Stefano Beccari

An analytical approach for the evaluation of the optimal combustion phase in spark ignition engines

It is well known that the spark advance is one of the most important parameters influencing the efficiency of a spark ignition engine. A change in this parameter causes a shift in the combustion phase, whose optimal position, with respect to the piston motion, implies the maximum brake mean effective pressure for given operative conditions. The best spark timing is usually estimated by means of experimental trials on the engine test bed or by means of thermodynamic simulations of the engine cycle. In this work, instead, the authors developed, under some simplifying hypothesis, an original theoretical formulation for the estimation of the optimal combustion phase. The most significant parameters involved with the combustion phase are taken into consideration; in particular, the influence of the combustion duration, of the heat release law, of the heat transfer to the combustion chamber walls, and of the mechanical friction losses is evaluated. The theoretical conclusion, experimentally proven by many authors, is that the central point of the combustion phase (known as the location of the 50% of mass fraction burnt, here called MFB50) must be delayed with respect to the top dead center as a consequence of both heat exchange between gas and chamber walls and friction losses.

PERFORMANCES IMPROVEMENT OF A S. I. CNG BI-FUEL ENGINE BY MEANS OF DOUBLE-FUEL INJECTION

Natural gas represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of Compressed Natural Gas (CNG) fuelled S.I. engine, above all in the bi-fuel version, i.e. capable to run either with gasoline or with natural gas. This characteristic, on the one hand, permits the vehicle to go even when natural gas is not available, on the other hand requires the engine to be designed to run safely with gasoline, i.e. with compression ratio lower than what natural gas would allow. Moreover the electronic control units are programmed to adopt rich mixture and poor spark advance when running with gasoline at medium-high loads, in order to prevent the engine from dangerous knocking phenomena: this causes an increase in fuel consumption and pollutant emissions. Starting from these considerations, the authors decided to investigate on the benefit attainable by means of a double-fuel injection, i.e. the injection of a certain amount of natural gas during the gasoline operation in order to increase the knocking resistance of the mixture and to run the engine with “overall stoichiometric” mixture even at full load, thus improving both engine efficiency and its environmental impact. To this purpose, the authors carried out an experimental campaign on the engine test bed, equipped with a fully instrumented series production bi-fuel spark ignition engine; the gasoline injection was managed by means of a real-time controlled ECU, while the simultaneous injection of natural gas was performed by means of IGBT transistors properly designed for fuel injection or spark timing control connected to a counter/timing PCI board. The results obtained fuelling the engine with both fuels in stoichiometric proportion with air show, with respect to the pure gasoline operation, considerable increase in fuel economy without remarkable power losses, while, with respect to the pure natural gas operation, only power improvements have been achieved: these advantages may lead the way to the adoption of the double-fuel injection in bi-fuel-engines. INTRODUCTION As is known gaseous fuels, such as Liquefied Petroleum Gas (LPG) and Natural Gas (NG), thank to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Moreover the use of natural gas, mainly constituted by methane, whose molecule has the highest hydrogen/carbon ratio, leads also to lower CO2 equivalent emissions. Some of the automobile producers already put on the market “bifuel” engines, which may be fed either with standard gasoline or with natural gas. These engines, endowed of two separate injection systems, are originally designed for gasoline operation, hence they do not fully exploit the good qualities of methane, such as its high knocking resistance [1], which would allow higher compression ratios. Moreover, when running with gasoline at mediumhigh loads, the engine is often operated with rich mixture and low spark advance in order to prevent from dangerous knocking phenomena: this produces both high hydrocarbon and carbon monoxide emissions (also due to the low catalyst efficiency caused by the rich mixture) and high fuel consumption. As example Figure 1 reports mixture strength (in terms of lambda values), HC and CO raw emissions acquired on the test bed running at full load a FIAT bi-fuel engine (whose characteristics are reported in Table 1) fuelled with gasoline: as can be seen the strong mixture enrichment operated by the series production ECU causes high HC and CO levels (for a comparison, the dashed lines represent the probable pollutant concentrations which would be measured with stoichiometric mixture). Figure 2 instead reports indicated and effective efficiencies (i.e. evaluated on the base of the IMEP and BMEP respectively) measured at WOT (Wide Open Throttle, i.e. full load) running the bi-fuel engine either with gasoline or with natural gas (best spark timing for both cases); due to the relatively low compression ratio, the engine may be fed with stoichiometric air-natural gas mixtures even at full load without knocking to occur, while, during gasoline operation, in order to prevent knocking, the mixtures adopted are those reported in Figure 1: this is the main cause of the strong difference in the measured efficiencies. SAE Technical Paper 2009-24-0058 Copyright

The Experimental Validation of a New Thermodynamic Method for TDC Determination

In-cylinder pressure analysis is becoming more and more important both for research and development purpose and for control and diagnosis of internal combustion engines; directly measured by means of a combustion chamber pressure transducers or evaluated by analysing instantaneous engine speed [1,2,3,4], incylinder pressure allows the evaluation of indicated mean effective pressure (IMEP), combustion heat release, combustion phase, friction pressure, etc...It is well known to internal combustion engine researchers that for a right evaluation of these quantities the exact determination of Top Dead Centre (TDC) is of vital importance: a 1° error on TDC determination can lead to evaluation errors of about 10% on the IMEP and 25% on the heat released by the combustion. In this paper the authors present the experimental validation of an original thermodynamic method for the correct evaluation of the “loss angle”, i.e. the angular phase shift between the TDC location and the pressure peak location. The validation has been carried out on a spark ignition engine comparing the results of the thermodynamic method, whose input is the in-cylinder pressure acquired in a “motored” cylinder (i.e. without combustion), with those obtained from a commercial available TDC sensor. The comparative tests aimed to characterize the precision of the proposed method.

