Reciprocating Engine Combustion Diagnostics

The main goal of studying internal combustion (IC) engines is to improve fuel conversion efficiency in the face of increasingly severe energy issues and global warming. The increasing competition among automotive OEMs together with the worsening of environmental pollution has led to the development of complex engine systems. Innovative control strategies are required to simplify and improve the engine management system (EMS), moving toward energy saving and complying with the stringent emission legislation. In this scenario, researchers are focusing on improving conventional combustion engines as well as the development of alternative combustion modes with the utilization of conventional as well as alternative fuels. Present chapter provides a brief introduction of conventional engines (spark ignition and compression ignition) as well as advanced low-temperature combustion engines. To improve the comprehension of engine performance and its combustion reactions, development of the comprehensive performance measurement technique, in-cylinder visualization technique, and numerical simulations is essential and strongly demanded. This chapter describes the need of combustion diagnostics and provides a brief overview of combustion diagnostics using in-cylinder pressure measurement and analysis. Additionally, the chapter presents the summary of the cylinder pressure signal processing methods for calculating combustion parameters and information extracted about engine combustion, performance, and emissions that can be used for further development, optimization, and calibration of modern engines.

Combustion diagnosis based on in-cylinder pressure signal is widely used to study and optimize the combustion in reciprocating engines. The development of more affordable sensors along with the enhancement of computation features in current engine management systems (EMS) makes the in-cylinder pressure sensing a suitable methodology for the onboard combustion diagnosis and control of the engine. Cylinder pressure is one of the most valuable parameter for analysis of the combustion process. The experimental cylinder pressure measurement setup and its major elements along with their functions are discussed in this chapter. Two fundamental measurement variables for combustion measurement and evaluation are (1) cylinder pressure and (2) cylinder volume, which provides the measures of work and energy release in the engine cylinder for combustion diagnosis. Direct measurement of cylinder pressure is often done by installation of the pressure transducer, which is a challenging job due to harsh conditions in the combustion chamber (high temperature and pressure). All the aspect pressure transducer including its construction and material requirement, specification, mounting, and installation are presented in this chapter. Although piezoelectric pressure sensors are the most widely used technology, however with advances in technology and miniaturization of electronic components, alternative techniques and technology are available for use in transducers for the evaluation of the combustion phenomena via pressure sensing. The alternatives to piezoelectric pressure transducer are discussed including ion current sensor, strain gauges, and optical sensors. The crank angle encoder is the key component of the in-cylinder pressure measurement chain. Different types of crank angle encoders for combustion measurement, working principle, and output signal from encoders are discussed. The most important task of the crank angle encoder is to provide referenced crank degree marks to the data acquisition system, which samples the cylinder pressure signal. The sampling rate of data acquisition system is governed by resolution of crank angle encoder. The resolution requirement of crank angle encoder is described in this chapter for different engine operating conditions.

In addition to in-cylinder pressure sensor, the other sensors are typically required for analyzing the processes involved during engine combustion cycle. For engine combustion diagnostics, the measurement of dynamic signals from engine subsystems (whose activity directly relates to the combustion and in-cylinder processes) is required. These subsystems also need high-speed, crank angle-based measurements to analyze their operation and effectiveness because it directly influences the engine combustion process. The most commonly measured signals for combustion analysis are low-pressure signals from intake and exhaust manifold, injection-related signals (injector needle lift, dynamic fuel pressure), temperature, mass flow, ignition-related signals (ignition timing signal, ion current signal), valve motion events, and oxygen signal for air-fuel ratio control. The present chapter deals with these additional sensors required for the combustion analysis and describes their function, mounting, and signal generation. Intake and exhaust pressure sensors are particularly important for measuring the gas exchange or low-pressure phase of the engine combustion cycle. These measurements are used in conjunction with valve lift and high-pressure cylinder measurement to derive accurate heat release calculations. Needle lift and line pressures establish the fuel injection rate and the dynamic behavior of fuel in the high-pressure fuel system at different engine operating conditions. A common requirement is to measure and evaluate the behavior of gas flow into the engine cylinder for detailed combustion analysis. Additionally, these measurements can provide data for engine simulation models that calculate cylinder residual charge and trapping efficiencies. Ignition signals are measured to relate the ignition and combustion events in spark ignition engines. The timing of the ignition of the fuel-air mixture is critical to achieve the efficient engine operation. Air-fuel ratio control is required to maintain the stoichiometric air-fuel ratio in conventional spark ignition engines. Chemical sensors are used for oxygen detection and air-fuel ratio control. Thus, the chapter describes these additional sensors required for engine combustion diagnosis.

