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Research

  • Experimental observations of engine piston damage induced by knocking combustion

    Year: 2017

    Author: Lorella Ceschini, Andrea Morri, Eleonora Balducci, Nicolò Cavina, Nahuel Rojo, Lucio Calogero, Luca Poggio

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    Abnormal combustion leads to a significant increase in combustion speed, pressure and temperature at the surfaces enclosing the combustion chamber. Severe and lasting knock or pre-ignition can permanently damage and, in many cases, destroy engine pistons, due to very high and localised thermomechanical stresses. The deleterious effects of abnormal combustions have led car manufacturers to set extremely precautionary thresholds in spark advance calibration (in terms of temperatures and pressures) of turbocharged spark ignition direct injection engines, often limiting engine performance and efficiency. Since the mechanisms of piston damage due to abnormal combustion are not currently fully understood, the aim of this study was to characterise its effects on Al forged pistons. The more suitable characterisation techniques were evaluated. The results highlighted that roughness measurements, as well as visual, optical and scanning microscopy analyses on specific zones of the top land and piston crown are useful techniques to qualitatively relate piston damage to combustion regime. Moreover, a significant quantitative relationship was observed between the MAPO (Maximum Amplitude Pressure Oscillations) index and residual piston hardness.

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  • Application of Acoustic and Vibration-Based Knock Detection Techniques to a High Speed Engine

    Year: 2017

    Author: Nicolò Cavina, Andrea Businaro, Matteo De Cesare, Federico Monti, Alberto Cerofolini

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    Knock control systems based on engine block vibrations analysis are widely adopted in passenger car engines, but such approach shows its main limits at high engine speeds, since knock intensity measurement becomes less reliable due to the increased background mechanical noise. For small two wheelers engines, knock has not been historically considered a crucial issue, mainly due to small-sized combustion chambers and mixture enrichment. Due to more stringent emission regulations and in search of reduced CO2 emissions, an effective on-board knock controller acquires today greater importance also for motorcycle applications, since it could protect the engine when different fuel types are used, and it could significantly reduce fuel consumption (by avoiding lambda enrichment and/or allowing higher compression ratios to be adopted). These types of engines typically work at high rotational speeds and the reduced signal to noise ratio makes knock onset difficult to identify. The paper shows how knock-related information can be extracted both from accelerometer and acoustic signals, and how the correlation with in-cylinder pressure based indexes can be optimized using advanced signal processing algorithms and specific calibration methodologies, for a wide engine speed range. An optimization procedure that has involved all the calibration parameters that make up sound and vibration-based knock indexes, has allowed to successfully apply knock detection techniques up to 13,000 rpm. Experimental results obtained on the engine test bench are shown throughout the paper, demonstrating the feasibility of both approaches, which provide similar signal-to-noise ratio levels, and can therefore be considered as possible alternatives.

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  • Investigation on Pre-Ignition Combustion Events and Development of Diagnostic Solutions Based on Ion Current Signals

    Year: 2017

    Author: Nicolò Cavina, Nahuel Rojo, Luca Poggio, Lucio Calogero, Ruggero Cevolani

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    Pre-ignition combustions are extremely harmful and undesired, but the recent search for extremely efficient spark-ignition engines has implied a great increase of the in-cylinder pressure and temperature levels, forcing engine operation to conditions that may trigger this type of anomalous combustion much more frequently. For this reason, an accurate on-board diagnosis system is required to adopt protective measures, preventing engine damage.Ion current signal provides relevant information about the combustion process, and it results in a good compromise between cost, durability and information quality (signal to noise ratio levels). The GDI turbocharged engine used for this study was equipped with a production ion current sensing system, while in-cylinder pressure sensors were installed for research purposes, to better understand the pre-ignition phenomenon characteristics, and to support the development of an on-board diagnostic system solely based on ion current measurements.In this work, pre-ignition events induced by heavy knocking operation have been analysed. The focus was mainly on ion current signal real-time processing, and on the possibility to correctly and rapidly detect pre-ignition events. In a previous work, destructive effects of this kind of combustion on engine components had been described.As shown in the paper, the development and implementation of an ion current based detection algorithm results to be very effective in identifying pre-ignition combustions, and it could allow an extremely fast reaction of the engine controller that can prevent further anomalous combustions once the first event has occurred. Moreover, pre-ignition phase information extracted from the ion signal and characteristic combustion angles obtained from pressure signal analysis are well correlated, further confirming the ion signal robustness and accuracy.

