Two-zone Thermodynamic Model Research Articles

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Impact of waste-plastic-derived diesel on the performance and emission characteristics of a diesel engine under low load conditions

In recent years, hybrid (e.g. diesel-renewable) power systems have been implemented in many isolated regions (e.g. remote areas and islands) to reduce dependency on diesel generation. Nevertheless, low load (up to 30% of the maximum rated power) at variable speed operations is considered flexible and relevant for maximum renewable energy penetration because of the high cost and complexity of the conventional mode with fixed speed operation in the hybrid system. Given the increasing emphasis on fossil fuels in power generation sectors, plastic-derived diesel can be a suitable alternative. In this study, plastic-derived diesel was used for engine testing. It was produced from plastic crude oil from high-density polyethylene, polypropylene and polystyrene in a 1:1:1 ratio by using a vacuum distillation technique via the pyrolysis process. The pyrochemical properties of plastic-derived diesel were tested after distillation. The present work focused on validating the two-zone thermodynamic model for investigating engine performance and emissions at variable speeds (i.e. 1200, 1500, 1800 and 2100 rpm) at 5, 10, 15, 20, 25 and 30% load conditions fuelled with ultra-low sulphur diesel and plastic-derived diesel blends (5, 10 and 20). The simulation and experimental results showed good agreement. The maximum variation of 17.2% was found between the experimental and simulated results at mentioned speeds and loads for all tested fuels. Compared with ultra-low sulphur diesel, plastic-derived diesel showed a significant improvement in brake power, torque, brake thermal efficiency and brake-specific fuel consumption. The maximum brake power variation was 9.85% for plastic-derived diesel (20%) at 1200 rpm at 5% load. Moreover, the minimum brake specific fuel consumption variation was 4.15% for ultra-low sulphur diesel at 1500 rpm and 25% load. Compared with the commercial ultra-low sulphur diesel, 20% of plastic-derived diesel blends exhibited maximum reductions of 4.75, 5.95, 4.45 and 4.35% in NOx, CO2, CO, and HC emissions, respectively, under any tested conditions. Plastic-derived diesel blends showed less particulate matter emission compared to ultra-low sulphur diesel. Therefore, up to at least 20% of the plastic-derived diesel blends tested in this study can be used as an alternative fuel for ultra-low sulphur diesel. Further study should be carried out to test more than 20% plastic-derived diesel blends and reach a conclusion.

Modelling and simulation of virtual NOx sensor for diesel engine using thermodynamic model

This paper presents a thermodynamic model for virtual NOx sensor which is capable of predicting nitrogen oxides (NOx) emissions in diesel engines. In-cylinder volume and pressure are the inputs from estimator. The proposed method is based on a two-zone thermodynamic model which divides the combustion chamber into burned and unburned gas zone. The contribution of proposed method arises from: In-cylinder volume and pressure are the inputs from estimator using slider crank mechanism. And the two-zone thermodynamic modeling framework is applied in a way that the thermodynamic equations can be solved in a closed form, which provides the basis for achieving high level of predictiveness using MATLAB which uses ADVISOR as prerequisite for the project. The model is validated using real data values. The model is suitable for two different direct injection diesel engines, i.e. a light and a heavy-duty engine. Also, it maintains a low pipe emission which minimizes fuel consumption.

“near wall” combustion model of spark ignition engine

This paper has illustrated a "near wall" combustion model for a spark ignition engine that was included in a two-zone thermodynamic model. The model has calculated cylinder pressure and temperature, composition, as well as heat transfer of fresh and combustion gas. The CO submodel used a simplified chemical equation to calculate the dynamics of CO during the expansion phase. Subsequently, the HC submodel is introduced, and the post-flame oxidation of un-burned hydrocarbon was affected by the reaction/diffusion phenomenon. After burning 90% of the fuel, the hydrocarbon reaction dominates at a very late stage of combustion. This modeling method can more directly describe the ?near wall? flame reaction and its contribution to the total heat release rate.

