Kalyan K. Srinivasan

Strategies for reduced NOX emissions in pilot-ignited natural gas engines

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 NO X 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 NO X 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 NO X 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.

Improving low load combustion, stability, and emissions in pilot-ignited natural gas engines

Abstract Stringent environmental policies and the ever-increasing demand for energy have triggered interest in novel combustion technologies that use alternative fuels as energy sources. Of these, pilot-ignited natural gas engines that employ small diesel pilots (∼1-5 per cent on an energy basis) to compression ignite a premixed natural gas-air mixture have received considerable attention. This paper discusses the effect of intake charge temperature and pilot injected quantity on the onset of ignition (ΔIGN) and combustion (ΔCOM) in a pilot-ignited natural gas engine with specific focus on early diesel pilot injection [begining of injection (BOI) at about 60° before top dead centre (BTDC)] for low-load operation. Both ΔIGN and ΔCOM had a strong influence on performance and emissions at 60° BTDC. At advanced BOI for both half and quarter-load operation, the best performance and hydrocarbon (HC) emissions could be obtained by optimally advancing ΔIGN relative to TDC and minimizing the cyclic variability in the ΔIGN. Furthermore, a clear dependence of ΔCOM on ΔIGN was observed with the optimally advanced and the least-variable ΔIGN producing the least ΔCOM variations. Engine performance, stability, and emissions were more sensitive to intake charge temperatures in comparison with pilot injected quantities. The best improvement in performance and emissions was obtained with increasing intake temperature at half load, where fuel conversion efficiency (FCE) increased from approximately 31 per cent to 38 per cent, coefficient of variation of indicated mean effective pressure (COVIMEP) decreased from about 11 per cent to 4 per cent, and HC emissions decreased from 72 to 23 g/kW h, while oxides of nitrogen (NOx) emissions increased from 16 to 142 mg/kW h. Performance and emissions trends at quarter load were similar to those observed at half load.

EFFECT OF PILOT INJECTION TIMING, PILOT QUANTITY AND INTAKE CHARGE CONDITIONS ON PERFORMANCE AND EMISSIONS FOR AN ADVANCED LOW-PILOT-IGNITED NATURAL GAS ENGINE

Abstract Diesel engines may be converted readily to operate primarily on natural gas using the injection of a diesel pilot to achieve ignition. Advanced low-pilot-ignited natural gas (ALPING) engines show significant potential to match diesel engines in their part-load and full-load efficiencies. Experiments were performed to study the effects of pilot injection timing (- 15 to - 60° ATDC), pilot quantity, intake manifold pressure and intake charge temperature on the performance and emissions from an ALPING engine under half-load (21 kW at 1700 r/min) and full-load (42 kW at 1700 r/min) conditions. Low NOx emissions (below 0.03 g/kW h at - 60° ATDC) with satisfactory fuel conversion efficiency (31 per cent) for half-load and NOx emissions of 0.2 g/kW h at -60° ATDC with fuel conversion efficiency of 40 per cent could be obtained for full-load engine operation. High HC emissions, 96 g/kW h at - 20° ATDC for half-load and 21 g/kW h at -60° ATDC for full-load operation, were recorded. The NOx emissions showed an interesting trend for varied injection timings with maximum NOx emissions occurring at -35° ATDC and minimum at - 60° ATDC injection timing. Increased pilot quantity, intake charge temperature and lower intake manifold pressures resulted in increased NOx emissions and fuel conversion efficiency, and decreased HC emissions.

The behaviourt of small- and large-scale variations of in-cylinder flow during intake and compression strokes in a motored four-valve spark ignition engine

Abstract An experimental study was conducted to investigate the small-scale and large-scale variations of flow within the cylinder of an internal combustion engine (turbulence and cyclic variation) during the intake and compression processes. Spatial fast Fourier transform (FFT) filtering techniques with various cut-off lengths were used to determine whether there existed a particular length scale that differentiated turbulence from cyclic variations. The flow field was measured using a particle image velocimetry (PIV) system, which was set with 1.25 mm grid spacing. From the experiments and analysis, it was found that there was no particular size of vortical structures dominating the flow field during intake and compression strokes over many cycles. It was also found that not only the ensemble-averaged variation but also the standard deviation of the large- and small-scale variations were greater during the early intake and late compression compared with late intake through the early compression process.

Sensitivity Analysis of NOx Formation Kinetics in Pilot-Ignited Natural Gas Engines

A sensitivity analysis of NOx formation in pilot-ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The NOx formation mechanism employed in this study incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to NOx formation, and identifies the rate-controlling NOx formation reactions in different high-temperature regions—the burning pilot spray, the premixed flame associated with the gaseous fuel-air mixture, and the burned combustion products. The formation rates for reactions involving NOx are also investigated to reveal the primary NOx formation paths. Results show two main NOx formation paths both in the pilot spray (also called the packets zone) and the burned zone. The rate-limiting reactions for the packets zone are O+N2=NO+N and N2+HO2=NO+HNO. Rate-limiting reactions for the burned zone are N2O+M=N2+O+M and N2+HO2=NO+HNO. Since the aforementioned reactions significantly influence the net NOx prediction, it is important that the corresponding reaction rates be determined accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in NOx modeling, a transient relative error is estimated to evaluate the validity of the assumption. The relative error in NOx prediction with and without the use of the steady-state assumption is small, of the order of 2%. This work also confirms that sensitivity analysis can provide valuable insight into the possible NOx formation pathways in engines and improve the ability of current prediction tools to obtain more reliable predictions.

