If a diesel engine is run without a thermostat which of the following tailpipe emissions would be more likely to increase?

• A. HC
• B. NOx
• C. Sulfur dioxide
• D. Nitrogen dioxide

Answer: (A)

When the engine isn’t allowed to reach its normal operating temperature, combustion stays less complete for longer. Warm-up temperature matters because the fuel-air mixture and combustion process work best when the engine and exhaust system see heat. In a cold diesel, there’s more unburned fuel and incomplete combustion, so hydrocarbon emissions rise in the exhaust.

NOx formation, on the other hand, requires high combustion temperatures. A cooler, slower-warming engine produces lower peak temperatures, so NOx (and thus NO2) doesn’t increase as much and may even decrease. Sulfur dioxide is determined mainly by the sulfur content of the fuel, not the engine temperature, so it isn’t affected by removing the thermostat. Nitrogen dioxide follows NOx trends, so it wouldn’t rise if NOx doesn’t rise.

So the emissions most likely to increase when the thermostat is missing are hydrocarbons, due to less complete combustion in a cold engine.