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Homogeneous charge compression ignition is considered a promising solution to face the increasing regulations imposed by the legislator in the transport sector, thanks to pollutant and CO2 emissions reduction. In this work, a quasi-dimensional multi-zone HCCI model integrated with 1D commercial software is developed and validated. It is based on the control mass Lagrangian approach and computes the mixture chemistry evolution through offline tabulation of chemical kinetics (tabulated kinetic of ignition). Thus, the simulation can predict mixture auto-ignition with reduced computational effort and high accuracy. Multi-zone schematization mimics the typical thermal stratification of HCCI engines, controlling the combustion evolution. The model is coupled to sub-models for pollutant emissions estimation. Initially, the tabulated chemistry approach is validated against a chemical kinetics solver applied to a constant-volume homogeneous reactor, considering various fuel blends. The model is then used to simulate the operations of four engines using different fuels (hydrogen, methane, n-heptane, and n-heptane/toluene/ethanol blend), under various boundary conditions. The model predictivity is demonstrated against pressure traces, heat release rate, and noxious emissions. The numerical results showed to adequately agree with measured counterparts (average relative error of 1.3% on in-cylinder pressure peak, average absolute error of 0.95 CAD on pressure peak angle, average relative error of 8.4% on uHCs emissions, absolute error below 1 ppm on NOx emissions) only adapting the thermal stratification to the engines under study. The methodology proved to be a reliable tool to investigate the operation of an HCCI engine, applicable in the development of new engine architecture. [ABSTRACT FROM AUTHOR]

• A comprehensive phenomenological uHC model is proposed. • New additions from previous application included flame penetration, non-piston crevice volumes and scavenging model. • The uHC model is implemented in a commercial software through user coding. • The model is tested on three engines with different size and combustion concepts. • The model is validated against experimental uHC concentrations, without caseby-case tuning. The development of lean-burn concepts for internal combustion engines, utilizing a homogeneous air/fuel charge, aims to improve multiple aspects simultaneously, including thermal efficiency, fuel consumption, nitric oxide, and carbon monoxide emissions. However, the adoption of this technology may result in a notable increase in the emission of unburned hydrocarbons (uHC) compared to stoichiometric engines. The sources of uHC are diverse and their relative significance depends on fuel characteristics, engine operating conditions, and geometric features of the combustion chamber. This concern becomes more pronounced when considering engines fueled with natural gas, as methane has a high global warming potential compared to other pollutant emissions. This research proposes an enhancement to an existing simulation model that describes the primary mechanisms involved in uHC formation, which are filling/emptying of the piston top-land crevice volume and the flame wall quenching, with subsequent uHC post-oxidation. The improvements include the calculation of flame penetration into crevice volumes, the description of uHC production from non-piston crevices, and the introduction of a novel scavenging model. The uHC model is integrated into commercial software (GT-Power) as a "user routine". Validation of the model is conducted with reference to three large bore engines (W31DF, W31SG, and W46SG) with bore sizes of 31 cm and 46 cm, fueled with natural gas, and operated under lean mixtures (λ > 2), employing different ignition methods (dual fuel mode or pre-chamber device). The uHC model is validated against engine-out measurements, exhibiting an average error of 10.0 % for W31DF, 19.4 % for W31SG, and 14.0 % for W46SG engines, respectively. The reliability of the uHC model is demonstrated by the absence of case-specific adjustments to its tuning constants, even when there are significant variations in engine load and geometric design of the crevice volumes. [ABSTRACT FROM AUTHOR]

• Experimental characterization of air/hydrogen mixture combustion for ultra-lean cases and engine-like thermodynamic states; • Development of a refined thermodynamic model of RCEM considering mass and energy exchange with crevices, and heat transfer; • Estimation of unburned and burned-gas related flame speed based on a simplified geometrical description of the flame front; • Model validation against experimental data from optical diagnostics. This work presents a thermodynamic inverse model to extract the main flame propagation characteristics of an ultra-lean air/hydrogen mixture burning in a rapid compression/expansion machine (RCEM). Differently from combustion vessels, RCEMs allow fundamental combustion studies under thermodynamic conditions of temperature and pressure relevant to internal combustion engine (ICE) operations. The presented thermodynamic model takes into account all the main phenomena occurring in an RCEM, such as the wall heat transfer, and mass, species, and energy exchange with the crevice volumes. Moreover, combining the inverse model with a simplified geometrical description of the flame front, the flame propagation velocities are derived from experimental data of pressure and piston position. The model is applied to investigate the combustion process of a hydrogen/air mixture with λ between 2.1 and 2.6 and thermodynamic states relevant to ICE operations (initial pressure and temperature of around 30 bar and 720 K, respectively). The methodology is validated against direct measurements of flame front velocities realized by optical access in the combustion chamber. The model and measurements underline that the flame speed related to the unburned gas progressively decreases as λ increases, passing from 4.4 m/s at λ = 2.1 down to 2.5 m/s at λ = 2.6. The model/experiment RMSE on the absolute flame velocity is of 0.6 m/s over twelve test points, supporting the consistency of the presented thermodynamic model. [ABSTRACT FROM AUTHOR]

