One of the key conjunctions in the IC engine is that made by the top compression ring to the cylinder liner. Although the compression ring has seen considerable improvements since its introduction into steam engines by John Ramsbottom in the 1850s, its multiple and often contradictory functions still remain a challenge today. The primary aim has always been to seal the combustion chamber and guard against leakage of combustion gasses. The ring is also required to conduct some of the generated heat away. These requirements call for good ring-bore conformance, but often at the expense of increased friction. A simplified inter-ring gas flow model, as well as the measured chamber pressure using a Kistler pressure transducer is carried out under motored conditions to obtain the net gas pressure acting behind the ring. This and the elastic pressure as the result of ring restoring tension force in fitment constitute the contact load, which is usually carried by a mixed-hydrodynamic regime of lubrication. The conjunction pressures are treated as a combination of hydrodynamic generated pressures and asperity-pair interactions. The former is obtained by solution of two dimensional Reynolds equation, whilst the latter is determined assuming a Gaussian distribution of asperities on the counterfaces. Surface topography of the bounding solids; the compression ring and the liner are measured and used as appropriate statistical functions in the Greenwood and Tripp model. Using an analytical flow model, pressure acting behind the ring (on the inner periphery of the ring) is obtained, leading to the calculation of ring-bore friction. A floating liner is used to measure friction of piston and ringpack in situ . The characteristic of the predicted variations are compared with the measured data. However, a quantitative comparison is not possible as the measured data corresponds to all the conjunctions of piston system, including the oil control rings as well as the piston skirt. The results show that variation of gas force behind the ring significantly changes the ring-bore contact, thus friction and the nature of interactions.

Recent developments in numerical techniques and computational processing power now permit time-dependent, multi-dimensional computational fluid dynamic (CFD) calculations with reduced chemical kinetic mechanisms (approx. 20 species and 100 reactions). Such computations have the potential to be highly effective tools for designing lean-burn, high BMEP natural gas engines that achieve high fuel efficiency and low emissions. Specifically, these CFD simulations can provide the analytical tools required to design highly optimized natural gas engine components such as pistons, intake ports, precombustion chambers, fuel systems and ignition systems. To accurately model the transient, multi-dimensional chemically reacting flows present in these systems, chemical kinetic mechanisms are needed that accurately reproduce measured combustion data at high pressures and lean conditions, but are of sufficient size to enable reasonable computational times. Presently these CFD models cannot be used as accurate design tools for application in high BMEP lean-burn gas engines because existing detailed and reduced mechanisms fail to accurately reproduce experimental flame speed and ignition delay data for natural gas at high pressure (40 atm and higher) and lean (0.6 equivalence ratio (ϕ) and lower) conditions. Existing methane oxidation mechanisms have typically been validated with experimental conditions at atmospheric and intermediate pressures (1 to 20 atm) and relatively rich stoichiometry. These kinetic mechanisms are not adequate for CFD simulation of natural gas combustion in which elevated pressures and very lean conditions are typical. This paper provides an analysis, based on experimental data, of the laminar flame speed computed from numerous, detailed chemical kinetic mechanisms for methane combustion at pressures and equivalence ratios necessary for accurate high BMEP, lean-burn natural gas engine modeling. A reduced mechanism that was shown previously to best match data at moderately lean and high pressure conditions was updated for the conditions of interest by performing sensitivity analysis using CHEMKIN. The reaction rate constants from the most sensitive reactions were appropriately adjusted in order to obtain a better agreement at high pressure lean conditions. An evaluation of this adjusted mechanism, “MD19”, was performed using Converge CFD software. The results were compared to engine data and a remarkable improvement on combustion performance prediction was obtained with the MD19 mechanism.

