N Ladommatos

Optical diagnostics for soot and temperature measurement in diesel engines

This paper reviews the optical techniques for in-cylinder combustion temperature measurement, particularly soot measurements in diesel engines. The review starts with the two-colour method for in-cylinder soot and combustion temperature measurement. The principle and implementation of the two-colour technique are described in detail. Both signal point and full-field temperature and soot measurements by the two-colour method are considered. In the second part, the soot diagnostics based on light scattering, especially the light extinction method for in-cylinder soot concentration measurements, are discussed. In the third part, optical techniques for spatially resolved two-dimensional measurements of soot particles in diesel engines are introduced. Since laser induced incandescence (LII) is a relatively new technique and is particularly suitable for the two-dimensional imaging of soot distribution, the operating principle and implementation of LII are discussed in detail. At the end of each part, examples are given to illustrate the understanding gained about diesel combustion as a result of the application of these optical techniques. This paper provides a comprehensive review for those who are interested in using optical diagnostics for in-cylinder soot and combustion temperature measurement in diesel engines.

The effects of exhaust gas recirculation on diesel combustion and emissions

Abstract An investigation was conducted with the aim of identifying and quantifying the effects of exhaust gas recirculation (EGR) on diesel engine combustion and exhaust emissions. Five effects of EGR were identified and investigated experimentally: the reduction in oxygen supply to the engine, participation in the combustion process of carbon dioxide and water vapour present in the EGR, increase in the specific heat capacity of the engine inlet charge, increased inlet charge temperature and reduction in the inlet charge mass flowrate arising from the use of hot EGR. The experimental methodology developed allowed each one of these effects to be investigated and quantified separately. The investigation was carried out on a high-speed, direct injection diesel engine, running at an intermediate speed and load. A limited number of tests were also conducted in an optically accessible diesel engine, which established the effects of EGR on local flame temperature. Finally, tests were conducted with simulated EGR being used additionally to the engine air supply. This contrasts with the conventional use of EGR, whereby EGR replaces some of the air supplied to the engine. It was found that the first effect of EGR (reduction in the oxygen flowrate to the engine) was substantial and resulted in very large reductions in exhaust NOx at the expense of higher particulate emissions. The second and third effects (participation of carbon dioxide and water vapour in the combustion process and increase in the charge specific heat capacity) were almost insignificant. The fourth effect (higher inlet charge temperature) increased both exhaust NOx and particulate emissions. The fifth effect (reduction in the inlet charge due to thermal throttling) reduced NOx but raised particulate emission. Finally, when EGR was used additionally to the inlet air charge (rather than displacing air), substantial reductions in NOx were recorded with little increase in particulate emission.

Optical diagnostics for in-cylinder mixture formation measurements in IC engines

Abstract In the last few years, there has been a very rapid increase in the number of applications of laser diagnostics in internal combustion (IC) engines, particularly the use of optical techniques for studying mixture preparation and in-cylinder mixture distribution. This is due to the importance of mixture formation in controlling the subsequent combustion and pollutant formation in IC engines. This paper attempts to review and discuss the current status of the applications of these optical techniques for in-cylinder mixture formation studies. The first half of this paper focuses on the three optical techniques used for the measurements of fuel vapour concentrations in IC engines. These are Laser Rayleigh Scattering (LRS), Spontaneous Raman Scattering (SRS), and Laser Induced Fluorescence (LIF). The second half of this paper is devoted to another two optical techniques. Laser Induced Exciplex Fluorescence (LIEF) and dual wavelength Laser Extinction/Absorption (LEA), which are capable of simultaneous measurements of fuel vapour and liquid droplets in IC engines. Simple theoretical descriptions are used to introduce the principle of each optical technique. Experimental consideration are given for the application of these techniques to IC engines. Much attention is devoted to providing readers with appropriate references to the technical literature, which will be of further assistance to readers interested in a particular application of each technique.

