In-cylinder Volume Research Articles

Numerical prediction of research octane numbers via a quasi-dimensional two-zone cylinder model

Sustainably produced synthetic fuels offer great potential for a fast reduction of the greenhouse gas emissions of the transport sector. For an immediate application within the existing infrastructure and vehicle fleet, synthetic fuels need to comply with existing standards such as the EN 228 for gasoline. Beyond these standards and with optimized fuel design, certain properties can be improved compared to conventional fuels. For this purpose, methods for evaluating the properties are needed.This work discusses the development of a simplified numerical quasi-dimensional two-zone cylinder model, combined with chemical kinetic models, for the estimation of octane numbers. The model emulates fuel specific operating conditions of the standardized procedure for the determination of octane numbers in the cooperative fuel research engine with variable compression ratios. The two zones represent the burned and unburned in-cylinder volume and are modeled with homogeneous reactors. The octane numbers are determined by the identification of the critical compression ratio, for which premature ignition occurs in the unburned reactor.A two-zone cylinder model is validated against experimental data for various primary reference fuels, blended with toluene, ethanol, isobutanol and ethyl tert-butyl ether. The successful application of different kinetic models is demonstrated and enables the application on a wide range of fuels. It is shown that the simulated research octane numbers are in good agreement with the experimental data.

Technologies integration to achieve brake thermal efficiency beyond 55% for HD diesel engines

This study focuses on the technologies integration to aim about 60% indicated thermal efficiency (ITE) with a single-cylinder HD diesel engine, roughly equivalent to 55% brake thermal efficiency (BTE) with multiple-cylinder engines. A new thermodynamic cycle concept and a new heat insulation structure were discussed as the key integrated technologies. Otto cycle is the best thermodynamic cycle for ideal thermal efficiency. However, significant increase in heat release rate around top dead center results in higher cooling loss and sometimes in deterioration of late combustion under a higher compression ratio without any improvement in ITE. The newly proposed cycle is the hybrid of isobaric cycle and following steep pressure increase up to peak firing pressure (PFP) constraint from the timing when in-cylinder volume change rate becomes significant, which suppresses the average gas temperature. For further cooling loss reduction, the new in-cylinder wall insulating concept was investigated. The in-cylinder insulating effect was much significant by applying not only on the piston crown but also on the cooling side of piston. Although the test engine hasn’t been optimized yet, ITE was almost reached at about 60% by means of the multiple-injector concept with compression ratio of 23.5:1.

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.

An applicable approach to mitigate pressure rise rate in an HCCI engine with negative valve overlap

Low-temperature combustion in a homogeneous-charge compression-ignition (HCCI) engine offers high thermal efficiency while cutting off emissions. However, HCCI’s feasibility is hampered by excessive peak pressure rise rates under high load, causing combustion noise and possible engine damage. This study considers extending the high-load limit in a boosted HCCI engine accommodating variable valve timing and fuel reforming during negative valve overlap. Three techniques are evaluated on a research engine: (i) exhaust valve timing retardation (ii) boost pressure adjustment and (iii) reduction of fuel subjected to reforming. Two load regimes are explored: a mid-load point with indicated mean effective pressure of 0.61 MPa; and high-load conditions achieved by 25% more fuelling. The former is often reported as boundary condition for HCCI’s, the latter is usually far beyond the acceptable pressure rise rate limit. Results indicate that strategies (i) and (iii) offer a trade-off-free solution for high-load extension. This can be realized as a supervisory, in-cylinder pressure based, control function. Independently of the pressure rise rate mitigation method considered, two key variables are crucial for closed-loop control: the in-cylinder volume at 50% fuel burnt and the combustion duration. They are closely coupled and can be real-time calculated using well-established control framework based on sensing the combustion timing. The expansion rate and differences in fuel mass subjected to reforming are secondary for pressure rise rate estimation and should be considered if greater accuracy is required.

