Characteristics and Control of Low Temperature Combustion Engines

Energy is a fundamental prime mover for the economic growth of any country and essential for sustainability of modern economy as well as society. Long-term availability of environment-friendly, affordable and accessible energy sources is desirable for economic growth in the future. Presently, humanity is at crossroads and requires the radical and novel approach for the utilization of energy. The goal of this book is to present a novel approach for internal combustion (IC) engines, which are one of the most important machines for transforming the energy of hydrocarbon fuels into useful mechanical work. Two different viewpoints exist on energy production using IC engines. A number of people think in terms of mobility advantages, while others associate with emissions of harmful exhaust gases and large-scale consumption of limited fossil fuel reserves. Irrespective of one’s viewpoint, the number of vehicles running on IC engine will be increasing in the future due to the rapid economic growth. Furthermore, on-board power requirement on vehicle will increase due to the growing number of accessories and electronic devices. These factors lead to the increase in worldwide fuel consumption and gaseous emissions. Therefore, an IC engine with alternative combustion mode having superior characteristics than conventional engines needs to be developed. Ideally, such an alternative combustion mode should be operated on renewable fuels and have a better fuel conversion efficiency and no harmful emissions. Present book deals with the detailed analysis of performance, combustion and emissions characteristics of novel low temperature combustion (LTC) concept using conventional as well as alternative fuels. This chapter first discusses the motivation for engine research in general and LTC mode in particular. The LTC mode is an alternative to conventional, well-known and frequently used combustion concepts, i.e. compression ignition (CI) and spark ignition (SI) combustion modes. Brief description of conventional combustion mode is also provided in this chapter. An overview of alternative engine concepts and alternative fuels is also presented in this chapter for setting the stage for the discussion of various LTC engines.

Current stringent emission legislation, market and environmental concern governs the automotive research for developing high-efficiency and low emission engines. Vehicle manufacturers meet the present emission norms using a combination of in-cylinder emission reduction methods and exhaust after-treatment devices. Proposed future emission norms are even more stringent; and thus, newer technologies are required to satisfy emission standards worldwide. Low temperature combustion (LTC) engines have potential to deliver higher fuel conversion efficiency and simultaneous reduction of NO _x and soot emissions to an ultralow level. The LTC engines can also reduce the heavy dependence on NO _x and soot after-treatment devices for meeting the emission norms. In this chapter, LTC principles along with different proposed LTC strategies are discussed in detail. The LTC strategies are broadly categorized into homogeneous charge compression ignition (HCCI) and partially stratified charge compression ignition (SCCI). The SCCI strategy is further classified as thermal stratification- and fuel concentration stratification-based combustion process. The partially premixed compression ignition (PPCI) and reactivity-controlled compression ignition (RCCI) are the two main engine combustion modes with fuel concentration stratification are discussed in this chapter. Thermally stratified compression ignition mode is also described in the present chapter.

In low temperature combustion (LTC) engines, premixed fuel-air mixture is created in the cylinder, and combustion starts by auto-ignition due to compression of the fuel–air mixture during compression stroke. The LTC process involves various physical processes (atomization, evaporation and mixing) and complex chemical reactions occurring in the cylinder. Fuel properties and fuel composition play an important role in all the physical and chemical processes involved in LTC process. Autoignition depends on the evolution of cylinder pressure and temperature with time and autoignition chemistry of the fuel–air mixture. Autoignition chemistry depends on the fuel composition and fuel-air mixture quality (equivalence ratio). This chapter discusses the autoignition characteristics (autoignition chemistry, impact of fuel molecular structure on autoignition, fuel autoignition quality), fuel effects on autoignition and several fuel indices developed for LTC engines. Fuel design and fuel properties/quality required for LTC engines are also discussed in the present chapter.

Mixing of fuel and oxidiser (air) for combustion is termed as charge preparation. Creation of premixed charge is the key feature of low temperature combustion (LTC) engines. Quality of fuel–air mixture governs the combustion process and its rate. Premixed charge is required prior to the start of combustion in LTC engines. Depending on LTC strategy, different quality of premixed charge (degree of homogeneity) is required for higher thermal efficiency and control of the combustion rate. The premixed charge can be created by injecting fuel outside the engine cylinder (external charge preparation) or inside the engine cylinder (internal charge preparation), depending on fuel properties and combustion strategy. This chapter presents the summary of premixing techniques used for charge preparation in LTC engines. Charge preparation strategies for well-mixed charge (more often external charge preparation) and partially stratified charge (internal charge preparation using direct injection) are discussed in detailed for gasoline-like fuels as well as diesel-like fuels. Dual fuel charge preparation (using high and low reactivity fuels) is typically used in RCCI combustion is also discussed in the present chapter.

Low temperature combustion (LTC) engines need different enabling technologies depending on the fuel and strategy used to achieve combustion of the premixed fuel–air mixture. Controlling the combustion rate is one of the major challenges in LTC engines, particularly in HCCI combustion engine. To achieve higher thermal efficiency, the desired phasing of combustion timings is essential even at moderate combustion rates. Present chapter describes the combustion control variables and control strategies used for LTC engines. Various methods demonstrated to control the LTC engines can be categorized in to two main strategies: (i) altering pressure–temperature history and (ii) altering fuel reactivity of the charge. Temperature history of the charge in the cylinder can be altered by several parameters such as intake conditions (temperature and pressure), EGR, variable valve timing (VVT), variable compression ratio (VCR), water injection, supercharging and fuel injection strategies. Fuel reactivity of charge in the cylinder can be altered by various parameters such as equivalence ratio (Φ), fuel stratification, fuel additives, ozone additions and dual fuel. All these combustion control strategies are discussed for utilizing gasoline-like fuels in HCCI, PPC and RCCI combustion mode engines.

