Alberto Vassallo

Analysis of Combustion Parameters and Their Relation to Operating Variables and Exhaust Emissions in an Upgraded Multivalve Bi-Fuel CNG SI Engine

The combustion propagation and burned-gas expansion processes in a bi-fuel CNG SI engine were characterized by applying a newly developed diagnostic tool, in order to better understand how these processes are related to the fuel composition, to the engine operating variables as well as to the exhaust emissions. The diagnostic tool is based on an original multizone heat-release model that is coupled with a CAD model of the burned-gas containing surface for the computation of the burning speed and the burned-gas mean expansion velocity. Furthermore, the thermal and prompt NO sub-models, embedded in the diagnostic code, were employed to study the effects of NO formation mechanisms and thermodynamic parameters on nitric oxide emissions. A previously developed multivalve bi-fuel SI engine has been upgraded to take a wide experimental database of in-cylinder pressure time-histories, engine performance and pollutant emissions throughout an extended air-fuel ratio interval for both fuels, i.e., gasoline and CNG (up to the lean-combustion stability limit) and an enlarged speed range for CNG (up to 6500 rpm). In particular, the CNG pressure reducer upstream of the injection system was replaced to deliver the gaseous fuel at higher pressures and the CNG injectors were replaced with new injectors purposely designed and realized with larger flow nozzle areas. Experimental tests have thus been carried out in a broad interval of speeds (n = 2000-5500 rpm), loads (bmep = 200-790 kPa), relative air-fuel ratios (RAFR = 0.80-1.60) and spark advances (SA ranging from 8 deg retard to 8 deg advance with respect to MBT timing). One of the main findings was that the ratio between the burning speed Sb and the laminar flame speed SL, at the point of the engine cycle where Sb peaks, scales with n. The scale factors were worked out for both gasoline and CNG operations. The effects of operating engine variables on flame propagation parameters were analyzed

Methods for Specific Emission Evaluation in Spark Ignition Engines Based on Calculation Procedures of Air-Fuel Ratio: Development, Assessment, and Critical Comparison

New computational procedures are proposed for evaluating the exhaust brake specific mass emissions of each pollutant species in internal combustion (IC) engines. The procedures start from the chemical reaction of fuel with combustion air and, based on the measured exhaust raw emissions THC, CH 4 , NO x , CO, O 2 , CO 2 , calculate the volume fractions of the compounds in the exhaust gases, including those that are not usually measured, such as water, nitrogen and hydrogen. The molecular mass of the exhaust gases is then evaluated and the brake specific emissions can be obtained if the exhaust flow rate and the engine power output are measured. The algorithm can also be applied to the evaluation of air-fuel ratio from measured raw volume emissions of IC engines. The new procedures take the effects of various fuel and combustion air compositions into account, with particular reference to different natural gas blends as well as to the presence of water vapor, CO 2 , Ar and He in the combustion air. In the paper, the algorithms are applied to the evaluation of air-fuel ratio and brake specific mass emissions in an automotive bi-fuel Spark Ignition (Sl) engine with multipoint sequential port-fuel injection. The experimental tests were carried out in a wide range of steady-state operating conditions under both gasoline and compressed natural gas operations. The specific emissions calculated from the new procedures are compared to those evaluated by applying Society of Automotive Engineers (SAE) and International Standards Organization (ISO) recommended practices and the air-fuel ratio results are compared to those obtained either from directly measured air and fuel mass flow rates or from Universal Exhaust Gas Oxygen (UEGO) sensor data. The sensitivity of the procedure results to the main engine working parameters, the influence of environmental conditions (in particular the effect of air humidity on NO x formation) and the experimental uncertainties are also determined.

Evaluation of Combustion Velocities in Bi-fuel Engines by Means of an Enhanced Diagnostic Tool Based on a Quasi-Dimensional Multizone Model

The burned-gas propagation process has been characterized in two bi-fuel engines by means of a combustion diagnostic tool resulting from the integration of an original multizone heat-release model with a CAD procedure for the burned-gas front geometry simulation. Burned-gas mean expansion speed U b , mean gas speed u g and burning velocity S b were computed as functions of crank angle and burned-gas radius for a wide range of engine speeds (n = 2000-5500 rpm), loads (bmep = 200-790 kPa), relative air-fuel ratios (RAFR = 0.80-1.60) and spark advances (SA ranging from 8 deg retard to 8 deg advance from MBT), under both gasoline and CNG operations. Finally, the influence of intake runner and combustion chamber geometries on flame propagation process was investigated. Main results show that S b is generally comparable for the engine running on both gasoline and CNG, at the same engine speed and load, under stoichiometric and MBT operations. In fact, higher temperatures and pressures of the unburned-gas ahead of the flame front under CNG fuelling compensate for natural gas lower laminar-burning speed S L at reference conditions. The tested intake runner sets showed to exert a minor effect on burned-gas propagation. On the contrary, combustion chamber shape and spark plug positioning strongly influenced combustion process. Finally, the ratio of S b to S L was analyzed as a function of engine operating variables during the rapid-burning interval.