A Novel Two-Zone Thermodynamic Model for Spark-Ignition Engines Based on an Idealized Thermodynamic Process

The thermodynamic model is a valuable simulation tool for developing combustion engines. The most widely applied thermodynamic models of spark-ignition engines are the single-zone model and the two-zone model. Compared to the single-zone model, the two-zone model offers more detailed in-cylinder thermodynamic conditions, but its governing equations are numerically stiffer, therefore it is restricted when applied in computationally intensive scenarios. To reduce the two-zone model’s stiffness, this paper isolates an idealized thermodynamic process in the unburned zone and describes this idealized thermodynamic process by an algebraic equation. Assisted with this idealized thermodynamic process, this paper builds a novel two-zone model for spark-ignition engines, whose governing equations are simplified to a set of two ordinary differential equations accompanied by a set of three algebraic equations. Benchmarked against the single-zone model and conventional two-zone model, the novel two-zone model is formed and validated by experimental results, and its stiffness is quantitatively evaluated by linearizing its governing equations at simulation steps. The results show that the novel two-zone model inherits the conventional two-zone model’s ability to estimate both zones’ state variables highly accurately while its simplified structure reduces its stiffness down to the level of the single-zone model, accelerating the computation speed.

Real-time capable virtual NOx sensor for diesel engines based on a two-Zone thermodynamic model

This paper presents a control-oriented thermodynamic model capable of predicting nitrogen oxides (NOx) emissions in diesel engines. It is derived from zero-dimensional combustion model using in-cylinder pressure as the input. The methodology is based on a two-zone thermodynamic model which divides the combustion chamber into a burned and unburned gas zone. The original contribution of proposed method arises from: (1) application of a detailed two-zone modeling framework, developed in a way that the thermodynamic equations could be solved in a closed form without iterative procedure, which provides the basis for achieving high level of predictiveness, on the level of real-time capable models and (2) introduction of relative air-fuel ratio during combustion as a main and physically motivated calibration parameter of the NOx model. The model was calibrated and validated using data sets recorded in two different direct injection diesel engines, i.e. a light and a heavy-duty engine. The model is suitable for real-time applications since it takes less than a cycle to complete the entire closed cycle thermodynamic calculation including NOx prediction, which opens the possibility of integration in the engine control unit for closed-loop or feed-forward control.

Near wall combustion modeling in spark ignition engines. Part B: Post-flame reactions

Reduced fuel consumption, low pollutant emissions and adequate output performance are key features in the contemporary design of spark ignition engines. Zero-dimensional numerical simulation is an attractive alternative to engine experiments for the evaluation of various engine configurations. Both flame front reaction and post-flame processes contribute to the heat release rate. The contribution of this work is to highlight and model the role of post-flame reactions (CO and hydrocarbons) in the heat release rate.The modeling approach to CO kinetics used two reactions considered to be dominant and thus more suitable for the description of CO chemical mechanism. Equilibrium concentrations of all the species involved were calculated by a two-zone thermodynamic model. The computed characteristic time of CO kinetics was found to be of a similar order to the results of complex chemistry simulations. The proposed model captured the ‘freezing’ effect (reaction rate is almost zero) for temperatures lower than 1800K and followed the trends of the measured values at exhaust. However, a consistent underestimation of CO levels at the exhaust was observed. The impact of the remaining CO on the combustion efficiency is considerable especially for rich mixtures. For a remaining 0.4% CO mass fraction, the impact on combustion inefficiency is 0.1%.Unburnt hydrocarbon, which have not reacted within the flame front before quenching, diffuse in the burnt gas and react. In this work, a global reaction rate models the kinetic behavior of hydrocarbon. The diffusion process was modeled by a relaxation equation applied on the calculated kinetic concentration. The relaxation time was calculated by the reduction of a general hydrocarbon diffusion equation. This modeling approach enabled the final part of combustion to be simulated. The HC consumption predicted by the model is consistent with the HC measurements at the exhaust. The hydrocarbon consumption at the final combustion stage varied from 1% to 6% depending on the operating point.

Modeling of in-cylinder pressure oscillations under knocking conditions: A general approach based on the damped wave equation

The modeling of the in-cylinder pressure oscillations under knocking conditions is tackled in this work. High frequency pressure oscillations are modeled by the explicit integration of a partial differential wave equation augmented with a time-dependent dissipation term. The general solution of such equation is determined by the Fourier method of separation of variables whereas the integration constants are obtained from the boundary and initial conditions. The integration space is a cylindrical acoustic cavity whose volume is that of the combustion chamber evaluated at the knock onset. The domain of integration is assumed to be formed by a finite set of small volumes having the shape of annulus sectors. This approach involves that knock region can assume more realistic shape of the kernels where abnormal combustion initiates. The initial conditions are evaluated by means of a two-zone thermodynamic model applied to low-pass filtered experimental pressure cycles. The damping coefficient and the knock region are model parameters to be assigned or identified experimentally by means of a proper least-squares optimization process. Experimental data obtained on a direct injection spark ignition engine, operating under knocking conditions at different speeds, have been used to validate the model both in time and frequency domains.

