True or False: Excess combustion air keeps diesel exhaust temperatures cooler than SI engines burning at stoichiometric ratios.

The statement is true because excess combustion air in a diesel engine, which operates with a leaner air-fuel mixture compared to spark ignition (SI) engines, typically results in more complete combustion of the fuel. This leads to a more efficient burn, producing lower exhaust temperatures. Diesel engines are designed to run with varying amounts of excess air, which helps to cool the combustion process. In contrast, SI engines, which ideally operate at a stoichiometric air-fuel ratio, can produce higher exhaust temperatures because they have less excess air to absorb and dissipate heat. This difference in operating conditions affects how the combustion process is managed, resulting in cooler exhaust temperatures for diesel engines when excess air is present.

Understanding the combustion dynamics of diesel engines in relation to air-fuel ratios is crucial for determining efficiency, emissions, and thermal management in heavy-duty applications.