Downsized Spark Ignition Engine Research Articles

Performance and energy balance analysis of the downsized SI engine with using a novel OPCVCR and synchronized twin spark plugs configurations

Recently, the exhaust emissions become one of the main issues in the downsizing spark ignition (SI) engine. It is necessary to develop clean combustion and high efficiency SI engines for further increasing the performance and brake thermal efficiency. In this study, a novel opposite piston type continuous variable compression ratio (OPCVCR) mechanism combined with synchronous twin spark plugs configurations was proposed and applied in the turbocharged downsizing SI engine. The purpose of this study was to propose a novel type of OPCVCR engine and analyze its combustion, performance and energy balance characteristics. The results showed that the peak combustion temperature (PCT) and peak combustion pressure (PCP) of the OPCVCR engine with twin spark plugs were increased when the compression ratio (CR) increased. The knocking combustion intensity of the downsized SI engine was increased when the CR increased, while the knocking combustion intensity can be alleviated effectively by introducing the external EGR. In addition, the energy utilization efficiency was increased with increasing the CR of the OPCVCR engine with twin spark plugs. Besides, the indicated specific fuel consumption (ISFC) of the base downsized SI engine could be decreased by 23.59% and the indicated thermal efficiency (ITE) of the base downsized SI engine could be increased 21.62% by adopting CR 16 and introducing 10% external exhaust gas recirculation (EGR), respectively.

Performance Estimation of a Downsized SI Engine Running with Hydrogen

Hydrogen is a carbon-free fuel that can be produced in many ways starting from different sources. Its use as a fuel in internal combustion engines could be a method of significantly reducing their environmental impact. In spark-ignition (SI) engines, lean hydrogen–air mixtures can be burnt. When a gaseous fuel like hydrogen is port-injected in an SI engine, working with lean mixtures, supercharging becomes very useful in order not to excessively penalize the engine performance. In this work, the performance of a turbocharged PFI spark-ignition engine fueled by hydrogen has been investigated by means of 1-D numerical simulations. The analysis focused on the engine behavior both at full and partial load considering low and medium engine speeds (1500 and 3000 rpm). Equivalence ratios higher than 0.35 have been considered in order to ensure acceptable cycle-to-cycle variations. The constraints that ensure the safety of engine components have also been respected. The results of the analysis provide a guideline able to set up the load control strategy of a SI hydrogen engine based on the variation of the air to fuel ratio, boost pressure, and throttle opening. Furthermore, performance and efficiency of the hydrogen engine have been compared to those of the base gasoline engine. At 1500 and 3000 rpm, except for very low loads, the hydrogen engine load can be regulated by properly combining the equivalence ratio and the boost pressure. At 3000 rpm, the gasoline engine maximum power is not reached but, for each engine load, lean burning allows the hydrogen engine achieving much higher efficiencies than those of the gasoline engine. At full load, the maximum power output decreases from 120 kW to about 97 kW, but the engine efficiency of the hydrogen engine is higher than that of the gasoline one for each full load operating point.

Numerical analysis of the effects of port water injection in a downsized SI engine at partial and full load operation

The water injection (WI) technique represents a viable way to improve the performance of spark ignition (SI) engines. The main benefit of using this technique is to take advantage of the high heat of vaporization of water. If injected into the intake port, its vaporization cools the air–fuel mixture entering the engine. This could lead to greater spark advances, with optimal combustion phasing, and to a reduction of the combustion temperature with consequent lower heat losses. Moreover, due to the vaporization cooling effect, the heat capacity ratio of the in-cylinder gases increases. These three main factors allow improving the thermal efficiency of the engine. In this paper, the numerical simulation has been utilized in order to assess the influence of water injection on the performance of a “downsized” turbocharged spark-ignition engine. Calculations have been carried out using a 1-D model validated with respect to the experimental data of the analyzed engine. A knock model has been implemented in the 1-D model in order to identify the knock-limited parameters in engine optimization. Threshold values for knock index and exhaust turbine inlet temperature have been imposed as optimization constraints. The model has been subsequently modified by adding water injection into the intake ports. A simple thermodynamic approach has been utilized in order to obtain a first estimation of effects of water evaporation on the temperature of the fresh charge entering the cylinder and on that of the unburned mixture at the end of the compression stroke. The authors analyzed the effects of water injection at various operating points. An optimization procedure of the engine parameters in case of WI operation is presented and its effects on combustion characteristics and engine performance are described in detail. The engine control parameters have been optimized in order to obtain minimum specific fuel consumption. The most remarkable results show that, at high load operation, it is possible to increase both spark advance and air-to-fuel ratio with a consequent improvement in terms of both brake mean effective pressure (BMEP) and brake specific fuel consumption (BSFC). In particular, at full load, the BSFC registered a 12% mean improvement in the engine speed range, while BMEP mean increase was about 9%. The obtained results show water injection technique could be a useful tool in performance improving of turbocharged spark-ignition engines.

