Clean air system

Abstract

The present invention provides a method for controlling exhaust gas emissions including platinum, wherein exhaust gas is brought into contact with an emissions control catalyst and the oxygen content of the exhaust gas is controlled in order to maintain fuel rich stoichiometry close to the stoichiometric value.

Description

FIELD OF THE INVENTION

The present invention is generally directed to a method for achieving pollutant free operation of automotive engines and other sources of carbonaceous and nitric oxide pollutants. More specifically, the present invention is directed at controlling emissions of platinum oxides from catalytic converters.

BACKGROUND OF THE INVENTION

Historically, great strides have been made in controlling automotive emissions. Early oxidation catalyst systems were very effective in controlling hydrocarbon and carbon monoxide emissions. However, it was soon recognized that control of nitrogen oxides (NOx) was also necessary. Because NOx control catalysts are typically ineffective under oxidizing conditions, two stage systems were used having a reducing first stage followed by an oxidizing second stage with added air. A major improvement was the development of three way catalysts and the system of dithering the fuel air ratio between rich and lean around the stoichiometric value. This allows both NOx reduction and oxidation of hydrocarbons and carbon monoxide, and has been the standard approach for many years. All these approaches have a serious flaw. Any period of lean operation will allow loss of platinum as platinum oxide. Thus it should be no surprise that European researchers have recently discovered widespread platinum contamination of the environment. This is a potentially serious problem since workers in a platinum refinery have developed serious allergic reaction to extremely low concentrations. Although a guard bed could be used to gather platinum, frequent replacement would likely be required.

Based on the foregoing, it is an objective of the present invention to provide a method and apparatus to eliminate loss of platinum from catalytic converters without impairing effectiveness.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for operating a catalytic converter system. In the method of this invention, a catalytic converter, such as used on present day automobiles, is operated on the fuel rich side of stoichiometric but with sufficient air to consume most of the hydrocarbons and carbon monoxide in the exhaust. It has now been found that as low or lower HC, CO and NOx emissions can be achieved using the method of the present invention as with dithering systems. Non-methane hydrocarbon emissions of less than five ppm are achievable with carbon monoxide representing the bulk of residual fuel values. In the absence of excess oxygen on the catalyst surface, platinum loss is negligible. Even methane is readily converted in a fuel rich environment. It has now been shown that in fuel rich oxidation of methane, no measurable loss of platinum could be detected after more than 2000 hours of operation. In contrast, nearly all the platinum was lost in less than 200 hours running fuel lean.

It has also been found that minor platinum emissions resulting from any failure of the control system can be captured by use of a guard bed of a material such as alumina down stream of the catalytic reactor. Note that available modern digital control systems have proven highly reliable.

In the present invention, the required control of exhaust gas stoichiometry is achievable such as by feedback of measured values from a universal oxygen sensor to a modern engine control computer. Test runs have demonstrated that high conversion levels of all three pollutants are achievable. High mass transfer efficiency catalysts, such as the short channel length catalysts of U.S. Pat. No. 5,051,241, allow high conversion of hydrocarbons and carbon monoxide even with low residual oxygen levels. Advantageously, stoichiometry is controlled to supply sufficient oxygen to convert all but ten to two hundred ppm of the fuel values present.

While the present invention has been described in detail with reference to providing a method for eliminating the loss of platinum from catalytic converters, other catalytic reactor configurations exhibiting the characteristics taught herein are contemplated. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred embodiments described herein.

Claims

1. A method for controlling exhaust gas emissions including platinum, comprising:

a) contacting the exhaust gas with an emissions control catalyst; and

b) controlling the oxygen content of the exhaust gas to maintain fuel rich stoichiometry close to the stoichiometric value.

2. The method of claim 1 wherein the catalyst is a three way catalyst.

3. The method of claim 1 wherein the catalyst is an oxidation catalyst.

4. The method of claim 1 wherein a universal oxygen sensor is used to measure exhaust gas stoichiometry.

5. The method of claim 4 wherein the measured exhaust gas stoichiometry is fed real time to a fuel flow control computer.

6. The method of claim 5 wherein the fuel control computer is programmed to maintain a constant fuel-air stoichiometry in transient operation.

7. The method of claim 6 wherein sufficient oxygen is present to consume all but between about ten and about two hundred ppm of the fuel values present.

8. A method for operating an automotive engine comprising:

a) controlling fuel flow in relation to air flow;

b) measuring exhaust gas stoichiometry using a universal oxygen sensor; and

c) adjusting fuel flow to maintain exhaust gas stoichiometry slightly on the rich side of stoichiometric.