Cycle-Resolved Detection of Combustion Start in SI Engines by Means Of Different In-Cylinder Pressure Data Reduction Techniques

An original technique for the detection of combustion start in SI engines on a cycle-by-cycle basis was proposed and applied to the analysis of pressure time-histories taken on a bi-fuel engine fueled by either gasoline or CNG. Such a technique locates the onset of combustion on the basis of the earliest release of chemical energy. It stems from the fact that, during the compression stroke, changes in the charge sensible energy, and thus in the cylinder pressure, are ruled only by work and heat exchanges with the combustion chamber walls. Hence, an imbalance in these three energies indicates the correspondent release of chemical energy, identifying combustion onset. The results of this technique were compared to those obtained through a direct analysis of in-cylinder pressure time-histories on logarithmic-coordinates p-V diagrams. More specifically, compression stroke appears like a segment on such diagrams and thus combustion onset can be defined by the detachment of in-cylinder pressure curve from linearity. The results of the different approaches for combustion start detection were compared on a wide range of working conditions of the bi-fuel SI engine under both gasoline and CNG fueling. The experimental matrix covered different engine speeds (N = 2000–4600 rpm), loads (bmep = 200–790 kPa), relative air-fuel ratios (RAFR = 0.80–1.60) and spark advances (SA ranging from 8 deg retard to 2 deg advance from MBT timing). 100 consecutive in-cylinder pressure traces were analyzed for each point in the test matrix. Particular attention was also given to the techniques applied for in-cylinder pressure filtering, which proved to be fundamental for accurate cycle-by-cycle investigation. Finally, on the basis of the experimental results obtained through the chemical-energy approach, two correlations for flame-development angle prediction are proposed, one for gasoline and the other for CNG. These correlations are based on cylinder-average thermodynamic properties at SA and can be usefully applied for triggering the flame propagation routines in indicated-cycle simulation codes.© 2006 ASME

Impact on Performance, Emissions and Thermal Behavior of a New Integrated Exhaust Manifold Cylinder Head Euro 6 Diesel Engine

The integration of the exhaust manifold in the engine cylinder head has received considerable attention in recent years for automotive gasoline engines, due to the proven benefits in: engine weight diminution, cost saving, reduced power enrichment, quicker engine and aftertreatment warm-up, improved packaging and simplification of the turbocharger installation. This design practice is still largely unknown in diesel engines because of the greater difficulties, caused by the more complex cylinder head layout, and the expected lower benefits, due to the absence of high-load enrichment. However, the need for improved engine thermomanagement and a quicker catalytic converter warm-up in efficient Euro 6 diesel engines is posing new challenges that an integrated exhaust manifold architecture could effectively address. A recently developed General Motors 1.6L Euro 6 diesel engine has been modified so that the intake and exhaust manifolds are integrated in the cylinder head. Extensive CAD/CAE/CAM analyses have been employed in order to guide the design of the overall surface and the water cooling jacket that surround the exhaust manifold of the new engine version, and thus to be able to improve the low-frequency thermal fatigue resistance of the head. The thus obtained prototype engine head has been tested on a highly-dynamic test bench at the Politecnico di Torino in order to characterize performance, emissions and thermal behavior in comparison to the baseline production engine. The results have generally been very promising and have shown the possibility of maintaining the same performance rating over the overall engine speed range as well as comparable emissions and brake specific fuel consumption in steady-state conditions. Furthermore, appreciably faster engine and aftertreatment warm-up have been recorded due to the higher heat fraction that is transferred to the coolant and to the more favorable exhaust gas enthalpy management. The latter benefit is in fact very interesting as far as the control of HC and CO emissions within the NEDC homologation is concerned

Methods for Specific Emission Evaluation in SI Engines Based on Calculation Procedures of Air-Fuel Ratio: Development, Assessment and Critical Comparison

