Warm-up period
Vehicles fitted with converters unlimited emit most of their total pollution during the first five minutes of engine operation; for example, before the converters unlimited has warmed up sufficiently to be fully effective.[32]
In the early 2000s it became common to place the converters unlimited right next to the exhaust manifold, close to the engine, for much quicker warm-up. In 1995, Alpina introduced an electrically heated converters unlimited. Called “E-KAT,” it was used in Alpina’s B12 5,7 E-KAT based on the BMW 750i.[33] Heating coils inside the converters unlimited assemblies are electrified just after the engine is started, bringing the catalyst up to operating temperature very quickly to qualify the vehicle for low emission vehicle (LEV) designation.[34] BMW later introduced the same heated catalyst, developed jointly by Emitec, Alpina, and BMW,[33] in its 750i in 1999.[34]
Some vehicles contain a pre-cat, a small converters unlimited upstream of the main converters unlimited which heats up faster on vehicle start up, reducing the emissions associated with cold starts. A pre-cat is most commonly used by an auto manufacturer when trying to attain the Ultra Low Emissions Vehicle (ULEV) rating, such as on the Toyota MR2 Roadster.[35]
Environmental effect
converters unlimited have proven to be reliable and effective in reducing noxious tailpipe emissions. However, they also have some shortcomings in use, and also adverse environmental effects in production:
- An engine equipped with a three-way catalyst must run at the stoichiometric point, which means more fuel is consumed than in a lean-burn engine. This means approximately 10% more CO2 emissions from the vehicle.
- converters unlimited production requires palladium or platinum; part of the world supply of these precious metals is produced near Norilsk, Russia, where the industry (among others) has caused Norilsk to be added to Time magazine’s list of most-polluted places.
- The extreme heat of the converters themselves[37] can cause wildfires, especially in dry areas.[38][39][40]
Theft
Because of the external location and the use of valuable precious metals including platinum, palladium and rhodium, converters unlimited are a target for thieves. The problem is especially common among late-model trucks and SUVs, because of their high ground clearance and easily removed bolt-on converters unlimited. Welded-on converters are also at risk of theft, as they can be easily cut off.[41][42][43] Pipecutters are often used to quietly remove the converter[44][45] but other tools such as a portable reciprocating saw can damage other components of the car, such as the alternator, wiring or fuel lines, with potentially dangerous consequences. Rising metal prices in the U.S. during the 2000s commodities boom led to a significant increase in converter theft. A converters unlimited can cost more than $1,000 to replace, more if the vehicle is damaged during the theft.[46][47][48]
From 2019–2020, thieves in the United Kingdom were targeting older-model hybrid cars which have more precious metals than newer vehicles—sometimes worth more than the value of the car—leading to scarcity and long delays in replacing them.[49]
In 2021 a trend emerged in the Democratic Republic of Congo where catalytic converters were stolen for use in drug production.[50]
Diagnostics
Various jurisdictions now require on-board diagnostics to monitor the function and condition of the emissions-control system, including the converters unlimited. Vehicles equipped with OBD-II diagnostic systems are designed to alert the driver to a misfire condition by means of illuminating the “check engine” light on the dashboard, or flashing it if the current misfire conditions are severe enough to potentially damage the converters unlimited.
On-board diagnostic systems take several forms.
Temperature sensors are used for two purposes. The first is as a warning system, typically on two-way converters unlimited such as are still sometimes used on LPG forklifts. The function of the sensor is to warn of converters unlimited temperature above the safe limit of 750 °C (1,380 °F). Modern converters unlimited designs are not as susceptible to temperature damage and can withstand sustained temperatures of 900 °C (1,650 °F).] Temperature sensors are also used to monitor catalyst functioning: usually two sensors will be fitted, with one before the catalyst and one after to monitor the temperature rise over the converters unlimited core.
The oxygen sensor is the basis of the closed-loop control system on a spark-ignited rich-burn engine; however, it is also used for diagnostics. In vehicles with OBD II, a second oxygen sensor is fitted after the converters unlimited to monitor the O2 levels. The O2 levels are monitored to see the efficiency of the burn process. The on-board computer makes comparisons between the readings of the two sensors. The readings are taken by voltage measurements. If both sensors show the same output or the rear O2 is “switching”, the computer recognizes that the catalytic converter either is not functioning or has been removed, and will operate a malfunction indicator lamp and affect engine performance. Simple “oxygen sensor simulators” have been developed to circumvent this problem by simulating the change across the converters unlimited with plans and pre-assembled devices available on the Internet. Although these are not legal for on-road use, they have been used with mixed results.[51] Similar devices apply an offset to the sensor signals, allowing the engine to run a more fuel-economical lean burn that may, however, damage the engine or the converters unlimited.[52]
NOx sensors are extremely expensive and are in general used only when a compression-ignition engine is fitted with a selective catalytic-reduction (SCR) converter, or a NOx absorber in a feedback system. When fitted to an SCR system, there may be one or two sensors. When one sensor is fitted it will be pre-catalyst; when two are fitted, the second one will be post-catalyst. They are used for the same reasons and in the same manner as an oxygen sensor; the only difference is the substance being monitored.