Engine performance and emissions of a six-stroke Gasoline Compression Ignition (GCI) engine with wide range of Continuously Variable Valve Duration (CVVD) control were numerically investigated at low engine load conditions. For the simulations, an in-house 3-D CFD code with high fidelity physical sub-models was used and the combustion and emissions kinetics were computed using a reduced kinetics mechanism for a 14-component gasoline surrogate fuel. Double injections were employed to effectively form the local fuel/air mixtures with optimal reactivity. Several valve timing and duration variations through the CVVD control were considered under both positive valve overlap (PVO) and negative valve overlap (NVO) conditions. Effects of intake-valve re-breathing between the first expansion and the second compression strokes were also investigated. Close attention was paid to understand the effects of two additional strokes of the engine cycle on the thermal and chemical conditions of charge mixtures that alter ignition, combustion and energy recovery processes. Double injections were found to be necessary to effectively utilize the additional two strokes for the combustion of overly mixed lean charge mixtures during the second power stroke (PS2). It was found that combustion phasing in both power strokes is effectively controlled by the intake valve closure (IVC) timing since it affects the effective compression ratio. Engine operation under NVO condition with fixed exhaust valve opening (EVO) and IVC timings tends to advance the ignition timing of the first power stroke (PS1) but has minimal effect on the ignition timing of PS2. Re-breathing was found to be an effective way to control the ignition timing in PS2 at a slight expense of the combustion efficiency. The operation of a six-stroke GCI engine could be successfully simulated and the operability range of the engine could be substantially extended by employing the CVVD technique. In addition, the control of valve timings could successfully control the thermodynamic and compositional conditions of in-cylinder mixtures that enable to control the combustion phasing.

An efficient computational fluid dynamics model for predicting high pressure dual-fuel combustion is one of the most essential steps in order to improve the concept, to reduce the number of experiments and to make the development process more coste-efficient. For Diesel and natural gas such a model developed by the authors is first used to analyze the combustion process with respect to turbulence chemistry interaction and to clarify the question whether the combustion process is limited by chemistry or the mixing process. On the basis of these findings a reduced reaction mechanism is developed in order to save up to 35% of computing time. The prediction capability of the modified combustion model is tested for different gas injection timings representing different degrees of premixing before ignition. Compared to experimental results from a rapid compression expansion machine, the shape of heat release rate, the ignition timing of the gas jet and the burnout are well predicted. Finally, misfiring observed at different geometric configurations in the experiment are analyzed with the model. It is identified that in these geometric configurations at low temperature levels the gas jet covers the preferred ignition region of the diesel jet. Since the model is based on the detailed chemistry approach, it can in future also be used for other fuel combinations or for predicting emissions.

The automotive industry is subjected to increasing pressure in order to improve fuel efficiency and reduce the CO 2 emissions of internal combustion (IC) engines. The power cylinder system (piston, piston ring, and liner) contributes significantly to the friction losses, engine oil consumption and gas leakage called blow-by. The role of cylinder bore shape in engine performance has been the subject of several studies in recent years. High bore distortion must be avoided because it can lead to ring conformability issues, which leads to inadequate sealing resulting in increased blow-by and oil consumption. It also leads to asperity contact between the piston skirt and cylinder bore increasing friction causing abnormally high surface wear. Although bore distortion cannot be eliminated, engine manufacturers strive to contain it within acceptable limits. Therefore, numerical analysis of the power cylinder with physically based mathematical models becomes very essential to the engine and component manufacturer in order to reduce engine development lead time and minimize the number of engine tests. The integrated ring-pack modeling methodology developed by the authors [1] is used to investigate the piston ring-pack performance. Although the modeling approach can be used for extensive parameter analysis of piston, piston rings and lubrication oil consumption, the influence of the bore distortion on the ring conformability and its impact on blow-by, friction and wear is highlighted in this study. Piston tilting, piston ring twist and surface roughness of the piston ring and liner have been taken into consideration.

