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Conventional three-way catalysts could not treat NOx in lean air-fuel ratio regions (under an atmosphere with excess oxygen). Therefore, engines were driven using the stoichiometric air-fuel ratio in high load regions with high NOx emission volume, such as when accelerating, and the like. To improve fuel efficiency, the major issue was developing a catalyst capable of treating NOx at lean air-fuel ratios.
A catalyst called a lean NOx catalyst had a new treatment principle. It stores NOx when the air-fuel ratio is lean and treats the stored NOx by reduction when the air-fuel ratio is rich.
The lean NOx catalyst is composed of a conventional three-way catalyst with additional alkaline substances to store the NOx. This material is a monolithic carrier that is coated with alumina and supports platinum (Pt), rhodium (Rh), various types of alkali, alkaline earths, and rare-earth oxide.
The NOx stored in the catalyst is reduced by reactions with large amounts of CO and HC generated by creating short rich air-fuel ratio bursts.
In actual driving, there are various driving patterns and the lean condition may continue for a long period. Therefore, based on driving conditions, the NOx storage capacity and the maximum storage capacity of the catalyst under those conditions are estimated and compared. When the NOx storage capacity is close to its maximum point, the air-fuel ratio is controlled to be rich for a very short time to reduce and treat the NOx, before being restored to the lean state.
The loss of fuel efficiency caused by this rich control can be kept below 1%. The region of lean air-fuel ratio could be expanded for both city and constant speed driving, enabling both fuel efficiency improvement and NOx treatment.
When the air-fuel ratio is controlled to a rich state in actual driving, the torque difference between the lean and rich states caused a shock perceptible to the driver. However, by improving the air-fuel ratio control method and ignition timing, the shock could be reduced to a sufficiently tolerable level.
Degradation factor: Exhaust gas containing SO2 reacts with the storage material after being oxidized on the precious metal. This forms sulfates that are difficult to reduce in a rich atmosphere, resulting in a low treatment rate due to reduced NOx storage capacity.
An evaluation of emissions performance based on the Japanese test method (10-15 mode) showed a treatment rate of 90% for a brand-new catalyst, and 60% for a catalyst after durability tests. The main cause of catalyst degradation was sulfur poisoning. It is therefore best to use low-sulfur fuel with this catalyst. |
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| Storage location | : | TOYOTA MOTOR CORPORATION, TOYOTA HERITAGE DIV., TOYOTA AUTOMOBILE MUSEUM (41-100, Yokomichi, Nagakute, Nagakute-cho, Aichi-gun, Aichi 480-1131) | |
| Year manufactured | : | 1994 | |
| Manufacturer | : | Toyota Motor Corporation | |
| Classification | : | Mass-production product | |
| Current status | : | In storage: not open to the public | |
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| Model / Manufacture |
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| Company name | : | Toyota Motor Corporation |  |
| Nickname | : | Lean NOx catalyst | |
| Technical applications | : | For lean-burn engines | |
| Year of manufacture | : | 1994 | |
| Development completion year | : | 1994 | |
| Collaboration | : | Cataler Corporation, Toyota Central R&D Labs., Inc. |
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| Installation model / Engine / Fuel |
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| Installation model | : | Toyota Carina | |
| Engine | : | Lean-burn engine, inline 4-cylinder DOHC, 4A-FE (1.6 L), 7A-FE (1.8 L) | |
| Fuel | : | At constant speed of 60 km: 29.5 km/L, 10･15 mode driving: 17.6 km/L |
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| Emissions control system (incl. catalyst) | : | Catalyst carrier: Monolith 1.7 L coated with alumina; Catalyst: Platinum, rhodium series; Storage material: Various types of alkali, alkaline earths, rare-earth oxide; Air-fuel ratio control (EFI): Air-fuel ratio feedback control that switches between the stoichiometric air-fuel ratio and lean air-fuel ratio; Deceleration control: Fuel cut on deceleration (improving fuel efficiency, preventing catalyst heating), independent helical port; Ignition timing control: Electronic spark advance system (ESA), swirl control valve; Catalyst steam-heated alarm device: Exhaust temperature sensor, exhaust temperature warning lamp, combustion pressure sensor; Fuel evaporation gas control device: canister purge control, blowby gas reduction device | |
| Effect | : | NOx treatment rate (10･15 mode): Approx. 90% for brand-new catalyst, approx. 60% for catalyst after durability test | |
| Points of interest, topicality | : | A three-way catalyst developed under a new concept where NOx is stored under lean burn, before being reduced and treated. | |
| Features | : | The mechanism of NOx treatment for lean-burn regions is the oxidation and storage of NOx as nitrates, which are then reduced and treated by reactions with HC and CO, etc. at rich air-fuel ratios. This catalyst was a conventional three-way catalyst with alkaline substances added as the NOx storage material. The loss of fuel efficiency in the rich state was kept below 1%. | |
| Reference materials | : | Toshiaki Tanaka, Kenji Kato, Shinichi Takeshima, Shinichi Matsumoto, Koji Yokota, Koichi Kasahara, Tetsu iguchi, "Development of NOx storage-reduction 3-way catalyst system", JSAE Transaction, Vol. 26, No.4, Oct. 1995 | |
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