S. R. Krishnan

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.

Modeling and Experiments of Dual-Fuel Engine Combustion and Emissions

The combustion and emissions of a diesel/natural gas dual-fuel engine are studied. Available engine experimental data demonstrates that the dual-fuel configuration provides a potential alternative to diesel engine operation for reducing emissions. The experiments are compared to multi-dimensional model results. The computer code used is based on the KIVA-3V code and consists of updated sub-models to simulate more accurately the fuel spray atomization, auto-ignition, combustion and emissions processes. The model results show that dual-fuel engine combustion and emissions are well predicted by the present multi-dimensional model. Significant reduction in NO x emissions is observed in both the experiments and simulations when natural gas is substituted for diesel fuel. The HC emissions are under predicted by numerical model as the natural gas substitution is increased. The capabilities and limitations of the combustion model to simulate premixed combustion of air and natural gas were identified. It was found that the combustion model previously developed for diesel combustion provides adequately accuracy when extended to model the present dual-fuel cases. However, the accuracy of the predictions deteriorates for small pilot quantities. A brief discussion is given of a new combustion modeling approach that is applicable to very low pilot diesel fuel cases.

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.

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.

Detailed characterization of diesel-ignited propane and methane dual-fuel combustion in a turbocharged direct-injection diesel engine

This paper presents an experimental analysis of dual-fuel combustion based on the performance, emissions, and in-cylinder combustion measurements with gaseous propane or gaseous methane as the primary fuel and diesel as the pilot fuel. Two different sets of experiments were performed on a 1.9 litre four-cylinder engine at a constant engine speed of 1800 r/min: first, constant-pilot-quantity experiments, allowing the primary fuel concentration and the brake mean effective pressure to vary; second, constant-brake-mean-effective-pressure experiments, allowing the percentage energy substitution of the primary fuel and the pilot quantity to vary. In the constant-pilot-quantity experiments, the apparent heat release rate profiles showed the influence of the preignition chemistry and gaseous fuel burn rates on the dual-fuel combustion phasing and duration, the fuel conversion efficiency, and the engine-out emissions. With a fixed pilot quantity, the nitrogen oxide emissions were either reduced or unaffected while the smoke levels were increased or unaffected with increasing primary fuel concentration. The carbon monoxide and total unburned hydrocarbon emissions decreased and the fuel conversion efficiency increased as the pilot quantity or the primary fuel concentration was increased. Overall, diesel–propane combustion yielded higher carbon monoxide emissions, lower total unburned hydrocarbon emissions, and slightly higher fuel conversion efficiencies than diesel–methane combustion did. In the constant-brake-mean-effective-pressure experiments, at a brake mean effective pressure of 2.5 bar, diesel–propane and diesel–methane combustion behaved very similarly, the primary differences being in the preignition chemistry and the ignition delay trends. At a brake mean effective pressure of 2.5 bar, the nitrogen oxide and smoke emissions were simultaneously reduced while the carbon monoxide and total unburned hydrocarbon emissions were increased. At a brake mean effective pressure of 10 bar (a baseline diesel fuel conversion efficiency of 38%), diesel–propane fueling was prone to rapid earlier combustion while diesel–methane combustion was slower. For diesel–methane combustion at a brake mean effective pressure of 10 bar, the fuel conversion efficiency decreased to 37.1% as the percentage energy substitution was increased to 51%. For diesel–propane combustion at a brake mean effective pressure of 10 bar, the fuel conversion efficiency increased to 39% as the percentage energy substitution was increased to 46%. At high-brake-mean-effective-pressure–high-percentage-energy-substitution and large-pilot-quantity–high-equivalence-ratio conditions, diesel–propane combustion showed an apparent departure from the classical three-phase dual-fuel combustion to a distributed volumetric combustion process that resembled a “diesel-regulated homogenous-charge-compression-ignition-like” combustion process.

Analysis of Ignition Behavior in a Turbocharged Direct Injection Dual Fuel Engine Using Propane and Methane as Primary Fuels

Dual fuel engine combustion utilizes a high-cetane fuel to initiate combustion of a low-cetane fuel. The performance and emissions benefits (low NOx and soot emissions) of dual fuel combustion are well-known. Ignition delay (ID) of the injected high-cetane fuel plays a critical role in quality of the dual fuel combustion process. This paper presents experimental analyses of the ID behavior for diesel-ignited propane and diesel-ignited methane dual fuel combustion. Two sets of experiments were performed at a constant engine speed (1800 rev/min) using a four-cylinder direct injection diesel engine with the stock electronic conversion unit (ECU) and a wastegated turbocharger. First, the effects of fuel–air equivalence ratios (Фpilot ∼ 0.2–0.6 and Фoverall ∼ 0.2–0.9) on IDs were quantified. Second, the effects of gaseous fuel percent energy substitution (PES) and brake mean effective pressure (BMEP) (from 2.5 to 10 bars) on IDs were investigated. With constant Фpilot (>0.5), increasing Фoverall with propane initially decreased ID but eventually led to premature propane auto-ignition; however, the corresponding effects with methane were relatively minor. Cyclic variations in the start of combustion (SOC) increased with increasing Фoverall (at constant Фpilot) more significantly for propane than for methane. With increasing PES at constant BMEP, the ID showed a nonlinear trend (initially increasing and later decreasing) at low BMEPs for propane but a linearly decreasing trend at high BMEPs. For methane, increasing PES only increased IDs at all BMEPs. At low BMEPs, increasing PES led to significantly higher cyclic SOC variations and SOC advancement for both propane and methane. Finally, the engine ignition delay (EID), defined as the separation between the start of injection (SOI) and the location of 50% of the cumulative heat release, was also shown to be a useful metric to understand the influence of ID on dual fuel combustion. Dual fuel ID is profoundly affected by the overall equivalence ratio, pilot fuel quantity, BMEP, and PES. At high equivalence ratios, IDs can be quite short, and beyond a certain limit, can lead to premature auto-igniton of the low-cetane fuel (especially for a reactive fuel like propane). Therefore, it is important to quantify dual fuel ID behavior over a range of engine operating conditions.

Computational Analysis of Combustion of High and Low Cetane Fuels in a Compression Ignition Engine

Past research has shown that the combustion of low cetane fuels in compression ignition (CI) engines results in higher fuel conversion efficiencies. However, when high-cetane fuels such as diesel are substituted with low-cetane fuels such as gasoline, the engine operation tends to suffer from high carbon monoxide (CO) emissions at low loads and combustion noise at high loads. In this paper, we present a computational analysis of a light-duty CI engine operating on diesel, kerosene and gasoline. These three fuels cover a range of cetane numbers (CNs) from 46 for diesel to 25 for gasoline. Similar to experiments, the model predicted higher CO emissions at low load operation with gasoline. Predictions of in-cylinder details were utilized to understand differences in combustion characteristics of the three fuels. The in-cylinder mass contours and the evolution of model predicted in-cylinder mixture in Φ–T coordinates were then used to explain the emission trends. From the analysis, overmixing due to early single injection was identified as the reason for high CO emissions with low load gasoline low temperature combustion (LTC). Additional simulations were performed by introducing techniques like cetane enhancement, adding hot exhaust gas recirculation (EGR), and variation of the injection scheme. Their effects on low load gasoline LTC were studied. Finally, it is shown that use of a dual pulse injection scheme with hot EGR helped to reduce the CO emissions for low load gasoline LTC while maintaining low NOx emissions.