Turbulent Flame Speed of a Gasoline surrogate in conditions representative of modern downsized Spark-Ignition engine

The downsized spark ignition (SI) engine is considered as one promising technology to reduce pollutants and greenhouse gas emissions. By boosting the intake air pressure, this operating mode achieves higher efficiency, thus reducing pollutant emissions. Modeling the combustion process in these drastic conditions (high pressure, high temperature and high dilution rate) remains challenging, however, since most of models of premixed turbulent combustion seem insufficiently robust because the turbulent combustion regime might be out of the classical flamelet regime. Two different setups, a spherical vessel and a one-shot engine, were used to study turbulence-flame interaction in conditions representative of a modern downsized SI engine, i.e. under high pressure, high temperature and high dilution and controlled turbulence. Turbulent flame speed were measured in a wide range of Damkohler and Karlovitz number. Several turbulent flame speed correlations from the literature are also tested and compared to the complete experimental dataset gathered on both setups.

Effects of flame propagation speed on knocking and knock-limited combustion in a downsized spark ignition engine

Engine knock remains the key factor that ultimately restricts the peak thermal efficiency in modern downsized spark ignition (SI) engines. The flame propagation speed can affect knock tendency by the residence time and thermodynamic state of end-gas, which are competing with each other. The investigations in this study focus on flame propagation speed impacts on engine knock characteristics and thermal efficiency under a high load/low speed operating condition using three-dimensional numerical simulations. The results show that knock intensity (KI) increases first and thereafter decreases with the increase of SI flame speed under knocking condition. Behind the nonmonotonic relationship, the control mechanisms of end-gas auto-ignition are different. The low to moderate speed SI combustions are linked to local hot spots. Higher KI is caused by the interaction between the pressure waves induced by these hot spots. Then with the acceleration of SI flame, a homogeneous autoignition event occurs along the reaction front, leading to more intense pressure fluctuations. At the high-speed SI combustion, local hot spots disappear gradually, and the end-gas autoignition is dominated by flame-induced sequential auto-ignition. The corresponding KI decreases due to the reduction of available fresh gases. A posteriori analysis is also performed applying theories proposed by Zeldovich and Bradley. It’s shown that an increase in the SI flame speed results in lower first and thereafter higher temperature gradient in end-gas, leading to a drastic change in the auto-ignition behaviors. In addition, the relative effects of knock suppression and combustion duration reduction on thermal efficiency improvement are clarified.

End-Gas Autoignition Mechanism in a Downsized Spark-Ignition Engine: Effect of Inhomogeneity

ABSTRACT It is generally accepted that knock in spark-ignition engines is caused by end-gas autoignition. However, limited studies regarding to the effect of the inhomogeneity of temperature and composition on end-gas autoignition are presented. The major objective of this paper is to quantitatively understand the flame propagation velocity and the end-gas autoignition mechanism affected by the reactivity gradient formed by thermal stratification and composition stratification in the combustion chamber of a downsized SI engine. The conclusions were obtained based on a knock combustion case under an initial condition of high temperature and high pressure. In the present work, fuel stratification has a significant influence on the propagation velocity of the main flame. The flame propagation velocity declined with an increase in the level of fuel stratification. However, the temperature stratification had little effect on the flame propagation velocity. It was found that the onset of autoignition advanced with increasing the level of temperature stratification, which demonstrated an approximately linear relationship. It can be found that a high level of fuel stratification induced deflagration because of multi-spot spontaneous combustion. On the contrary, a very intense knock with detonation wave was observed under a slight level of temperature stratification. Under this condition, the temperature gradient near the nascent AI kernel was small and contributed to sequential spontaneous combustion. In fact, both the fuel stratification and temperature stratification are essentially the reactivity stratification. Therefore, similar results also can be conducted from fuel stratification. This work will give a new insight into suppressing knock mechanisms by different strategies.

