K. K. Srinivasan

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.

First and second law analysis of a Stirling engine with imperfect regeneration and dead volume

Abstract This article discusses the thermodynamic performance of an ideal Stirling cycle engine. This investigation uses the first law of thermodynamics to obtain trends of total heat addition, net work output, and thermal efficiency with varying dead volume percentage and regenerator effectiveness. Second law analysis is used to obtain trends for the total entropy generation of the cycle. In addition, the entropy generation of each component contributing to the Stirling cycle processes is considered. In particular, parametric studies of dead volume effects and regenerator effectiveness on Stirling engine performance are investigated. Finally, the thermodynamic availability of the system is assessed to determine theoretical second law efficiencies based on the useful exergy output of the cycle. Results indicate that a Stirling engine has high net work output and thermal efficiency for low dead volume percentages and high regenerator effectiveness. For example, compared to an engine with zero dead volume and perfect regeneration, an engine with 40 per cent dead volume and a regenerator effectiveness of 0.8 is shown to have ∼60 per cent less net work output and a 70 per cent smaller thermal efficiency. Additionally, this engine results in approximately nine times greater overall entropy generation and 55 per cent smaller second law efficiency.

Numerical Evaluation of the Effects of Compression Ratio and Diesel Fuel Injection Timing on the Performance and Emissions of a Fumigated Natural Gas–Diesel Dual-Fuel Engine

AbstractVarious solutions have been proposed for reducing the exhaust emissions and improve the well-known soot and nitrogen oxide (NO) trade-off in diesel engines, without making serious modifications to the engine, one of which is the use of natural gas as supplement to liquid diesel fuel. In these types of engines, referred to as fumigated, natural gas–diesel dual-fuel compression ignition (CI) engines, gaseous fuel is fumigated and premixed with the aspirated air during the induction stroke. Natural gas is a clean-burning fuel with a relatively high auto-ignition temperature, which is a serious advantage in dual-fuel combustion. Previous research studies have shown that the natural gas–diesel fuel dual-fuel combustion in a CI engine environment, compared with conventional diesel fuel operation, suffers from high specific fuel consumption and high carbon monoxide (CO) and unburned hydrocarbon (HC) emissions. Compression ratio (CR) and diesel fuel injection timing (IT) are two engine parameters that can...

Performance and Emissions Characteristics of Diesel-Ignited Gasoline Dual Fuel Combustion in a Single-Cylinder Research Engine

Diesel-ignited gasoline dual fuel combustion experiments were performed in a single-cylinder research engine (SCRE), outfitted with a common-rail diesel injection system and a stand-alone engine controller. Gasoline was injected in the intake port using a port-fuel injector. The engine was operated at a constant speed of 1500 rev/min, a constant load of 5.2 bar IMEP, and a constant gasoline energy substitution of 80%. Parameters such as diesel injection timing (SOI), diesel injection pressure, and boost pressure were varied to quantify their impact on engine performance and engine-out ISNOx, ISHC, ISCO, and smoke emissions. Advancing SOI from 30 DBTDC to 60 DBTDC reduced ISNOx from 14 g/kWhr to less than 0.1 g/kWhr; further advancement of SOI did not yield significant ISNOx reduction. A fundamental change was observed from heterogeneous combustion at 30 DBTDC to “premixed enough” combustion at 50–80 DBTDC and finally to well-mixed diesel-assisted gasoline HCCI-like combustion at 170 DBTDC. Smoke emissions were less than 0.1 FSN at all SOIs, while ISHC and ISCO were in the range of 8–20 g/kWhr, with the earliest SOIs yielding very high values. Indicated fuel conversion efficiencies were ∼ 40–42.5%. An injection pressure sweep from 200 to 1300 bar at 50 DBTDC SOI and 1.5 bar intake boost showed that very low injection pressures lead to more heterogeneous combustion and higher ISNOx and ISCO emissions, while smoke and ISHC emissions remained unaffected. A boost pressure sweep from 1.1 to 1.8 bar at 50 DBTDC SOI and 500 bar rail pressure showed very rapid combustion for the lowest boost conditions, leading to high pressure rise rates, higher ISNOx emissions, and lower ISCO emissions, while smoke and ISHC emissions remained unaffected by boost pressure variations.Copyright

Comprehensive uncertainty analysis of a Wiebe function-based combustion model for pilot-ignited natural gas engines

Abstract This paper presents a comprehensive uncertainty analysis of a Wiebe function-based combustion model for advanced low-pilot-ignited natural gas (ALPING) combustion in a single-cylinder research engine. The sensitivities, uncertainty magnification factors (UMFs), and uncertainty percentage contributions (UPCs) of different experimental input variables and model parameters were investigated. First, the Wiebe function model was validated against experimental heat release/mass burned fraction profiles and cylinder pressure histories for three pilot injection timings (start of injection (SOI)): −20°, −40°, and −60° after top dead center (ATDC). Second, the sensitivities and UMFs of predicted cylinder pressure histories were determined. Finally, crank angle-resolved uncertainties were quantified and mapped as ‘uncertainty bounds’ in predicted pressures, which were compared with measured pressure curves with error bars for cyclic variations. The Wiebe function-based combustion model with a quadratic interpolation equation for the specific heat ratio (γ) provided reasonable cylinder pressure and heat release/mass burned fraction predictions for all SOIs (better for −20° and −60° ATDC SOIs compared with −40° ATDC). Uncertainty analysis results indicated that γ (parameters in the quadratic interpolation equation), compression ratio, mass and lower heating value of natural gas trapped in the cylinder, overall trapped mass, and ignition delay were important contributors to the overall uncertainty in predicted cylinder pressures. For all SOIs, γ exhibited the highest UPC values (80–90 per cent) and therefore, γ must be determined with the minimum possible uncertainty to ensure satisfactory predictions of cylinder pressure histories. While the importance of γ in single-zone combustion models is well recognized, the specific contribution of the present analysis is quantification of the crank angle-resolved UPCs of γ and other model parameters to the overall model uncertainty. In this paper, it is shown that uncertainty analysis provides a unique methodology for quantitative validation of crank angle-resolved predictions from any type of engine combustion model with the corresponding experimental results. It is also shown that uncertainties in both predicted and measured cylinder pressures and heat release rates must be considered while validating any engine combustion model.

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