Desmond E. Winterbone

Chemical species tomography by near infra-red absorption

The spatial distribution of chemical species can be a critical determinant of the performance of chemical reactors. One such reactor is the combustion chamber of the Internal Combustion engine. This paper presents a design for the measurement of hydrocarbon concentration distribution within a running engine using near infra-red absorption tomography. The fundamentals of the technique, and design parameters for the equipment are discussed. By utilising micro-optic components, a minimally invasive system is feasible and by utilising advanced laser/photodetector combinations, good temporal performance is anticipated.

Near infra-red chemical species tomography of sprays of volatile hydrocarbons

We report an All-Opto-Electronic tomography system that is sensitive to hydrocarbon vapour distribution, or liquid spray distribution, with temporal resolution of over 3000 frames per second. A tomography system comprising 32 channels has been built and tested. For chemical sensitivity to saturated hydrocarbons, we exploit the principle of Near Infra-Red (NIR) absorption at 1700 nm relative to a reference wavelength, using laser diode sources whose technology is based on that of the communications industry. Images are obtained from a laboratory set-up incorporating both gaseous injection and a liquid Gasoline Direct Injection (GDI) system. The performance of a prototype system on a running GDI engine is reported. The difficulty in performing concentration measurements of the gaseous fuel within the liquid spray region is shown, and means to improve this performance are discussed. However, it has been found possible to image the liquid spray cone using attenuation of the reference beam. These images correlate well with other techniques [1].

Near-infrared absorption tomography system for measurement of gaseous hydrocarbon distribution

The spatial distribution of chemical species can be a critical determinant of the performance of chemical reactors. One such reactor is the combustion chamber of the Internal Combustion engine, in which the spatial variation of air-fiiel ratio has a significant influence on both fuel efficiency and emissions performance. We report the development of a fibre-based Near Infra-Red Absorption Tomography system, in order to measure the distribution of hydrocarbons in-cylinder. The technique exploits the specific (but weak) hydrocarbon absorption of 1.7 µm radiation, which wavelength has only recently become accessible for the present application by the availability of solid-state all-optoelectronic components. A custom-specified InGaAsP/InP laser diode has been supplied, delivering 3mW at 1.700µm, with about lnm tunability. A standard telecommunications laser diode is used to provide a reference wavelength at 1.55 µm, which is not absorbed by any species in the combustion environment. Along each of 32 absorption paths through the subject, both wavelengths are launched simultaneously via a single-mode optical fibre and GRIN lens. The transmitted light is collected by a large-core fibre and measured by an extended-sensitivity InGaAs photodiode. The attenuation at each individual wavelength is measured by modulating the intensity of the laser sources in a frequency-division multiplexed scheme. The logarithm of the ratio of the two measurements yields the path integral of the hydrocarbon absorption, and hence, of concentration. Single-channel characterisation shows that the technique is readily calibrated for temperature and pressure effects, over the region 70- 150°C and 1- 10bar. Tomographic reconstruction of different gaseous hydrocarbon flows has been achieved. Design considerations will be discussed concerning the deployment of the technique to a running engine, to achieve image rates over 10,000 per second.

Chapter 11 – Chemistry of Combustion

The mechanism by which a fuel can contain the energy that is released during a combustion process is discussed. It describes how combustion is an adiabatic process and that the temperature rise occurs because the breaking down of the fuel bonds liberates energy that then has to be absorbed by the gases in the mixture. It is shown that the enthalpies of formation and reaction for various hydrocarbon fuels can be evaluated by considering the bond energies in the compounds.

