In both the Otto cycle (used in gasoline engines) and the diesel cycle (used in diesel engines), preheating the air-fuel mixture before the compression stroke is not effective and is generally not employed. Here's why:
Premature Ignition: In both cycles, combustion occurs through the process of compression ignition. In the Otto cycle, the air-fuel mixture is ignited by a spark plug, while in the diesel cycle, fuel is injected into the highly compressed air, causing autoignition. Preheating the mixture excessively before the compression stroke can lead to premature ignition, where the fuel ignites too early in the compression stroke. This can cause engine knocking, increased pressures, and can lead to damage to engine components.
Reduced Efficiency: Preheating the air-fuel mixture would increase its temperature before compression. According to the ideal gas law, as temperature increases, pressure also increases for a given volume and number of moles. This means that more work would be required to compress the mixture to the desired pressure for combustion. As a result, the net work output of the engine decreases, leading to reduced efficiency.
Loss of Charge Density: Preheating the intake air reduces its density. In the Otto and diesel cycles, the power output of the engine is directly proportional to the mass of the air-fuel mixture trapped in the cylinder. By preheating the air, its density decreases, which means that a lower mass of air enters the cylinder during each intake stroke. This leads to a reduced charge density and, consequently, a decrease in power output.
Cooling Requirements: Preheating the air would also increase the cooling requirements of the engine. The combustion process already generates a significant amount of heat, and the engine's cooling system is designed to manage that heat. Further increasing the temperature of the intake air would necessitate more robust cooling systems, which could add complexity and cost to the engine design.
Overall, preheating the air-fuel mixture in the Otto and diesel cycles is not an effective approach due to the potential for premature ignition, reduced efficiency, decreased charge density, and increased cooling requirements. Therefore, these cycles rely on the compression of ambient air during the intake stroke to achieve optimal combustion and power output.