An experimental and theoretical study is presented to study the effect of surface texturing in the form of circumferential oil grooves on improving the tribological properties of piston ring-cylinder liner tribosystem. Tests were performed on a reciprocating test rig with actual piston rings and cylinder liner segments, and a numerical model has been developed. A comparison was made between the performance of the textured cylinder liners and un-textured cylinder liners. It was found that with the smaller oil groove area density, the reduction in friction force is more obvious, Parabolic and triangular oil grooves are more efficient in friction reducing, and the prediction results by numerical model match the experimental results well in most case.

Hydrogen utilization in spark ignition engines could reduce urban pollution including particulate matter as well as greenhouse gas (carbon dioxide) emission. However, backfiring, which is an undesirable combustion process of intake charge in hydrogen fuelled spark ignition (SI) engine with manifold based injection, is one of the major technical issues in view of safety as well as continuous engine operation as ignition process could proceed instantaneously due to less ignition energy requirement of hydrogen. Backfiring occurs generally during suction stroke as the hydrogen-air charge interacts with residual gas resulting in flame growth and propagation towards upstream of engine’s intake manifold resulting in stalling of engine operation and high risk of safety. This work is aimed at analysis of backfiring in a hydrogen fuelled SI engine. The results indicate that backfiring is mainly function of residual gas temperature, start of hydrogen injection timing and equivalence ratio. Any hot-spot present in the cylinder would act as ignition source resulting in more chances of backfiring. In addition to this, CFD analysis was carried out in order to assess flow characteristics of hydrogen and air during suction stroke in intake manifold. Furthermore, numerical analysis of intake charge velocity, flame speed (deflagration), and flame propagation (backfiring) towards upstream of intake manifold was also carried out. Some notable points of backfiring control strategy including exhaust gas recirculation (EGR) and retarded (late) hydrogen injection timing are emerged from this study for minimizing chance of backfiring. This study results are useful for development of dedicated spark ignition engine for hydrogen fuel in the aspects of elimination of backfiring.

The dynamics of valve train is influenced by stiffness, size and mass distribution of its components and initial valve clearance and so on. All the factors should be taken into consideration correctly by dynamic model and described qualitatively and quantitatively through mathematical variables. This paper proposes a new simplified method for valve train components, namely mode matching method (MMM) for camshaft, pushrod, rocker arm, valve and valve spring. In this method the amount of lumped masses for each flexible component is determined based on its natural frequencies and the considered frequency range. As a result, the dynamic model of each component is required to match its low order modes within the considered frequency range. The basis of this method is that the contributions of each component to valve train vibration are mainly in the low order modes. The numerical model of valve train is verified by an experiment conducted on a motor driven valve train system.

In this paper, a numerical method based on finite volume method for diesel engine is presented. The production of combustion is assumed to be a grey medium and the scatter of particle is neglected. In this paper the unstructured finite-volume method (UFVM) for radiation has been formulated and implemented in an in-house fluid flow solver GTEA. For its comparison and validation of the present FVM on unstructured grids, a test case with 3D complex geometries is chosen, which was used by S. W. Baek et.al. [1] . The results obtained by the presented FVM agree well with the exact solutions. Radiation heat transfer in TBD620 diesel engines is studied with present numerical method. It is shown that the radiation has little effect on average pressure and temperature, but it changes the temperature distribution in cylinder.