Modelling and validation of a free piston expander-linear generator for waste heat recovery system

In this study, an air driven free piston expander-linear generator (FPE-LG) test rig and the FPE-LG simulation model based on GT-Suite/Simulink have been established. The simulation model is validated by experimental results. Afterwards, the performance of FPE-LG under different conditions and the influence of intake temperature on FPE-LG are conducted. Since the simulation results are in good agreement with the experiment results, this proves the rationality of the simulation model. When intake pressure and temperature rise, the in-cylinder volume increases. Furthermore, the displacement, velocity and acceleration increase with intake temperature. The peak voltage output of FPE-LG linearly increases when increasing the intake temperature and pressure. The peak voltage output is more sensitive to intake pressure than temperature. The higher the intake pressure is, the more obvious the intake temperature effects the peak voltage output. The maximum voltage output can reach up to 23.6 V. The peak power output demonstrates an initial increasing and then reducing trend with the external load resistance. And the maximum peak power output can reach up to 22.23 W when the external load resistance is 9 Ω and intake temperature is 373 K.

Effects of combustion timing on pressure rise rates in a residual effected HCCI engine

Realization of a low temperature combustion concept in homogeneous charge compression ignition (HCCI) engines is a cutting-edge technology that offers clean combustion in parallel with high thermal efficiency. Low combustion temperature prevents from NOx for-mation whereas homogeneous mixture assures smokeless exhaust. However, achieving the production feasibility by HCCI technology is hampered by high pressure rise rates and the resulting combustion noise at a high load operation. This paper explores combustion tim-ing parameters that are capable of maintaining permissible levels of pressure rise rates under a high load regime. On the basis of exper-imental data collected at a high load HCCI operation, pressure rise level was correlated with combustion duration. Furthermore, com-bustion duration has been found to scale with in-cylinder volume, for which 50% of mass fraction burned appeared. The results showed quantitatively limitations of engine load, pointing out on required combustion timings to achieve acceptable combustion harshness de-pending on engine load.

Estimation of the polytropic index for in-cylinder pressure prediction in engines

Compression and expansion strokes in internal combustion engines can be characterized by the well-known polytropic process, PVn = constant, which describes the relationship between the in-cylinder volume and the pressure. This process is widely used to estimate the in-cylinder pressure during the compression stroke before combustion occurs; the polytropic index (n) is a key parameter for calculating the pressure accurately when volume data are given. However, the polytropic index changes with different engine operating conditions. Therefore, using the same value for the polytropic index under different operating conditions can decrease the accuracy of the pressure prediction during the compression stroke. It can also affect pressure modelling during combustion in which the pressure at the end of the compression stroke is used as an initial condition. Therefore, it is important to obtain an appropriate polytropic index to predict the in-cylinder pressure precisely.This study focused on a methodology to estimate the polytropic index for real-time applications. Heat loss from the in-cylinder gas was considered a major factor affecting the polytropic index. Therefore, a model of the polytropic index (n) could be established as a simple linear equation, ‘n=nisen+QlossT1×S’, where the heat loss (Qloss) is the main variable. nisen means the polytropic index with isentropic condition and T1 is the temperature of the in-cylinder gas when the compression starts. S which indicates the slope of the linear equation is inversely proportional to the compression ratio during the compression stroke. The main variable, heat loss, was modelled to complete the whole model of the polytropic index, and Woschni’s correlation was adopted to estimate the heat loss from the in-cylinder gas. The index model was established with 220 steady-state experimental cases using a 1.6-L compression-ignition (CI) engine. In addition, the pressure at the end of the compression stroke could be predicted using the modelled index with the polytropic process. The model could be validated against 1.6-L CI engine which has different compression ratio and in-cylinder geometry compared with the test engine. This study can contribute to a quantitative understanding of which parameters affect the in-cylinder pressure during the compression stroke. It will also be helpful for establishing a whole in-cylinder pressure prediction model for internal combustion engines; the pressure information obtained from such a model could be used for emissions modelling and combustion control.

Instantaneous in-Cylinder Volume Considering Deformation and Clearance due to Lubricating Film in Reciprocating Internal Combustion Engines

A new methodology for predicting the real instantaneous in-cylinder volume in the combustion chamber of a reciprocating internal combustion engine is implemented. The mathematical model developed as part of this methodology, takes into consideration the deformations due to pressure and inertial forces, via a deformation constant adjusted through ANSYS®, using a high-precision CAD model of a SOKAN SK-MDF300 engine. The deformation constant was obtained from the CAD model using the computational tool ANSYS® and the pressure data was obtained from the engine running at three regimes: 1500, 2500, and 3500 rpm. The results were compared with previous models reported in the literature, showing that the deformation constant obtained has a smaller variation among cycles, which leads to a more precise value of the mechanical deformations. Furthermore, to have a more accurate model of the instantaneous volume variation, a factor taking into consideration the lubricant film behavior is introduced to calculate volumetric variation due to geometrical clearances. The influence of the introduced volumetric variation was evaluated through a process of combustion diagnosis, evidencing the improvement in the predictive capacity of thermodynamic modeling and, therefore, the correct prediction of heat release rate.