New computational procedures are proposed for evaluating the exhaust brake specific mass emissions of each pollutant species in IC engines. The procedures start from the chemical reaction of fuel with combustion air and, basing on the measured exhaust raw emissions THC, CH4 , NOx , CO, O2 , CO2 , calculate the volume fractions of the compounds in the exhaust gases, including those that are not usually measured, such as water, nitrogen and hydrogen. The method also takes the effects of various fuel and combustion air compositions into account, with particular reference to different natural gas blends as well as to the presence of water vapor, CO2 , Ar and He in the combustion air. The molecular mass of the exhaust gases is then evaluated and the brake specific emissions can be obtained if the exhaust flow rate and the engine power output are measured. The methods stem from the extension of the different procedures that are used in the literature to evaluate α from measured raw volume emissions of IC engines running on conventional fuels. In the present study, a new algorithm is developed so as to generalize and refine all the mentioned α evaluation procedures, keeping conventional and alternative fuel compositions into account. First, the algorithm is applied to the evaluation of α in an automotive bi-fuel SI engine running on gasoline and CNG under a wide range of operating conditions. The α evaluation tests were carried out with a carefully controlled multipoint sequential injection system for both gasoline and CNG fueling. The results are compared to those obtained from the directly measured air and fuel mass flow rates as well as from more conventional UEGO sensor data. The algorithm is then applied to the evaluation of the brake specific mass emission of each pollutant species under gasoline and CNG engine operations for different steady-state working conditions. The sensitivity of results to the main engine working parameters, the influence of environmental conditions (in particular the effect of air humidity on NOx formation) and the experimental uncertainties are determined. The specific emissions calculated from the proposed algorithm are finally compared to those obtained by applying SAE and ISO recommended practices.Copyright

Analysis of Cyclic Variability in a Bi-Fuel Engine by Means of a ‘Cycle-Resolved’ Diagnostic Technique

The paper investigates cyclic variability in a fast-burn engine running on both gasoline or CNG by applying a new diagnostic technique based on a quasi-dimensional multizone model. Two different procedures were proposed for the ‘cycle-resolved’ calibration of the heat transfer correlation in the multizone model. The first procedure relates the cycle-resolved unreleased energy of the charge at the end of the flame propagation to the combustion efficiency determined from the average exhaust gas composition. The second procedure evaluates the coefficient in the heat transfer correlation through the application of the overall energy balance to the ensemble-cycle combustion and keeps them unchanged for all cycles. Both methods gave similar results, though the second procedure showed to be more physically consistent and in better agreement with the experimental results reported in the literature. The experimental matrix covered different engine speeds (n = 2000–4600 rpm), loads (bmep = 200–790 kPa), relative air-fuel ratios (RAFR = 0.80–1.60) and spark advances (SA ranging from 8 deg retard to 2 deg advance from MBT), for both CNG and gasoline operations, 100 consecutive in-cylinder pressure cycles were analyzed for each point in the test matrix and the sensitivity to cyclic variability of pressure, burn-rate and flame front position related parameters was analyzed. Main results showed that maximum pressure derivative, delay from SA of detected combustion start, NO exhaust concentration and maximum burning speed were the most sensitive parameters to cyclic variability. Strong correlations were found to hold between PFP and burned-gas temperature peak value, as well as between peak values of HRR and burning speed. On the contrary, some seemingly reasonable correlations were not assessed: for example, delay from SA of detected combustion start is related neither with PFP value nor with combustion duration. Finally, the results from mean cycle and cycle-resolved calculations were compared. Though they were usually in good agreement, in the case of NO emission and combustion interval calculation. cycle-resolved approach results in improved accuracy.© 2005 ASME

Flame Propagation Speed in SI Engines: Modeling and Experimental Assessment

The necessity for further reductions of in-cylinder pollutant formation and the opportunity to minimize engine development and testing time highlight the need of cycle simulation tools that have to accurately predict the effects of fuel, design and operating variables on engine performance. To develop reliable tools for indicated cycle simulation in SI engines, a correct prediction of heat release is required, which, in turn, involves the evaluation of in-cylinder turbulence generation and flame-turbulence interaction. This can be pursued by the application of a combustion fractal model coupled with semiempirical correlations of available geometrical and thermodynamical mass-averaged quantities. However, in the literature there is a lack of comparisons between the flame propagation speed obtained through these correlations and the experimental data determined under operating conditions that are significant for IC engines running on both conventional and alternative fuels. The present paper develops a new correlation that takes account of the effects of turbulence shrinking on the flame front as well as of the turbulent transfer of both species and heat across the flame front. The procedure has been applied to calculate the burning speeds in the cylinder of a naturally-aspirated bi-fuel engine for a wide range of engine speeds (N = 2000–4600 rpm), loads (bmep = 200–790 kPa), relative air-fuel ratios (RAFR = 0.80–1.30) and spark-advances (SA ranging from 8 deg retard to 2 deg advance with respect to MBT), under both gasoline and CNG operations. The computed burning speeds were compared to those obtained with the correlations currently available in the literature and to the experimental flame propagation data. These latter were extracted from measured in-cylinder pressure by means of a diagnostics technique previously developed by the authors. The results indicate that the burning speeds calculated through the authors’ procedure are in better agreement with the experimental outcomes than those derived from the correlations that are currently available in the literature.Copyright