Reliable TDC position determination: a comparison of different thermodynamic methods through experimental data and simulations

It is known to internal combustion researcher that the correct determination of the crank position when the piston is at Top Dead Centre (TDC) is very important, since an error of 1 crank angle degree (CAD) can cause up to a 10% evaluation error on indicated mean effective pressure (IMEP) and a 25% error on the heat released by the combustion: the TDC position should be then known within a precision of 0.1 CAD. This task can be accomplished by means of a dedicated capacitive sensor, which allows a measurement within the required 0.1 degrees precision. Such a sensor has a substantial cost and its use is not really fast; a different approach can be followed using a thermodynamic method, whose input is the pressure curve sampled during the compression and expansion strokes of a “motored” (i.e. without combustion) cylinder. In this work the authors compare an original thermodynamic method with other ones available in literature, by means of both experimental and simulated pressure curves. A zero dimensional thermodynamic model was employed to obtain an extensive collection of numeric pressure curves by changing engine geometry (e.g. compression ratios from 10 to 20 were adopted), operative conditions and wall heat transfer laws. The in-cylinder mass leakage has been taken into account in the model. Moreover, in order to assess the reliability and robustness of each method, the typical measurement errors and disturbances related to indicating analysis have been taken into account. The capability of the investigated methods to provide the correct TDC position in presence of the above mentioned errors has been evaluated. INTRODUCTION In-cylinder pressure analysis is of great importance in RD Method n. 2: Tazerout, Le Corre, Rousseau [2]; Method n. 3: Stas [3]; Method n. 4: Nilsson, Eriksson [4]. In the following section a brief description is given for each of the methods considered.

Calibration of a Knock Prediction Model for the Combustion of Gasoline-Natural Gas Mixtures

Gaseous fuels, such as Liquefied Petroleum Gas (LPG) and Natural Gas (NG), thank to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Moreover natural gas, which is mainly constituted by methane, whose molecule has the highest hydrogen/carbon ratio, leads also to lower ozone depleting emissions. The authors in a previous work (1) experienced the simultaneous combustion of gasoline and natural gas in a bi-fuel S.I. engine, exploiting so the high knock resistance of methane to run the engine with an ‘overall stoichiometric’ mixture (thus lowering fuel consumption and emissions) and better spark advance (which increases engine efficiency) even at full load: the results showed high improvements in engine efficiency without noticeable power losses with respect to the pure gasoline operation. With the aim to provide a knock prevision submodel to be used in engine thermodynamic simulations for a knock-safe performance optimization of engines fuelled by NG/gasoline mixtures, the authors recorded the in-cylinder pressure cycles under light knocking condition for different engine speed, loads and natural gas fraction (i.e. the ratio between the injected natural gas mass and the total fuel mass), and used the gas pressure data to calibrate a classical knock-prediction model: as shown, the results obtained allow to predict the onset of knocking in a S.I. engine fuelled with a gasoline-natural gas mixture with any proportion between the two fuels, with a maximum error of 5 CAD.© 2009 ASME

A refined model for knock onset prediction in spark ignition engines fueled with mixtures of gasoline and propane

In the last decade, gaseous fuels, such as liquefied petroleum gas (LPG) and natural gas (NG), widely spread in many countries, thanks to their prerogative of low cost and reduced environmental impact. Hence, bi-fuel engines, which allow to run either with gasoline or with gas (LPG or NG), became very popular. Moreover, as experimentally demonstrated by the authors in the previous works, these engines may also be fueled by a mixture of gasoline and gas, which, due to the high knock resistance of gas, allow to use stoichiometric mixtures also at full load, thus drastically improving engine efficiency and pollutant emissions with respect to pure gasoline operation without noticeable power loss. This third operation mode, called double fuel combustion, can be easily introduced in series production engine, since a simple electronic control unit (ECU) programing is required. The introduction into series production would require the availability of proper models for thermodynamic simulations, nowadays widely adopted to reduce research and development efforts and costs. To this purpose, the authors developed a quite original knock onset prediction model for knock-safe performances optimization of engines fueled by propane, gasoline, and their mixtures. The ignition delay model has been properly modified to account for the negative temperature coefficient (NTC) behavior exhibited by many hydrocarbon fuels such as gasoline and propane. The model parameters have been tuned by means of a considerable amount of light knocking in-cylinder pressure cycles acquired on a modified cooperative fuel research (CFR) engine, fueled by gasoline–propane mixtures. The adoption of many different compression ratios (CRs), inlet mixture temperatures, spark advances (SAs), and fuel mixture compositions allowed to use a very differentiated set of pressure and temperature curve, which gives the calibrated model a general validity for using different kinds of engines, i.e., naturally aspirated or supercharged. As a result, the model features a maximum knock onset prediction error around four crank angle degrees (CAD) and a mean absolute error always lower than 1 CAD, which is a negligible quantity from an engine control standpoint.

