Jerald A. Caton

On the destruction of availability (exergy) due to combustion processes — with specific application to internal-combustion engines

The destruction of availability (exergy) during combustion processes is examined for an adiabatic, constant volume system. This is an analytical examination and did not involve experimental measurements. The fraction of the fuels availability that is destroyed due to the irreversible processes is obtained as a function of temperature, pressure, and equivalence ratio for octane–air mixtures. In general, the destruction of the fuels available energy due to the combustion process decreases for operation at higher temperatures. In addition, the effect of equivalence ratio on the destruction of availability is significant and depends on the particular operating conditions. Specifically, for the conditions of this study, the destroyed availability due to the combustion process ranged between about 5 and 25% of the original reactant availability. The implications of these results to combustion processes in internal combustion engines are described.

Comparison of Nitric Oxide Removal by Cyanuric Acid and by Ammonia

Abstract Selective, non-catalytic techniques for removing nitric oxide (NO) from the exhaust gases of combustion processes include the addition of cyanuric acid, ammonia, or urea to the hot exhaust. This paper compares the effects of temperature and exhaust gas composition on the cyanuric acid (CA) and the ammonia (NH3) nitric oxide reduction processes and examines the decomposition of dry urea. The experiments were conducted in an electrically heated quartz flow reactor using mixtures of N2, 02, H2, H2, O, CO, and NO that simulated exhaust gases from overall lean hydrocarbon combustion processes Comparison of the CA and the NH3 nitric oxide reduction processes shows that the effects of the exhaust O2, H2, O, and CO concentrations on the NO reduction level and the temperature range over which the NO reduction occurs are different for each process. The comparison also shows that the by-products of each process are different for some conditions. These differences indicate that the detailed chemical mechanis...

Evaluation of the equivalence ratio-temperature region of diesel soot precursor formation using a two-stage Lagrangian model

Abstract The two-stage Lagrangian (TSL) reacting-jet model of Broadwell and Lutz is applied to n-heptane fuel jets to understand soot formation at diesel engine operating conditions. The model employs a diffusion-flame reactor and homogeneous core reactor with jet entrainment rates determined by empirical correlations. Detailed chemical kinetics, consisting of 696 species and 3224 reactions, are used for predictions of n-heptane oxidation and soot precursor formation up to seven-ring polycyclic aromatic hydrocarbons. Boundary conditions are based on realistic diesel operating conditions, mixing rates, and flame lift-off lengths. TSL soot precursor simulations are compared with closed-reactor (Senkin) predictions over a range of temperatures and equivalence ratios. Results show that the equivalence ratio-temperature region of soot precursor formation varies from the closed-reactor predictions and depends upon parameters such as ambient oxygen concentration, injection pressure, nozzle orifice size, and flame lift-off. The lack of a unique equivalence ratio-temperature region for soot precursor formation implies that the soot formation process depends upon the equivalence ratio-temperature path followed during jet mixing, and the residence time along the path.

Removal of nitric oxide from exhaust gas with cyanuric acid

Abstract Addition of gaseous isocyanic acid (HNCO) to the exhaust of combustion systems or chemical process has been proposed as a method for reducing nitric oxide (NO) emissions. The HNCO selectively reduces NO in the exhaust through a multistep chemical reaction mechanism. This article presents an experimental investigation of the proposed NO reduction process using cyanuric acid as the source of HNCO. At elevated temperature cyanuric acid decomposes and forms HNCO. The effects of temperature, exhaust gas composition, cyanuric acid concentration (i.e., HNCO concentration), and surfaces were examined. The experiments were conducted in an electrically heated quartz flow reactor using either exhaust from a diesel engine or simulated exhaust gas. The results demonstrate that gas phase NO reduction approaching 100% can be obtained. The lowest temperature for which gas phase NO reduction is observed is 950 K. The exhaust gas composition is the primary factor in determining the specific temperature range over which the NO reduction occurs, as well as the magnitude of the NO reduction, for a fixed cyanuric acid input. Three species in the exhaust gas that have a strong influence on the NO reduction process are O 2 , H 2 O, and CO. The results also demonstrate the cyanuric acid, HNCO, and N 2 O can be emitted when the NO reduction occurs in the gas phase. Finally, the results show that surfaces can have a major effect, either shifting the NO reduction to lower temperatures or causing a net production of NO.

The Selective Non-Catalytic Removal (SNCR) of Nitric Oxides From Engine Exhaust Streams: Comparison of Three Processes

Three processes for the selective non-catalytic removal (SNCR) of nitric oxides from engine exhaust gases are compared. The three processes are similar but each uses a different chemical agent: ammonia, urea, or cyanuric acid. A number of operating conditions have been studied. In particular, results for the removal of nitric oxide are significantly different for the three processes as the oxygen concentration varies. Ammonia, urea, and cyanuric acid were found to be most effective at low, intermediate, and high oxygen concentrations, respectively. The implications of these results for a range of engines and engine applications are discussed.

