Which exhaust constituent is most likely to increase if a diesel engine runs without a thermostat?

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

Answer: (A)

When a diesel engine runs without a thermostat, the engine tends to operate at a cooler average temperature because coolant can circulate freely. This lower temperature makes fuel less completely combusted, so more unburned fuel exits with the exhaust as hydrocarbons. Hydrocarbons rise when combustion is incomplete or inefficient, which is more likely in cooler-running conditions. In contrast, NOx formation grows with higher combustion temperatures, so cooler operation would typically reduce NOx (and its related NO2) rather than increase it. Sulfur dioxide depends on the sulfur content of the fuel, not the engine temperature. So the exhaust component most likely to increase is hydrocarbons.