Pedro Luis Curto-Risso

Theoretical and simulated models for an irreversible Otto cycle

We show in this work that a finite-time-thermodynamics model of an irreversible Otto cycle is suitable to reproduce performance results of a real spark ignition heat engine. In order to test our model we have developed a computer simulation including a two-zone combustion model and compared the evolution of the performance parameters of the simulated engine as functions of the rotational speed (ω) with those obtained from a simple theoretical scheme including chemical reactions. A theoretical Otto cycle with irreversibilities arising from friction, heat transfer through the cylinder walls, and internal losses properly reproduces simulation results by considering extreme temperatures and mass inside the cylinder as functions of ω. Furthermore we obtain realistic values for the parameters characterizing global irreversibilities, their evolution with ω, and a clearer understanding of their physical origin not always well established in theoretical models.

Optimizing the operation of a spark ignition engine: Simulation and theoretical tools

By performing quasidimensional computer simulations and finite-time thermodynamic analysis we study the effect of spark advance, fuel ratio, and cylinder internal wall temperature in spark ignition engines. We analyze the effect of these parameters on the power output and efficiency of the engine at any rotational speed, ω. Moreover, we propose the optimal dependence on ω of the spark advance angle and the fuel ratio with the objective to get maximum efficiency for any fixed power requirement. The importance of engine power-efficiency curves in order to perform this optimization procedure and also of the evaluation of macroscopic work losses in order to understand the physical basis of the optimization process is stressed. Taking as reference results from simulations with constant standard values of spark advance, fuel ratio, and cylinder internal wall temperature, the optimized parameters yield to substantial increases in engine performance parameters.

Fluctuations in the Energetic Properties of a Spark-Ignition Engine Model with Variability

We study the energetic functions obtained in a simulated spark-ignited engine that incorporates cyclic variability through a quasi-dimensional combustion model. Our analyses are focused on the effects of the fuel-air equivalence ratio of the mixture simultaneously over the cycle-to-cycle fluctuations of heat release (QR) and the performance outputs, such as the power (P) and the efficiency (QR). We explore the fluctuant behavior for QR, P and n related to random variations of the basic physical parameters in an entrainment or eddy-burning combustion model. P and n show triangle shaped first return maps, while QR exhibits a structured map, especially at intermediated fuel-air ratios. Structure disappears to a considerable extent in the case of heat release and close-to-stoichiometry fuel-air ratios. By analyzing the fractal dimension to explore the presence of correlations at different scales, we find that whereas QR displays short-range correlations for intermediate values of the fuel ratio, both P and n are characterized by a single scaling exponent, denoting irregular fluctuations. A novel noisy loop-shaped P vs. n plot for a large number of engine cycles is obtained. This plot, which evidences different levels of irreversibilities as the fuel ratio changes, becomes the observed loop P vs. n curve when fluctuations are disregarded, and thus, only the mean values for efficiency and power are considered.

Effects of Direct Fuel Injection Strategies on Cycle-by-Cycle Variability in a Gasoline Homogeneous Charge Compression Ignition Engine: Sample Entropy Analysis

In this study we summarize and analyze experimental observations of cyclic variability in homogeneous charge compression ignition (HCCI) combustion in a single-cylinder gasoline engine. The engine was configured with negative valve overlap (NVO) to trap residual gases from prior cycles and thus enable auto-ignition in successive cycles. Correlations were developed between different fuel injection strategies and cycle average combustion and work output profiles. Hypothesized physical mechanisms based on these correlations were then compared with trends in cycle-by-cycle predictability as revealed by sample entropy. The results of these comparisons help to clarify how fuel injection strategy can interact with prior cycle effects to affect combustion stability and so contribute to design control methods for HCCI engines.

Validating and Comparing with Experiments and Other Models

The objective of this chapter is to present and compare the performance parameters and the evolution of some process variables obtained by means of quasi-dimensional simulations with experimental results and with the corresponding estimations of other simulation or theoretical models. First, we summarize some of the basic performance parameters that are suitable for calculation. Second, some calculated parameters and functions obtained either from zero-dimensional or quasi-dimensional schemes are compared with engine test bench measurements. Finally, we analyze the evolution of a four-stroke Otto engine from a finite-time thermodynamics framework in order to elucidate to what extent a purely theoretical thermodynamic scheme is capable of reproducing realistic simulation results. It will be concluded that quasi-dimensional simulation models are capable to improve theoretical formulations in order to better approach theoretical predictions to experimental results.

Thermodynamic Engine Optimization

In this chapter, we show how the capabilities of numerical simulations and theoretical finite-time thermodynamics tools complement each other to optimize the performance of a spark ignition engine. Numerical simulations enable us to check the influence of specific parameters in engine operation, whereas theoretical techniques allow us to suggest possible optimization criteria and help to understand their consequences from a physical viewpoint in terms of the main sources of irreversibility. The analysis of power-efficiency curves plays a central role, because they provide a direct way to obtain maximum efficiency for any particular power requirement. In this optimization process, we consider both design and operation parameters such as the location of the ignition kernel, stroke-to-bore ratio, spark advance, fuel–air (equivalence) ratio, and cylinder wall temperature.

Physical Laws and Model Structure of Simulations

In this chapter, we present the Newton’s second law and the first principle of thermodynamics as a set or ordinary differential equations for the pressure and the temperature inside the combustion chamber. The solution of these equations with the appropriate initial conditions in each stroke allows to follow the evolution of the thermodynamic properties of the gas mixture performing the engine cycle. The solution of such equations requires the formulation of some additional submodels for the engine: heat transfer through cylinder walls, frictions, working fluid properties, combustion and chemical reactions, and several others. We present those basic models and how are they engaged in the structure of a computer simulation scheme. Special attention is paid to the use of alternative fuels. Detailed chemical reactions for the combustion of such fuels are presented in Appendix F. We include an up-to-date bibliographic survey on advanced submodels that could help the reader to build a particular advanced simulation scheme.

Cycle-to-Cycle Variability

In this chapter we shall analyze how to simulate, within a quasi-dimensional combustion model, the cyclic variability experimentally observed in spark ignition engines. Explicit qualitative comparison with experimental results will be shown. Moreover, we report a nonlinear dynamics analysis of cycle-by-cycle variations in heat release for the simulated engine with noisy components. Our approaches are based on nonlinear scaling properties of heat release fluctuations mainly, by means of correlation dimension, monofractal, and multifractal methods, and also by means of wavelet decomposition. We characterize the fluctuations for several fuel–air ratio values, \(\phi \), from lean mixtures to over stoichiometric situations by computing very long time series. Finally, we study the behavior of energetic functions when the presence of cyclic variability is considered. The fluctuating behavior of the net heat release, the power output, and the fuel conversion efficiency are simultaneously evaluated and analyzed.