Combustion characteristics of an engine determine its performance and emission characteristics. This chapter presents the combustion characteristic of LTC engines using different gasoline-like fuels (ethanol and methanol) vis-à-vis conventional fuels. First, combustion kinetics of ethanol and methanol in premixed charge compression ignition engine is discussed using reaction pathway and sensitivity analysis. Combustion kinetics in LTC engine using hydrocarbon fuels is discussed in Chap. 2. Ignition and heat release characteristics of LTC combustion process are described by analysis of ignition delay, in-cylinder pressure, pressure rise rate, ringing intensity, heat release rate, start of combustion, combustion phasing, combustion duration and combustion efficiency. Effect of different engine operating conditions on combustion characteristics of ethanol and methanol vis-à-vis conventional fuels in HCCI, PPC and RCCI combustion is discussed in the present chapter. Combustion stability and cyclic variation analysis of combustion parameters using statistical and nonlinear dynamic methods are also discussed in the last section of this chapter.

Low temperature combustion (LTC) engines use emerging combustion strategies that offer the potential for higher fuel conversion efficiency along with ultralow NO_x and PM emissions. This chapter presents the performance characteristics of LTC engine employing gasoline-like fuels. The LTC operating range is typically constrained by several limiting factors such as combustion noise, combustion instability, maximum cylinder pressure, oxygen availability, excessive reactivity and emission limits. Present chapter describes the operating range (in terms of engine speed and load) achieved by different LTC strategies (HCCI, PPC and RCCI) within operating limitations. Performance parameters such as thermal efficiency, fuel consumption and exhaust gas temperature are discussed for utilization of different fuels in LTC engines. Experimental data showed that high octane fuels can be best utilized in compression ignition engines with higher fuel conversion efficiency (~ 57% indicated thermal efficiency). Maximum indicated mean effective pressure (IMEP) up to 20 bar with well-mixed HCCI combustion and IMEP up to 25 bar in PPC strategy can be achieved, while keeping the NO_x and soot level below Euro VI limits and ringing intensity (RI) in acceptable level. The dual-fuel RCCI combustion also showed more than 58% indicated thermal efficiency.

The main motivation for the study of low temperature combustion (LTC) mode is its potential of significant reduction in the nitrogen oxides (NO_x) and particulate matter (PM) emissions as compared to conventional engine combustion modes. This chapter presents regulated and unregulated emission characteristics of LTC engines using ethanol and methanol fuel vis-à-vis conventional fuels (gasoline and diesel). Formation of NO_x, unburned hydrocarbons, carbon monoxide (CO) and PM from LTC engine at various engine operating conditions are discussed in detail. This chapter also discusses the particle number and size distribution analysis of LTC engines using gasoline-like fuels. Total particle number emission of soot particles emitted from different LTC strategies (HCCI, PPC and RCCI) is presented. Detailed discussion on effect of various engine operating parameters on exhaust emission is presented in this chapter. Unregulated emissions (aldehydes, alkanes, alkenes, alkynes, etc.) from LTC engines are also presented and discussed in this chapter.

Low temperature combustion (LTC) is an engine combustion mode that yields ultralow NO _x and soot emission levels along with high fuel conversion efficiency. Typically, LTC engines use premixed fuel–air mixture, and combustion is mainly governed by chemical kinetics. The LTC strategies such as partially premixed combustion (PPC) and reactivity-controlled compression ignition (RCCI) have some level of direct control on combustion phasing due to direct injection of fuel in the engine cylinder. However, homogeneous charge compression ignition (HCCI) combustion strategy lacks the direct control on combustion phasing. In HCCI combustion, ignition timings are kinetically controlled and affected by pressure and temperature history of the charge in the engine cylinder (indirect control). Therefore, HCCI combustion requires the combustion feedback control for its very operation. The present chapter describes the closed-loop combustion control in LTC engines. First, the need of closed-loop combustion control and control variables are discussed. Then, combustion feedback sensors and combustion control actuators are described in detail. Typically, cylinder pressure sensor and ion current sensors are used for sensing of combustion phasing in HCCI combustion. The last section presents the combustion control methods and various controllers used in different LTC strategies such as HCCI, PPC and RCCI, for closed-loop combustion control.

Research towards the development of internal combustion engine having high fuel conversion efficiency and ultralow emissions is driven by stringent emission legislations, degradation of ambient environmental conditions, depletion of fossil resources, energy security and global warming. The low temperature combustion (LTC) engines are one of the potential options to fulfil the objective of high fuel conversion efficiency along with ultralow emissions of NOx and particulate matter. The LTC engines are radically different from conventional spark ignition and compression ignition engines. Research has been conducted on various LTC concepts using conventional and alternative fuels on both light-duty (LD) and heavy-duty (HD) engines. Performance, combustion and emissions characteristics along with different control strategies of LTC engines are discussed in previous chapters of the present book. Summary of main findings regarding performance, combustion and emissions characteristics of various LTC strategies is presented in this chapter, and recommendations for further work are also outlined.