Homogeneous charge compression ignition (HCCI) combustion is widely regarded an attractive option for future high efficiency gasoline engines. HCCI combustion permits operation with a highly dilute, well mixed charge, resulting in high thermal efficiency and extremely low NO x and soot emissions, two qualities essential for future propulsion system solutions. Because HCCI is a thermo-kinetically dominated process, full understanding of how combustion chamber boundary thermal conditions affect the combustion process are crucial. This includes the dynamics of the effective chamber wall surface temperature, as dictated by the formation of combustion chamber deposits (CCD). It has been demonstrated that, due to the combination of CCD thermal properties and the sensitivity of HCCI to wall temperature, the phasing of auto-ignition can vary significantly as CCD coverage in the chamber increases. In order to better characterize and quantify the influence of CCDs, a numerical methodology has been developed which permits calculation of the crank-angle resolved local temperature profile at the surface of a layer of combustion chamber deposits. This unique predictor-corrector methodology relies on experimental measurement of instantaneous temperature underneath the layer, i.e. at the metal-CCD interface, and known deposit layer thickness. A numerical method for validation of these calculations has also been devised. The resultant crank-angle resolved CCD surface temperature and heat flux profiles both on top and under the CCD layer provide valuable insight into the near wall phenomena, and shed light on the interplay between the dynamics of the heat transfer process and HCCI burn rates.

The piston compression ring-cylinder liner contact experiences a transient regime of lubrication. This comprises hydrodynamic, mixed (partial) or boundary interactions. The regime of lubrication is influenced by contact kinematics, loading, mechanical and topographical properties of the bounding solid surfaces, as well as lubricant rheology and its supply to feed the conjunction. Ideally, a sufficient volume of lubricant would exist at the conjunctional inlet, which acts as a meniscus, from which a film of lubricant is entrained into the contact area. However, often there is an insufficient volume at the inlet to the contact or unfavourable kinematic conditions exist, such as at the dead centre reversals in the piston system. Such conditions are generally regarded as the underlying reasons for mixed or boundary regimes of lubrication. Whilst, these are the main reasons for failure to form a coherent lubricant film at dead centre reversals, poor hydrodynamic lubrication can also occur elsewhere in the piston cycle due to the lack of an inlet meniscus. This may be as the result of poor ring-bore conformability, where a meniscus cannot be formed in an increasing clearance space. Numerical analyses reported in open literature often assume an idealised fully flooded or drowned inlet. Other analyses assume starved inlet boundaries, usually based on a lubricant availability model, which itself is based on an assumed supply of lubricant on free surfaces ahead of the contact. It is, however, important to establish the limiting clearance space which would allow the lubricant to adhere to the adjacent boundary solids and, thus form a meniscus bridge. The current study is aimed at establishing inlet meniscus conditions. This would depend on the lubricant surface tension, affected by the free surface energy of liquid-vapour interface and the contact angle made between the lubricant and the bounding solids. It also depends on the solid surfaces, any coatings and their topography. In general, the results indicate the dependence of the meniscus force on surface material and topography, through measurement of contact angles made by the various compression rings against the various cylinder liner surfaces, using an especially developed rig and in conjunction with a goniometer. Obviously, the measurements do not exactly replicate those existing in a fired engine. Nonetheless, this initial study provides a good insight into oil-surface combination in separating gaps, which is far more representative than the usually assumed idealised inlet conditions.

This study investigates the effects of biogas composition on combustion stability for a purely biogas fueled HCCI engine. Biogas is one of the most promising renewable fuels for Combined Heat and Power systems driven by internal combustion engines. However, the high content of CO 2 in biogas composition leads to low thermal efficiencies in spark ignited and dual fuel compression ignited engines. The study is divided into two parts: first experimental results on a biogas-fueled HCCI engine are used to validate a numerical model, and second the model is used to investigate how biogas composition impacts combustion stability. In the first part of the study, experimental analysis of a 4 cylinder, 1.9 L Volkswagen TDI Diesel engine running with biogas in HCCI mode has shown high gross indicated mean effective pressure (close to 8 bar), high gross indicated efficiency (close to 45%) and ultra-low NO x emissions below the US2010 limit (0.27 g/kWh). An inlet absolute pressure of 2 bar and inlet temperature of 473 K (200°C) were required for allowing HCCI combustion with a biogas composition of 60% CH 4 and 40% CO 2 on a volumetric basis. However, slight changes in inlet pressure and temperature caused large changes in cycle-to-cycle variations at low equivalence ratios and large changes in ringing intensity at high equivalence ratios. A numerical model is validated against these experimental results. In the second part of the study, the numerical results for varied biogas composition show that at high load limit, higher contents of CH 4 in biogas composition allow advanced combustion and increased burning rates of the biogas air mixture. Higher contents of CO 2 in biogas composition allow lowered ringing intensities with moderate decrease in the indicated efficiency and power output. NO x emissions are not highly affected by biogas composition, while CO and HC emissions tend to increase with higher contents of CO 2 . According with the numerical results, biogas composition is an effective strategy to control the onset of combustion and combustion phasing of HCCI engines running biogas, allowing more stabilized combustion at low equivalence ratios and safe operation at high equivalence ratios. The main advantages of using biogas fueled HCCI engines in CHP systems are the low sensitivity of power output and indicated efficiency to biogas composition, as well as the ultra low NOx emissions achieved for all tested compositions.