Investigation on heat transfer evaluation for a more efficient two-zone combustion model in the case of natural gas SI engines

Two-zone model is one of the most interesting engine simulation tools, especially for SI engines. However, the pertinence of the simulation depends on the accuracy of the heat transfer model. In fact, an important part of the fuel energy is transformed to heat loss from the chamber walls. Also, knock appearance is closely related to heat exchange. However, in the previous studies using two-zone models, many choices are made for heat transfer evaluation and no choice influence study has been carried out, in the literature. The current study aims to investigate the effect of the choice of both the heat transfer correlation and burned zone heat transfer area calculation method and provide an optimized choice for a more efficient two-zone thermodynamic model, in the case of natural gas SI engines. For this purpose, a computer simulation is developed. Experimental measurements are carried out for comparison and validation. The effect of correlation choice has been first studied. The most known correlations have been tested and compared. Our experimental pressure results, supported for more general and reliable conclusions, by a literature survey of many other studies, based on measured heat transfer rates for several SI engines, are used for correlation selection. It is found that Hohenberg's correlation is the best choice. However, the influence of the burned zone heat transfer area calculation method is negligible.

Role of mixture richness, spark and valve timing in hydrogen-fuelled engine performance and emission

The use of hydrogen as an engine fuel has a great potential for reducing exhaust emissions. With the exception of a little amount of hydrocarbon emissions originating from the lubricating oil, NOx is the only pollutant emitted. The special properties of hydrogen compel much more study on hydrogen internal combustion engines (ICEs). Studying and analyzing the behavior of hydrogen ICE and its sensitivity to controllable parameters can help designers to have better understanding over hydrogen characteristics and its combustion in an ICE. In this paper, firstly a quasi-dimensional two-zone thermodynamic model of an SI hydrogen ICE is developed and validated by experimental data. The model is used as an engine simulator. Spark advance (SA), air to fuel ratio and valve timing are selected as the main effective and controllable parameters on engine emissions and performance characteristics. Valve timing parameter is defined as the intake and exhaust valves’ lift, opening time and duration. Secondly, the effects of variation of the mentioned three parameters on emission and performance characteristics of the modeled engine are illustrated. Finally, the reasons of the engine behavior and characteristics under variations of these parameters are fully discussed.

Safe operating conditions determination for stationary SI gas engines

Knock is a major problem when running combined heat and power (CHP) gas engines because of the variation in the network natural gas composition. A curative solution is widely applied, using an accelerometer to detect knock when it occurs. The engine load is then reduced until knock disappears. The present paper deals with a knock preventive device. It is based on the knock prediction following the engine operating conditions and the fuel gas methane number, and it acts on the engine load before knock happens. A state of the art about knock prediction models is carried out. The maximum of the knock criterion is selected as knock risk estimator, and a limit value above which knock may occur is defined. The estimator is calculated using a two-zone thermodynamic model. This model is specifically based on existing formulas for the calculation of the combustion progress, modified to integrate the effect of the methane number. A chemical kinetic model with 53 species and 325 equilibrium reactions is used to calculate unburned and burned gases composition. The different parameters of the model are fitted with a least squares method from an experimental data base. Errors less than 8% are achieved. The knock risks predicted for various natural gases and operating conditions are in agreement with previous work. Nevertheless, the knock risk estimator is overestimated for natural gases with high concentrations of inert gases such as nitrogen and carbon dioxide. The definition of a methane number limit based on the engine manufacturer's recommendation is then required to eliminate unwarranted alerts. Safe operating conditions are thus calculated and gathered in the form of a map. This map, combined with the real time measurement of the fuel gas methane number, can be integrated to the control device of the CHP engine in order to guarantee a safe running towards fuel gas quality variation.