Methodology for Optical Engine Characterization by Means of the Combination of Experimental and Modeling Techniques

Optical engines allow for the direct visualization of the phenomena taking place in the combustion chamber and the application of optical techniques for combustion analysis, which makes them invaluable tools for the study of advanced combustion modes aimed at reducing pollutant emissions and increasing efficiency. An accurate thermodynamic analysis of the engine performance based on the in-cylinder pressure provides key information regarding the gas properties, the heat release, and the mixing conditions. If, in addition, optical access to the combustion process is provided, a deeper understanding of the phenomena can be derived, allowing the complete assessment of new injection-combustion strategies to be depicted. However, the optical engine is only useful for this purpose if the geometry, heat transfer, and thermodynamic conditions of the optical engine can mimic those of a real engine. Consequently, a reliable thermodynamic analysis of the optical engine itself is mandatory to accurately determine a number of uncertain parameters among which the effective compression ratio and heat transfer coefficient are of special importance. In the case of optical engines, the determination of such uncertainties is especially challenging due to their intrinsic features regarding the large mechanical deformations of the elongated piston caused by the pressure, and the specific thermal characteristics that affect the in-cylinder conditions. In this work, a specific methodology for optical engine characterization based on the combination of experimental measurements and in-cylinder 0D modeling is presented. On one hand, the method takes into account the experimental deformations measured with a high-speed camera in order to determine the effective compression ratio; on the other hand, the 0D thermodynamic analysis is used to calibrate the heat transfer model and to determine the rest of the uncertainties based on the minimization of the heat release rate residual in motored conditions. The method has been demonstrated to be reliable to characterize the optical engine, providing an accurate in-cylinder volume trace with a maximum deformation of 0.5 mm at 80 bar of peak pressure and good experimental vs. simulated in-cylinder pressure fitting.

Development of a pressure-based technique to control IMEP and MFB50 in a 3.0L diesel engine

A pressure-based technique for the control of IMEP (Indicated Mean Effective Pressure) and MFB50 (crank angle at which 50% of fuel mass fraction has burned) has been developed, assessed and tested by means of MiL (Model-in-the-Loop) on a 4 cylinder 3.0L Euro VI diesel engine.The activity was carried out in the frame of a research project in collaboration with FPT Industrial.The developed controller is of the closed-loop type. It receives, as input, the desired targets of IMEP and MFB50 for each cycle and cylinder and performs a cycle-by-cycle and cylinder-to-cylinder correction of the injected fuel quantity of the main pulse (qmain) and of the start of injection of the main pulse (SOImain), in order to reduce the deviation between the actual and target values of IMEP and MFB50, respectively. The method is referred to as “pressure-based” since it requires the measurement of the in-cylinder pressure trace for each cylinder in order to extract the actual values of IMEP and MFB50. In fact, the actual IMEP value can be estimated by integrating the pressure signal with respect to the in-cylinder volume. At the same time, the actual MFB50 value can be extracted from the heat release curve, which is obtained from the in-cylinder pressure trace by using a single-zone heat release model. The proposed control technique has been developed in Simulink environment, and has been assessed and tested on an engine emulator which is constituted by a GT-power model of the 3.0L diesel engine.The controller has been tested in transient operation over a load ramp profile at different engine speeds and over a WHTC interval, and demonstrated to have a good potential for IMEP and MFB50 control, since it is characterized by a fast response and a limited overshoot behavior.

