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Research

  • Statistical Analysis of Knock Intensity Probability Distribution and Development of 0-D Predictive Knock Model for a SI TC Engine

    Year: 2018

    Author: Nicolo Cavina, Alessandro Brusa, Nahuel Rojo, Enrico Corti

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    Knock is a non-deterministic phenomenon and its intensity is typically defined by a non-symmetrical distribution, under fixed operating conditions. A statistical approach is therefore the correct way to study knock features. Typically, intrinsically deterministic knock models need to artificially introduce Cycle-to-Cycle Variation (CCV) of relevant combustion parameters, or of cycle initial conditions, to generate different knock intensity values for a given operating condition. Their output is limited to the percentage of knocking cycles, once the user imposes an arbitrary knock intensity threshold to define the correlation between the number of knocking events and the Spark Advance (SA).
    In the first part of the paper, a statistical analysis of knock intensity is carried out: for different values of SA, the probability distributions of an experimental Knock Index (KI) are self-compared, and the characteristics of some percentiles are highlighted.
    The innovative contribution of this work is to correlate such KI probability curves with mean combustion parameters (like maximum in-cylinder pressure or combustion phase) through an analytical function. In this way, KI distributions can be predicted by a fully deterministic combustion model, ignoring CCV. In the final part of the paper such relations are implemented in a 1-D environment and tested using a combustion model, previously calibrated via Three Pressure Analysis (TPA) for knock-free operating conditions. Validation is carried out by comparing experimental and simulated KI distributions.

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  • Model-Based Test Bench Conditioning Systems Control

    Year: 2018

    Author: Enrico Corti, Michele Taccioli, Fabrizio Ponti

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    Engine test benches are crucial instruments to perform tests on internal combustion engines. Possible purposes of these tests are detecting engine performance, checking the reliability of engine components or making a proper calibration of engine control systems managing the actuations. Since many factors affect tests results in terms of performance, emissions and components durability, an engine test bench is equipped with several conditioning systems (oil, water and air temperature, air humidity, etc.), in order to maintain the controlled variables to the target values, throughout the test duration.
    The conditioning systems are often independently controlled by means of dedicated Programmable Logic Controllers (PLC), but a centralized model-based management approach could offer several advantages in terms of promptness and accuracy. This work presents the application of such control methodology to oil, water and HVAC (Heating, Ventilating and Air Conditioning) conditioning systems, where each actuator is managed coupling model-based open-loop controls to closed-loop actions. The main advantage of integrating the management of several actuators is that the control actions can be coordinated, similarly to what has been achieved in engine management systems with torque-based control: the risk of conflicts in the control actions on different actuators can be reduced, while the introduction in the control loops of other actuators is easier.
    The control methodology has been validated on an engine test bench where the automation system has been developed on an open software Real-Time compatible platform, allowing the integration of the conditioning system control with all other functionalities concerning the test management. The paper shows the plant layout, details the control strategy and finally analyzes experimental results obtained on the test bench, highlighting the benefits of the proposed management approach.

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  • 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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