The effects of carbon dioxide in exhaust gas recirculation on diesel engine emissions

Abstract The investigation was conducted on a high-speed direct injection diesel engine and was concerned with the effects of exhaust gas recirculation (EGR) on diesel engine combustion and emissions. In particular, the effects of carbon dioxide (CO2), a principal constituent of EGR, on combustion and emissions were analysed and quantified experimentally. The use of CO2 to displace oxygen (O2) in the inlet air resulted in: reduction in the O2 supplied to the engine (dilution effect), increased inlet charge thermal capacity (thermal effect), and, potentially, participation of the CO2 in the combustion process (chemical effect). In a separate series of tests the temperature of the engine inlet charge was raised gradually in order to simulate the effect of mixing hot EGR with engine inlet air. Finally, tests were carried out during which the CO2 added to the engine air flow increased the charge mass flowrate to the engine, rather than displacing some of the O2 in the inlet air. It was found that when CO2 displaced O2 in the inlet charge, both the chemical and thermal effects on exhaust emissions were small. However, the dilution effect was substantial, and resulted in very large reductions in exhaust oxides of nitrogen (NO x ) at the expense of higher particulate and unburned hydrocarbon (uHC) emissions. Higher inlet charge temperature increased exhaust NO x and particulate emissions, but reduced uHC emissions. Finally, when CO2 was additional to the inlet air charge (rather than displacing O2), large reductions in NOx were recorded with little increase in particulate emissions.

Control of oxides of nitrogen from diesel engines using diluents while minimising the impact on particulate pollutants

The investigation was conducted on a high-speed direct-injection diesel engine and was concerned with the effects of exhaust gas recirculation (EGR) on diesel engine combustion and emissions. In particular, the effects on combustion and emissions of carbon dioxide (CO2) and water vapour (H2O), principal constituents of EGR, were analysed and quantified experimentally. The effects of increased inlet temperature and thermal throttling of the inlet charge, both arising from the use of hot EGR, were also investigated. Finally, tests were carried out during which the CO2 added to the engine air flow increased the charge mass flow rate to the engine, rather than displacing some of the oxygen (O2) in the inlet air. It was found that when CO2 or H2O displaced O2 in the inlet charge, both the chemical and thermal effects on exhaust emissions were small. However, the dilution effect was substantial, and resulted in very large reductions in exhaust NOx at the expense of higher particulate and unburnt hydrocarbon emissions. Higher inlet charge temperature increased exhaust NOx and particulate emissions, but reduced unburnt hydrocarbon emissions. Reduction in the inlet charge due to thermal throttling reduces NOx emissions but raises all the other pollutants. Finally, when CO2 was additional to the inlet air charge (rather than displacing O2), large reductions in NOx were recorded with little increase in particulate emissions.

Understanding of controlled autoignition combustion in a four-stroke gasoline engine

Abstract Controlled autoignition (CAI) combustion has recently emerged as a viable alternative combustion process to the conventional spark ignition or compression ignition process for internal combustion engines, owing to its potential for high efficiency and extremely low NOx and particulate emissions. Since CAI combustion is a process dominated by chemical kinetics of the fuel-air mixture, an engine simulation model with detailed chemical kinetics has been developed and applied to a four-stroke gasoline engine fuelled with isooctane. After calibration and validation, the engine simulation model was used to study the effects of the intake temperature, exhaust gas recirculation (EGR), the air-fuel ratio, the compression ratio and the engine speed on CAI combustion in a four-stroke gasoline engine. The characteristics of CAI combustion investigated include the autoignition timing, the partial burning and knocking combustion and NO emission. Results show that CAI combustion could be achieved within a limited speed and load range. The lower end of the CAI combustion range was affected by partial burning, and the higher end of its operation was limited by knocking combustion. Among the engine parameters investigated, the intake charge temperature and EGR had the greatest effect on the CAI combustion process. The effect of EGR was further analysed in terms of its thermal (increase in heat capacity), dilution, chemical and charge heating effects by means of a series of simulation studies. It was found that the charge heating effect caused advanced ignition timing, faster heat release rate and moderate reduction in the CAI combustion duration. The thermal effect (increased heat capacity) retarded ignition, extended combustion duration and slowed down heat release rate. The dilution effect also resulted in longer combustion duration and slower burning but it did not affect the ignition timing. The chemical effect was found to accelerate the combustion process when the percentage of EGR was large.