A comparison of high-temperature reaction and soot processes of conventional diesel and methyl decanoate

This paper aims to improve a knowledge base of methyl decanoate, a long alkyl-chain biodiesel surrogate fuel gaining popularity in engine combustion research. To this end, a comparative study on diesel and methyl decanoate combustion has been conducted with a focus on high temperature flame structures and soot distributions in an optically accessible single-cylinder light-duty common-rail diesel engine. The in-cylinder pressure trace and apparent heat release rate curves were well matched for both fuels when the same amount of fuel energy was supplied, which confirmed very similar combustion phasing. Planar laser induced fluorescence of hydroxyl radicals (OH-PLIF) and planar laser induced incandescence (PLII) as well as line-of-sight integrated chemiluminescence imaging of cool-flame signals and electronically excited OH (OH∗) were performed for various crank angles to capture the temporal and spatial development of diesel and methyl decanoate flames. The results show that both the cool-flame and OH radical signals are higher during methyl decanoate combustion with their wider distributions and larger in-cylinder volume fraction when compared to that of diesel, suggesting enhanced low- and high-temperature reactions due to oxygen in fuel. The oxygenated methyl decanoate with no aromatics in its molecular structure shows a lower soot formation rate than diesel as evidenced by delayed appearance of LII signals and lower overall intensity. This difference is significant even if the lower sooting propensity of methyl decanoate and thus less attenuation in the laser beam is considered. The rate of soot oxidation is also higher for methyl decanoate not only due to oxygen in fuel but also higher OH radicals surrounding smaller soot pockets compared to diesel.

An Alternative Method to MFB50 for Combustion Phase Detection and Control in Common Rail Diesel Engines

Although injection timing does not change at every cycle in compression ignition engines, the variations in combustion phase may occur. The variations in combustion phase are directly related to emission dispersion, torque variation, and fuel economy; therefore, the variations should be minimized by combustion phase detection and control. In this study, we propose a novel combustion indicator: location of 20% of initial heat release after the peak (IHRap20) that is an alternative to MFB50 to detect and control combustion phase in real time. IHRap20 is extracted from IHR, which is a simplified heat release model derived by the product of in-cylinder volume and the gradient of difference pressure between firing pressure and motoring pressure. IHRap20 is confirmed with engine experiment that it is highly correlated to MFB50 with RMSE of 0.2790° crank angle and R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 0.9873 under various operating conditions such as injection timing, mass air flow, boost pressure, and rail pressure changes. In addition, IHRap20 is able to detect combustion phase in real time with fewer number of in-cylinder pressure data and computational load compared to MFB50. Furthermore, IHRap20 is more robust to in-cylinder pressure sensor noise than MFB50. Consequently, IHRap20 is promising for real-time combustion phase detection.

Numerical study on convective heat transfer of a spark ignition engine fueled with bioethanol

Spark ignition (SI) engine performances depend strongly on the in-cylinder heat transfer from the combustion gas to the wall in the combustion chamber, especially the convective heat transfer. Thus, the model governing the in-cylinder gas side convective heat transfer of an engine is very important in prediction of the engine performances and it can be developed through the evaluation of the coefficient of heat transfer. This study aimed to contribute to the knowledge in this field for the case of bioethanol fueled engine. Simulation approach was applied in this study, where MATLAB was used as the simulation tool. Two optimized choice of correlations were applied for the evaluation of heat transfer coefficient, which are the Hohenberg's correlation and Eichelberg's correlation. In both correlations, same parameters are required, including the in-cylinder volume, mass burned fraction, in-cylinder pressure, and in-cylinder temperature. All those parameters vary with the crank angle and were determined from the simulation of combustion process with engine geometries from the real engine. According to the graph of the heat transfer coefficient against the crank angle obtained from both correlations, it can be concluded that the convective heat transfer from the gas to the combustion chamber wall in bioethanol fueled engine behaves in a similar manner with gasoline, natural gas and diesel fueled engine. The convective heat transfer changes proportionally with the in-cylinder pressure and in-cylinder temperature. From the comparison graphs, bioethanol fueled engine has a higher in-cylinder convective heat transfer from the start of the combustion process until its peak, but is lower during the end of the combustion process if compared to gasoline fueled engine.