LPG (Liquefied Petroleum Gas) has been widely used as an alternative fuel for gasoline and diesel vehicles in light of clean fuel and diversity of energy resources. But conventional LPG vehicles using carburetors or MPI fuel injection systems can’t satisfy the emissions regulations and CO 2 targets of the future. Therefore, it is essential to develop LPG engines of spark ignition or compression ignition type such that LPG fuel is directly injected into the combustion chamber under high pressure. A compression ignition engine using LPG is the ideal engine with many advantages of fuel economy, heat efficiency and low CO 2 , even though it is difficult to develop due to the unique properties of LPG. This paper reports on numerical and experimental studies related to LPG fuel for a compression ignition engine. The numerical analysis is conducted to study the combustion chamber shape with CATIA and to analyze the spray and fluid behaviors with FLUENT for diesel and LPG (n-butane 100%) fuels. In one experimental study, a constant volume chamber is used to observe the spray formation for the chamber pressure 0 to 3MPa and to analyze the flame process, P-V diagram, heat release rate and emissions through the combustion of LPG fuel with the cetane additive DTBP (Di-tert-butyl peroxide) 5 to 15 wt% at 25MPa of fuel injection pressure. In engine bench tests, experiments were performed to find the optimum injection timing, lambda, COV and emissions for the LPG fuel with the cetane additive DTBP 5 to 15 wt% at 25MPa fuel injection pressure and 1500 rpm. The penetration distance of LPG (n-butane 100%) was shorter than that of diesel fuel and LPG was sensitive to the chamber pressure. The ignition delay was in inverse proportion to the ambient pressure linearly. In the engine bench tests, the optimum injection timing of the test engine to the LPG fuel with DTBP 15 wt% was about BTDC 12° CA at all loads and 1500 rpm. An increasing of DTBP blending ratio caused the promotion of flame and fast burn and this lead to reduce HC and CO emissions, on the other hand, to increase NO x and CO 2 emissions.

Direct injection (DI) of compressed natural gas (CNG) under high pressure conditions is a topic of great interest, owing to its potential for improving SI engine performance and fuel consumption. However, relevant technical difficulties have yet to be resolved in order to stabilize combustion process, especially for stratified engine operating conditions. The present paper is focused on experimental and numerical investigations of the jet formation and fuel-air mixing process in a research optical-access single-cylinder engine. The engine is based on the multi-cylinder engine under development within the European Community (EC) VII Framework Program (FP) InGAS Integrated Project, and features a centrally mounted poppet-valve injector on a pent-roof combustion chamber with a bowl in piston. Experimental investigations were made by means of the planar laser-induced fluorescence technique, and revealed a cycle-to-cycle jet shape variability. In particular, for specific cylinder pressure values at the start of injection, the jet can adhere to chamber walls for a relevant number of cycles, leading to an ‘umbrella-like’ shape. This can change the mixing capabilities of the combustion chamber and cause instabilities in the combustion process. The mentioned behaviour is strongly dependent not only on the injection and cylinder pressures, but also on important design parameters, such as needle cone angle and in-chamber injector protrusion. For this reason, in order to obtain a deep insight into the injected gas behaviour on an average cycle basis, the experimental investigation was supported by a numerical analysis. Simulations were carried out by an optimized variable-density finite-volume numerical model which was built within the Star-CD environment. A previously developed and validated ‘virtual injector’ model was implemented. The outcomes of the numerical model were compared to laser-induced fluorescence images, for both stratified- and homogeneous-charge engine operating conditions and a good agreement was obtained, substantiating the reliability of the applied computational model. Then, the effects of the injector protrusion in the combustion chamber and of injection timing were analyzed, and their impact on jet stability and mixture-formation process was analyzed.

