• Energy Research

The modeling of the in-cylinder pressure oscillations under knocking conditions is tackled in this work. High frequency pressure oscillations are modeled by the explicit integration of a partial differential wave equation augmented with a time-dependent dissipation term. The general solution of such equation is determined by the Fourier method of separation of variables whereas the integration constants are obtained from the boundary and initial conditions. The integration space is a cylindrical acoustic cavity whose volume is that of the combustion chamber evaluated at the knock onset. The domain of integration is assumed to be formed by a finite set of small volumes having the shape of annulus sectors. This approach involves that knock region can assume more realistic shape of the kernels where abnormal combustion initiates. The initial conditions are evaluated by means of a two-zone thermodynamic model applied to low-pass filtered experimental pressure cycles. The damping coefficient and the knock region are model parameters to be assigned or identified experimentally by means of a proper least-squares optimization process. Experimental data obtained on a direct injection spark ignition engine, operating under knocking conditions at different speeds, have been used to validate the model both in time and frequency domains.

The present paper describes some experiments aiming to point out the link between oil consumption and reverse blowby. Some tests have been carried out on a motored single cylinder diesel engine. The reverse blowby gas mass flow has been evaluated by a thermodynamical model that utilizes both the measured combustion and second land pressures, and the blowby gas mass flow. Oil consumption has been measured in real time using a CO 2 -tracer method, whereas the blowby has been measured by a fast response orifice meter. The first ring lifting has also been recorded. It has been observed that, under certain engine operating conditions, both blowby and oil consumption assume quite constant levels. On the contrary, under other operating conditions, they vary in a cyclical way. However, in both cases, a relationship between blowby, reverse blowby and oil consumption can be recognized. The results obtained in the present research lead us to conclude that reverse blowby is one of the most significant causes of oil transport into the combustion chamber.

The effect of external EGR on knock was evaluated using a CFR engine. Combustion pressure was sampled on a time basis. A band pass filter in the time domain was applied to the pressure cycles. Five knock indices were calculated for each combustion cycle. The problem to quantify knock intensity was focused. At this extent, measurements were carried out on standard iso-octane-n-heptane blends in the test conditions used for the determ of the Motor Method Octane Number (MON). Knock intensity was varied acting on compression ratio. For each index, the conditions of absence of knock were determined using motored cycles. The indices were compared and one of them, showing the lowest C.O.V., was selected for further measurements. The effect of EGR on test fuels having different composition was evaluated varying the compression ratio, at fixed ignition timing. In this way, the same level of detonation, obtained in the absence of EGR, was realized with different amounts of external EGR. Percent variation of compression ratio was used to compare the ability of fuels, having different octane number, to tolerate compression ratio increase in presence of EGR. In particular, the tests were carried out on a matrix of twelve fuels with three levels of EGR. The results show that with all tested fuels the percent increase of compression ratio is strongly dependent on EGR.

In this work the authors present a model to simulate the in-cylinder pressure oscillations due to knock. Pressure oscillations are predicted by the explicit integration of a Partial Differential Wave Equation (PDWE) similar, in its structure, to the so-called “Equation of Telegraphy”. This equation differs mainly from the classical wave formulation for the presence of a loss term. The general solution of such equation is obtained by the Fourier method of variables separation. The integration space is a cylindrical acoustic cavity whose volume is evaluated at the knock onset. The integration constants are derived from the boundary and initial conditions. A novel approach is proposed to derive the initial condition for the derivative of the oscillating component of pressure. It descends, conceptually, from the integration of the linearized relation between the derivative of pressure versus time and the expansion velocity of burned gas. In practice, the required calculation parameters are evaluated by means of a two zone thermodynamic processing of single, low-pass filtered pressure cycles. The damping constant, the size and position of the knocking volume of unburned gases at knock onset are the model parameters to be assigned or identified. The model was validated using a set of experimental data obtained using a four cylinder Direct Injection SI engine at different operating conditions. The position of the unburned gases at knock onset was identified through a mean square optimization process based on hybrid genetic algorithms. Even if a simple cylindrical geometry was adopted to include the mass of unburned gases, simulations reproduced experimental measurements with fairly good accuracy.