Combustion diagnostic research on reciprocating engines often requires the observation and recording of rapidly changing variables such as in-cylinder pressure, diesel fuel injection pressure, diesel injector needle position, ignition current, etc. The magnitude of these variables often changes significantly in milliseconds. Typically, data needs to be recorded on a crank angle basis in engine cycle-resolved measurements, which requires a high-speed data acquisition system. The present chapter describes the general principles of data acquisition system along with the functions of its elements. Generally, in any data acquisition system, the essential sequence of operation includes input signal generation by transducers, signal conditioning, multiplexing, analog-to-digital conversion (sampling of data), and data processing. All these operations involved in data acquisition are described with a specific focus on engine combustion measurement. Typical analog and digital signals involved in engine combustion measurement along with their signal conditioning are also presented. Data sampling needs to be carried out properly to circumvent the loss of signal that can lead to misinterpretation of actual phenomena during the combustion process. This chapter describes the issues involved in the sampling of data and sampling rate requirement for different engine combustion modes and operating conditions. Additionally, offline and online processing of measured combustion data is also briefly discussed.

In modern reciprocating engines, combustion diagnosis is essential for the estimation of the combustion quality and control of combustion phasing for superior performance. Post-processing of measured cylinder pressure signal is conducted for combustion diagnostics. Four-step signal processing (phasing with respect to crank angle, absolute pressure referencing (pegging), smoothing/filtering, and cycle averaging) is often used before further processing of cylinder pressure data for calculation of different combustion parameters. All the aspects of signal processing of the cylinder pressure signal (typically measured using a piezoelectric transducer) are discussed in this chapter. The measured pressure signal from the piezoelectric sensor needs to be accurately referenced in both magnitude (absolute pressure value) and crank angle position (time). Phasing of cylinder pressure signal with a crank angle position is performed with respect to TDC position. Different methods of crank angle phasing of the cylinder pressure signal with (capacitive or microwave probe) and without additional hardware are presented. Several pegging methods including the inlet and outlet manifold pressure-based referencing, two-point referencing, and least-square method based on a variable polytropic coefficient are also discussed. To carry out combustion analysis with good accuracy, it is essential to eliminate high-frequency noise from measured pressure signal because signal noise gets amplified when the signal derivative is used during post processing. Different methods for filtering the high-frequency noise for pressure signal are discussed for online as well as offline combustion analysis application. For an accurate heat release computation, typically mean values of input parameters (fuel mass flow, engine speed, air mass flow, etc.) are used, and hence, it is important to use averaged in-cylinder pressure data so that a representative thermodynamic cycle can be analyzed. Additionally, point-to-point variations (because of signal noise) diminishes by averaging of the signal, thus, improving the combustion analysis accuracy. The methods for determination of an optimal number of cycles for ensemble averaging of cylinder pressure data is also described in this chapter.

Cylinder pressure-based combustion analysis provides a clear understanding of the combustion process by which engine performance improvements can be realized in an expedient and quantitative manner. Cylinder pressure-based combustion diagnostics can help to identify the performance-limiting componentry and can provide direction for the redesign of that componentry. However, effective analysis requires exacting care to be taken during engine instrumentation, data acquisition, and interpretation of results by processing experimental data. This chapter presents the methods for analysis of cylinder pressure data and discusses the interpretation of results in different graphical representations of measured data. Engine torque is one of the important factors that characterize the performance and running status of the engine. Indicated torque estimation methods based on cylinder pressure measurement are presented. Typical methods of monitoring engine performance are based on the indicated mean effective pressure (IMEP), and a large amount of research has been focused on ways to estimate IMEP from more easily measurable signals. This chapter describes offline and online (real-time) calculation of IMEP from indicating system based on commercially available hardware and software. Engine performance maps are discussed to explain the torque, power, and fuel consumption characteristics of the reciprocating engines.

In-cylinder pressure is directly related to the combustion process in reciprocating engines. Thus, the cylinder pressure measurement and its analysis can provide a large amount of information that can be utilized for further development, optimization, and tuning of engines. Combustion parameters can be quantified using thermodynamic analysis of cylinder pressure data. Presently, online estimation of combustion parameters is required for closed-loop control of the combustion process in reciprocating engines. Methods for the burn rate and heat release rate analysis are discussed in this chapter for offline and real-time estimation. Various heat release rate estimation models and sources of error in the calculation of heat release are summarized. For accurate computation of heat release, tuning of measurement parameters is required. Different methods for estimation of various combustion parameters such as the start of combustion (SOC), end of combustion (EOC), combustion duration, etc. are discussed. The SOC is an important parameter to assess ignition delay in diffusion-controlled combustion concepts, flame kernel growth in spark-ignited concepts, or success of combustion initiation in homogeneous charge compression ignition (HCCI) concepts. Different techniques for detecting the SOC and EOC for online and offline applications are described. Thermal stratification of the unburned charge before ignition plays a significant role in governing the heat release rates, particularly in HCCI engine. Understanding the conditions affecting thermal stratification is essential for actively managing HCCI burn rates and expanding its operating range. Recently developed thermal stratification analysis applied to calculate the unburned temperature distribution before ignition is also discussed in this chapter.

