Heat release rate and emissions regimes of stratified pilot-ignited direct-injection natural gas combustion

Abstract

Natural gas (NG) is an attractive fuel for heavy-duty internal combustion engines because of its potential for reduced CO2, particulate, and NOX emissions and lower cost of ownership. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a main direct injection of NG. Recent studies have demonstrated that increased NG premixing is a viable strategy to increase PIDING indicated efficiency and further reduce particulate and CO emissions while maintaining low CH4 emissions. However, it is unclear how the combustion strategies relate to one another, or where they fit within the continuum of NG stratification. The objective of this work is to present a systematic evaluation of pilot combustion, NG combustion, and emissions behavior of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions. A sweep of the relative injection timing, [Formula: see text], of NG and pilot diesel was performed in a heavy-duty PIDING engine with [Formula: see text] = 140–220 bar, [Formula: see text] = 0.47–0.71, and a constant NG energy fraction of 94%. Apparent heat release rate and emissions analyses identified interactions between the pilot fuel and NG, and qualitatively characterized the impact of NG stratification on combustion and emissions. Changes in the [Formula: see text] resulted in six distinct PIDING combustion regimes, for all considered injection pressures and equivalence ratios: (i) RIT-insensitive premixed, (ii) stratified-premixed (early-cycle injection), (iii) NG jet impingement transition, (iv) stratified-premixed (late-cycle injection), (v) variable premixed fraction, and (vi) minimally-premixed. Parametric definitions for the bounds of each regime of combustion were valid for the wide range of [Formula: see text] and [Formula: see text] investigated, and are expected to be relevant for other PIDING engines, as previously identified regimes agree with those identified here. This conceptual framework encompasses and validates the findings of previous stratified PIDING investigations, including optimal ranges of operation that provide significantly increased efficiency and lower emissions of incomplete combustion products.

Optical characterization of stratified-premixed natural gas direct-injection combustion regimes

“…Previously investigated operating conditions from the thermodynamic engine configuration shown with solid line. 24 highlight the difference in NG mixture development as it pertains to combustion, approximate phasing of 50% indicated heat release, CA 50 , is also shown in Figure 7. For all conditions with late-cycle injections (top and middle plot of Figure 7), NG stratification has not fully-developed by the time of combustion.…”

“…16,17 Near the transition between early-and late-cycle NG injection (SOI NG 'SOI NG, transs B ) poor engine performance (combustion stability and emissions) occurs. 16,24 Two early-cycle PIDING regimes were distinguished by RIT insens: ( s A , RIT insens: ' À 538): (i) The ''RIT-Insensitive Premixed Regime'' ( s 1 ) for RIT \ RIT insens: , emissions were not significantly affected by changes in RIT and combustion and emissions behavior was observed to be consistent with port-injected dual-fuel combustion, and (ii) the ''Stratified-Premixed Regime'' (early-cycle) ( s 2 ) for RIT . RIT insens: , where RIT had a significant influence on combustion and emissions.…”

“…Operating conditions for the thermodynamic configuration correspond to previously published measurements. 24…”

“…Combustion transitions to the variable premixed fraction regime when RIT is reduced below RIT Ã = 0 and t NG begins to increase. 24 In Figure 8, X 0 fuel for reacting and non-reacting operation (X 0 fuel and X 0 fuel, NR , respectively) are compared to AHRR for minimally-premixed and variable-premixed Figure 8. Comparison of AHRR, X 0 fuel , and X 0 fuel, NR to assess relative phasing of the start of NG combustion, u SOC, NG and fuel consumption at the ICOS, u SOC, ICOS (*) for minimally-premixed (RIT Ã .…”

“…0, due to pilot quenching by the NG jets. 24 Despite the increased t NG for the variable premixed fraction conditions (RIT Ã \ 0), the OH*-CL in the center of the combustion chamber remains weak throughout the cycle. This is an indication of the incomplete premixing of the NG, which is also indicated by X 0 fuel in Figures 7 and 8 for all conditions shown in Figure 9.…”