Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream three-way catalyst. The LNT stores exhaust components such as, for example, NOx and oxygen, when the engine is operating at a lean air-fuel ratio, and releases and reduces (purges) them when the engine is operating at a rich or stoichiometric air-fuel ratio. Over time, the ability of the LNT to store exhaust components can decrease due to factors such as sulfur deposits (SOx) from the fuel. Therefore, when the LNT efficiency is sufficiently reduced, a SOx purge has to be performed. Typically, the catalyst is heated and engine air-fuel ratio is changed to rich for SOx release and reduction. Since SOx purges result in fuel economy penalties, it is desirable not to purge unnecessarily. Thus, in order to maintain adherence to emission standards and obtain fuel economy benefits of a lean burning engine, it is desirable to monitor the efficiency of the LNT.
One method of determining the efficiency of the LNT is by correlating it to the oxidant storage capability, as described in U.S. Pat. No. 5,713,199. The amount of stored oxidant is calculated from a quantity of fuel required to purge it from the LNT wherein the purge is performed during high load operating conditions.
The inventors herein have recognized a disadvantage with this approach. Namely, when estimates of this method are performed under high load operating conditions, i.e., at high space velocity, the amount of time that the reductant used to purge stored oxidants is present in the LNT is reduced. Therefore, not all of the oxidants are purged during high-speed engine operation, resulting in an inaccurate estimate of the total LNT storage capacity. In other words, the reductant does not have sufficient time to react with stored oxidants when the LNT purge is performed under high load conditions. This will cause inaccurate capacity estimates and reductant wastage.