Development And Research On The Combustion System Of Natural Gas Engine Based On Stoichiometric Operation Mode

In order to meet Euro ? emission legislation for heavy-duty engines,the natural gas engine with lean burn operation has to be equipped with selective catalytic reduction(SCR)which increases the engine cost obviously.In contrast,the stoichiometric operation with exhaust gas recirculation(EGR)could meet the emission legislation with only three-way catalyst(TWC).In the current study,through combining the methods including experiments,thermodynamic analysis and numerical simulations,the advantage and potential of stoichiometric operation in improving the fuel economy of natural gas engine for meeting Euro ? emission legislation were explored.The effects of late intake valve closing(LIVC)strategy and combustion chamber optimization on the engine combustion characteristics and thermal efficiency improvement with stoichiometric operation were studied in depth,which is of great importance for achieving high efficiency and clean utilization of natural gas and the combustion system development of natural gas engine for meeting the forthcoming China ? emission legislation.The fuel economy of the natural gas engine achieved with stoichiometric operation and lean burn for meeting Euro ? emission legislation were systematically compared first,and the strategy of air and EGR dilution was proposed for further optimizing the combustion phasing with lean burn.The results indicate that,lean burn with air and a small fraction of EGR achieves an obviously higher thermal efficiency,especially at high loads.Nevertheless,the fuel economy of stoichiometric operation does not show significant difference with that of the lean burn with consideration urea consumptions of SCR,in addition,the use of stoichiometric operation could reduce the after-treatment cost obviously,therefore,it should be the main technical route to meet Euro ? emission legislation for the natural gas engine.The effects of the key parameters with LIVC strategy on the performance of stoichiometric operation natural gas engine were then studied.The results indicate that,the increase of the geometric compression ratio(GCR)with LIVC could obtain higher theoretical thermal efficiency,which could effectively increase the indicated thermal efficiency.However,since the essential EGR dilution leads to a large spark advance,the effect of the retarded intake valve closure timing on lowering the compression temperature around the spark timing becomes much more significant,regardless of the variation of effective compression ratio(ECR),in addition,the increased GCR results in a higher heat transfer rate around top dead center(TDC)due to the increased surface/volume ratio,consequently,the combustion rate is inevitably lowered with LIVC strategy,and this effect would be more obvious at higher engine speed and load,and also with higher EGR rate.On the other hand,the adverse effect of the increased ECR on heat transfer loss which leads to a reduction of thermal efficiency is much more obvious compared with its positive effect on combustion rate which leads to an increase of thermal efficiency,even within knock limit.The synthetic effect of these two factors restricts the thermal efficiency improvement in a certain extent.Through designing the cross chamber,nebula chamber,and reentrant chambers with different diameter ratios between chamber throat and cylinder bore(DR),the effects of three representative categories of chamber geometries on the performance of stoichiometric operation natural gas engine were studied.The results indicate that,the optimized reentrant chamber and the cross chamber show quite similar performances in increasing the combustion rate and thermal efficiency,followed by the nebula chamber.In addition,for the reentrant chamber,the combustion duration could be reduced continuously with the reduction of DR,which is beneficial for the thermal efficiency improvement,however,excessively smaller DR could obviously increase the knock tendency and heat transfer loss,which leads to a thermal efficiency reduction.On the other hand,the combustion duration with the spark timing set at the optimum point shows a good linear relationship with the DR.This function is greatly valuable to optimize the reentrant chamber since it could be obtained via few engine tests and then used for well predicting the effects of reentrant chamber structure variation on the thermal efficiency combined with only one-dimensional simulation.The threedimensional calculation results indicate that,the asymmetrical turbulence distribution and hence asymmetrical flame propagation,which reduce the flame development in a certain direction,should be one of the main reasons for increasing the combustion duration with nebula chamber.The cross chamber has similar issues;and its grooves could further reduce the flame surface.Its higher combustion rate than that of the nebula chamber can be mainly attributed to the stronger squish effect which results in a stronger turbulence during initial combustion stage.The reentrant chamber has the highest turbulence intensity before TDC and hence a higher flame surface density.Furthermore,it shows a more symmetrical flame propagation,and also a larger flame surface development during the late period of combustion.However,the effect of its turbulence on flame propagation reduces obviously after TDC since the high intensity region rapidly separates from the flame surface.