Internal-combustion engines – Additional air supply
Reexamination Certificate
2001-10-04
2003-05-13
Yuen, Henry C. (Department: 3747)
Internal-combustion engines
Additional air supply
Reexamination Certificate
active
06561139
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, relates to a method and apparatus for reducing hydrocarbon and carbon monoxide emissions from an engine by effecting a secondary reaction between residual reactable combustion product components and supplemental air following a primary combustion event.
2. Discussion of the Related Art
Much effort has been expended in recent years to lower engine emissions to reduce urban smog. Urban smog, a severe global environmental problem, is formed by the sunlight-induced photochemical reaction of hydrocarbons (HC) with oxides of nitrogen (NOx). Because HC and NOx are both emitted by internal combustion engines, smog reduction efforts have focused on reducing these emissions. Carbon monoxide (CO), another undesired byproduct of combustion, is also an emission of concern to many researchers and engine designers. A discussion of available techniques for reducing these emissions and the problems with those techniques requires an understanding of how they are formed.
Referring to
FIG. 1
, the relationship between exhaust product concentrations versus equivalence ratio (ER) is graphically illustrated for a spark-ignited combustion of a homogeneous mixture of fuel and air. ER is the ratio of the stoichiometric air-fuel ratio of the air/fuel charge, divided by the actual air/fuel ratio of the charge. For example, a fuel-lean mixture having an air-fuel ratio of 29.4 and a stoichiometric air-fuel ratio 14.7 has an equivalence ratio of 0.5. A stoichiometric air-fuel ratio has an equivalence ratio of 1.0; a fuel-lean mixture has an ER value of less than 1, and a fuel-rich mixture has an ER value of greater than 1.
Curve
30
plots NOx vs. ER. NOx are formed when available oxygen and nitrogen react with one another at elevated temperatures. Generally speaking, NOx concentrations increase as the ER rises above about 0.6. However, the curve
30
also illustrates that, as the ER continues to increase beyond 1.0, the NOx concentrations fall sharply, even though the combustion temperature does not drop as sharply. This effect is due principally to the consumption of available oxygen through the reaction of the fuel and air as represented by the downwardly sloping curve
32
. That is, at ERs significantly above 1.0, more fuel is available in the combustion chamber for reaction with a given volume of atmospheric oxygen. Because hydrocarbons react with oxygen more readily than nitrogen reacts with oxygen, a greater percentage of the available oxygen is consumed through combustion, leaving relatively little remaining oxygen in the combustion chamber to react with nitrogen. As a result, NOx emissions are sharply reduced as ERs rise above a stoichiometric air/fuel ratio.
Curves
34
,
36
, and
38
also show that CO, H
2
, and HC, increase steadily and non-linearly with ER due to the fact that insufficient air is present in the combustion chamber at high ERs to assure complete reaction of fuel with air during the combustion event. As a result, after combustion ceases at high ERs, the resultant combustion products have a relative high percentages of unburnt and partially burnt fuel products. Because these products are capable of oxidation under the appropriate conditions, they will hereafter be referred to as “residual reactable combustion product components.” Residual reactable combustion product components form a large percentage of the undesired HC and CO emissions.
Hence, it can be seen that HC and CO emissions are proportionaly related at ERs above the stoichiometric air fuel ratio. Traditional emission reduction techniques attempted to employ fuel injection and air supply techniques to control the ER to be relatively close to 1.0 and to employ engine after-treatment in the form of a three-way converter to further reduce HC, CO, and NOx emissions. When operated very close to the stoichiometric air-fuel ratio (ER=1), the three-way catalyst has the unique ability to reduce and oxidize HC, CO, and NOx with impressive efficiency, hence reducing HC, CO, and NOx emissions to a level that the engine can reasonably be considered to be “clean” or “non-polluting.” The typical clean engine emits pollutant concentrations that are measured in the range of parts-per-million. Most modern automotive engines and derivatives of them can be considered to be non-polluting by this standard.
In contrast, many non-automotive engines, particularly relatively small utility engines and derivatives of them, are usually considered “dirty” or “polluting” because they do not incorporate active measures to reduce HC, CO and NOx emissions to the levels enjoyed by clean engines. Typical uses for these engines include, but are not limited to: lawn mowers; line trimmers, chain saws, generator sets, welding machines; cement mixers, chipper/shredder machines, mini-bikes, motorcycles, jet skis, outboard engines, and low-cost automotive engines for emerging nations. These engines are “rich-burn” engines, typically operating at an ER value of about 1.2 or even higher. Hence, 20% of the fuel admitted to the engine passes through the engine without being combusted. The engines are factory-calibrated to run rich because they perform well at this condition and also run cooler with reduced propensity for destructive combustion knock. This, in turn, reduces a manufacturer's warranty exposure. These engines typically produce low NOx emission levels because they operate at such a high ER.
HC and CO emissions of levels produced by utility engines and other rich-bum engines are not readily oxidized using a catalytic converter. That is, catalysts typically employed by non-polluting engines would be overwhelmed by the quantity of residual reactable combustion components emitted by a typical rich-bum utility engine. That engine is passing 20% excess fuel to the catalyst, not the trace amounts characteristic of a modern automotive engine. The reaction of 20 percent of the engine's fuel flow within a catalytic converter generates a sizeable exothermic reaction, raising the exhaust gas temperature sharply. This high temperature can destroy the typical catalytic converter in short order.
An attempt to “lean out” the polluting utility engine to near stoichiometric air-fuel ratio in order to reduce HC and CO emissions would also be fraught with difficulty. As briefly discussed above, this type of engine experiences compromised performance when operated at the stoichiometric air-fuel ratio. Power density, final engine weight, and cost also suffer when traditional clean technologies are employed. Design improvements to offset some of these problems would require increased compression ratio, high quality valve, valve seat, and valve guide materials, improved heat rejection schemes (likely liquid-cooling), and/or electronic ignition systems that incorporate combustion knock sensing. All of these design changes are relatively expensive to design and to implement. They also undesirably add to the weight and/or cost of the engine and the machine powered buy it. Weed trimmers, for instance, are too light-weight and inexpensive to be economically powered by a large, heavy, expensive engine.
Finally, even if a “dirty” engine were reconfigured to run well at an ER that is sufficiently near an ER of 1 to reduce HC and CO emissions sufficiently for practical implementation of an oxidation catalyst, the resulting engine would produce high NOx levels that would also have to be dealt with by a three way catalyst or otherwise.
While the combustion characteristics of stratified charge engines differ from that of a homogenous charge spark-ignited engine, the underlying fundamental principals are quite similar, as are the difficulties encountered when attempting to reduce HC, CO, and NOx emissions. Similarly, while reducing HC, CO, and NOx emissions without employing fuel injectors and/or a three way catalyst and/or other extreme or expensive measures is especially difficult yet desirable in a rich-bum, spark-ignited utility engine, it
Ali Hyder
Boyle Fredrickson Newholm Stein & Gratz S.C.
Evan Guy Enterprises, Inc.
Yuen Henry C.
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