Methodological analysis of variable geometry turbine technology impact on the performance of highly downsized spark-ignition engines

New generation of spark ignition (SI) engines are expected to represent most of the future market share in a context of powertrain hybridization. Nevertheless, the current technology has still critical challenges in front to meet incoming CO2 and pollutant emissions standards, so new technologies are emerging to improve engine efficiency. In parallel to combustion concepts, a key required trend is downsizing based on high engine boosting. New turbocharger technologies, such as variable geometry turbines (VGT), become suitable for its application under the demanding operating conditions of SI engines. In this work, a methodology for the analysis of the VGT usage in comparison with traditional waste-gate (WG) turbine is presented. From experimental data obtained in engine test cell, a theoretical analysis aimed at ensuring full control on turbine boundary conditions, such as combustion variability, compressor map or engine calibration, was conducted. Taking advantage of highly validated and physically representative 1-D gas-dynamics and turbocharger models, the engine performance is discussed as a function of the turbine technology at full and partial load in a wide range of engine speed at the same time as the altitude impact is addressed. In all, it was found that VGT technology shows less limitations in extreme working conditions, such as low- and high-end torque regions, where the WG technology represents a limitation in terms of the maximum power output. Full load differences become more even more evident in altitude working conditions. When it comes to partial loads, differences in fuel consumption are minor, but potentially beneficial for VGTs.

A methodology for the assessment of the knock mitigation potential of a port water injection system

The introduction of innovative spark ignition engines featuring extreme downsizing, high compression ratio and Miller Cycle has led to an increasing interest in water injection as one of the most promising countermeasures to reduce the knock risk. In this paper, the potential of a Port Water Injection (PWI) system integrated in a state-of-the-art turbocharged downsized spark ignition engine was investigated through both numerical simulations and experiments. A 3D-CFD water spray model, properly calibrated against experimental data, was used to predict water spray evaporation and to evaluate the engine oil dilution risk. On the basis of the 3D CFD analysis, two new indexes were specifically defined and correlated with experimental data to evaluate the effectiveness of water injection in terms of knock mitigation and the risk of engine oil dilution.After assessing their reliability by means of a sensitivity analysis on both calibration parameters and hardware configurations, the proposed indexes were successfully exploited to perform a preliminary design and optimization of the water injection system.

Effect of relative humidity on water injection technique in downsized spark ignition engines

Combustion knock is a major barrier to achieving high thermal efficiency in spark ignition engines. Water injection was recently identified as a potential way of overcoming this barrier. To evaluate its general applicability, experiments were performed on a downsized three-cylinder spark ignition engine, varying the humidity of the intake air, the water injection timing, and the engine speed. The minimum quantity of injected water required to maintain a given load (and thus level of engine performance) was determined under each set of tested conditions. The knock-suppressing effects of water injection were found to be related to changes in the fuel–air mixture’s specific heat ratio (kappa) rather than evaporative cooling, and to therefore depend on the total quantity of water in the cylinder rather than the relative humidity per se. The total quantity of water in the cylinder was also shown to be a key determinant of advancement in combustion phasing and particulate emissions under various conditions.

Effects of reactivity inhomogeneities on knock combustion in a downsized spark-ignition engine

The reactivity inhomogeneities are inevitably generated in the cylinder with the implementation of knock suppression strategies in downsized engines with direct-injection spark ignition. However, the mechanism of its influence on combustion has not yet been fully understood. A fundamental study was conducted by large eddy simulation to comprehensively study the effects of reactivity inhomogeneity formed by an independent variable of fuel stratification or temperature stratification on the in-depth mechanism of end-gas autoignition in a downsized spark ignition engine. The turbulent flame propagation is determined by an improved G-equation turbulent combustion model, and the detailed chemistry mechanism of a primary reference fuel is employed to observe the detailed reaction process in the end-gas autoignition process. Knock suppression results are observed in the temperature stratification case. In a large temperature gradient case, the premature autoignition timing caused by higher local temperature provides sufficient fresh gas for spontaneous combustion, leading to a better power performance compared to slight temperature gradient cases. Results of fuel stratification show that the knock is suppressed and gradually disappears with increasing ΔΦ. However, the subsequent reduction of peak pressure and the lengthening of combustion duration results in a terrible drop in power performance. The impact of flame propagation velocity dominates the pressure evolution compared to the effect of a highly local rich mixture reducing the ignition delay time under present conditions. This work will give an underlying understanding of the knock mechanism through different strategies.