Due to the vast resources of natural gas (NG), it has emerged as an alternative fuel for SI internal combustion engines in recent years. The need to have better fuel economy and less emission especially that of greenhouse gases has resulted in development of NG fueled engines. Direct injection of natural gas into the cylinder of SI internal combustion engines has shown great potential for improvement of performance and reduction of engine emissions especially CO2 and PM. Direct injection of NG into the cylinder of SI engines is rather new thus the flow field phenomena and suitable configuration of injector and combustion chamber geometry has not been investigated completely. In this study a numerical model has been developed in AVL FIRE software to perform investigation of direct natural gas injection into the cylinder of spark ignition internal combustion engines. In this regard, two main parts have been taken into consideration aiming to convert an MPFI gasoline engine to direct injection NG engine. In the first part of study multidimensional numerical simulation of transient injection process, mixing and flow field have been performed via different validation cases in order to assure the numerical model validity of results. Adaption of such a modeling was found to be a challenging task because of required computational effort and numerical instabilities. In all cases present results were found to have excellent agreement with experimental and numerical results from literature. In the second part, using the moving mesh capability, the validated model has been applied to methane injection into the cylinder of a direct injection engine. Five different piston head shapes have been taken into consideration in investigations. An inwardly opening multi-hole injector has been adapted to all cases. The injector location has been set to be centrally mounted. The effects of combustion chamber geometry have been studied on mixing of air-fuel inside cylinder via quantitative and qualitative representation of results. Based on the results, suitable geometrical configuration for a NG DI engine has been discussed.

The effect of spray penetration distance on fuel impingement on piston bowl of a 7.4 kW diesel engine for biodiesel-diesel blend (B20) was studied using modeling and CFD simulation. As the peak inline fuel pressure increased from 460 bar with base diesel to 480 bar with B20, the spray penetration distance (fuel jet) increases. It is observed from the study that the jet tip hits on piston bowl resulting to fuel impingement which is one of durability issues for use of biodiesel blend in the diesel engine. In addition to this, the simulation of effects of different injection pressures up to 2000 bar on spray penetration distance and wall impingement were also studied. The penetration distance increases with increase the in-line fuel pressure and it decreases with decrease nozzle hole diameter. The fuel impingement on piston bowl of the engine with high injection pressure (typically 1800 bar) can be avoided by decreasing the nozzle diameter from 0.19 mm to 0.1 mm. Increase in swirl ratio could also reduce fuel impingement problem.

Both the piston ring and cylinder bore experience wear throughout their life cycle. Therefore change occurs in the geometrical profile and topography of the ring. In addition, coating on the ring is also subject to wear, thus altering the physical/mechanical property of the contacting surface. These changes affect the tribological performance of the ring-bore conjunction. Geometrical changes often alter the ring contacting profile, which affects the entrainment of the lubricant into the conjunction. This can affect the regime of lubrication, thus mechanisms that contribute to friction. Wear of surfaces also plays some role in boundary friction in terms of topographical changes as well as surface properties such as hardness and asperity shear strength. Most analysis does not take into account changes in tribological conditions which occur as the result of these salient changes in practice. The paper intends to bridge this gap in the fundamental knowledge and provide explanations for some in-field experiences noted with wear of a compression ring in a typical engine test. The method of solution used is based on the average flow factors which are indicative of entrainment of the lubricant through the rough ring-bore conjunction. This approach was initiated by Patir and Cheng, for which realistic topographical parameters according to the stage of the wear process is included. Changes in friction and fuel energy consumed are predicted.

For increasing the thermal engine efficiency, faster combustion and low cycle-to-cycle variation are required. In PFI engines the organization of in-cylinder flow structure is thus mandatory for achieving increased efficiency. In particular the formation of a coherent tumble vortex with dimensions comparable to engine stroke largely promotes proper turbulence production extending the engine tolerance to dilute/lean mixture. For motorbike and scooter applications, tumble has been considered as an effective way to further improve combustion system efficiency and to achieve emission reduction since layout and weight constraints limit the adoption of more advanced concepts. In literature chamber geometry was found to have a significant influence on bulk motion and turbulence levels at ignition time, while intake system influences mainly the formation of tumble vortices during suction phase. The most common engine parameters believed to affect in-cylinder flow structure are: 1. Intake duct angle; 2. Inlet valve shape and lift; 3. Piston shape; 4. Pent-roof angle. The present paper deals with the computational analysis of three different head shapes equipping a scooter/motorcycle engine and their influence on the tumble flow formation and breakdown, up to the final turbulent kinetic energy distribution at spark plug. The engine in analysis is a 3-valves pent-roof motorcycle engine. The three dimensional CFD simulations were run at 6500 rpm with AVL FIRE code on the three engines characterised by the same piston, valve lift, pent-roof angle and compression ratio. They differ only in head shape and squish areas. The aim of the present paper is to demonstrate the influence of different head shapes on in-cylinder flow motion, with particular care to tumble motion and turbulence level at ignition time. Moreover, an analysis of the mutual influence between tumble motion and squish motion was carried out in order to assess the role of both these motions in promoting a proper level of turbulence at ignition time close to spark plug in small 3-valves engines.

