Scott B. Fiveland

Compression Ratio Influence on Maximum Load of a Natural Gas Fueled HCCI Engine

This paper discusses the compression ratio influence on maximum load of a Natural Gas HCCI engine. A modified Volvo TD100 truck engine is controlled in a closed-loop fashion by enriching the Natural Gas mixture with Hydrogen. The first section of the paper illustrates and discusses the potential of using hydrogen enrichment of natural gas to control combustion timing. Cylinder pressure is used as the feedback and the 50 percent burn angle is the controlled parameter. Full-cycle simulation is compared to some of the experimental data and then used to enhance some of the experimental observations dealing with ignition timing, thermal boundary conditions, emissions and how they affect engine stability and performance. High load issues common to HCCI are discussed in light of the inherent performance and emissions tradeoff and the disappearance of feasible operating space at high engine loads. The problems of tighter limits for combustion timing, unstable operational points and physical constraints at high loads are discussed and illustrated by experimental results. Finally, the influence on operational limits, i.e., emissions peak pressure rise and peak cylinder pressure, from compression ratio at high load are discussed. (Less)

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.

Experimental and Simulated Results Detailing the Sensitivity of Natural Gas HCCI Engines to Fuel Composition

Natural gas quality, in terms of the volume fraction of higher hydrocarbons, strongly affects the auto-ignition characteristics of the air-fuel mixture, the engine performance and its controllability. The influence of natural gas composition on engine operation has been investigated both experimentally and through chemical kinetic based cycle simulation. A range of two component gas mixtures has been tested with methane as the base fuel. The equivalence ratio (0.3), the compression ratio (19.8), and the engine speed (1000 rpm) were held constant in order to isolate the impact of fuel autoignition chemistry. For each fuel mixture, the start of combustion was phased near top dead center (TDC) and then the inlet mixture temperature was reduced. These experimental results have been utilized as a source of data for the validation of a chemical kinetic based full-cycle simulation. Results reported here clearly demonstrate the ability of a thermo-kinetic, single-zone model to capture the fuel composition effects seen in the experiments. The uncertainty that exists in both the experiment and simulation is discussed in light of the model predictions. This uncertainty is used to quantify what reasonable level of accuracy can be expected between a model and experiment under HCCI operation. Finally, the simulation has been further exercised to compute the sensitivity of ignition timing to changes in hydrocarbon composition outside what has been experimentally tested.

A Computationally Efficient Method for the Solution of Methane - Air Chemical Kinetics With Application to HCCI Combustion

The Rate-Controlled Constrained-Equilibrium (RCCE) method is applied to the numerical solution of methane-air combustion. The RCCE method offers a reduction in computation time for complex chemically reacting systems because the rate equations for a small number of slowly evolving constraints need to be solved. The current work focuses on presenting both the principles of the RCCE method and its application to methane-air Homogeneous Charge Compression Ignition (HCCI) combustion. This work takes into consideration some of the previously unexplored numerical issues associated with solving the RCCE equation set. Application of the RCCE method is first demonstrated in constant and variable volume adiabatic environments and compared to the integration of the full set of kinetic rate equations for each species. Results presented here show a reduction in computational time. For large molecules, which require larger chemical mechanisms, it is expected that the computational time associated with the RCCE method should continue to improve over direct integration. The latter part of this work uses a thermo-kinetic HCCI model coupled with the RCCE method to simulate the combustion process in a methane-fueled internal combustion engine operating under HCCI conditions. This is the first known application of this method to HCCI simulations. Results are compared in light of HCCI experiments.

Phenomenological Autoignition Model for Diesel Sprays Using Reduced Chemical Kinetics and a Characteristic Scalar Dissipation Rate

• Research focused on liquid sprays & atomization, turbulent reacting flows, combustion & chemical kinetics, and emissions formation • Expertise in internal combustion engines with an emphasis on advanced combustion strategies • Experimental expertise in engine combustion, optical measurements of sprays & combustion, and soot measurements & characterization • Simulation expertise in both multi-dimensional turbulent combustion modeling and simplified phenomenological models of the combustion & emissions formation processes

Development and Use of a Segregated-Solver for Detailed Modeling of End-Gas Detonation in a Lean-Burn Spark-Ignited Engine

End-gas detonation occurs in a spark-ignited engine when the advancing flame front compresses the end-gas mixture to its autoignition temperature. The rapid energy release results in shock waves which are undesirable due to resulting combustion noise and boundary layer breakdown leading to reduced engine performance and incipient engine damage. In a spark-ignited engine, end-gas knock can result from improper combinations of compression ratio, spark timing or inlet thermodynamic conditions (i.e. manifold temperature, pressure, and equivalence ratio). These variables exhibit very complex interactions, which require costly high dimensional experimental designs for proper evaluation. As a result, detailed modeling tools are needed to predict the onset of the end-gas detonation regime for engine design applications. Developing a solver to predict the end-gas detonation of gases ahead of the flame front in an operating engine is not trivial. In theory, the model would need to simultaneously resolve both the detailed fluid mechanics as well as describe the fuel decomposition using detailed chemistry. Calculations for this type can take weeks or months depending on the number of dimensions that are resolved. Since hundreds of computations may be necessary to optimize a given configuration, it is necessary to be able to not only compute the onset of auto-ignition and other parameters accurately, but efficiently. The objective of this work was to develop an efficient methodology that could be utilized to effectively predict detonation in an internal combustion spark-ignited engine. This paper presents the computational methodology, a review of the combustion tool capability, and a comparison to experiments. The work clearly demonstrates the existence of inhomogeneities in the temperature field and discusses their impact on the prediction of end-gas knock.Copyright

Numerical Simulation and Experiments of Reformed Fuel Blends in a Lean-Burn Spark-Ignited Engine

Premixed, lean burn combustion research has focused for years on extending the lean flammability limit while maintaining both stables ignition and turbulent flame propagation. Operating with a leaner air-fuel mixture results in a lower temperature conversion of reactants to products (i.e. reduced NOx) while maintaining thermal efficiency. The lean limit, at some level, is dependent on both the fuel transport and chemical properties. This work sets out to numerically explore the effect of reformed fuels on both fundamental flame stability and the performance/emissions tradeoffs of the engine. The numerical simulations were conducted for a range of reformed fuel blends (10–40%) as well as mixture equivalence ratios (0.35–0.6). The laminar flame speed results clearly define the regime of stable flame propagations for equivalence ratio/reformed fuel blend combinations. Subsequently, a validated and predictive quasi-dimensional engine simulation is used to simulate the performance/emissions characteristics of the complete engine system operating on the reformed fuel blends (10–50%) for a range of ignition timings, and air-fuel ratios. The performance trends define not only the misfire and detonation limits associated with the air-fuel blends but also the thermal efficiency/NOx tradeoffs.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

Strategies for Reduced NO

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 percent 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 effect of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling was studied. With appropriate control of the above variables, 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 1–2 percent. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. Heat release schedules obtained from measured cylinder pressure data are also presented. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.Copyright