During vehicle cold start, emissions, mass flow rates, and catalytic converter space velocities vary by orders of magnitude. Therefore, catalytic exhaust control systems must be designed to operate at high efficiency almost from the moment of engine start-up. Catalysts must reach their operating temperature as quickly as possible. Therefore, the utility of different methods for improving the warm-up characteristics of catalytic systems is illustrated.
A very elegant method to speed the warm-up is the use of the engine itself as a “preheater” for the catalytic converters. High exhaust gas enthalpy to raise exhaust system mass up to its operating temperature is obtained by the use of extreme spark retard, stochiometric mixtures, and fully opened throttle. Intensive studies to investigate the effects of concurrent changes of spark timing and air/fuel mixtures on exhaust gas temperature, enthalpy, NOx and HC emissions are discussed.
Finally, NOx catalyst characteristics are dealt with, because the NOx catalyst is the first in a dual-bed catalytic system. The NOx catalyst should have high activity, low ignition temperature, and good warm-up performance. If the NOx catalyst has a fast warm-up rate, this would result even in a significant improvement in the warm-up characteristic of the HC/CO bed.