The paper presents the validation of a 1D compressor model (1DCM) applied to the simulation of deep-surge operation. The compressor is described following an enhanced map-based approach, where proper "virtual pipes" are placed upstream and downstream the compressor to deal with the mass and energy storage and wave propagation effects. The proposed methodology, which takes into account main flow and thermal loss mechanisms, is based on the employment of "extended" compressor maps obtained through a steady version of the 1DCM. The tuning and validation of the 1DCM have been carried out comparing its results with the experimental data. Preliminarily, the steady version of the 1DCM is tuned against to the measured map for various rotational speeds. Subsequently, it is used to derive the extended map, including both direct and reverse flow branches. Finally, the unsteady version of the 1DCM is validated against experimental data denoting a satisfactory agreement, especially in terms of pulse frequency, amplitude and global shape. Summarizing, the proposed model, combining the reduced computational effort typical of 1D simulation with the adoption of advanced features such as "virtual pipe" and extended compressor map, shows the capability to capture the phenomenology of the compressor surging. [ABSTRACT FROM AUTHOR]

• Multi-zone phenomenological model able to handle effects of flame propagation and fuel auto-ignition depending on local fuel reactivity. • Auto-igntion process described by a tabulated-chemistry approach to preserve computational effort. • Experimental campaign on a large-bore research engine supplied with natural gas and light fuel oil under various operating conditions, different in terms of load, air/fuel proportion, and injection timing/duration. • Demonstrated capability of the model in predicting engine performance and combustion evolution with good accuracy. Increasingly stringent pollutant and CO 2 emission standards require engine manufacturers to investigate innovative solutions. Among these techniques, low-temperature combustion (LTC) concepts have a large potential to simultaneously reduce NOx emissions and fuel consumption. A promising manner to realize LTC consists of adopting ultra-lean mixtures, where the combustion evolution is controlled by a proper spatial distribution of fuels with different chemical reactivities. In this context, depending on the proportion and stratification of the fuels, the heat release can primarily depend on chemistry progression, leading to a Reactivity Controlled Compression Ignition (RCCI) mode, or on flame propagation, locally initiated by a high reactivity fuel. In this work, the combustion characteristics of a large-bore research engine are experimentally investigated. Natural gas is supplied into the intake port, while light fuel oil (LFO) is directly injected in the cylinder. An experimental campaign is carried out including sweeps of engine load, air/fuel proportion, LFO amount, valve timing, and intake air temperature. Global engine operating parameters as well as cylinder pressure traces are recorded and analyzed. Based on the available experimental data, a phenomenological model handling both chemistries of fuels with different reactivities and flame propagation is developed and validated. The model is based on a multi-zone approach, where auto-ignition chemistry is solved by a tabulated method to preserve the computational effort. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, using a fixed tuning constant set. A dedicated correlation is built to reproduce the expected in-cylinder distribution of the directly injected liquid fuel. Global performance and combustion parameters are predicted with an average error below 5%. The model demonstrates to correctly describe the behavior of the tested engine under different operating conditions and to capture the physics behind such advanced combustion concepts. [ABSTRACT FROM AUTHOR]

It is well known that the downsizing philosophy allows the improvement of the brake specific fuel consumption (BSFC) at part load operation for spark ignition (SI) engines. On the other hand, the BSFC is penalized at high load because of the knock occurrence and of further limitations on the turbine inlet temperature (TIT). Knock control forces the adoption of a late combustion phasing, causing a deterioration of the thermodynamic efficiency, while the TIT control requires the enrichment of the air-to-fuel ratio (A/F), with additional BSFC drawbacks. In this work, two promising techniques are investigated by a 1D approach with the aim of improving the fuel economy of a turbocharged SI engine at full load knock-limited operation. The first technique is the recirculation of low-pressure cooled exhaust gas (EGR), while the second is the injection of liquid water at the intake ports. Proper “in-house developed” sub-models are used to describe the turbulence, combustion and knock phenomena. The effects of the above techniques are studied in six operating points at full load and different speeds for various A/F levels and inert content, by varying the EGR rate and water-to-fuel ratio. The presented results highlight that both the solutions involve significant BSFC improvements, especially in the operating conditions at medium engine speeds. In fact, the introduction of inert gas in the cylinder contributes to reduce the knock tendency, resulting in the possibility to advance the combustion phasing and reduce, or even avoid, the mixture over-fuelling. The heat subtracted by the water evaporation enhances the above effects, resulting in an improved fuel economy. However, the BSFC advantages are limited by the maximum allowable in-cylinder pressure, TIT, turbocharger speed, and boost level. The developed numerical procedure is able to take into account the complex interactions among different driving parameters affecting the engine behaviour. It can be hence very useful to define a numerical engine pre-calibration and to realistically predict the EGR- and water injection-related BSFC advantages. [ABSTRACT FROM AUTHOR]