In this paper, a new hydraulic variable valve actuation system is proposed. Using this system, the engine valve opening and closing timings and lift are flexibly controlled with two rotary spool valves actuated by the engine crankshaft. High degree of flexibility with less control complexity and high repeatability are the advantages of this system over other camless valvetrains; however, in this system, there is a trade-off between its robustness and power consumption. A numerical model of the system is developed to study the system functionality at different operating conditions. To validate the developed model, the simulation results for a random operating condition are compared with those from the experiments. A sensitivity analysis is done to study the effects of variations in different design parameters on system robustness and power consumption. The results prove that increasing engine valve return-spring stiffness and actuator piston area will reduce the mechanism sensitivity to engine cycle-to-cycle variations; however, this results in poor energy efficiency. Therefore, a neat energy recovery strategy is developed to recuperate a portion of the energy used to compress the engine valve return-spring during valve opening interval. The results show that more than 90% of the extra energy wasted for the sake of system robustness could be regenerated through the proposed energy recovery system.

The regime of lubrication changes in a transient manner in many load bearing conjunctions. This is particularly true of any conjunction which is subjected to changes in contact kinematics as the result of stop-start or motion reversals and loading. One such conjunction in the IC engine is the piston-bore contact. A repercussion of these transient events under otherwise perceived steady operating condition is the underlying changes in the mechanisms giving rise to engine efficiency, such as parasitic losses, mainly due to friction. Understanding the nature of these losses is the prelude to any form of palliation. A single cylinder motocross motorbike engine’s cylinder barrel is redesigned to accept wet liners with various incorporated instrumentation. The paper describes one such barrel which incorporates an instrumented floating liner for the purpose of measurement of in-cylinder friction. The principle and design of the floating liner is described. A series of tests are carried out in order to ensure the operational integrity and repeatability of the device. The basic test includes motorised running of the engine without the cylinder head installed. This renders simplified motion of the liner, subject to resistance by friction only. In a sense, under this type of motion, the liner should undergo a form of simple harmonic motion, which is verified using a number of suitably positioned accelerometers. Some more representative tests are reported under motorised conditions with the cylinder head installed. Thus, the effect of chamber pressure is introduced. However, with no combustion pressure, heat output and resulting side forces, a better understanding of tribological conditions is accrued owing to the reduced physical interactions. The results show the dominance of a mixed regime of lubrication at the dead centre reversals.

Variable displacement lubricant pumps allow oil flow to be matched to engine requirements over the whole operating range, reducing energy losses through excessive pumping work. An experimental investigation has been performed on-engine to understand the effects of such devices. Significant instrumentation was fitted to the production, EURO IV specification, 2.4L Diesel engine to assess the impacts of lubricant flow on thermal state. The reduced oil flow was measured as a reduction in engine oil pressure with the production pump supplying 4–6bar whereas the variable flow device provided pressures as low as 1–2bar. The reduction in oil flow significantly reduced the oil pump energy consumption, measured as a change in indicated work, resulting in a 4% benefit in fuel economy over both hot and cold start NEDC. The reduced oil flow also impacted oil and metal temperatures: during engine warm-up, oil temperatures were approximately 4°C colder with the lower flow as a result of less work input from the oil pump. Conversely, cylinder liner temperatures were 2–6°C hotter both during warm-up and fully warm conditions as a result of reduced piston cooling from piston cooling jets. The changes in thermal state were reflected by changes in emissions with a 3% increase in NO x and a 3–5% reduction in HC and CO. The calibration of the variable flow device follows a fuel consumption/NO x trade-off that is more favourable to fuel economy than conventional control parameters. However, these benefits are strongly linked to engine duty cycle with larger benefits at higher engine speeds.