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  • Knock Control Based on Engine Acoustic Emissions: Calibration and Implementation in an Engine Control Unit

    Year: 2017

    Author: Nicolò Cavina, Andrea Businaro, Matteo De Cesare, Luigi Paiano

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    In modern turbocharged downsized GDI engines the achievement of maximum thermal efficiency is precluded by the occurrence of knock. In-cylinder pressure sensors give the best performance in terms of abnormal combustion detection, but they are affected by long term reliability issues and still constitute a considerable part of the entire engine management system cost. To overcome these problems, knock control strategies based on engine block vibrations or ionization current signals have been developed and are widely used in production control units. Furthermore, previous works have shown that engine sound emissions can be real-time processed to provide the engine management system with control-related information such as turbocharger rotational speed and knock intensity, demonstrating the possibility of using a multi-function device to replace several sensors. In this paper, an innovative knock controller based on engine sound emissions is assessed by real-time implementation of the algorithm in a standard Engine Control Unit. The effectiveness of the technology has been proved by closing the spark advance control loop on a turbocharged GDI engine, and by comparing the controller performance with the traditional accelerometer-based system.

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  • Combustion Indexes for Innovative Combustion Control

    Year: 2017

    Author: Vittorio Ravaglioli, Fabrizio Ponti, Matteo De Cesare, Federico Stola, Filippo Carra, Enrico Corti

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    The continuous development of modern Internal Combustion Engine (ICE) management systems is mainly aimed at combustion control improvement. Nowadays, performing an efficient combustion control is crucial for drivability improvement, efficiency increase and pollutant emissions reduction. These aspects are even more crucial when innovative combustions (such as LTC or RCCI) are performed, due to the high instability and the high sensitivity with respect to the injection parameters that are associated to this kind of combustion. Aging of all the components involved in the mixture preparation and combustion processes is another aspect particularly challenging, since not all the calibrations developed in the setup phase of a combustion control system may still be valid during engine life.The most important quantities used for combustion control are engine load (Indicated Mean Effective Pressure or Torque delivered by the engine) and center of combustion (CA50), i.e. the angular position in which 50% of fuel burned within the engine cycle is reached. All these quantities can be directly evaluated starting from in-cylinder pressure measurement; however, the use of in-cylinder pressure sensors would significantly increase the cost of the whole engine management system. Due to these reasons, over the past years, many methodologies have been developed by the authors of this paper in order to evaluate combustion characteristics using low-cost sensors or sensors that are already present on-board. The approaches considered in this paper are based on engine speed fluctuations and engine block vibration. These measurements are performed through the magnetic pick-up facing the toothed wheel already present on-board and a low-cost accelerometer mounted on engine block. Each of these measurements allows estimating a combustion characteristic that can be used for combustion control, such as IMEP, pressure peak position, CA50. The paper presents how the combination of the information that can be extracted from the low or zero cost sensors employed enables the control of innovative combustions, as for example dual-fuel RCCI combustion.

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  • Real-Time Processing of Engine Acoustic Emission for Diesel Injectors Diagnostic and Recentering

    Year: 2017

    Author: Ponti, V. Ravaglioli and M. De Cesare

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    Diesel engine control strategies use complex injection patterns which are designed to meet the increasing request for engine-out emissions and fuel consumption reduction. As a result of the large number of tuneable injection parameters in modern injection systems (such as start and duration of each injection), injection patterns can be designed with many degrees of freedom. Each variation of the injection parameters modifies the whole combustion process and, consequently, engine-out emissions.

