| According to the characteristic of spray of traditional diesel engine, a Double Reflex combustion system with spary impingement was designed based on wall-guiding, stratified-diffusion and space-dispersed on a DI Diesel Engine. NOx was reduced by low combustion temperature, Particulate Emission was reduced by premixed combustion and technique of Double Reflex with Spary Impingement. The reduction of NOx and PM was realized to solve the "paradox of Diesel Engine". The characteristics of this new combustion system are low cylinder pressure, rapid mixing, long ignition delay period and low combustion temperature. Double Reflex with Spary Impingement means that guide bellied arcs and little shoulders were set on surface of chamber wall, liquid spray blowout from injector and hit on surface of chamber wall, then stratified-diffusion spray was obtained, abbreviated as the Double Reflex, this combustion system is named as the Double Reflex combustion system. The reflected spray distributes at squish zone and concave zone of combustion chamber, big swirl of spray is obtained and expand to more domains. Thus, the spray to the wall can entrain more air, produce more mixture in limited ignition delay period, generate multiple ignition points and shorten duration of combustion. After the spray burst into flame, upper and lower spray make radiant heat transfer. By the effect of centrifugal force, fuel oil drops atomized insufficiently from upper spray can continue to burn in the lower spray. By the effect of reversed squish of piston, uncompleted combustion products from lower spray can continue to burn in the upper spray, too. Moreover, this reflection achieves better breaking for spray, at the same time, more homogeneous mixture can be achieved in rapid because of impingement and diffusion.The experimental investigation on Double Reflex combustion system is carried out on a4100ZL diesel engine.The experimental test was to optimize the structural parameters for combustion system of the engine, including chamber structure, swirl ratio of head, spray angle,number and diameter of Nozzle, pressure and timing of fuel oil injection. The effect of Double Reflex combustion system on combustion and engine performance was researched to obtain the best performance indexes, and comparison to the original engine was carried out. The results show that NOx was reduced from712×10-6to487×10-6at2100r·min-1/307N·m (operating point of peak torque), from593×10-6to369×10-6at3000r·min-1/267N-m (operating point of rated power),that’s31.6%and37.7%separately. The BSFC and Smoke were improved at low speed and rated load. BSFC was reduced form240.5g.(kw-h)-1to225.4g·(kw-h)-1, that’s6.3%, Smoke was reduced from3.67BSU to2.1BSU, Pressure in cylinder was obviously reduced for128bar to108bar, that’s15.6%at peak torque point.The optimized Double Reflex combustion system is studied in detail and the conclusions can be given as following.(1) Research of DI diesel engine Performance based on controlling of ROHR under high speed operating condition ensuring the best emissions and fuel economy.The main difficulty of improving the combustion performance was the control of SOC and the combustion rate under operating conditions of high speed and heavy load, that’s the control of the instantaneous ROHR. The SOC of diesel engine was postponed by reducing the geometric compression ratio,and the combustion rate of the cylinder mixture was improved by the geometric method (Double Reflex technology) for achieve the effect of reducing NOx emissions and maintaining a high indicated thermal efficiency, that is to effective control of the combustion phase and the center of gravity of the instantaneous ROHR. Double Reflex combustion system with lower compression ratio can maintain the combustion rate under the high-speed and ensure optimal emissions and economy performance of diesel engine as well as the power does not be reduced.Based on the above ideas, the experiment is carried out at speed of3000r·min-1(2) Research of low cylinder pressure and low fuel consumption for Double Reflex combustion system of DI diesel engine based on single peak ROHR characteristics.The results show that:The instantaneous ROHR of Double Reflex combustion system shows a trend of single peak. When the compression ratio is reduced,the peak cylinder pressure of Double Reflex combustion system was reduced at all operating point range.The fuel consumption rate characteristics is analyzed according to the cumulative ROHR, the percentage of total heat release, the single peak ROHR and the indicated thermal efficiency, the results shows that the BSFC is at a very low level.(3) Experimental research on rapid combustion of Double Reflex combustion system of DI diesel engine based on new concept of low compression ratio.The results showed that:When the compression ratio is reduced, the SOC was postponed for about2～3℃A, which ensures the dynamic engine performance in the same level and makes the cylinder pressure reduced greatly, the highest cylinder combustion temperature and NOx emissions reduced,too. The instantaneous ROHR showed the single peak characteristic at medium and high speed at external characteristic, the combustion of new engine had the same combustion rate at crank angle of θ5～θ70with accumulate ROHR of5%～70%as the original engine.Both phenomenon perfectly demonstrated the Double Reflex combustion system of DI diesel engine with low compression ratio can maintain rapid combustion rate indeed.(4) Research of low soot emissions through advanced injection timing.As the static fuel supply was increased to18℃A BTDC, the smoke emission would be reduced to0.21BSU at the operating point of rated power and peak torque.At the same time of experimental research on the Double Reflex combustion system, the related numerical simulations on spray field at a large plate and in the combustion chamber are carried out. The fuel hits on the plate with guide arc and little shoulders, reflects in the form of vapor and forms the stratified wall jet. Taking the reflector as a center, the spray forms a high concentrated fuel zone, and constantly spreads out to form a lean mixture. In the combustion chamber, a large vortex is formed influenced by squish flow before the TDC. The vortex will be more larger when guide arc and little shoulders are adopted, and low-speed region of the bottom is smaller than that of the virtual original engine. After the piston reaches TDC, a medium-scale vortex is formed in the Double Reflex combustion chamber as the piston down, which benefits the fully mixing of fuel oil and gas. For the Double Reflex combustion chamber, the fuel locates on the sidewall of combustion chamber, and does not reach the concave, the distribution of A/F ratio on the chamber wall is small, which indicates that fuel is evaporated and lean mixture is formed.For the distribution of particles, the same conclusion can also be got. After the injection at19℃A ATDC, for the original engine and virtual original engine, there is a lot of droplets not evaporated in time on whole sidewall and concave wall, but for the Double Reflex engine there are particles not evaporated only at the touch points on combustion chamber wall.So it can also be drawn that fuel evaporation rate of the Double Reflex combustion chamber is more fast,that also proves the theory of rapid mixing. |
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