THE EFFECT OF AROMATIC HYDROCARBONS AND OXYGENATES ON DIESEL ENGINE EMISSIONS

Abstract Tests were conducted in a Cooperative Fuel Research (CFR) diesel engine aimed at discerning the effects of fuel aromatic and oxygenate compounds on exhaust emissions. The base fuel was heptane to which were added increasing amounts of monoaromatic toluene and diaromatic methylnaphthalene. Blends of heptane and toluene containing oxygenated compounds (methanol, ethanol, heptanol and diglyme) were also tested. The results indicate that both toluene and methylnaphthalene increase smoke, oxides of nitrogen (NOx) and unburned hydrocarbon (UHC) emissions substantially. The results also showed that increases in ignition delay could account for some of these rises in exhaust emissions. The oxygenated fuel blends tested caused reductions in NOx and smoke emissions. The exception was heptanol, which showed no reduction in smoke emission. This may be associated with the fact that heptanol had the lowest amount of molecular-bound oxygen.

Particle image velocimetry measurement of in-cylinder flow in internal combustion engines : Experiment and flow structure analysis

Abstract A cross-correlation digital particle image velocimetry (PIV) system has been developed and applied to study the in-cylinder flow in a single-cylinder engine with a production-type cylinder head. The PIV system set-up and its optimization are described in the first part of the paper. Two-dimensional velocity distributions measured over 100 cycles are analysed to obtain ensemble-averaged mean and fluctuating velocities, the turbulent length scale, vorticity and strain rate distribution in the measurement plane. In particular, a spatial filtering scheme is developed in order to obtain the cycle-resolved velocity measurements. The cycle-resolved analysis shows that the low-frequency velocity fluctuation component (i.e. cyclic variation) is mainly responsible for the spatial variation in velocity and turbulent kinetic energy distributions. The integral length scale calculated from the PIV data is between 6 and 10 mm and the strain rate is estimated to be within 1000 s-1 in most areas of the measurement plane.

Effects of exhaust gas recirculation temperature on diesel engine combustion and emissions

Abstract When hot exhaust gas is mixed with inlet air, the charge to a diesel engine is modified in three ways: the charge temperature increases, the total charge mass is reduced and the charge composition changes. This paper is concerned with the effects on combustion and pollutant emissions of only the first two items. The last item has been investigated by the authors previously and reported in references [1-4]. The investigation was conducted on a high-speed direct-injection diesel engine at about 40 per cent of full load, 2000 r/min and constant fuelling rate. The investigation included in-cylinder heat release and exhaust gas analysis. It was found that increasing inlet charge temperature, at constant inlet charge mass and composition, increased oxides of nitrogen (NO x ) and particulate exhaust emissions but reduced unburnt hydrocarbon (UHC) emissions. When the inlet charge mass was reduced at constant inlet temperature, the exhaust particulates and UHC emissions increased substantially, principally because the oxygen available for combustion was reduced. In contrast, NO x emissions increased only slightly, probably because the effects on NO x of a higher combustion peak temperature tended to be offset by substantially lower oxygen availability.

In-cylinder studies of the effects of CO2 in exhaust gas recirculation on diesel combustion and emissions

Abstract This paper reports the results of the effects of CO2 in exhaust gas recirculation (EGR) on diesel combustion and emissions. The experiments were carried out on a specially designed single-cylinder diesel engine. In-cylinder measurements were obtained from the optically accessible swirl chamber using high-speed shadowgraphy, the two-colour method and laser extinction. Furthermore, in-cylinder pressure measurements from the combustion chambers were used to derive the heat release rates during combustion. Two experiments were carried out on the effects of CO2 on diesel combustion and pollutant formation. In the first series of experiments, CO2 was used to replace some of the oxygen in the intake mixture, which simulated the dilution effect of conventional EGR. This so-called replacement EGR method was characterized by the typical NOx and smoke trade-off, where NOx reduction was accomplished at the expense of exhaust smoke. In the second series of tests, CO2 was added to the intake charge so that the oxygen concentration in the combustion chamber was not affected. In this additional EGR method, CO2 was found to suppress both NOx and smoke emissions. The mechanisms of these two different EGR modes on diesel combustion and emissions were examined using the above in-cylinder measurement techniques and exhaust emission analysis.