| The increase of energy consumption and pollutions produced by vehicles has lead to energy crisis and harmed environment deeply recently twenty years. The fossil oil resource is limited or finite, so this indicates that all the vehicles will be drived by electric power in the future, the power maybe is traditional electric or fuel cell. But all of these techniques are not developed; they are underway now, so it is sure that internal combustion engine still will be the main power for all of the vehicles in the coming several decades. The requests to the engine have changed from powerful, economy, hardness to high flexibility, high efficiency, low emission and low noise in new period; especially high efficiency and low emission are new challenges for engine development.Lean burn, stratified combustion, gasoline direct injection (GDI) and controlled auto-combustion (CAI) are the main directions for gasoline engine study. The stratified combustion is the most important technique in all of these directions, because whether lean burn or GDI, the main method is form stratification in combustion chamber to optimized combustion. Studies had indicated that stratified combustion has deep potential for higher power output and better fuel economy, these advantages mainly because it is similar in operation to a diesel cycle, where an engine functions throttleless with load controlled by AFR alone. And this is main reason to put emphasis upon the stratified combustion in this thesis.Combustion is the most important process in courses of engine running, so the development of engine technology relies upon the advancement of combustion technology. We must have a profound understand on fuel injection and evaporation, mixing with air before we organize stable and effective combustion in combustion chamber. The main task of study on combustion process discovers and predominate the law using combustion instrumentation and diagnostics. We haven't clear physicochemical patterns of combustion flow field until now, since the lag of combustion diagnostics. So this is a big hindrance to further investigations of combustion mechanics. Furthermore, the shortages of combustion mechanics embarrass the Research and Development (R&D) of efficient engine.The principle of laser diagnostics is that we obtain the results through energy distribution inner measurands and the electromagnetic results from energy levels transiton excitated by laser. This technique has many advantages, such as high-sensitivity, non-intrusion measurement, high resolution of time and dimension and so on. Laser diagnostics can measure temperature and components of combustion, velocity of flame propagation at formidable combustion conditions.The needs of engine technique development are the research gist of this study. Stratified combustion and CAI have deeper potential to get lean burn, economy and good emission than SI combustion. In order to investigate the effects of charge stratification to combustion, at first the formative process of mixture should be controlled to form stratified or homogeneous charge, second a new technique was put forward and used for studying stratified and homogeneous combustion. The laser diagnostics and the application on engine combustion was studied in this thesis, the main contents include:(1) A unique research engine was required to facilitate study the stratified and homogenous charge and their effects upon both combustion and exhaust emissions. A small Lister-Petter engine was carefully selected, to which a bespoke cylinder head was fitted. The valves were set at side of the cylinder head, there are two advantages for this disposal, one is high-swirl was formed in combustion chamber, this will benefit to charge stratification, and the other one is there is no other parts on the top of combustion chamber, so a optical window was designed to observe parameters in combustion chamber. And two side optical windows were devised for laser diagnostics. At the same time install holes were designed for pressure sensor and spark plugs. Infinitely variable timing of valves was achieved by worm camshaft. (2) The volumetric efficiency of this optical engine was improved from 27% to 55% by optimized the valves lift and the shape of exhaust port. The stratified and homogeneous combustion were achieved through skipped injection and spark strategy. Eight cycles as a period, for seven consecutive cycles the engine would be motored with fuel being injected to a known AFR in order to develop a homogenous charge within the cylinder; this would form the residual gas for the firing cycle. During the eighth cycle the fuel injection would continue or fuel would be cut off altogether, the former strategy retains a homogeneous charge structure and the latter to promote stratification. The spark plug was fired at eighth cycle of every period, so the stratified and homogenous combustion were achieved.