Analysis of Errors in Heat Release Calculations Due to Distortion of the In-Cylinder Volume Trace from Mechanical Deformation in Optical Diesel Engines

Analysis of Errors in Heat Release Calculations Due to Distortion of the In-Cylinder Volume Trace from Mechanical Deformation in Optical Diesel Engines

Experimental and computational analysis of the combustion evolution in direct-injection spark-controlled jet ignition engines fuelled with gaseous fuels

Jet ignition and direct fuel injection are potential enablers of higher-efficiency, cleaner internal combustion engines (ICEs), where very lean mixtures of gaseous fuels could be burned with pollutants formation below Euro 6 levels, efficiencies approaching 50 per cent full load, and small efficiency penalties operating part load. The lean-burn direct-injection (DI) jet ignition ICE uses a fuel injection and mixture ignition system consisting of one main-chamber DI fuel injector and one small jet ignition pre-chamber per engine cylinder. The jet ignition pre-chamber is connected to the main chamber through calibrated orifices and accommodates a second DI fuel injector. In the spark plug version, the jet ignition pre-chamber includes a spark plug which ignites the slightly rich pre-chamber mixture which then, in turn, bulk ignites the ultra-lean stratified main-chamber mixture through the multiple jets of hot reacting gases entering the in-cylinder volume. The paper uses coupled computer-aided engineering and computational fluid dynamics (CFD) simulations to provide better details of the operation of the jet ignition pre-chamber (analysed so far with downstream experiments or stand-alone CFD simulations), thus resulting in a better understanding of the complex interactions between chemistry and turbulence that govern the pre-chamber flow and combustion.

A quasi-dimensional combustion model for performance and emissions of SI engines running on hydrogen–methane blends

The development of a predictive two-zone, quasi-dimensional model for the simulation of the combustion process in spark ignited engines fueled with hydrogen, methane, or hydrogen–methane blends is presented. The code is based on a general-purpose thermodynamic framework for the simulation of the power cycle of internal combustion engines. Quasi-dimensional modelling describes the flame front development assuming a simplified spherical geometry, as well as infinitesimal thickness. The flame front subdivides the in-cylinder volume into a zone of unburned mixture, and a second zone of burned gases. As far as the combustion process is concerned, attention is paid to the description of the physical and chemical phenomena controlling the flame development and the formation of combustion products. First of all, an empirical correlation has been defined for estimating the laminar burning velocity. The equation, tailored for arbitrary fuel blendings and equivalence ratios, has been validated against detailed experimental data. Furthermore, the influence of turbulence on flame evolution has been implemented according to a fractal-based model. Then, a physical and chemical computing environment for evaluating both gaseous mixtures' thermodynamic properties, and equilibrium species concentrations of combustion products has been developed and coupled to the code. The validation has been performed by comparing numerical pressure traces against literature experimental data, on a standard CFR single-cylinder engine. A unique set-up of the model parameters has been obtained, suitable for both pure hydrogen and pure methane fuelings; finally, the predictive capabilities of the model have been applied to analyze different fuel blends and equivalence ratios: the comparison against experimental pollutant emissions (NO and CO) shows a reasonable accuracy.

Investigation of piston bowl geometry and speed effects in a motored HSDI diesel engine using a CFD against a quasi-dimensional model

Abstract The present work investigates the effect of varying the combustion chamber geometry and engine rotational speed on the gas flow and temperature field, using a new quasi-dimensional engine simulation model in conjunction with an in-house developed computational fluid dynamics (CFD) code served to validate the predicted in-cylinder flow field and gas temperature distribution calculated by the quasi-dimensional model, for three alternative piston bowl geometries and three rotational speeds. This CFD code can simulate three-dimensional curvilinear domains using the finite volume method in a collocated grid; it solves the generalized transport equation for the conservation of mass, momentum and energy, and incorporates the standard k–e turbulence model with some slight modifications to introduce the compressibility of a fluid in generalized coordinates. On the other hand, the quasi-dimensional model solves the general transport equation for the conservation of mass and energy by a finite volume method throughout the entire in-cylinder volume, while for the estimation of the flow field a new simplified three dimensional air motion model is used. To compare these two models the in-cylinder spatial and temporal temperature distribution, the mean cylinder pressure diagram, as well as the mean in-cylinder radial and axial velocity are examined, for the three piston bowl geometries and the three speeds, for a high speed direct injection (HSDI) diesel engine operating under motoring conditions. From the comparison of calculated results, it becomes apparent that the two models predict similar in-cylinder temperature distributions and mean air velocity fields at each crank angle, for all cases examined. Thus, it is shown that the quasi-dimensional model with the proposed simplified air motion model is capable of capturing the physical effect of combustion chamber geometry and speed on the in-cylinder velocity and temperature field, while needing significantly lower computing time compared to the more detailed and accurate CFD model. On the other hand, the CFD model is more suitable when detailed simulation of the in-cylinder geometry is required and the way the corresponding transport phenomena are affected.