Are combustion temperatures in a diesel engine higher or lower than those in a gasoline engine?

• A. Higher
• B. Similar
• C. Much higher
• D. Lower

Answer: (D)

Combustion temperature is governed by the reacting mixture’s heat release and how much heat is absorbed by the surrounding air and diluent. Diesel engines burn with a much leaner mixture, meaning a lot more air for the same amount of fuel. That excess air (mostly nitrogen) acts as a heat sink and dilutes the flame, so the peak temperature reached during combustion is lower. Gasoline engines typically run closer to stoichiometric or slightly richer mixtures with less diluent, so the flame temperature tends to be higher. Even though the air in a diesel is highly compressed and hot before ignition, the extensive dilution during combustion keeps the maximum temperature lower than in a gasoline engine.