Auer / Douglas Advances in Energy Systems and Technology

Volume 4
1. Auflage 2013
ISBN: 978-1-4831-9128-7
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 228 Seiten

Advances in Energy Systems and Technology, Volume 4 covers articles about energy systems and technology. The book discusses articles about atmospheric carbon dioxide (CO2), including anthropogenic sources of CO2, the effects of increased atmospheric CO2 on climate, and the effects of climate changes and direct CO2 impacts. The text also describes automobile pollution control with regard to complex chemical and physical processes that take place during combustion in automobile engines and the reduction of the levels of pollution emitted by internal combustion engines. The statistical perspective on world oil resources, as well as the historical perspective on electricity and energy use and on the relationship of electricity to gross national product, is also considered. The book further explores the relationship between economic activity and energy use and uninterrupted trend toward increasing electrification in the United States. Professional workers in the field of energy systems and technology as well as those of university students at the graduate or advanced undergraduate level will find the book useful.

Autoren/Hrsg.

Auer, Peter

Douglas, David

Leseproben

Automobile Pollution Control

E.L. Resler, Jr.,     Cornell University, Ithaca, New York

Publisher Summary

This chapter discusses control of automobile pollution. The principal pollutants that the automobile emits are carbon monoxide (CO), which is a deadly poison; unburned hydrocarbons (HC), which are irritants and sources of smog; and NOx, which is an important link in a chemical chain leading to smog in combination with HC and sunlight. Automobile pollution control is difficult as HC and CO pollution results when the temperatures during combustion are too low, while NOx results when the temperatures are too high. The difficulty with simultaneous control of the pollutants is reflected in the catalysts that might be used to control them. All popular means to control pollutants are very expensive and add considerable complexity to the automobile, confounding both the consumer and repair mechanic and making customer maintenance of the vehicle nearly impossible. Prestratification is a very simple and inexpensive way to control pollution and to increase economy. NOx can be controlled in this manner so that three-way catalysts and electronic carburetion are not required. The prestratification with air is more effective than adding an air pump that delivers air to the exhaust manifold just aft of the exhaust valve. The use of prestratification for pollutant control is currently being applied in combustion control to develop a multifuel engine capability.

I. Introduction

II. Cylinder Temperature Distribution (Caused by Flame-Front Dynamics)

III. Effect of Cylinder Temperature Distribution on Otto Cycle Theoretical Description

IV. Kinetics of NOx Formation

A. General Kinetics

B. Equilibrium Species Concentrations

V. NOx Pollution from Automobiles

A. Lean Burning Engines (? < 1)

B. Peak NO in Lean Mixtures (Equilibrium).

C. Engines with “Correct” Air-Fuel Mixtures (? = 1)

D. Rich Mixtures (? > 1)

E. Simplified General Case (All ?)

VI. Kinetics of NO Formation

A. Extended Zel’dovich (Linearized)

B. Finite Rate Chemistry

C. Finite NO Chemistry in an Engine

VII. Cornell Experiments with Automobile Pollution Controls

A. Piston Chamber Control

B. Stratification

C. Air Stratification

D. Prestratified Exhaust Gas Recirculation (SEGR)

VIII. Conclusions

References

I INTRODUCTION

A great deal of the pollution that plagues our civilization is due to the combustion products of the fuels we use to satisfy our energy needs. Since a large fraction of the fuel we consume is used to provide transportation and since our favorite mode of transportation in many instances is the private automobile, automobile pollution is a major contributor to the overall problem. Because the pollution accompanies the vehicle, it is large where the vehicle density is large, and the stringent regulations regarding its emissions are meant to cope with the pollution levels in our large population centers.

The principal pollutants that the automobile emits are carbon monoxide (CO), a deadly poison; unburned hydrocarbons (HC), irritants and sources of smog; and NO (NO and NO2), thought to be an important link in a chemical chain leading to smog in combination with HC and sunlight and possibly injurious to both humans and plants. Automobile pollution control is difficult since HC and CO pollution results when the temperatures during combustion are too low, while NO results when the temperatures are too high. The difficulty with simultaneous control of the pollutants is reflected in the catalysts that might be used to control them. To lower the CO and HC pollution one requires a catalyst that promotes the oxidation of the pollutant, while in the case of NO one wishes to effect the opposite, that is to reduce the NO to N2. Some difficulties are expected to arise from these circumstances.