Potential of a two-stage variable compression ratio downsized spark ignition engine for passenger cars under different driving conditions

With the aim of reducing pollutant emissions from internal combustion engines (ICE), the application of stoichiometrically operated spark ignition (SI) engines, for light-duty vehicles, has been overcoming the compression ignition (CI) engines market share throughout the past years. The ability of a substantial reduction of the primary harmful emissions (HC, CO, and NOx) through the use of the simple three-way catalyst (TWC) is the main reason for that. Nonetheless, with increasing attention to CO2 emissions, the development of highly efficient downsized SI engines turn to be of enormous interest. The synergies of multiple systems such as direct injection, turbocharger, and variable valve actuation are able to lead the SI efficiencies closer to those of CI engines. However, to enable high load operation on such downsized engines, the compression ratio (CR) must be reduced due to knock limitations, reducing the partial-load operations efficiency. The implementation of two-stage variable compression ratio (VCR) systems enables the extraction of high thermal efficiency with high CR at lower loads and extended knock-free high load operation with low CR. In this study, the evaluation of a two-stage VCR system applied to a state-of-the-art downsized SI engine was made through standard driving cycle simulations. The VCR mechanism is composed of an eccentric element in the small end of the connecting rod, which is rotated to increase/decrease the effective connecting rod length, achieving the CRs of 12.11:1 and 9.56:1. The engine was run in an eddy-current dynamometer test bench throughout the essential operating range to obtain the brake specific fuel consumption (BSFC) map. The VCR mechanism CR switching delay was also experimentally characterized to derive a function of the operating conditions. The measured map was entered into the map-based driving cycle simulation with a sub-model to account for the isolated effects of the transient period encompassing the compression ratio switching. The results show that slow CR transitions lead to fuel consumption penalties, which suggests the need for optimizing the control strategies of the VCR system. Even though this penalty, once the gear up-shift speed is optimized for each driving cycle, the VCR system still enables fuel consumption reductions up to 3% on the WLTC driving cycle, up to 4% on the proposed urban driving cycles and up to 3% on highway driving cycles with respect to the fixed CR.

Numerical analysis of knocking characteristics and heat release under different turbulence intensities in a gasoline engine

In this work, different knocking characteristics and heat release have been numerically studied in a downsized spark-ignition engine, with addressing turbulence intensity and spark-ignition timing. The results show that knock intensity mainly depends on the heat release of hot-spots, and the heat release is not only associated with unburned mass fraction but also closely related to subsequent autoignition development processes, e.g. end-gas autoignition mode. Under low turbulence intensity, local mixture autoignition is induced by a single hot-spot, accompanied by an autoignition reaction wave propagation; whereas under high turbulence intensity, a nearly homogeneous autoignition event characterizing quasi constant-volume combustion is observed. Meanwhile, compared with the single hot-spot autoignition, the homogeneous one can lead to more intensive local overpressure and thus stronger knock intensity. Further analysis on the underlying reasons shows that the higher heat release rate under homogeneous autoignition conditions is the main reason for the stronger knock intensity. Besides, the dependency of knock intensity and heat release on unburned mass fraction is found effective only for the same autoignition scenarios. Current study can provide insights into knocking suppression with consideration of intake system optimization in modern downsized gasoline engines.

Experimental and numerical study on the influence of cooled EGR on knock tendency, performance and emissions of a downsized spark-ignition engine

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.

The LES and LEM Study of End-Gas Auto-Ignition Mechanism in a Downsized Spark Ignition Engine: Effect of Turbulence

ABSTRACT The effect of turbulence on the end-gas auto-ignition process of different primary reference fuels (PRFs) under engine-relevant conditions is numerically investigated through 3-D LES (large eddy simulation) and 1-D LEM (linear eddy model) simulations. Spontaneous ignition in the end-gas and pressure oscillations under the knock condition of a downsized spark ignition engine are qualitatively captured through the LES simulation. In order to take a further insight into the effect of full-scale turbulence-chemistry interaction on the auto-ignition process of the end-gas, 1-D LEM simulations are performed in a 1-D domain composed of a homogeneous core region and a linearly thermal distributed boundary layer. The effects of turbulent fluctuating velocity, fuel composition, turbulence length scale and initial temperature on the ignition process are investigated in the present study. It is found that the turbulence has a non-monotonic influence in the auto-ignition (AI) formations and expansion process of the PRF/air mixtures. Increased turbulence intensity delays the formation of the initial AI and shrinks the kernel size at the onset of AI due to the enhanced heat and radicals dissipation. However, after the first AI kernel forms, the intensified turbulence accelerates the overall combustion as a result of expanded preheating zone and more homogeneous unburned mixtures caused by turbulent stirring. In addition, the dissipation and homogenization effects of turbulence on the high-octane fuel are more prominent than on the low-octane fuel. The analysis of cases with different turbulent integral length scales reveals that the dissipation effect of the turbulence with the similar length scale of the nascent AI kernel is more prominent while the small-scale turbulence is more apt to shift the combustion mode after the first auto-ignition flame forms. At a high initial temperature of 900 K falling in the NTC regime, the first AI kernel of PRF10 appears in the boundary layer with a lower temperature instead of the core region. As the turbulence intensifies, the effect of NTC on the end-gas auto-ignition vanishes.