In the last few years, a significant research effort has been made for developing and enhancing Direct Injection (DI) for compressed natural gas (CNG) engines. Several research projects have been promoted by the European Community (EC) in this field with the objective of finding new solutions for the automotive market and also of encouraging a fruitful knowledge exchange among car manufacturers, suppliers and technical universities. This paper concerns part of the research activity that has been carried out by the Politecnico di Torino, AVL List GmbH and Siemens AG within the EC VII Framework Program (FP) InGAS Collaborative Project (CP), aimed at optimizing the control phase of a new injector for CNG direct injection, paying specific attention to its behavior at small injected-fuel amounts, i.e., small energizing times. The CNG injector which was developed within the research project proved to be suitable to be used in a DI SI engine, featuring a pent-roof combustion chamber head and a bowl in piston, with reference to both homogeneous and stratified charge formation. Fuel flow measurements made by AVL on the four-cylinder engine revealed a good linearity between injection duration and fuel mass-flow rate for injection durations above a reference value. In order to improve the injector characterization at short injection durations, an experimental and numerical activity was designed. More specifically, a multidimensional CFD model of the actual injector geometry was built by Politecnico di Torino, and purposely-designed simulation cases were carried out, in which the needle-lift time-history was defined on the basis of experimental measurements made by Siemens. The numerical model was validated on the basis of experimental data concerning the total injected-fuel amount under different conditions. Then, the model was applied in order to evaluate the dynamic flow characteristic by taking also the inner geometry of the injector valve group into account, so as to establish a correlation to the needle lift measurements done by Siemens for injector characterization. In the paper this dynamic behavior of the injector is analyzed, under actual operating conditions, and its impact on the nozzle injection capability is discussed. The simulation results did not show significant oscillations of the stagnation pressure upstream of the nozzle throat section, and thus the resultant mass-flow rate profile is almost proportional to the needle-lift one. As a consequence, in order to characterize the injector flow behavior in the nonlinear region (short injection duration), the measurement of needle lift is sufficient.

The paper proposes a cost-saving analytical methodology using empirical based models to efficiently evaluate design alternatives in the optimization of a CNG converted diesel engine. The procedure is performed in five steps. Firstly, a database of different combustion chambers that can be obtained from the original piston is obtained. The chambers in the database differ for the shape of the bowl, the value of the compression ratio, the offset of the bowl and the size of the squish region. The second step of the procedure is the selection, from the first database, of the combustion chambers able to resist to the mechanical stresses due to the pressure and temperature distribution at full load. For each combination of suitable combustion chamber shape and ignition timing, a CFD simulation is used to evaluate the combustion performance of the engine. Then, a post-processing procedure is used to evaluate the detonation tendency and intensity of each combination. All the tools developed for the application of the method have been linked in the ModeFrontier optimization environment in order to perform the final choice of the combustion chamber. The overall process requires not more of a week of computation on the 4 processor servers considered for the optimization. Moreover, the selected chambers can be obtained from the original piston of the engine. Therefore, the conversion cost of the engine is quite small compared with the case of a completely new piston. The procedure can be applied to diesel engines to be converted to either CNG dedicated or dual fuel combustion. The main aspects and challenges to be taken into account in both cases are also analyzed.

Nowadays, environmental concerns are posing a great challenge to DI Diesel engines. Increasingly tightening emission limits require a higher attention on combustion efficiency. In this scenario, computational fluid-dynamics can prove its power guaranteeing a deeper understanding of mixture formation process and combustion. A high efficiency Diesel engine can be developed only mastering all the parameters that can affect the combustion and, therefore, NOx and soot emissions. In this work, the development of an engine in order to fulfill Tier 4i emission standard will be presented. Originally, the engine was a two-valve engine supplied with a DPF. Since no SCR aftertreatment is supplied, NOx emission target are achieved through external exhaust gas recirculation and retarding the start of injection. In order to fulfill Tier 4i emissions, the main concern is on soot emission and, thus, the combustion chamber has been re-designed, through CFD simulations, leading to a better interaction between the flow field, the fuel spray and the piston bowl geometry. Particularly, through intake phase simulations, performed with the CFD code Fire v2009 v3, different intake ducts, with different swirl ratio, have been simulated in order to provide a flow field as realistic as possible for the combustion simulations. Through combustion process simulations, performed with the CFD code Kiva, by varying different parameters the interaction between the swirl flow field, generated by the intake duct, the reverse squish motion, and motions aerodynamically generated by spray has been investigated leading to the definition of a new engine lay-out. The study shows how, given the need of retarded injection for limiting NOx emission, the decrease of swirl ratio, when combined with a proper piston bowl design, allows a significant decrease of soot emissions and the achievement of Tier 4i emission standard.

