Design Of The Variable Valve Timing System For Heavy Duty Diesel Engine And Its Experimental Study

The enhancement of fuel efficiency and simultaneous suppression of NOx andsoot formation in a high-load modern diesel engine has been a big challenge.Professor Su and his research team presented a new concept of mixing and chemistrytime-scale control and combustion path control and put forward the HighDensity-Low Temperature Combustion (HD-LTC) theory, which is realized by twostage turbo charge and late intake valve closing and showed higher thermal efficiencyand very low emission potential. Late intake valve closure can lower the effectivecompression ratio, the pressure and compressed gas temperatures near the top deadcenter; increase ignition delay and mixing time and control the ignition andcombustion process of premixed combustion.To carry out the high density-low temperature combustion, the author developeda two mode late intake valve closing actuation (IVCA)controlled by a slide valve.Two mode means the valve can either work on the original cam lift or get anadditional lift created by a hydraulic piston adding to the original cam lift to retard theintake valve close timing to satisfy different work conditions. The simulation of theworking process showed this system can postpone the intake valve closure timingeffectively, the intake hole and back hole1can strongly affect the intake valve closetiming, while back hole2mainly affect the velocity when the valve is closing. It wasalso found that lesser intake hole diameter and suitable back hole2open timing canget preferable intake valve close timing coherence for different speeds. Throughexperimental study, the author finally confirmed the best parameters: intake holediameter1.3mm, back hole1diameter1.0mm, back hole2diameter2.0mm, backhole2opening position1.1mm. The two mode working system has simple structureand is easy to equip, especially suitable for the diesel engines already in use.Optimized and adjusted late intake valve closing system is suitable for differentworking conditions, and has good cylinder-to-cylinder coherence. The goodadaptability and coherence of the late intake valve closing system offer greatpractical value. The study of the effects of IVCA on diesel engine Combustioncharacteristics showed IVCA is the most effective way to lower in-cylinder pressureat high loads. At medium loads IVCA has obvious effect on combustion progresscontrol and emission control. The IVCA lowered effective compression ratio, thepressure and compressed gas temperatures near the top dead center; increaseedignition delay and mixing time; achieved more homogeneous mixture and reduced local rich regions—which is better for depressing soot emissions. IVCA reducedintake gas; lowered the temperature before ignition; enhanced equivalence ratio—which directly resulted in NOx emission reduction, and improved the NOx-soottrade-off. IVCA has distinguished effects on heavy duty diesel engines, when theintake valve close timing is retarded, thermal efficiency is enhanced because of Millercycle and pumping loss decrease, a3.7%and7.3%thermal efficiency improvementcan be achieved at1600rpm and1900rpm50%load respectively.Based on the engine thermal efficiency and emissions, the author finallyaccomplished the IVCAworking MAP in the working plane through experiments.