Large-eddy simulation analysis of knock in a direct injection spark ignition engine

Downsized spark ignition engines running under high loads have become more and more attractive for car manufacturers because of their increased thermal efficiency and lower CO2 emissions. However, the occurrence of abnormal combustions promoted by the thermodynamic conditions encountered in such engines limits their practical operating range, especially in high efficiency and low fuel consumption regions. One of the main abnormal combustion is knock, which corresponds to an auto-ignition of end gases during the flame propagation initiated by the spark plug. Knock generates pressure waves which can have long-term damages on the engine, that is why the aim for car manufacturers is to better understand and predict knock appearance. However, an experimental study of such recurrent but non-cyclic phenomena is very complex, and these difficulties motivate the use of computational fluid dynamics for better understanding them. In the present article, large-eddy simulation (LES) is used as it is able to represent the instantaneous engine behavior and thus to quantitatively capture cyclic variability and knock. The proposed study focuses on the large-eddy simulation analysis of knock for a direct injection spark ignition engine. A spark timing sweep available in the experimental database is simulated, and 15 LES cycles were performed for each spark timing. Wall temperatures, which are a first-order parameter for knock prediction, are obtained using a conjugate heat transfer study. Present work points out that LES is able to describe the in-cylinder pressure envelope whatever the spark timing, even if the sample of LES cycles is limited compared to the 500 cycles recorded in the engine test bench. The influence of direct injection and equivalence ratio stratifications on combustion is also (MAPO) analyzed. Finally, focusing on knock, a Maximum Amplitude Pressure Oscillation analysis (MAPO) is conducted for both experimental and numerical pressure traces pointing out that LES well reproduces experimental knock tendencies.

Potential for onboard hydrogen production in an direct injection ethanol fueled spark ignition engine with EGR

Exhaust gas recirculation, EGR, can be used to produce hydrogen gas (H2) via on board catalytic steam reforming, aiming at controlling emissions and mitigating loss of performance in downsized spark ignition engines. In the most straightforward design, the H2-rich syngas produced by a reactor placed in the EGR line is mixed with the intake air. The EGR and reactor operate in a closed loop, starting in the exhaust and delivering syngas into the intake pipe. Here, a thermodynamic model is developed to evaluate the potential for onboard H2 production in an direct injection ethanol fueled spark ignition engine with EGR. Chemical composition is predicted by assuming equilibrium at engine exhaust and at reactor outlet. Total engine mass flow rate, exhaust temperatures and pressures were adjusted to experimental data. The results indicated a potential to produce a molar concentration of 1% of H2 in the intake mixture, for an engine speed of 5000 rpm. Decreasing engine speed causes a decrease in H2 due to the lower engine exhaust temperature. Hot EGR presents a higher potential for heat recovery, but decreases substantially the heating value of the fuel by recirculation inert gases.

Effects of applying a Miller cycle with split injection on engine performance and knock resistance in a downsized gasoline engine

Downsizing a spark ignition (SI) engine with boost pressure is a proven way of increasing the thermal efficiency and reducing CO2 emissions. However, the occurrence of abnormal combustion problems such as knock and super knock could seriously damage the downsized SI engine and limit its efficiency improvement. In this study, the effects of the Miller cycle on anti-knock and engine power are experimentally investigated using a single-cylinder gasoline engine, the speed corresponded to 1600 rpm and air–fuel ratio corresponded to 1. The Miller cycle provides a good anti-knock performance because of its low in-cylinder temperature induced by the lower effective compression ratio, however, it has a poor dynamic output. To increase the intake charge of the Miller cycle, boost pressure was applied. The results showed a clear improvement in both the thermal efficiency and torque of the engine. Furthermore, the knock tendency of the engine was reduced. However, compared with the Miller cycle without boost pressure, the knock tendency with boost pressure was significantlyhigher. Therefore, to improve both engine power and knock resistance, this study combined split injection to the Miller cycle with boost pressure. The result indicates that the combined method can effectively decrease the knock tendency and increase the engine torque. The best secondary start of injection timing for the Miller cycle with boost pressure and split injection is 100 CAD BTDC. In conclusion, the Miller cycle with intake boost pressure and split injection has a considerable potential for achieving both a better knock resistance and higher engine power.

