Typical engine exhausts are extremely complex and have to be taken into account when designing test units. Feeds having 20 or more components, each precisely regulated, are not unusual. A typical fuel mixture may contain: inerts, such as nitrogen or argon, CO₂, CO, water vapor, air, sulfur oxides (typically SO₂), various NOx, hydrocarbons (propane, vaporized VOCs). These have to be fed at different points upstream of the reactor so as to prevent premature reactions. Additionally, ammonia may be included to simulate SCR in diesel catalysts.