A detailed analysis of knocking event can help improving engine performance and diagnosis strategies. The paper aim is a better understanding of the phenomena involved in knocking combustion through the combination of CFD and signals analysis tools. CFD simulations have been used in order to reproduce knock effect on the in-cylinder pressure trace. In fact, the in-cylinder pressure signal holds information about waves propagation and heat losses: for the sake of the diagnosis it is important to relate knock severity to knock indexes values. For this purpose, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking condition. The CFD model validation phase is crucial for a correct representation of both regular and knocking combustions: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data. The simulation results allow relating the combustion characteristics to their effect on the in-cylinder pressure signal. It is then possible to highlight critical issues regarding typical knock detection methodologies, while disclosing novel approaches. One of the main results is the validation of knock detection strategies based on the low-frequency content of the pressure signal. These strategies can be used together with standard high-frequency based techniques in order to improve the detection robustness.

A numerical analysis of homogeneous natural gas/diesel/air mixture, that could be formed when small amounts of diesel pilot are used in a diesel dual fuel engine or when using natural gas/diesel dual fuel homogeneous charge compression ignition combustion, was studied to provide insight on future experimental investigations to be conducted by the institution. Use was made of Curran’s normal-heptane mechanism in CHEMKIN to deduce its effects on lean fuel/air mixtures with respect to ignition timing and carbon monoxide (CO) and oxides of nitrogen (NO x ) emissions. For the cases considered, increase in the amount of n-heptane addition resulted in advanced ignition closer to TDC thereby increasing peak cylinder pressure and gross IMEP. Increase in the amount of n-heptane addition also resulted in early decomposition of the fuel consequently advancing the production of OH radicals that oxidised the CO. For the same fuel/air mixture strength, increase in the amount of n-heptane addition increased NO x production due to increased peak temperature, residence time and availability of O 2 . When using leaner mixtures and small amounts of n-heptane to promote/advance ignition, however, peak cylinder temperatures were reduced and hence comparatively reducing engine-out NO x emissions.

A production common rail type injector has been investigated via numerical simulation and experimentation. The functioning principle of the injector has been carefully analysed so as to obtain a mathematical model of the device. A zero-dimensional approach has been used for modelling the injector, thus considering the variables as function of time only. The analysis of the hydraulic part of the injector resulted in the definition of an equivalent hydraulic scheme, on which basis both the equations of continuity in chambers and flow through nozzles were written. The moving mechanical components of the injector, such as needle, pressure rod and control valve have been modelled using the mass-spring-damper scheme, thus obtaining the equation governing their motion. An electromagnetic model of the control valve solenoid has also been realized, in order to work out the attraction force on the anchor, generated by the electric current when flowing into its coil. The model obtained has been implemented using the Matlab® toolbox Simulink® and solved by means of the NDF ( Numerical Differentiation Formulas ) implicit scheme of the second order accuracy, suitable for problems with high level of stiffness. The experimental investigation on the common-rail injection system was performed on a test bench at some standard test conditions. Electric current flowing through the injector coil, oil pressure at the injector inlet, injection rate, needle lift and control valve lift were gauged and recorded during several injection phases. The mean reflux-flow rate and the mean quantity of fuel injected per stroke were also measured. Temperature and pressure of the feeding oil, as well as pressure in the rail were continuously controlled during the experimental test. The numerical and experimental results were compared. The model was then used to investigate the effect of control volume feeding and discharge holes and of their inlet fillet, as well as the effect of the control volume capacity, on the injector performance.