Cyclic combustion stability is an important factor that impacts the overall engine performance. Large cycle-to-cycle variations undermine combustion stability, and it brings serious problems such as engine roughness, efficiency loss, higher emissions, compromised power, etc. Thus, investigation of combustion stability is necessary during engine development and calibration process to achieve desired stable operating condition through appropriate control strategy. The manifestation and origin of combustion variability are discussed in this chapter along with the characterization of combustion variability in terms of partial burn and misfire regimes. To analyze the engine combustion stability, traditional statistical measures of cyclic variability such as coefficient of variation (COV), frequency distribution, and histograms, lowest normalized value, autocorrelation, and cross-correlation are discussed. The conventional statistical analysis assumes that each measurement is an independent, random event. Typically, the coefficient of variation in mean effective pressure is used as measure of combustion stability. Chaotic time series analysis methods such as Poincare sectioning, mutual information, data symbolization, symbol sequence histograms, time irreversibility, modified Shannon entropy, and return maps are discussed for analyzing cycle-resolved combustion data series obtained from different engine operating conditions and combustion modes. Studies demonstrated that cyclic combustion variability has at least some nonlinear deterministic structure depending on engine operating conditions and combustion modes. Nonlinear dynamics methods such as recurrence plot and its quantification, multifractal analysis, 0–1 test, and wavelets are described for engine combustion stability analysis. It is demonstrated that the engine combustion process has different levels of complexity depending on the engine operating conditions as well as fuel.

Knock is an abnormal and stochastic combustion phenomenon which needs detailed analysis because it governs the power density of engine, fuel consumption (engine efficiency), and engine durability, as well as noise and emission characteristics. Typically, compression ratio of spark ignition (SI) engine is limited by knock characteristics or knock propensity. This chapter discusses the knock fundamentals including modes of knock, onset of knock, characteristic knock frequencies, and super-knock. The super-knock is an extremely intense knock phenomenon which limits the engine turbocharging and downsizing proposed for improving the fuel conversion efficiency in SI engine. Accurate and repeatable measurement of engine knock is an important aspect of knock analysis and control. In-cylinder pressure-based techniques are considered as the most reliable method for knock detection; however, installation of pressure transducers in the combustion chamber is both difficult and expensive. This chapter presents the detailed cylinder pressure-based knock detection and analysis methods. Cylinder pressure- and heat release-based knock intensity indices (in time and frequency domain) along with their signal processing methods are discussed. Different methods of knock characterization/detection including statistical methods, stochastic method, and wavelets are also discussed. To fulfill the requirement of a low-cost and nonintrusive alternative method, knock detection using ion current sensors, engine vibrations, and microphones is used. Reduction of combustion noise is required as the part of engine development process due to customer demands. The multiple degrees of freedom in engine control and calibration provides more scope to influence combustion noise, which is required to be measured first, to control effectively. This chapter presents the discussion on the combustion noise estimation from the in-cylinder pressure measurements. Different combustion noise metrics are discussed along with their calculation algorithms and signal processing techniques.

In-cylinder pressure-based heat release analysis is one of the most commonly used tools for engine combustion investigations. Heat release rate calculation based on the first law of thermodynamics relies mainly on two parameters: (1) cylinder pressure and (2) cylinder volume as a function of crankshaft position. The phasing of cylinder pressure and volume (by exact determination of TDC position) is of vital importance because it causes a large error in the calculation of IMEP and heat release. Total cylinder volume determination is directly affected by the value of compression ratio used for calculations. The TDC determination can be pursued by a dedicated TDC sensor (additional hardware) or by thermodynamic methods based on in-cylinder pressure measurement. The different methods for TDC position determination by thermodynamic analysis of measured in-cylinder pressure are presented in this chapter. Compression ratio depends on displacement volume and combustion chamber volume at TDC position. Displacement volume can be calculated within tight limits, but volume at TDC is difficult to determine because it consists of different crevices including the top-land region between piston, first ring and cylinder, head gasket, injectors and valve seats interstices, as well as the spark plug thread. This chapter presents different methods for compression ratio estimation based on in-cylinder pressure traces and evaluates for their accuracy and convergence speed. Blowby and gas flow through the cylinder-piston-ring crevices are phenomena that affect the in-cylinder pressure, the temperature, and the amount of charge during combustion, which affects the performance and exhaust emissions. The models for the estimation of blowby losses are also discussed. Variations in residual gas fraction, wall temperature, and air-fuel mass significantly contribute to cyclic combustion variability in addition to performance and emission characteristics. Thus, estimation of these parameters is necessary for detailed combustion analysis. This chapter presents the methods for determination of residual gas fraction, cylinder wall temperature, and air-fuel mass using in-cylinder pressure measurement.