponding to an optimal HRR curve that can achieve high efficiency low emission combustion process. The optimal HRR curve constrained by combustion noise, exhaust emission and fuel economy. Combustion noise depends on the maximum in-cylinder pressure, rate of pressure rise and the coincidence degree of both. In order to reduce combustion noise level, it should control the maximum in-cylinder pressure, rate of pressure rise and the coincidence of both by control the optimal HRR. At light duty operating points, using sigal HRR can achieve perfect combustion process. The start of optimal HRR should fix by temperature, high temperature duration, BSFC, and mixture formation control difficult degree. At heavy duty operating points, using sigal HRR will postpone the start of optimal HRR and makes mixture formation control become more difficult. So it commonly uses multiple HRR at heavy duty operating points.For reduce emissions of diesel engine in the premise of ensuring efficiency. This dissertation focused on the effect of air-fuel mixture formation characteristic of direct injection diesel engine on NO produce rule that under the high injection pressure condition. It must produce some prompt NO in diesel oil unhomogenous charge combustion process and the produce mechanism of prompt NO is different from thermal NO. Prompt NO mainly produce in the rich mixture that the equivalence ratio equals about 1.5, the prompt NO depend on temperature less than thermal NO that it can produce less than 2100K, and the reaction rate quicker than thermal NO. Prompt NO produce mainly due to the intermediate such as O,OH, HCN,CN and NH et al. Prompt NO reaction activation energy is low, reaction rate is quick, less depend on temperature, directly relate to equivalence ratio and the produce of prompt NO can promote the produce of thermal NO. With the increasingly stringent emission regulations, control the produce of prompt NO become more important for reduce total NOX in diesel engine. This dissertation use in-cylinder virtual sampling method to study the transient equivalence ration distribution and the mixture process of the crucial parts in the chamber. And moreover, analyzed the effect of equivalence ration distribution and temperature field distribution on prompt NO, thermal NO and soot and also constructed the prompt NO reaction rateΦ-T diagram. Aforementioned study proposed a theoretical basis for NO reduction; it is significant for control the diesel engine emission and achieves high efficiency combustion process.