Numerical study of combustion characteristics and emissions of a diesel–methane dual-fuel engine for a wide range of injection timings

Computational fluid dynamics simulations are performed to investigate the combustion and emission characteristics of a diesel/natural gas dual-fuel engine. The computational fluid dynamics model is validated against experimental measurements of cylinder pressure, heat release rate, and exhaust emissions from a single-cylinder research engine. The model predictions of in-cylinder diesel spray distribution and location of diesel ignition sites are related to the behavior observed in measured and predicted heat release rate and emissions. Various distributions of diesel fuel inside the combustion chamber are obtained by modifying the diesel injection timing and the spray included angle. Model predictions suggest that the distribution of diesel fuel in the combustion chamber has a significant impact on the characteristics of heat release rate, explaining experimental observations. Regimes of combustion in the dual-fuel engine are identified. Turbulent flame speed calculations, premixed turbulent combustion regime diagram analysis, and high-temperature front propagation speed estimation indicated that the dual-fuel combustion in this engine was supported by successive local auto-ignition and not by turbulent flame propagation.

Sensitivity Analyses of NOx Formation in Micro-Pilot Ignited Natural Gas Engines

A sensitivity analysis of NOx formation in micro-pilot ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The model’s NOx formation mechanism incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to NOx formation, and identifies the rate controlling NOx formation reactions. The formation rates for reactions involving NOx are also investigated to reveal the primary NOx formation paths. Results show that there are two main NOx formation paths both in the packets zone and the burned zone. The rate limiting reactions for the packets zone are identified as: O + N2 = NO + NN2 + HO2 = NO + HNO Rate limiting reactions for the burned zone are: N2O + M = N2 + O + MN2 + HO2 = NO + HNO Since the aforementioned reaction significantly influence the net NOx prediction, it is important that the corresponding reaction rates be determined fairly accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in NOx modeling, a transient relative error is estimated to evaluate its use. The relative error in NOx prediction with and without this assumption is of the order of 2 percent. Clearly, sensitivity analysis can provide valuable insight into understanding the possible NOx formation pathways in engines and improve the status of current prediction tools to obtain better estimates.Copyright

The Advanced Low Pilot Ignited Natural Gas Engine: A Low NOx Alternative to the Diesel Engine

High nitrogen oxides (NOx ) and particulate matter (PM) emissions restrict future use of conventional diesel engines for efficient, low-cost power generation. The advanced low pilot ignited natural gas (ALPING) engine described here has potential to meet stringent NOx and PM emissions regulations. It uses natural gas as the primary fuel (95 to 98 percent of the fuel energy input here) and a diesel fuel pilot to achieve compression ignition. Experimental measurements are reported from a single cylinder, compression-ignition engine employing highly advanced injection timing (45°–60°BTDC). The ALPING engine is a promising strategy to reduce NOx emissions, with measured full-load NOx emissions of less than 0.25 g/kWh and identical fuel economy to baseline straight diesel operation. However, unburned hydrocarbons were significantly higher for ALPING operation. Engine stability, as measured by COV, was 4–6 percent for ALPING operation compared to 0.6–0.9 percent for straight diesel.Copyright

The Advanced Injection Low Pilot Ignited Natural Gas Engine: A Combustion Analysis

The Advanced Low Pilot Ignited Natural Gas (ALPING) engine is proposed as an alternative to diesel and conventional dual fuel engines. Experimental results from full load operation at a constant speed of 1700 rev/min are presented in this paper. The potential of the ALPING engine is realized in reduced NOx emissions (less than 0.2 g/kWh) at all loads accompanied by fuel conversion efficiencies comparable to straight diesel operation. Some problems at advanced injection timings are recognized in high unburned hydrocarbon (HC) emissions (25 g/kWh), poor engine stability reflected by high COVimep (about 6 percent), and tendency to knock. This paper focuses on the combustion aspects of low pilot ignited natural gas engines with particular emphasis on advanced injection timings (45°–60°BTDC).Copyright

Modeling of NOx Emissions Using a Super-Extended Zel’dovich Mechanism

A kinetic model for NOx production has been developed to predict NOx emissions. The reaction scheme is a modified super-extended Zel’dovich mechanism (SEZM), which includes 43 reactions and 20 species instead of just the three reactions typically used in the extended Zel’dovich mechanism. The NOx emissions predicted by both mechanisms are compared using two separate models. First, a theoretical investigation of the two mechanisms is made for an SI engine using prescribed temperature and pressure histories. Then each of the two mechanisms is combined with a phenomenological combustion model for a single-cylinder Caterpillar 3400 series diesel engine to calculate the NOx emissions. The predictions from both mechanisms are compared with experimental results. It is shown that the SEZM can predict NOx emissions more accurately than the extended Zel’dovich mechanism. Results show that the SEZM increases the predicted NOx by about 25 percent. The difference between the two models is more pronounced for lean combustion, in which NO2 and NH play an important role in the NOx formation. In addition, the effects of several parameters on diesel engine NOx production are investigated. The super-extended Zel’dovich mechanism for NOx formation is expected to be more appropriate for lean combustion, such as in diesel or natural gas engines and other engines that typically operate at lean conditions.Copyright