The paper focuses on the modelling, control and analysis of a micro-combined heat and power plant based on a light-duty (1.6 liter, four-cylinder) spark-ignition internal combustion engine fuelled by methane and (green) hydrogen blends. The engine is modelled and simulated using advanced predictive one-dimensional commercial codes with the combustion process tailored to reflect the characteristics of methane–hydrogen mixtures. The electrical machine connected to the engine is described through established equivalent electrical circuit equations for a three-phase permanent magnet synchronous machine with sinusoidal flux. The recoverable thermal power from engine waste heat is estimated using a model that combines simulation results and literature data. A robust model-based control system, adaptable to variations in hydrogen content, is designed and numerically validated. The paper provides a comprehensive characterisation of the system under different steady-state and dynamic power loads, as well as varying hydrogen concentrations. A novel primary energy savings index is introduced to evaluate the cogeneration benefits when green hydrogen is mixed with fossil fuels for engine fuelling. The proposed micro-combined heat and power system can also function as a programmable power source, mitigating the non-programmability of renewable energy sources. The developed model enables the analysis of hybrid energy grids scenarios, promoting the sustainable use of mixed renewable and fossil fuels.

In the present work a novel predictive Wiebe-Based combustion Model (WBM) is proposed for simulation of the combustion process in a normally aspirated 1.6 L spark ignition (SI) engine. Unlike other approaches presented in literature, the novelty consists of: the considered set of Wiebe parameters, that is the angle at 50% of burned fuel, the combustion duration between 10% and 90% of burned fuel, and the form factor m; the nonlinear feature of the used correlations; the set of the involved engine variables, including particularly the laminar burning speed of the air/fuel mixture at combustion start. Based on a wide experimental database a Turbulent entrainment Combustion Model (TCM) is also set up, validated and embedded in a 1D simulation model of the engine. The parameters of the Wiebe function fitting the Mass Burned Fraction (MBF) development are estimated for each engine operating condition and then correlated to main engine variables. To assess to what extent the simpler WBM can be used in place of the TCM, simulations of the validated 1D engine model were carried out with both WBM and TCM and their performances compared in a wide range of engine operating conditions in terms of Brake Mean Effective Pressure (BMEP), Brake Specific Fuel Consumption (BSFC) and Carbon monoxide concentration (CO).

Several methods have been proposed to use pressure signal for air/fuel ratio estimation, knock detection and optimal spark timing selection. In this paper some of these methods were compared, and their accuracy and effectiveness was checked. In order to avoid the misleading effects of measurement errors, the comparison was performed using a database of test conditions obtained by means of the WAVE code (Ricardo). New correlations physically based were introduced to evaluate the trapped air mass and the Exhaust Gas Recycling (EGR), cylinder per cylinder. These correlations can give a very important contribution to balance the air-fuel ratio in each cylinder and to improve EGR control strategies.

A system for both ignition and ion current measurement was designed and set up at Istituto Motori. Particular attention was paid to the problem of dissipating the residual energy stored in the ignition coil, reducing the electromagnetic interferences and especially improving the response of the measurement system. In order to assess the capability of the ion current signal to give reliable and accurate information for knock detection, a number of tests were carried out at full load on a commercial PFI four-cylinder engine, at various air/fuel ratios and spark timings. Some knock indices based on the ionization signal, both band pass filtered and non-filtered, were introduced, in particular: the Amplitude of the Second Ionization Peak (ASIP), the Mean not-filtered Ionization Current signal (MIC), the Maximum Amplitude of Ionization Current signal Oscillation (MAICO), the Integral of Modulus of filtered Ionization Current signal Oscillation (IMICO). The thresholds of the MAPO (Maximum Amplitude of Pressure Oscillation) knock index, based on the band pass-filtered pressure signal, were used to classify no-knocking and light-knocking cycles. A criterion based on the Percentage of Knocking Cycles was proposed to evaluate the MAPO-equivalent thresholds of the considered indices to classify the knock type. In particular, thresholds for the MAICO and IMICO indices were found, allowing a fairly good classification of no-knocking and light-knocking cycles based on ion current measurement.