Calibration of a Knock Prediction Model for the Combustion of Gasoline-LPG Mixtures in Spark Ignition Engines

Gaseous fuels, such as liquefied petroleum gas (LPG) and natural gas (NG), thanks to their good mixing capabilities, allow complete and cleaner combustion than gasoline in spark ignition (SI) engines, resulting in lower pollutant emissions and particulate matter. In a previous work the authors showed that the simultaneous combustion of gasoline and LPG improves an SI engine efficiency with respect to pure gasoline operation with any significant power loss. The addition of LPG to the gasoline-air mixture produces an increase in knock resistance that allows running the engine at full load with overall stoichiometric mixture and better spark advance. In order to predict both performance and efficiency of engines fed by LPG-gasoline mixtures, a specific combustion model and in particular a knock prediction sub-model is required. Due to the lack of literature works about this matter, the authors investigated the knock resistance of LPG-gasoline mixtures. As a result, a reliable knock prediction sub-model has been obtained. The model can be easily implemented in thermodynamic simulations for a knock-safe engine performance optimization. The authors recorded light knocking in-cylinder pressure cycles on a cooperative fuel research (CFR) engine fueled by LPG-gasoline mixtures in different proportions. The tests were performed varying the compression ratio, the spark advance, and the inlet mixture temperature. The collected data have been used to calibrate and then compare two classical knock-prediction models. The models have been calibrated with a heterogeneous set of experimental data in order to predict knock occurrence in SI engines of different kinds. The results show that the models predict the knock onset position with a maximum error of around 6 crank angle degrees (CAD).

A comprehensive model for the auto-ignition prediction in SI engines fuelled with mixtures of gasoline and methane based fuel

The introduction of natural gas (NG) in the road transport market is proceeding through bifuel vehicles, which, endowed of a double-injection system, can run either with gasoline or with NG. A third possibility is the simultaneous combustion of NG and gasoline, called double-fuel (DF) combustion: the addition of methane to gasoline allows to run the engine with stoichiometric air even at full load, without knocking phenomena, increasing engine efficiency of about 26% and cutting pollutant emissions by 90%. The introduction of DF combustion into series production vehicles requires, however, proper engine calibration (i.e., determination of DF injection and spark timing maps), a process which is drastically shortened by the use of computer simulations (with a 0D two zone approach for in-cylinder processes). An original knock onset prediction model is here proposed to be employed in zero-dimensional simulations for knock-safe performances optimization of engines fueled by gasoline-NG mixtures or gasoline-methane mixtures. The model takes into account the negative temperature coefficient (NTC) behavior of fuels and has been calibrated using a considerable amount of knocking in-cylinder pressure cycles acquired on a Cooperative Fuel Research (CFR) engine widely varying compression ratio (CR), inlet temperature, spark advance (SA), and fuel mixture composition, thus giving the model a general validity for the simulation of naturally aspirated or supercharged engines. As a result, the auto-ignition onset is predicted with maximum and mean error of 4.5 and 1.4 crank angle degrees (CAD), respectively, which is a negligible quantity from an engine control standpoint. [DOI: 10.1115/1.4041675]

Supercharging the Double-Fueled Spark Ignition Engine: Performance and Efficiency

Internal combustion engine development focuses mainly on two aspects: fuel economy improvement and pollutant emissions reduction. As a consequence, light duty spark ignition (SI) engines have become smaller, supercharged, and equipped with direct injection and advanced valve train control systems. The use of alternative fuels, such as natural gas (NG) and liquefied petroleum gas (LPG), thanks to their lower cost and environmental impact, widely spread in the automotive market, above all in bifuel vehicles, whose spark ignited engines may run either with gasoline or with gaseous fuel. The authors in previous works experimentally tested the strong engine efficiency increment and pollutant emissions reduction attainable by the simultaneous combustion of gasoline and gaseous fuel (NG or LPG). The increased knock resistance, obtained by the addition of gaseous fuel to gasoline, allowed the engine to run with stoichiometric mixture and best spark timing even at full load. In the present work, the authors extended the research by testing the combustion of gasoline–NG mixtures, in different proportions, in supercharged conditions, with several boost pressure levels, in order to evaluate the benefits in terms of engine performance, efficiency, and pollutant emissions with respect to pure gasoline and pure NG operation. The results indicate that a fuel mixture with a NG mass percentage of 40% allows to maximize engine performance by adopting the highest boost pressure (1.6 bar), while the best efficiency would be obtained with moderate boosting (1.2 bar) and NG content between 40% and 60% in mass. [DOI: 10.1115/1.4036514]