Final answer:
Running a diesel engine without a thermostat is likely to increase HC emissions due to incomplete combustion. NOx emissions might also increase due to variable temperatures affecting combustion, whereas changes in SO2 and NO2 emissions depend more on fuel content and combustion conditions than on engine temperature regulation.
Step-by-step explanation:
If a diesel engine is run without a thermostat, the tailpipe emissions likely to increase would be hydrocarbons (HC). The thermostat in an engine plays a crucial role in regulating the engine's operating temperature. Without it, the engine may not reach its optimal operating temperature, leading to incomplete combustion of the fuel. This incomplete combustion results in higher emissions of unburnt hydrocarbons, which are a component of diesel exhaust.
NOx (nitrogen oxides) emissions might also increase because NOx formation is exacerbated by high combustion temperatures, and without a thermostat, an engine might operate at varying temperatures outside the optimal range, sometimes too high, favoring NOx formation.
Sulfur dioxide (SO2) and Nitrogen dioxide (NO2) are more directly related to the sulfur content of the fuel and the specific combustion conditions, including the presence of catalytic converters designed to reduce such emissions.
Oxygen is not a pollutant and does not contribute to smog or air pollution, so the increase or decrease in oxygen would not be a concern in terms of tailpipe emissions from a diesel engine running without a thermostat.