Various solutions are being proposed to improve the performance of spark-ignition internal combustion engines. A very effective approach is the downsizing technique, which allows the reducing of the Brake Specific Fuel Consumption (BSFC) at part load, while maintaining the required performance at high load. On the other hand, the above technique may cause substantial BSFC detrainments at high load because of the onset of knocking combustions. In the present work, a turbocharged spark ignition engine equipped with a fully flexible valve system is numerically investigated by a 1D model (GT-Power™). Proper “user routines” are used to simulate the turbulent combustion process and the knock phenomenon. In a first stage, the engine model is validated against experimental data under both high and part load operations, in terms of overall performance and combustion evolution. The validated model is then integrated in a multipurpose commercial optimizer (modeFRONTIER™) with the aim to identify the engine calibrations that maximize the load and minimize the BSFC under high load knock-limited operations at a speed of 3000 rpm. The effects of different intake valve strategies are compared. The optimized operating parameters are the waste-gate valve opening and the air-to-fuel ratio, while the combustion phasing is automatically adjusted to avoid the knock onset. Proper constraints are assigned for the boost pressure, turbocharger speed, and turbine inlet temperature. The adopted optimization process shows the capability to reproduce the experimentally-identified calibration with satisfactory accuracy. In addition, the results underline the BSFC advantages related to an early intake valve closure strategy with respect to a Full Lift one, due to a better combustion phasing and a reduced mixture over-fuelling. The developed automatic procedure allows for a “virtual” engine calibration on a completely theoretical basis and proves to be very helpful in reducing the engine development costs and time-to-market. [ABSTRACT FROM AUTHOR]

Nowadays the turbocharging technique is playing a fundamental role in improving automotive engine performance and reducing fuel consumption and the exhaust emissions, in spark-ignition and compression ignition engines, as well. To this end, one-dimensional (1D) modeling is usually employed to compute the engine-turbocharger matching, to select the boost level in different operating conditions, and to estimate the low-end torque level and the transient response. However, 1D modeling of a turbocharged engine requires the availability of the turbine and compressor characteristic maps. This leads to some typical drawbacks: (1 performance maps of the turbocharger device are usually limited to a reduced number of rotational speeds, pressure ratios, and mass flow rates because of turbine/compressor matching limits; (2) as a consequence of previous issue, unphysical extrapolation of maps' data is commonly required; and (3) heat transfer conditions may strongly differ between test bench measurements and actual operation, where turbocharger is coupled to an internal combustion engine. To overcome the above problems, in the present paper a numerical procedure is introduced: It solves 1D steady flow equations inside the turbine components with the aim of accurately reproducing the experimentally derived characteristic maps. The steady procedure describes the main phenomena and losses arising within the stationary and rotating channels constituting the turbine. It is utilized to directly compute the related steady maps, starting from the specification of a reduced set of geometrical data. An optimization process is employed to identify a number of tuning constants included in the various loss correlations. The procedure is applied to the simulation of five different turbines: three waste-gated turbines, a twin-entry turbine, and a variable geometry turbine. The numerical results show good agreement with the experimentally derived maps for all the tested devices. The model is, hence, used to evaluate the turbine performance in the whole operating domain. [ABSTRACT FROM AUTHOR]

In recent years, the development of hybrid powertrain allowed to substantially reduce the CO2 and pollutant emissions of vehicles. The optimal management of such power units represents a challenging task since more degrees of freedom are available compared to a conventional pure-thermal engine powertrain. The a priori knowledge of the driving mission allows identifying the actual optimal control strategy at the expense of a quite relevant computational effort. This is realized by the off-line optimization strategies, such as Pontryagin minimum principle—PMP—or dynamic programming. On the other hand, for an on-vehicle application, the driving mission is unknown, and a certain performance degradation must be expected, depending on the degree of simplification and the computational burden of the adopted control strategy. This work is focused on the development of a simplified control strategy, labeled as efficient thermal electric skipping strategy—ETESS, which presents performance similar to off-line strategies, but with a much-reduced computational effort. This is based on the alternative vehicle driving by either thermal engine or electric unit (no power-split between the power units). The ETESS is tested in a "backward-facing" vehicle simulator referring to a segment C car, fitted with a hybrid series-parallel powertrain. The reliability of the method is verified along different driving cycles, sizing, and efficiency of the power unit components and assessed with conventional control strategies. The outcomes put into evidence that ETESS gives fuel consumption close to PMP strategy, with the advantage of a drastically reduced computational time. The ETESS is extended to an online implementation by introducing an adaptative factor, resulting in performance similar to the well-assessed equivalent consumption minimization strategy, preserving the computational effort. [ABSTRACT FROM AUTHOR]