FL1-3: A Two-Zone Thermodynamic Model for Hydrogen-Fueled S.I. Engines(FL: Fuels and Lubricants,General Session Papers)

Because of significantly different physical and chemical properties of hydrogen, compared to hydrocarbons, running an engine on hydrogen or modeling the combustion of hydrogen in engines is not straightforward. This paper focuses on modeling the power cycle of a hydrogen-fueled spark-ignition engine. The authors have investigated the laminar burning velocity u_1 of hydrogen/air mixtures, with allowance for residuals, as this is a fundamental parameter in turbulent burning velocity models. The correlation for u_1 was evaluated in a quasi-dimensional engine code by a comparison to experimental data on a single cylinder hydrogen engine. A two-equation combustion model was used to allow for flame brush thickness effects. Several models for the turbulent burning velocity were used to provide values for the turbulent 'entrainment' velocity. All models were able to correctly predict the effects of varying compression ratio and ignition timing. The ability to recover the effects of changes in equivalence ratio, one of the features in which a hydrogen engine differs clearly from hydrocarbon-fueled engines, is shown to be the benchmark to distinguish between models.

Numerical Study of Friction and Heat Transfer Influence on Spark Ignition Engine Manifolds

A model has been developed in order to analyse the influence of friction and heat transfer between the exhaust gases and the inside wall of the exhaust manifold on spark ignition engine performance. One-dimensional unsteady compressible flow equations are used to describe instantaneous evolution of the flow inside the manifolds. Special attention is paid to friction and convective heat transfer source terms. A high-order total variation diminishing (TVD) scheme is applied to compute a second-order accurate numerical solution of the unsteady flow in engine manifolds. The combustion process in the spark ignition engine cylinder is described by a two-zone thermodynamic model. A series of numerical tests has been carried out. The influence of friction and convective heat transfer between the exhaust flow and the inside wall manifolds has been studied by analysing computed and experimental engine performance curves. Also, the instantaneous pressure at the exhaust manifold has been studied. The manifold pressure predicted by the developed model is successfully compared with experimental data from a spark ignition engine. From these comparisons it has been found that friction and heat transfer in exhaust manifolds play an important role in engine performance. Furthermore, it has been checked that the performance is not very sensitive to which friction factor correlation is chosen, whereas the engine performance is rather dependent on the constant factor set in the heat transfer correlation.

Method for predicting the performance of an internal combustion engine fuelled by producer gas and other low heating value gases

Gasification can be seen as a process in which solid biomass is converted into a mixture of combustible gases, which complete their combustion in a second device, usually a reciprocating internal combustion engine (RICE). Downdraft moving-bed gasifiers coupled with RICE are a good choice for moderate quantities of available biomass, the equivalent of up to 500 kW of electric power. The producer gas must be mixed with additional air in order to have a stoichiometric mixture to fuel the RICE. To predict the engine performance working with such a fuel, a so-called Engine Fuel Quality (EFQ) parameter has been developed by the authors. This parameter considers the combined effect of stoichiometric air–fuel ratio and stoichiometric mixture heating value, both depending on the producer gas composition. The estimation of engine power made by using the EFQ parameter indicates that power at full load is reduced at about two-thirds of the maximum obtained with a conventional liquid fuel. A more detailed prediction of engine performance requires the use of computer simulation of several types. The authors have used successfully a two-zone thermodynamic model to predict engine performance. Model results include the fraction of mass burned, the pressure and temperature evolution, and pollutant emissions. The detailed results for power confirm the first order prediction based on the EFQ parameter.

Comparisons between thermodynamic and one-dimensional combustion models of spark-ignition engines

A one-dimensional combustion model, employing a constant eddy diffusivity and a one-step chemical reaction, has been developed and applied to study the flame propagation in a spark-ignition engine. Calculations have been made at 1600 and 4200 rev min −1 under fuel rich conditions and compared with available engine pressure data. One- and two-zone thermodynamic models have also been developed and applied to study the combustion process in the engine. The thermodynamic models have been compared with the one-dimensional model results and comparisons include the average mixture temperature, the temperatures of the burned and unburned gases and the flame surface area. These comparisons indicate that the one-dimensional model predictions are very sensitive to the eddy diffusivity and reaction rate data. The two-zone thermodynamic model predicts, first, a monotonically increasing flame surface area with time and, then, a monotonically decreasing surface area, whereas the one-dimensional model always predicts a monotonically increasing flame surface area. The average mixture temperature predicted by the one-zone thermodynamic model is higher than those of the two-zone and one-dimensional models during the compression stroke, while that of the one-dimensional model is higher than the temperatures predicted by the one- and two-zone models during the expansion stroke. The one-dmensional model predicts an accelerating flame even when the front approaches the cold cylinder wall. This yields a faster fuel consumption rate than those predicted by the one- and two-zone thermodynamic models which predict smoother burned fuel mass profiles.