1 – State of Equilibrium

This chapter focuses on equilibrium thermodynamics but the effects of making this assumption are explicitly borne in mind. The majority of processes met by engineers are in thermodynamic equilibrium, but some important processes have to be considered by non-equilibrium thermodynamics. Most of the combustion processes that generate atmospheric pollution include non-equilibrium effects, and carbon monoxide (CO) and oxides of nitrogen (NO x ) are both the result of the inability of the system to reach thermodynamic equilibrium in the time available. There are four kinds of equilibrium: stable, neutral, unstable, and metastable equilibrium. The type of equilibrium in a mechanical system can be judged by considering the variation in energy due to an infinitesimal disturbance. If the energy (potential energy) increases then the system will return to its previous state, if it decreases it will not return to that state. Equilibrium can be defined; if the properties of an isolated system change spontaneously there is an increase in the entropy of the system; when the entropy of an isolated system is at a maximum the system is in equilibrium; and if for all the possible variations in state of the isolated system, there is a negative change in entropy then the system is in stable equilibrium.

Chapter 15 – Combustion and Flames

The basic mechanisms in combustion processes are discussed. The concepts of premixed and diffusion flames, and the differences between spark ignition (gasoline) and compression ignition (diesel) combustion are considered. Then a rigorous development of the thermodynamics of the combustion process is undertaken. This introduces reaction order, explosion limits and multiplication factors in reactions. Premixed flames are considered in some detail and the laminar flame speed is defined. Data are presented showing the variation of laminar flame speed with fuel and equivalence ratio. It is shown that the laminar flame speed is too slow for modern engines and turbulent flame speed is introduced. Diffusion flames, found in boilers, gas turbines and diesel engines, are also considered.

Chapter 21 – Fuel Cells

Various types of fuel cells are described, before considering the equations that define their operation. Fuel cells are not limited by the Carnot efficiency which applies to conventional heat engines, but it is shown that they only achieve their extremely high intrinsic efficiencies at very low output. The equations defining the chemical transfers occurring in electric cells are developed initially, and then a new property, electrochemical potential, is introduced to enable further analysis to take place. It is shown that, like combustion, the fuel cell only produces energy when it transforms chemical bonds with a view to achieving equilibrium. Fuel cells are then analysed using the techniques of irreversible thermodynamics to show that some energy is lost under load propelling ions through the cell. Finally, the more practical aspects of fuel cells are considered including the many losses that can occur. Also discussed are the combustion related methods of obtaining hydrogen as the fuel.

Chapter 1 – Introduction and Revision

This is a revision chapter that defines the Laws of Thermodynamics and introduces systems and properties: it is the foundation for the remainder of the book. Thermal equilibrium and temperature are defined, before examining work and heat transfers. This leads to the concept of processes and cycles, culminating in the First Law and internal energy and enthalpy. Finally, the First Law is developed into the unsteady flow energy equation.

Chapter 10 – Thermodynamics of Combustion

This chapter introduces basic combustion processes. Fossil fuels are discussed and octane and cetane numbers are introduced. Different fuels are then discussed in some detail. Chemical equations for combustion processes are developed and the concepts of stoichiometry, rich and lean mixtures are introduced. A coherent method of calculating the temperature rise due to combustion based on the energy equation is derived: this can be applied to both closed and open systems. Heats of reaction and formation, including Hess’ Law are developed, and common values are tabulated. These concepts are then applied to a number of numerical examples, including adiabatic, constant pressure combustion, closed system combustion with both work and heat transfer and incomplete combustion. These examples lay the foundation for the later chapters.

Chapter 3 – Engine Cycles and their Efficiencies

The reversible Carnot cycle is compared to a range of engine cycles. It is shown that the efficiency of a Carnot cycle is the highest achievable, but that its power output is zero. Rankine cycles and the logical use of vapour based cycles are introduced and their efficiencies are compared to equivalent Carnot cycles – the concept of mean temperature of energy addition and rejection is used. The sensitivity of a ‘gas’ based Carnot cycle to non-isentropic compression and expansion is demonstrated. Otto, diesel and dual combustion cycles are defined and the method of analysing them as air-standard cycles is shown. The Joule (Brayton) cycle, as used in gas turbines, is introduced. Analysis of all these cycles shows that their thermal efficiencies can be related to a lower temperature ratio than the Carnot cycle – the reason why these cycles can never achieve the Carnot efficiency. Finally, reversed heat engine cycles, found in refrigerators and heat pumps, are discussed.