    Aging of the injection system usually affects injection location within the cycle as well as the amount of injected fuel (compared to the target value), especially for small pre-injections. Since Diesel combustion is very sensitive to injection pattern variations, aging of injectors strongly affects engine behavior, both in terms of efficiency and pollutant emissions production. Moreover, such variations greatly affect other quantities related to the effectiveness of the combustion process, such as noise radiated by the engine.
    This work analyses the effects of pre-injection variations on combustion, pollutant emissions and noise radiated by the engine. In particular, several experimental tests were run on a 1.3L Common Rail Diesel engine varying the amount of fuel injected in pre-injections. Torque delivered by the engine and center of combustion (MFB50) were kept constant using a specifically designed closed-loop combustion controller. During the tests, noise radiated by the engine was measured by properly processing the signal coming from a microphone faced to the engine block. The investigation of the correlation between the combustion process and engine noise can be used to set up a closed-loop algorithm for detecting and recentering injectors’ drifts over time.

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  • A Control-Oriented Knock Intensity Estimator

    Year: 2017

    Author: Enrico Corti, Claudio Forte, Gian Marco Bianchi, Lorenzo Zoffoli

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    The performance optimization of modern Spark Ignition engines is limited by knock occurrence: heavily downsized engines often are forced to work in the Knock-Limited Spark Advance (KLSA) range. Knock control systems monitor the combustion process, allowing to achieve a proper compromise between performance and reliability. Combustion monitoring is usually carried out by means of accelerometers or ion sensing systems, but recently the use of cylinder pressure sensors is also becoming frequent in motorsport applications. On the other hand, cylinder pressure signals are often available in the calibration stage, where SA feedback-control based on the pressure signal can be used to avoid damages to the engine during automatic calibration.
    A predictive real-time combustion model could help optimizing engine performance, without exceeding the allowed knock severity. Several knock models are available in the literature: most of those proposed for real-time applications are single zone or two-zone models, grounded on more complex CFD simulations. However, since the knock phenomenon is influenced by several factors, the real-time determination of KLSA requires the model to be adapted to the engine actual behavior. The approach proposed in the present paper, is based on the constant adaptation of a two-zone model to measured cylinder pressure data: typical results of the indicating analysis, available cycle-by-cycle and cylinder-by-cylinder, are used as inputs for the model, with the aim of predicting KLSA for the current running condition, without exceeding the maximum allowed knock intensity.
    The approach has been applied to indicating data referring to non-knocking, light and heavy knocking conditions, showing a good prediction capability.

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  • Investigation of Water Injection Effects on Combustion Characteristics of a GDI TC Engine

    Year: 2017

    Author: Nicolo Cavina, Nahuel Rojo, Andrea Businaro, Alessandro Brusa, Enrico Corti, Matteo De Cesare

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    This paper presents simulation and experimental results of the effects of intake water injection on the main combustion parameters of a turbo-charged, direct injection spark ignition engine. Water injection is more and more considered as a viable technology to further increase specific output power of modern spark ignition engines, enabling extreme downsizing concepts and the associated efficiency increase benefits. The paper initially presents the main results of a one-dimensional simulation analysis carried out to highlight the key parameters (injection position, water-to-fuel ratio and water temperature) and their effects on combustion (in-cylinder and exhaust temperature reduction and knock tendency suppression). The main results of such study have then been used to design and conduct preliminary experimental tests on a prototype direct-injection, turbocharged spark ignition engine, modified to incorporate a new multi-point water injection system in the intake runners. The experiments allowed to validate the model results, demonstrating the effectiveness of the proposed technology, and to further investigate on the mechanisms that allow controlling thermal load and knocking tendency by varying the water-to-fuel ratio.

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