The complexity of modern hybrid powertrains poses new challenges for the optimal control concerning, on one hand, the thermal engine to maximize its efficiency, and, on the other hand, the vehicle to minimize the noxious emissions and CO2. In this context, the engine calibration has to be conducted by considering simultaneously the powertrain management, the vehicle characteristics, and the driving mission. In this work, a calibration methodology for a two-stage boosted ultra-lean pre-chamber spark ignition (SI) engine is proposed, aiming at minimizing its CO2 and pollutant emissions. The engine features a flexible variable valve timing (VVT) control of the valves and an E-compressor, coupled in series to a turbocharger, to guarantee an adequate boost level needed for ultra-lean operation. The engine is simulated in a refined 1D model. A simplified methodology, based on a network of proportional integral derivative (PID) controllers, is presented for the calibration over the whole operating domain. Two calibration variants are proposed and compared, characterized by different fuel and electric consumptions: the first one aims to exclusively maximize the brake thermal efficiency, and the second one additionally considers the electric energy absorbed by the E-compressor and drained from the battery. After a verification against the outcomes of an automatic optimizer, the calibration strategies are assessed based on pollutant and CO2 emissions along representative driving cycles by vehicle simulations. The results highlight slightly lower CO2 emissions with the calibration approach that minimizes the E-compressor consumption, thus revealing the importance of considering the engine calibration phase, the powertrain management, the vehicle characteristics, and its mission. [ABSTRACT FROM AUTHOR]

Abstract Cooled exhaust gas recirculation (EGR) is a viable technique to mitigate the knock occurrence, to improve the fuel consumption and to reduce the nitrogen oxides (NOx) emissions of spark-ignition engines. This work aims at investigating the effects of a low-pressure cooled EGR system on the performance and exhaust emissions of a small-size turbocharged SI engine through numerical and experimental analyses. First, the experiments are carried out at a speed of 3000 rpm and different engine loads. The standard engine calibration is applied at the reference test conditions. Then, the EGR system is activated and the load is controlled by adjusting the plenum pressure and the spark timing. The experimental results are used to validate a 1D engine model, developed in GT-Power™ software. The latter is integrated with "user-defined" sub-models for an accurate description of the in-cylinder processes, namely turbulence, combustion and heat transfer. Maximum EGR benefits over fuel consumption are achieved at low load, thanks to the reduction of the pumping losses. At high load, minor fuel consumption improvements are obtained, mainly arising from a slight increased knock resistance. Furthermore, increasing EGR rate results in a sensible NOx reduction at each engine load, with a slight penalty on the unburned hydrocarbon emission. [ABSTRACT FROM AUTHOR]

Nowadays various technical solutions have been proposed in order to improve the performance of spark-ignition internal combustion engines both at part and full load operations, especially in terms of Brake Specific Fuel Consumption (BSFC). Among the most advanced technical solutions, a fully flexible valve control system (VVA – Variable Valve Actuation) appears a very robust and reliable approach to attain the above aim. In fact advanced valve strategies, such as Early Intake Valve Closure (EIVC) and Late Intake Valve Closure (LIVC), proved to be an effective way to decrease the fuel consumption: at part load through a reduction of the pumping work and, at high load, through a knock mitigation and an over-fueling reduction. In this paper, a comparative numerical study is realized to evaluate the influence of the intake valve strategy on the performance of a small-size turbocharged spark-ignition engine. The analyzed engine is equipped with a fully flexible VVA on the intake side, based on the “lost motion” principle and able to realize both EIVC and Full Lift strategies, while the virtual modification of the intake cam profile allows for the actuation of LIVC profiles. First, a 1D model of the tested engine is developed in GT-Power™ framework. It is integrated with in-house developed sub-models for the description of in-cylinder phenomena, including turbulence, combustion, knock and heat transfer. The adopted approach is validated against 3D turbulence results, measured global performance parameters and in-cylinder pressure cycles. The consistency of the proposed approach, without requiring any case-dependent tuning, is demonstrated at various speeds, loads and intake valve strategies. The validated engine model is used to perform a parametric analysis for different intake valve closure angles in two representative operating points at full and part load. The results point out that both EIVC and LIVC induce an improved fuel consumption with respect to a conventional Full Lift valve strategy. EIVC proves to be more effective at part load than LIVC, while similar BSFC advantages are obtained at high load. The proposed approach, based on refined sub-models for in-cylinder phenomena description, shows the capability to predict the effects of advanced valve strategies, making the implementation of a “virtual” calibration of a VVA engine possible. [ABSTRACT FROM AUTHOR]