The combustion of air/fuel mixtures in internal combustion engines, such as those found in automobiles, produces an exhaust gas stream comprised of various gaseous components. Some of these components, such as hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NO.sub.x), may be termed noxious components. Those skilled in the art will appreciate that oxides of nitrogen refers to both NO and NO.sub.2. Environmental concerns have led to ever stricter regulations concerning the maximum allowed emissions of these particular components.
Attempts to eliminate or control these noxious components have most recently involved the use of three-way catalysts (TWC). Such catalysts typically oxidize HC and CO, while simultaneously reducing NO.sub.x. The exhaust gas stream is typically passed through a combination of noble metals coated onto a stabilized alumina substrate carried on a monolithic ceramic or metallic cellular core. The three-way catalysts are commonly designed so that the exhaust stream or emissions pass therethrough along a straight uninterrupted axial flow.
However, optimizing both the oxidation of HC and CO and the simultaneous reduction of NO.sub.x, requires close control of the air/fuel (A/F) ratio entering the internal combustion engine. Optimum reduction of all three components occurs when the A/F ratio is close to stoichiometric, i.e. .about.14.65 kilograms of air to 1 kilogram of gasoline or .lambda.=1. It will be appreciated that .lambda. is the excess air/fuel factor and is defined by dividing the quantity of air and fuel actually supplied, by the theoretical stoichiometric air/fuel requirement.
In order to achieve and maintain the desired A/F ratio, exhaust gas oxygen (EGO) sensors and closed loop control circuits have been used in conjunction with three-way catalysts. Such emission control systems generally measure the oxygen concentration of the exhaust gas and adjust the relative amounts of air and fuel supplied to the engine in response thereto. EGO sensors provide a feedback adjustment whenever the air/fuel ratio is other than a predetermined level, i.e. .lambda.=1, to indicate whether the mixture is richer (.lambda.&lt;1) or leaner (.lambda.&gt;1) than .lambda.=1. This level is known as the closed loop control point and, within narrow limits, may be selected as desired, i.e. other than .lambda.=1.
U.S. Pat. No. 5,083,427 to Anderson, commonly assigned, discloses an apparatus system to control unwanted emissions from an automotive engine. The system has a low mass three-way filter catalyst, a high mass three-way main catalyst downstream of the filter catalyst, a continuous universal exhaust gas oxygen sensor (UEGO) positioned between the filter and main catalysts to indicate the level of oxygen in the exhaust stream exiting the filter catalyst, and highly sophisticated proportional control means for adjusting in closed loop the A/F ratio entering the engine in interactive response to a deviation of the sensed oxygen level from a target level. Anderson does not teach that hydrogen must be removed from the exhaust gas stream.
The universal exhaust gas oxygen sensor employed by Anderson has a very fast response, typically 60 milliseconds, or 16.7 Hz. Without a filter catalyst, the sensor will not provide optimum control because it will respond to high frequency (i.e. &gt;10 Hz) "chemical noise" rather than to the mean oxygen level in the exhaust. According to Anderson, the filter catalyst buffers these fluctuations to 4 Hz or less, thereby providing the UEGO sensor with exhaust gas more representative of the chemical mean. The Anderson filter catalyst buffers the sensed oxygen concentration by masking the effect of unpredictable oxygen deviations, (i.e. individual cylinder events, PCV and air distribution) and compensating for predictable oxygen level deviations.
Although such systems have been found to be useful, their accuracy and effectiveness is impaired with the use of various alternative automotive fuels. Alternative fuels may be generally defined as those other than traditional gasoline and diesel fuels. Illustrative examples are natural gas, methanol, methane, propane (LPG), ethanol and combinations of these fuels such as methanol/gasoline or gasoline/ethanol fuel mixtures. Natural gas is herein defined as fuel consisting mostly of methane with small and varying amounts of ethane, propane, butane and other inert gases.
Experiments have indicated that the combustion of such alternative fuels generally results in a significant lean shift of the closed loop control point. See Hepburn, J. S., "A COMPARISON BETWEEN THE COMBUSTION OF ISOOCTANE METHANOL, AND METHANE IN PULSE FLAME COMBUSTORS WITH CLOSED LOOP A/F CONTROL", SAE Technical Paper 920799, International Congress and Exposition, Detroit, Mich., Feb. 24-28 1992.
These shifts in the closed loop oxygen sensor control point are believed to be due to the difference in the gas diffusion rates of low molecular weight components present in the exhaust gas stream. As compared to gasoline exhaust streams, the exhaust gas streams produced from the combustion of alternative fuels in internal combustion engines typically contain higher concentrations of low molecular weight components like hydrogen.
It is believed that the low molecular weight of the hydrogen gas allows it to preferentially diffuse through the protective spinel sheath surrounding the electrode of the typical EGO sensor. As a result, the relative hydrogen concentration at the inner surface of the sheath is greater than the relative hydrogen concentration at the outer surface of the sheath. The higher relative hydrogen concentration at the electrode surface causes the EGO sensor to detect a richer gas mixture than is actually present in the bulk gas surrounding the protective spinel sheath. As a result, the EGO sensor experiences a "shift". Such shifts occur in both switching type sensors, (i.e. EGO, HEGO) and continuous type sensors (i.e. UEGO). They occur under both steady-state and transient conditions and are unrelated to the high frequency chemical noise cited by Anderson.
In general, lean shifts result in inaccurate control of the air/fuel mixture entering the engine. That is, because the EGO sensor believes the air/fuel mixture to be richer in fuel than it is, the closed loop control system directs a reduction in the amount of fuel entering the engine and a concurrent increase in air. The air/fuel mixture entering the engine thus becomes leaner than what it actually should be, i.e. a lean shift.
For fuels with high hydrogen to carbon ratios, such as alternative fuels like methanol and natural gas, the magnitude of the lean shift can be large, (ca 0.2-0.3 delta A/F units). Depending on the particular calibration employed, lean A/F shifts of 0.1 A/F units or less can result in reductions in NO.sub.x conversion efficiency of 20 percentage points or more.
However, it is believed that such effects may be seen to a lesser extent even with the combustion of traditional fuels like gasoline. In gasoline the effect may be responsible for less than optimum control of A/F ratios.
There is thus a need for an emission control system which will eliminate the lean shift in a closed loop control point caused by the preferential diffusion of hydrogen through the protective spinel coating of an EGO sensor. Most desirably, such an emission control system would provide for optimum reduction of noxious components like NO.sub.x, HC and CO, while exhibiting long term durability and little or no loss in efficiency.