The in situ profiles of the piston skirt and cylinder bore surface are subject to thermo-elastic global deformation due to differential operating temperatures and forces. In operation, a lubricant film is entrained into and pressurized within the gap between these profiles. This film not only supports the prevailing contact load, but also inhibits direct interaction of surfaces, thus reducing friction and thereby improving fuel efficiency. The reduction of reciprocating mass in motorsport applications has been achieved through the use of partial circumferential skirts for a number of years now. The response of the shape to both mechanical and structural loadings differs from the classic full circumferential skirt studies. This paper provides a ‘snapshot’ into how the inherent piston side load is supported by the piston skirt. It highlights the importance of the operational temperature on the skirt profile, conjunctional gap and the lubricant film. Additionally, it shows how a given piston skirt shape and its structural stiffness perform in operation.

Dresser Waukesha has developed a more robust piston capable of operating under abuse conditions beyond those typically seen for large industrial natural gas engines. The results of the modeling and experimental work used to develop and validate the new robust piston will be presented in this paper. Simulation tools were first used to gain a comprehensive understanding of the causes of piston scuffing when operated under abuse conditions. With this understanding, an improved design was developed and the design changes were compared to the original piston. The performance of the new design was then simulated and compared to the original design. To investigate the robustness of the new design, the effect of reducing the major diameter clearance beyond the print specification was studied. Wear loads were predicted to be insensitive to size variations and were predicted to be acceptable even to the limit of a cold piston/liner interference fit. The predicted robustness of the new design was confirmed with experimental testing.

During the last years, the integration of computational CFD tools in the internal combustion (IC) engine design process has continuously been increased, allowing to save time and cost as the need of experimental prototypes has diminished. Numerical analyses of IC engine flows are rather complex from both the conceptual and operational sides. In fact, such flows involve a variety of unsteady phenomena, and the right balance between numerical solution accuracy and computational cost should be always reached. The present paper is focused on computational modeling of natural gas (NG) direct injection (DI) processes from a poppet-valve injector into a bowl-shaped combustion chamber. At high injection pressures, the efflux of gas from the injector and the mixture formation processes include compressible and turbulent flow features, such as rarefaction waves and shock formation, which are difficult to be accurately captured by the numerical simulation, particularly when combustion chamber geometry is complex and piston and intake/exhaust valve grids are moving. A three-dimensional moving grid model of the combustion engine chamber, originally developed by the authors, was enhanced by increasing the accuracy in the sonic section proximity of the critical valve seat nozzle, in order to precisely capture the expansion dynamics the methane undergoes inside the injector and immediately downstream from it. The enhanced numerical model was validated by comparing numerical results to Schlieren experimental images for nitrogen injection into a constant-volume bomb. Then, numerical studies were carried out in order to characterize the fuel jet properties and the evolution of mixture-formation for a centrally-mounted injector configuration in both cases of a pancake test chamber and the real-shaped engine chamber. Finally, the fluid properties computed by the model in the throat-section of the critical nozzle were taken as reference data for developing a new effective ‘virtual injector’ model, which allows the designer to remove the whole computational domain upstream from the sonic section of the nozzle, keeping the flow properties practically unchanged. The outcomes of such a virtual injector model were shown to be in very good agreement with the results of the enhanced complete injector model, confirming the reliability of the proposed novel approach.

A two-pronged experimental and computational study was conducted to explore the formation, transport, and vaporization of a wall film located on the piston surface within a four-valve, pent roof, direct-injection spark-ignition (DISI) engine, with the fuel injector located between the two intake valves. Negative temperature swings were observed at three piston locations during early injection, thus confirming the ability of fast-response thermocouples to capture the effects of impingement and heat loss associated with fuel film evaporation. Computational Fluid Dynamic (CFD) simulation results demonstrated that the fuel film evaporation process is extremely fast under conditions present during intake. Hence, the heat loss measured on the surface can be directly tied to the heating of the fuel film and its complete evaporation, with the wetted area estimated based on CFD predictions. This finding is critical for estimating the local fuel film thickness from measured heat loss. The simulated fuel film thickness and transport corroborated well temporally and spatially with measurements at thermocouple locations directly in the path of the spray, thus validating the spray and impingement models. Under the strategies tested, up to 23% of fuel injected impinges upon the piston and creates a fuel film with thickness of up to 1.2 μm. In summary, the study demonstrates the usefulness of heat flux measurements to quantitatively characterize the fuel film on the piston top and allows for validation of the CFD code.

