K. C. Midkiff

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.

Performance and heat release analysis of a pilot-ignited natural gas engine

Abstract The influence of engine operating variables on the performance, emissions and heat release in a compression ignition engine operating in normal diesel and dual-fuel modes (with natural gas fuelling) was investigated. Substantial reductions in NOx emissions were obtained with dual-fuel engine operation. There was a corresponding increase in unburned hydrocarbon emissions as the substitution of natural gas was increased. Brake specific energy consumption decreased with natural gas substitution at high loads but increased at low loads. Experimental results at fixed pilot injection timing have also established the importance of intake manifold pressure and temperature in improving dual-fuel performance and emissions at part 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.

ANALYSIS OF DIESEL PILOT-IGNITED NATURAL GAS LOW-TEMPERATURE COMBUSTION WITH HOT EXHAUST GAS RECIRCULATION

Abstract Earlier efforts demonstrated the low NOx(<0.2 g/kWh) and high efficiency (>40%) benefits of the low temperature, advanced low pilot injection natural gas (ALPING) combustion concept that utilized advanced injection (about 60° BTDC) of small diesel pilots (2–3% on an energy basis) to compression-ignite a premixed natural gas-air mixture. At these injection timings, combustion was accompanied by increased unburned hydrocarbons (HC) (mostly methane) and variations in torque fluctuations. In this article, hot exhaust gas recirculation (EGR) is proposed as a potential strategy to reduce HC emissions and torque fluctuations at low (quarter and half) loads. It is shown that the addition of hot EGR leads to a combination of one or more of the following effects on the intake mixture entering the cylinder: oxygen depletion, increased temperatures due to mixing with exhaust gases, dilution due to introduction of high specific heat species, and active recycling of unburned hydrocarbons to effect reburn in subsequent cycles. In particular, hot EGR addition extends the ALPING operation regime from 50°–60° BTDC to 60°–70° BTDC, increases low-load efficiencies by more than 5 percentage points, substantially improves combustion stability, and drastically reduces HC emissions (by more than 70%) with little associated penalty in NOx emissions.

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

The Advanced (injection) 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 NO x emissions (to less than 0.2 g/kWh) 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) and poor engine stability reflected by high COV IMEP (about 6%). This paper focuses on the combustion aspects of low pilot ignited natural gas engines with particular emphasis on advanced injection timings (45°-60° BTDC). Ignition phasing at advanced injection timings (∼60° BTDC), and combustion phasing at retarded injection timings (∼15° BTDC) are recognized as important combustion parameters that profoundly impact the combustion process, HC emissions, and the stability of engine operation.

Effect of hot exhaust gas recirculation on the performance and emissions of an advanced injection low pilot-ignited natural gas engine:

Abstract The development of the advanced injection low pilot-ignited natural gas (ALPING) combustion concept that employs very small diesel pilots (1–5 per cent by energy) to compression ignite a premixed natural gas-air mixture to achieve very low NO x (0.2 g/kWh) and high efficiencies (about 41 per cent) has been described in a previous work. However, at part loads the ALPING combustion mode suffers from higher HC emissions (mostly unburned methane) and poor engine stability. To resolve this problem, tests were carried out employing various levels of hot EGR (0–26 per cent) at different loads on a single-cylinder research engine at a constant speed of 1700 r/min. Experimental results compared with baseline ALPING mode (0 per cent EGR) for quarter load operation are presented in the current paper. The results show that, at 60° BTDC injection timing, the application of hot EGR reduced HC emissions by up to 70 per cent without any significant NO x emissions penalty. The fuel conversion efficiencies were improved by 8 percentage points, while COVi.m.e.p. and CO emissions decreased 20 percentage points and 40 per cent, respectively. To identify the upper limits of hot EGR substitution, engine knock tests, which were conducted to identify audible knock limits, are also presented for a representative case (half load). The progress made by this project better positions ALPING combustion as a potentially viable approach to meet the regulatory and economic challenges of the future.

Development of a multizone model for direct injection diesel combustion

Abstract Diesel engines have attracted considerable attention in recent years because of the increasingly restrictive ‘engine-out’ emission standards being adopted by regulatory agencies. The cutting-edge technologies of emissions reduction in engines fall into three categories: preprocessing, improved combustion processing and post-processing. An engine cycle simulation was developed to investigate and, thus, find possible avenues of reducing emissions through modifying the combustion process. This simulation includes models for fresh air charging, fuel and air mixing, wall heat transfer, diesel droplet evaporation, ignition delay and mixture combustion with species equilibrium reactions. These models, together with a thermo-dynamic analysis of the cylinder gas, yield instantaneous cylinder conditions, overall indicated engine performance and a prediction of the engine-out NOx and soot emissions. The engine parameters and operating conditions used in the work presented here were chosen to be representative of a Caterpillar 3401 single-cylinder diesel engine. Experimental investigations were also conducted with the engine, and the combustion model has been verified by comparing the experiment results to the simulation results.

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.

Ignition in Pilot-Ignited Natural Gas Low Temperature Combustion: Multi-Zone Modeling and Experimental Results

In previous research conducted by the authors, the Advanced Low Pilot-Ignited Natural Gas (ALPING) combustion employing early injection of small (pilot) diesel sprays to ignite premixed natural gas-air mixtures was demonstrated to yield very low oxides of nitrogen (NOx ) emissions and fuel conversion efficiencies comparable to conventional diesel and dual fuel engines. In addition, it was observed that ignition of the diesel-air mixture in ALPING combustion had a profound influence on the ensuing natural gas combustion, engine performance and emissions. This paper discusses experimental and predicted ignition behavior for ALPING combustion in a single-cylinder engine at a medium load (BMEP = 6 bar), engine speed of 1700 rpm, and intake manifold temperature (Tin ) of 75°C. Two ignition models were used to simulate diesel ignition under ALPING conditions: (a) Arrhenius-type ignition models, and (b) the Shell autoignition model. To the authors’ knowledge, the Shell model has previously not been implemented in a multi-zone phenomenological combustion simulation to simulate diesel ignition. The effects of pilot injection timing and Tin on ignition processes were analyzed from measured and predicted ignition delay trends. Experimental ignition delays showed a nonlinear trend (increasing from 11 to 51.5 degrees) in the 20°–60° BTDC injection timing range. Arrhenius-type ignition models were found to be inadequate and only yielded linear trends over the injection timing range. Even the inclusion of an equivalence ratio term in Arrhenius-type models did not render them satisfactory for the purpose of modeling ALPING ignition. The Shell model, on the other hand, predicted ignition better over the entire range of injection timings compared to the Arrhenius-type ignition delay models and also captured ignition delay trends at Tin = 95°C and Tin = 105°C. Parametric studies of the Shell model showed that the parameter Ap3 , which affects chain propagation reactions, was important under medium load ALPING conditions. With all other model parameters remaining at their original values and only Ap3 modified to 8 × 1011 (from its original value of 1 × 1013 ), the Shell model predictions closely matched experimental ignition delay trends at different injection timings and Tin .Copyright