Abstract
By harnessing gasoline’s low reactivity for partially premixed combustion promotion, gasoline compression ignition (GCI) combustion shows the potential to produce markedly improved NOx-soot trade-off with high fuel efficiency compared to conventional diesel combustion. However, at low-load conditions, gasoline’s low reactivity poses challenges to attaining robust combustion with low unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions. Increasing the in-cylinder charge temperature by using variable valve actuation (VVA) can be an effective means to address these challenges. In this numerical investigation, VVA strategies, including (1) early exhaust valve opening (EEVO), (2) positive valve overlap (PVO), and (3) exhaust rebreathe (ExReb), were investigated at 1375 RPM and 2 bar brake mean effective pressure in a heavy-duty GCI engine using a market-based gasoline with a research octane number (RON) of 93. The total residual gas level was kept over 50% to achieve an engine-out NOx target of below 1.5 g/kWh. For a complete engine system analysis, one-dimensional (1-D) system-level modeling and three-dimensional (3-D) computational fluid dynamics (CFD) analysis were close-coupled in this study. Performance of the VVA strategies was compared in terms of in-cylinder charge and exhaust gas temperatures increase versus brake-specific fuel consumption (BSFC). The EEVO strategy demonstrated in-cylinder charge and exhaust temperature increase up to 130 and 180 K, respectively. For similar in-cylinder charge temperature gains, the ExReb strategy demonstrated 11% to 18% lower BSFC compared to the EEVO strategy. This benefit primarily originated from a more efficient gas-exchange process. The PVO strategy, due to the valve–piston contact constraint, required excessive exhaust back-pressure valve (BPV) throttling for hot residuals trapping, thereby incurring higher BSFC compared to ExReb. In addition, the ExReb strategy demonstrated the highest potential for exhaust temperature increase (up to 673 K) among the three strategies. This was achieved by ExReb’s maximum air-fuel ratio reduction from high internal residuals mass and BPV throttling. Finally, the ExReb profile was optimized in terms of the peak lift, the duration, and the location for maximizing the fuel-efficiency potential of the strategy.
Highlights
Regulatory pressures and market competition continue to drive commercial applications toward lower-criteria pollutants and lower greenhouse gas emissions [1]
The results showed, increasing t(hTseoIiV) C (Tivc) beyond 400 K dramatically improved the combustion efficiency and the gasoline compression ignition (GCI) combustion robustness by enabling an advanced combustion phasing (CA50)
Three different exhaust rebreathe strategies—ExReb1, the ExReb2, and the ExReb3—were simulated for the int.exhaust gas recirculation (EGR) portion sweep up to 50% and compared in terms of the pumping mean effective pressure (PMEP) penalty
Summary
Regulatory pressures and market competition continue to drive commercial applications toward lower-criteria pollutants and lower greenhouse gas emissions [1]. Severe combustion efficiency loss at low-load conditions reduces exhaust gas temperature, potentially degrading the EAT system performance To address these key technical challenges for low-load GCI operations, effective thermal management strategies that can appreciably increase the in-cylinder charge and the exhaust gas temperatures are desirable. Ding et al [24] experimented with combinations of EEVO, late intake valve closing (LIVC), and cylinder deactivation (CDA) for exhaust gas temperature increase at the 800 RPM and 0.26 bar brake mean effective pressure (BMEP) condition in a medium-duty diesel engine. Building on this CFD model, a combustion strategy de velopment was performed, targeting an engine-out NOx (EO-NOx) below 1.5 g/kWh. FFiigguurree 44F..igCCuoormemp4p.aaCrrioissomonnpoaofrficsyocynlilniondfdecreyrpliprnedrseessrusruperr,eeas, spauppraepr,aearnpetnphtaerhaeentatrtehlreeeaaltesearesrelaetaresaet(eAraH(AteRHR(AR),HRaR)n,Rda)n,edmaniedsmsiiosnssiobnesbtwetweeenenthetmeheiesxsepixoepnreismrbimeentewtnsetaesnnadnthdmemoedxopedleerrliemrseueslnuttslstasatanBtd2Bm525aondadneldBr7eB5s7u.5l.ts at B25 and B75.
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