Diesel engine technology advancements in performance and requirements for meeting emission regulations have developed new challenges for piston system durability. Increased combustion pressure and exhaust gas recirculation have severely increased the demand of the top piston ring to top piston groove surface interactions. This more demanding environment has resulted in new needs for improved durability and compatibility of the bottom flank of the top piston ring and the bottom side to the top piston groove. Developments in materials and treatments of these surfaces require accurate analysis and assessment of the worn regions. With this demand, new measurement techniques have been deployed to accurately map the bottom flank of the top piston ring and the bottom side of the top piston groove. The new methods allow for volumetric wear measurements and regional analysis for wear step magnitudes to assist in better material wear characterization as well as piston ring design influence.

Motivated by the need to achieve greater efficiency in the product development cycle, engine manufacturers in the heavy-duty industry are relying more and more on analytical tools to help resolve performance issues, minimize testing costs and reduce time to market. This is particularly the case in the area of power cylinder development, since the physical phenomena occurring within the cylinders are difficult to observe and quantify. However, analytical tools are useful only if they are able to accurately represent the physical systems that they are supposed to simulate. It therefore becomes critical to establish a correlation between model predictions and empirical measurements. To this end, a correlation study was initiated comparing power cylinder analytical modeling results to empirical measurements in a John Deere 6135H 13.5L I-6 engine. The engine test measurements that were carried out included cylinder pressure, piston land pressure, engine blowby, and lube oil consumption. The correlation parameters considered in the study included piston land gas pressures, power cylinder contribution to engine blowby, and lube oil consumption mechanisms.

On the basis of ongoing research conducted on the clarification of processes responsible for lubricant degradation in the environment of piston grooves in EGR diesel engines, an experimental investigation was aimed to develop a kinetic model which can be used for the prediction of lubricant oxidative degradation correlated to endurance test conducted on engines. Knowing that base oils are a complex blend of paraffins and naphtenes with a wide range of sizes and structures, their chemistry analysis during the oxidation process can be highly convoluted. In the present work, investigations were carried out with the squalane (C 30 H 62 ) chosen for its physical and chemical similarities with the lubricant base oils used during the investigations. Thermo-oxidative degradation of this hydrocarbon was conducted at atmospheric pressure in a tubular furnace, while varying temperature and duration of the tests in order to establish an oxidation reaction rate law. The same experimental procedures was applied to squalane doped with two different phenolic antioxidants usually present in engine oil composition: 2,6-di- tert -butyl-4-methylphenol (BHT), and octadecyl-3-(3,5-di- tert -butyl-4-hydroxyphenyl)propionate (OBHP). Thus, the effect of both antioxidants on the oxidation rate law was investigated. Data analysis of the oxidized samples (FTIR spectroscopy, gas chromatography/mass spectrometry GC/MS) allowed to rationalize the thermo-oxidative degradation of squalane. The resulting kinetic modelling provides a practical analytical tool to follow the thermal degradation processes, which can be used for prediction of base oil hydrocarbon ageing. If experiments confirmed the role of phenolic additives as an affective agent to lower oxidation rates, the main results lay in the observation of a threshold temperature where a reversed activity of these additives was observed.

A mathematical model for cylinder-piston system considering the coupling between dynamic behavior and tribological behavior is presented in this paper. The three-dimensional finite element method is used to compute the engine block vibrations. The lubrication of cylinder-piston pair was described by an average Reynolds equation considering the piston second-vibration and asperity contact of rough surface. The corresponding computing program developed can be used to calculate the engine block vibrations, the entire piston trajectory, the Piston slap forces, friction forces and oil film thickness etc. as functions of crank angle under engine running conditions. The numerical simulation results show that the coupling between engine block dynamical behavior and lubrication has big influence for the secondary motion of piston and the engine block vibration in cylinder-piston system. It is essential to consider the engine block vibration in the analysis of cylinder-piston tribology.