Effects of the compression ratio on nitric oxide emissions for a spark ignition engine: Results from a thermodynamic cycle simulation

Abstract Past experimental work has not provided consistent results for the effects of the compression ratio on nitric oxide emissions. A thermodynamic cycle simulation for spark ignition engines, which included a formulation using multiple zones for the combustion process, was used to determine the effects of the compression ratio on nitric oxide emissions. An important feature of this simulation is the use of an adiabatic zone to capture the high temperatures in the core region of the cylinder better during combustion. This study was completed for a commercial, 5.71 spark ignition V-8 engine operating at a part load condition at 1400r/min with an equivalence ratio of 1.0 and ‘MBT’ (maximum brake torque) spark timing. The effects of the compression ratio on nitric oxide emissions are shown to be the result of a complex set of interactions involving the combustion gas temperature, pressure, species concentrations, chemical kinetics and other thermodynamic quantities. For the base case condition, the nitric oxide concentration first increases and then decreases with increasing compression ratio. This is shown to be a result of the relative values of the nitric oxide moles formed, the combustion gas volume and the moles of the exhaust gas. The computed results are shown to agree qualitatively with experimental values from the literature.

Relation between group combustion and drop array studies

Abstract Spray combustion modeling requires a knowledge of the strength of the mass and heat sources for each drop in the spray. Since sprays involve a large number of drops, interactive transport processes must be accounted for in estimating the source strengths of the drops. Earlier approaches (called array studies) considered a finite number of drops (2 to 9), and accounted for the three dimensional variation of temperature and species mass fraction profiles in order to determine an evaporation/combustion correction factor. This correction factor is defined as the ratio of the average strength of each drop in the array to the strength of the drop if it is kept in an isolated environment. The current approach utilizes group combustion theory which involves a large number of drops, assumes top hat profiles in the interstitial space between the drops, and accounts for their variation in the radial direction. The correction factor is again determined. This paper reports the results of the current group combustion approach and compares these results with those from array studies when the group combustion approach is extended to an array consisting of as few as 2 to 9 drops. The change in transport rate due to proximity of other drops is accounted for in the present group combustion approach. An unexpected result is the close agreement of the results obtained from group combustion approach with those results from array studies. A simple algebraic expression is given for the correction factor in terms of l a ratio and total number of drops for a 2 to 9 drop array.

In situ measurements of nitric oxide in coal-combustion exhaust using a sensor based on a widely tunable external-cavity GaN diode laser

A diode-laser-based sensor has been developed to measure nitric oxide mole fractions using absorption spectroscopy. The sensor is based on sum-frequency mixing of a 395 nm external-cavity diode laser (ECDL) and a 532 nm laser in a beta-barium-borate crystal. Using a new tuning scheme, the GaN ECDL wavelength was modulated over 90 GHz without mode hops. The sensor was applied for measurements of the NO mole fraction in the exhaust of a laboratory-scale, 30 kW(t) coal-fired boiler burner. Absorption measurements were successfully performed despite severe attenuation by scattering from ash particles in the exhaust stream and on the exhaust-section windows. A detection limit (1sigma) of 4.5 ppm m/(square root)Hz at 700 K was demonstrated in coal- combustion exhaust at a maximum detection rate of 5 Hz.

Utilizing a Cycle Simulation to Examine the Use of EGR for a Spark-Ignition Engine Including the Second Law of Thermodynamics

The use of exhaust gas recirculation (EGR) for a spark-ignition engine was examined using a thermodynamic cycle simulation including the second law of thermodynamics. Both a cooled and an adiabatic EGR configuration were considered. The engine was a 5.7 liter, automotive engine operating from idle to wide open throttle, and up to 6000 rpm. First, the reduction of nitric oxides is quantified for the base case condition (bmep = 325 kPa, 1400 rpm, φ = 1.0 and MBT timing). Over 90% reduction of nitric oxides is obtained with about 18% EGR for the cooled configuration, and with about 26% EGR for the adiabatic configuration. For constant load and speed, the thermal efficiencies increase with increasing EGR for both configurations, and the results show that this increase is mainly due to decreasing pumping losses and decreasing heat losses. In addition, results from the second law of thermodynamics indicated an increase in the destruction of availability (exergy) during the combustion process as EGR levels increase for both configurations. The major reason for this increase in the destruction of availability was the decrease in the combustion temperatures. Complete results for the availability destruction are provided for both configurations.Copyright

An Assessment of the Thermodynamics Associated With High-Efficiency Engines

Recent advancements have demonstrated new combustion modes that exhibit low nitric oxide emissions and high thermal efficiencies. These new combustion modes involve various combinations of stratification, lean mixtures, high levels of EGR, multiple injections, variable valve timings, two fuels, and other such features. Although the exact combination of these features that provides the best design is not yet clear, the results (low emissions with high efficiencies) are of major interest. The current work is directed at determining some of the fundamental thermodynamic reasons for the relatively high efficiencies and to quantify these factors. Both the first and second laws are used in this assessment. An automotive engine (5.7 liter) which included some of the features mentioned above (e.g., high compression ratios, lean mixtures, and high EGR) was evaluated using a thermodynamic cycle simulation. These features were examined for two operating conditions: a moderate load, moderate speed condition (“A”), and a lower load, lower speed condition (“B”). By the use of lean operation, high EGR levels, high compression ratio and other features, the net indicated thermal efficiency increased from 35.6% to 48.2% (condition “A”), and from 30.3% to 44.6% (condition “B”). These increases are explained in a step-by-step fashion. One of the major reasons for these improvements was the lower heat losses associated with the advanced conditions. Other thermo-dynamic features are described.Copyright