Splash blended ethanol in a spark ignition engine – Effect of RON, octane sensitivity and charge cooling

Downsized spark ignition engines have the benefit of high thermal efficiency; however, severe engine knock is a challenge. Ethanol, a renewable gasoline alternative, has a much higher octane rating and heat of vaporization than conventional gasoline, therefore, ethanol fuels are one of the options to prevent knock in downsized engines. However, the performance of ethanol blends in modern downsized engines, and the contributions of the research octane number (RON), octane sensitivity (defined as RON-MON) and charge cooling to suppressing engine knock are not fully understood. In this study, eight fuels were designed and tested, including four splash blended ethanol fuels (10vol.%, 20vol.%, 30vol.% and 85vol.% ethanol, referred to as E10, E20, E30 and E85), one match blended fuel (E0-MB) with no ethanol content but the same octane rating as E30, and three fuels (F1-F3) with different combinations of RON and octane sensitivity. The experiments were conducted in a single-cylinder direct-injection spark ignition (DISI) research engine. Load and spark timing sweep tests at 1800rpm were carried out for E10-E85 to assess the combustion performance of these ethanol blends. In order to investigate the impact of charge cooling on combustion characteristics, the results of the load sweep for E0-MB were compared to those of E30. Load sweep tests were also carried out for F1-F3 to understand the impacts of RON and octane sensitivity on suppressing engine knock. The results showed that at the knock-limited engine loads, splash blended ethanol fuels with a higher ethanol percentage enabled higher engine thermal efficiency through allowing more advanced combustion phasing and less fuel enrichment for limiting the exhaust gas temperature under the upper limit of 850°C, which was due to the synergic effects of higher RON and octane sensitivity, as well as better charge cooling. In comparison with octane sensitivity, RON was a more significant factor in improving engine thermal efficiency. Charge cooling reduced engine knock tendency through lowering the unburned gas temperature.

Cycle-resolved visualization of pre-ignition and abnormal combustion phenomena in a GDI engine

Downsized spark ignition engines are highly efficient but promote abnormal combustion phenomena like pre-ignition and knocking. This limits the maximum load of SI engines and consequently the scale of downsizing. In this paper, cycle resolved visualization and thermodynamic analysis were used to investigate abnormal combustion phenomena such as pre-ignition at localized surfaces and due to moving hot-particles, as well as post-spark discharge secondary flames near the walls and knock. Tests were performed on an optically accessible single cylinder ‘wall guided’ GDI engine equipped with self-lubricant rings in order to avoid oil residuals in the combustion chamber and to exclude them as cause of pre-ignitions. Cycle resolved visualization allowed obtaining information on hot-particles trajectory and provided insight into the morphology and speed of flame fronts (both spark-ignited and secondary ones). Optical data were correlated to in-cylinder pressure and rate of heat release curves. Secondary flames were detected both in normal and knocking cycles. Under heavy knocking conditions, when secondary flames were identified, their occurrence was correlated to the onset of knock; more advanced start of pressure oscillations corresponded to prior evidence of non-spark ignited sites near the walls. A distribution map of secondary flames showed higher probability of occurrence near the cylinder walls and towards the injector location, suggesting that fuel deposits play a major role in the initiation of abnormal combustion phenomena.

Analysis of an Increase in the Efficiency of a Spark Ignition Engine Through the Application of an Automotive Thermoelectric Generator

We have analyzed the increase of the overall efficiency of a spark ignition engine through energy recovery following the application of an automotive thermoelectric generator (ATEG) of our own design. The design of the generator was developed following emission investigations during vehicle driving under city traffic conditions. The measurement points were defined by actual operation conditions (engine speed and load), subsequently reproduced on an engine dynamometer. Both the vehicle used in the on-road tests and the engine dynamometer were fit with the same, downsized spark ignition engine (with high effective power-to-displacement ratio). The thermodynamic parameters of the exhaust gases (temperature and exhaust gas mass flow) were measured on the engine testbed, along with the fuel consumption and electric current generated by the thermoelectric modules. On this basis, the power of the ATEG and its impact on overall engine efficiency were determined.