A curve of pollutant level emitted from an automobile as a function of its air-fuel ratio (weight of air divided by the weight of fuel delivered by the carburetor to the engine) has the general features shown in Fig 1. In Fig 1, ? is the equivalence ratio, defined as the air-fuel () ratio for stoichiometric combustion (combustion products consisting of CO2 and H2O only) divided by the actual operating air-fuel ratio. The apparent feature is that lean mixtures (? < 1) have lower CO and HC emission levels than rich mixtures (? > 1), which might be expected since rich mixtures are oxygen-poor, so that incomplete combustion products necessarily result. Even though there is excess oxygen (O2) at lean mixtures ? < 1, there still is appreciable pollution due to some incomplete combustion. This results from the fact that the walls, valves, and pistons in contact with the hot driving gases of combustion must be cooled to maintain their integrity: The flame front that engulfs and burns the combustible mixture is quenched near these relatively cold surfaces. Thus the HC and CO pollutants reside in a so-called “quench layer” adjacent to the boundaries of the gaseous combustion products. This assumes, of course, that the combustion processes elsewhere in the engine cylinder have been completed. Reducing ? lowers the HC, CO, and NO for ? < 1, but there is a lower limit to this effect where the CO and HC again increase. When the mixture is too lean, the temperature after combustion is too low to allow a flame front to propagate, and misfire results. The CO and HC curves go through a minimum near the “lean burn limit.” The NO, which is the major oxide of nitrogen produced in the engine, is less at lean mixtures (? < 1) because the temperature is less. Lean mixtures, while less polluting, result in less power for an engine of given size, since the engine derives its power by converting the energy of the fuel to heat. The NO pollutant peaks before ? = 1, where the temperature of the combustion products is at its peak value; however, the diminishing supply of O2 restricts the production of NO (or evidently the H and C both have a higher affinity for O than N). Above ? = 1, the decline in NO persists owing to both falling temperature and even less available O2. The curve in Fig. 1 is an experimental fact. No scales are indicated. If we were to include the amount of pollution on the ordinate of the figure and make a few calculations we would soon discover that the magnitudes involved are not simply explained. The exhausted pollutants depend on the history of events after ingestion through the air intake. To adequately address pollution control of the exhaust emissions we must explore further the factors involved and how they relate to the pollutant efflux.

Fig. 1 Pollutant level (ordinate) of a spark ignition engine versus equivalence ratio ?.

After much investigation and study it is now generally accepted that the pollutant emissions are governed by chemical kinetic processes. That is, even if mixing and combustion were perfect, the rates of reactions are such that the rate of cooling during the power stroke would have to be taken into account to determine the measured pollutant efflux. The dynamics of the flame-front propagation also affect the temperature profile in the working cylinder, and this temperature gradient must be accounted for if detailed agreement with actual fact is desired. These effects are discussed below. In practice there are many variables which must be controlled to accomplish pollution reduction, so many in fact that application to various engines or even the same engine under different operating conditions is possible but in many cases impractical. Our purpose here will be to discuss some of the more important effects and indicate some controls and simple calculation procedures that have proved useful.

II CYLINDER TEMPERATURE DISTRIBUTION (CAUSED BY FLAME-FRONT DYNAMICS)

The fuel in a spark ignition engine is converted into combustion products across a flame front that originates at, and is initiated by, the spark plug. The energy of the fuel is released across this front in the structure of the front. The detailed structure of the front does not concern us. Account for its speed of propagation is regulated with spark advance, and the engine’s combustion chamber design insures that it is not quenched before combustion is complete. The structure is thin compared with the dimensions of the combustion chamber so that an adequate assumption for our purposes is that the heating value of the fuel is released across an infinitesimally thin...