One of the fundamental distinctions between a spark-ignition (SI) and a compression-ignition (CI) engine is the air/fuel ratio. Whereas CI engines always run with a lean mixture, SI engines may run lean, rich or stoichiometric (neither lean nor rich). The air/fuel ratio has a major influence over both the quantity and type of harmful emissions, and determines the ability of a catalytic converter to clean them up. Catalytic converters on engines running with a lean mixture are only really effective against carbon monoxide and hydrocarbons. If the engine runs rich, the catalytic converter will be good at reducing oxides of nitrogen but will show little effect against carbon monoxide or hydrocarbons. If the engine runs with a stoichiometric mixture, the catalytic converter will exhibit high conversion efficiency over all three major pollutant groups, and will therefore be known as 'three-way'. The following graph illustrates this
The type of mixture that an SI engine runs on is influenced by the degree of sophistication of both the engine itself and also the engine management system. The type of use the engine is put to is also an important factor. In general, modern SI engines with engine management systems fitted and used in transient conditions are set up to run stoichiometric. By comparison an equally advanced engine used in conditions of steady speed and load would be set to run lean. A more basic engine without engine management used in transient conditions would probably be running rich most of the time.
The following table identifies the appropriate catalytic converter depending on the air/fuel ratio which is present.
| Air / fuel ratio | Catalytic converter type |
| Lean | 2 way |
| Stoichiometric | 3 way |
| Rich | 2 way plus secondary air |
Further details of these types of catalytic converters can be found below.
3-Way
This type of catalytic converter, when fitted to an engine running with a stoichiometric air/fuel ratio, can achieve the following conversion efficiencies:
| Carbon monoxide | up to 95% |
| Hydrocarbons | up to 90% |
| Oxides of nitrogen | up to 95% |
The substrate used may be of the ceramic or metal type, usually with 400 cells per square inch. The coating will contain rhodium together with either platinum, palladium or a combination of the two.
2-Way
A two-way catalyst, when fitted to a managed engine set to run lean can achieve the following conversion efficiencies:
| Carbon monoxide | up to 99% |
| Hydrocarbons | up to 95% |
The substrate may be ceramic or metal, usually with 400 cells per square inch. The coating will contain platinum, palladium, or a mixture of the two.
2-Way plus secondary air
No catalytic converter can promote oxidation reactions in an exhaust environment where oxygen is in short supply. This will be the case if the engine in question is running with a rich air/fuel ratio. Here, the solution is to introduce air into the exhaust system upstream of the catalytic converter, which transforms the exhaust environment so that it is similar to that found with a lean-burn engine.
A 2-way catalytic converter with a secondary air system can achieve the following conversion efficiencies:
| Carbon monoxide | up to 99% |
| Hydrocarbons | up to 95% |
In these conditions the catalytic converter may experience extreme temperature peaks, and for this reason a metal substrate is preferable to a ceramic one, because of the ability of the metal type to resist localised overheating.
There is a variety of methods for introducing the secondary air. In some cases a mechanical or electrical pump is used. However, it is usually possible to find a 'natural' method which does not require any external energy source. The most popular 'natural' methods of secondary air induction are as follows:
Pulse air valve
A reed valve is fitted in a suitable location in the exhaust. This uses the pulsing nature of the exhaust gases to draw in air at times when the exhaust gas pressure is below atmospheric.
Venturi or Coanda system
These systems rely on physical phenomena associated with moving gases to create low pressure regions which can then be used to draw in an air supply.
Either of these 'natural' methods is easier to apply in practice if the average level of pressure in the exhaust is as low as possible, and for this reason the catalytic converter substrate is usually of the metal variety, which causes less obstruction to the moving exhaust gas.
