Study On Combustion And Emissions Of CNG/Gasoline Combined Injection Engine

Compressed natural gas?CNG?has been regarded as the most mature alternative fuel for vehicles due to its good fuel properties.Compressed natural gas is a clean fuel.In theory,it only produces CO₂ and water after complete combustion with oxygen.Adding it to a traditional gasoline engine as a blended fuel has obvious emission reduction advantages.Through the direct injection in the CNG cylinder,the precise control of CNG is achieved,and the lean burn limit is widened.At the same time,it can effectively avoid the problems such as tempering and the reduction of charging efficiency caused by the injection of the intake port,and it can be improved combustion performance through the organization of the mixed gas layering..The combined injection mode of CNG in-cylinder direct injection?NDI?and gasoline intake port injection?GPI?is an effective way to reduce emissions of traditional gasoline engines and improve performance.In this paper,the CNG/gasoline combined injection method is used to explore the changes in combustion and emissions of a engine under stoichiometric air-fuel ratio and lean burn conditions.The main work content and research conclusions are as follows:1.The combined injection system including CNG high-pressure direct injection and gasoline port injection was established,and the precise injection of CNG and gasoline was achieved based on the dSAPCE control platform.A high-pressure CNG is formed in the cylinder to accumulate at the spark plug,and a layered mixture of gasoline mixture is distributed around the periphery.The engine control software is debugged to achieve flexible control of parameters such as excess air coefficient,direct injection timing,ignition timing,intake manifold pressure and other parameters to ensure the smooth running of the experiment.The experiment proves that the combustion performance of the engine is best when the direct injection time is 120°CA BTDC with different CNG blending ratios under the stoichiometric air-fuel ratio.2.The influence of the CNG direct-injection ratio on the combustion performance of the engine in different operating conditions under stoichiometric air-fuel ratio was studied.Without changing the total energy into the cylinder,the best combustion performance was obtained under different operating conditions when 20%of CNG was directly injected.But it is more conducive to improving the combustion performance under low speed and small load.Compared with the traditional PFI gasoline engine,the reasonable GPI+NDI mode further reduces the CO,HC,and NOx emissions.In particular,the effect of reducing the number of particles is huge.3.Taking C20 under low speed and low load as an example,the influence of the combination of the direct injection timing of CNG and the ignition advance angle on the combustion and emission performance was studied.When 120°CA BTDC and15°CA BTDC are used as the direct injection timing and ignition timing,the engine hachieved the largest torque,the smallest BSFC and the highest effective thermal efficiency.With the same blending ratio and total energy,HC and NOx emissions mainly depend on changes in ignition timing,but the number of particles depends on a reasonable direct injection and ignition strategy.4.The influence of CNG direct injection strategy on the combustion performance of internal combustion engine under lean conditions was studied.When?reaches 1.4,pure gasoline mixture can no longer organize stable combustion.Under the premise of keeping the total energy into the cylinder,through injecting a proper amount of CNG and enriching it at the spark plug,the wide lean burn limit of CNG ensures stable combustion and flame propagation after the mixture is ignited and the flame spreads out to ignite the lean gasoline mixture outside,which widens the lean burn limit.When the proper injection timing is used,the lean burn stability is further improved,and the COV_IMEP is controlled below 1.5.Under different excess air coefficients,compared to the pure gasoline PFI mode,the GPI+NDI combined injection mode is more conducive to improving the performance index in lean burn conditions.