(3) 3-pentanone in iso-octane has the distinct advantage of being a combination that has been used in many past studies, and as such its limitations are both well understood and documented, so they are selected for this PLIF study. One spherical lens and two cylindrical lenses were used to form a laser sheet in combustion chamber, which is 400mm wide and 1-1.2mm thick. In the interest of decrease the noise of fluorescence from oil, there kinds of low fluorescence raw oil were studied and the PIB oil was selected in this study. The background and reference images were used to decrease noise; and the effects of temperature and pressure was studied, the temperature calibration factor was gotten and "charge density" was used to calibrate the effect of different pressures.(4) Fuel stratification of different boost pressure was studied based on PLIF technique, and combined with pressure data and transient emission analysis, For the first time in this field of research one can heve absolutely confidence in the correlation between stratification pattern and combustion affect since, for each operating condition, the complete data suite was recorded simultaneously and therefore every single datum pertains to the same combustion cycle. Using intake manifold pressure as a means to govern the dilution ratio of air to charge in the cylinder, and therefore the level of stratification, it has been found that as the global AFR of the combustion increased, the combustion performance of the stratified charge condition was far superior to tis homogenous charge counterpart, and that the stratified charge structure could maintain reasonable combustion stability in lean conditions where the homogenous charge case would suffer severe misfire. The reduction in AFR in the vicinity of the spark event ensured rapid kernel development, the overall bulk combustion duration benefited from the richer mixtures and the reduction in propagation length required in order to consume the vast majority of fuel. Absolute NOx and uHC emission levels were significantly higher when operating under stratified charge conditions, however, the vast improvement in IMEP when operating in this regime effectively counteracted this trend, when the specific emissions figures were calculated.(5) Transient temperature distribution is crucial for combustion study, especially for heat and mass transfer, physical and chemical action of overflow combustion. It is very difficult to get the transient temperature distribution. In this thesis the author try to measure the 2D gas temperature distribution using laser induced phosphorescence technique at a small optical engine. According to the excitated property, intensity ratio method was used. A rod heated by a cartridge heater was coated with a mixture (50:50) % of phosphor and high temperature ceramic cement to a thickness of approximately 200um. The calibration was conducted by measuring the surface temperature of solid objects in a test cell. The Dy: YAG phosphor powder was excited by 355nm wavelength of a Nd: YAG laser. The median particle diameter was 4um. The phosphorescence was captured an ICCD camera connected with an image doubler. The relationship between the intensity ratios of two wavelengths with temperature was calculated. (6) 2D gas phase temperature distribution was measured using phosphor thermometry. Because the frequency of Nd: YAG laser must be 10 or 20Hz, the trigger signal of laser comes from the encoder fixed on the crankshaft, so the engine speed was fixed at 600rpm, then the trigger frequency for laser is 10Hz, but because capture frequency of ICCD is less than 1Hz, so in order to keep the synchronous between laser fire and the gate ICCD, a divider was used to get the 0.625Hz trigger for the gate of ICCD. The temperature distribution of compression stroke was measured and compared with mean temperature calculated from pressure data, the maximum error is 8.35%, this suggest that this technique can be used for 2D gas phase temperature measurement. And this technique was used for 2D combustion temperature measurement in this optical engine. The flame propagation was studied by temperature distribution, three phases of combustion marked from temperature distribution is consistent with the result gotten from fuel burned mass percent. The mean temperature of LIP have a good consistency with the mean temperature calculated from pressure at pre combustion and after combustion, but the error is more than 10% at main combustion because of chemiluminescence of combustion.Main innovations of thesis are:(1) For the first time the LIP was used for 2D gas phase temperature distribution measurement, the feasibility of this technique for entgine was validated through the temperature distribution of compression stroke; the SI homogeneous combustion was studied using LIP technique, the results indicate that the three periods marked by temperature distribution is consistent to the results marked by burned fuel mass fraction.(2) "Charge indensity" was used for correct the effect of pressure variety to fluorescence intensity, and factor of temperature to fluorescence intensity was fixed by experiment. (3) The quantitative distribution of Lambda for stratified and homogeneous combustion was studied using PLIF in this thesis, combined with analysis of heat release rate and transient emission, the results indicated that the stratified combustion have better performance than homogeneous combustion. |
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