The objective of this work is to investigate the effect of premixed fuel ratio on the combustion and emission characteristics in diesel engine by the experimental and numerical method. In order to investigate the effect of various factors such as the mixing ratio, EGR rate, and engine load on the exhaust emissions from the premixed charge compression ignition diesel engine, the injection amount of premixed fuel is controlled by electronic port injection system. The range of mixing ratio between dual fuels used in this study is between 0 and 0.85, and the exhaust gas is recirclulated until 30 percent of EGR rate.

Recent four value HSDI Diesel engines are able to control the swirl intensity, in order to enhance the in-cylinder flow field at partial load without decreasing breathing capabilities at full load. Making reference to a current production engine, the purpose of this paper is to envestiage the influence of port design and flow-control strategies on both engine permeability and in-cylinder flow field. Using previously validated models, 3-D CFD simulations of the intake and compression strokes are performed in order to predict the in-cylinder flow patterns originated by the different configurations. The comparison between the two configurations in terms of airflow at full load indicates that Geometry 2 can trap 3.03% more air than Geometry 1, while the swirl intensity at IVC is reduced (−30%). The closure of one intake valve (the left one) is very effective to enhance the swirl intensity at partial load: the Swirl Ratio at IVC passes from 0.7 to 2.6 for Geometry 1, while for Geometry 2 it varies from 0.4 to 2.9.

We present theoretical models to simulate spark discharge and ignition. Two models are presented. The first model considers simplified fluid mechanics with chemistry effects. The second model utilizes more sophisticated flow with no chemistry. The simplified model incorporates physical models of breakdown and chemical kinetics of combustion with species and energy equations solved using a control volume method. A three-step mechanism is used to simulate chemical kinetics for fuel combustion and nitrogen chemistry. Dissociation and ionization in the plasma are included by assuming local thermal equilibrium. The second model simulates a spark discharge in air with more complete physics of flow using a high order numerical method, which shows the evolution of the shock wave and a torus-like plasma kernel.

The flow field within a Clark turbocharger compressor has been thoroughly analyzed by numerical simulation. The numerical simulation was validated with experimental data collected from field test sites. The numerical results are found to closely match the experimental data. Once validated, the numerical simulation was used to investigate the flow field within the turbocharger compressor under a variety of operating conditions. Several regions of flow recirculation were identified that restrict flow through the turbocharger.

Based on both experimental observations and available numerical methods, an innovative 2D approach for determining droplet size during the atomization process has been developed. Based on experimental evidences (see [1] and [2]) atomization of turbulent high speed jets is assumed to occur in a two stage process: ligaments detachment and droplets formation. The simulation method here proposed wants to take the advantages typical of the two most effective methods in spray investigation. It joins LES (i.e Large Eddy Simulations) approach and Linear Stability Analysis: the first one is used to solve the liquid-air fluid dynamics interaction and in particular the instabilities leading to ligament formation. The second one is finally adopted to compute the droplet size spectrum from ligament break-up. Therefore dynamics of ligament formation is directly computed while droplet formation is modelled by using a Linear Stability Analysis. The numerical simulation adopts a VOF (i.e. Volume of Fluid) method to track liquid-gas interface. Turbulence effects on liquid surface are accounted for by adding a turbulent flow field at the nozzle exit which represents a part of the boundary condition of the computational domain. A physical criterion is then applied to detach ligaments from liquid jet surface which will reduce in diameter during simulation. The droplet formation is then computed by applying the linear stability analysis to the ligaments, assumed being circular and subject to circulation. An extensive validation and sensitivity analysis has been carried out in order to assess method advantages and limits. The experimental results of Wu et al. [3] and Horoyasu et al. [4] were used as test cases. A sensitivity analysis has been performed under typical HSDI Diesel engine injection conditions. The method proved to exhibit promising attitude in the reconstruction of the droplet size spectrum depending on injection parameter or conditions.