Strategies for Reduced NOx Emissions in Pilot-Ignited Natural Gas Engines

Abstract

The performance and emissions of a single-cylinder natural gas fueled engine using a pilot ignition strategy have been investigated. Small diesel pilots (2–3% on an energy basis), when used to ignite homogeneous natural gas-air mixtures, are shown to possess the potential for reduced NOx emissions while maintaining good engine performance. The effects of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling were studied. With appropriate control of the above variables, it was shown that full-load engine-out brake specific NOx emissions could be reduced to the range of 0.07–0.10 g/kWh from the baseline diesel (with mechanical fuel injection) value of 10.5 g/kWh. For this NOx reduction, the decrease in fuel conversion efficiency from the baseline diesel value was approximately one to two percentage points. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. The nature of combustion under these conditions was analyzed using heat release schedules predicted from measured cylinder pressure data. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.

Multi-zone modelling of partially premixed low-temperature combustion in pilot-ignited natural-gas engines

“…However, increasing the naturalgas equivalence ratios w NG at fixed T in and Q inj led to only higher peak heat release rates without affecting the T pPEAK_AVG values significantly. These results are in agreement with experimentally measured trends [18,20] for NO x emissions, which were relatively unaffected as w NG was increased (at fixed Q inj ) to attain engine loads as high as 12 bar BMEP.…”

“…While the higher T pPEAK_AVG may lead to slightly higher NO x emissions at higher w NG , this increase will probably be modest compared with the influences of T in and Q inj on NO x . This is largely because, as w NG was increased to 0.56, the T pPEAK_AVG value attained was still lower than the critical NO x formation temperature of 2000 K. Consequently, engine-out NO x emissions with ALPING combustion remained very low even when the engine load was increased by increasing w NG , a fact that was demonstrated experimentally in previous studies [18,20].…”

“…Dual-fuel experiments [18][19][20] performed with a common-rail low-pilot-injection system established that engine-out NO x emissions can be reduced to about 0.2 g/kW h over a wide range of engine loads when very small diesel sprays (providing 2-3 per cent of total fuel energy) are injected early in the compression stroke (about 60u before top dead centre (before TDC)) to ignite premixed natural-gasair mixtures. The penalty on fuel conversion efficiencies was minimal despite the remarkable reduction in NO x emissions; however, HC and carbon monoxide (CO) emissions increased substantially at advanced injection timings.…”

Natural Gas Fired Reciprocating Engines for Power Generation: Concerns and Recent Advances

“…The amount of NOx resulting from such a method of ignition though very low is limited by the NOx formation resulting from the diesel pilot itself. Krishnan et al [29] have tried using very advanced timing for the diesel pilot in order to reduce the NOx formation. Others have tried to reduce the amount of the diesel pilot as a fraction of the total fuel introduced in to the cylinder with promising results [30].…”

An Experimental Investigation on the Combustion and Emissions Performance of a Natural Gas–Diesel Dual Fuel Engine at Low and Medium Loads

“…The 2 Copyright © 2015 by ASME diesel injection timing in [6,7] was fixed. During the studies of pilot-ignited dual fuel combustion, Alla et al [8], Singh et al [2], Krishnan et al [9] and Srinivasan et al [10] found that a change in diesel injection timing had a significant effect on the combustion performance of dual fuel combustion. The combustion performance of dual fuel combustion could also be affected by intake conditions [2,9], exhaust gas recirculation [11], and cetane number of diesel fuel [12].…”