The combustion phasing of Homogeneous Charge Compression Ignition combustion is incredibly sensitive to intake temperature. Controlling the intake temperature on a cycle-to-cycle basis is one-way to control combustion phasing, however accomplishing this with an intake air heater/intercooler is unfeasible. One possible way to control the intake temperature is through the direct injection of fuel. The direct injection of fuel during the intake stroke cools the charge via evaporative cooling. Some heat is absorbed from the incoming air, lowering the in-cylinder temperature, while some heat is absorbed from the piston/cylinder walls if the spray reaches the walls. The amount of heat that is absorbed from the air vs. the walls depends on the spray penetration length. The available spray penetration length can be controlled by the injection timing during the intake stroke. Therefore, if a high latent heat of vaporization fuel is used, the intake valve closing temperature will become very sensitive to injection timing, allowing for cycle-to-cycle control of combustion phasing.
Ethanol is a fuel with a high latent heat of vaporization and therefore possesses a large charge cooling potential. Wet ethanol, a mixture of ethanol and water, offers an even higher charge cooling potential. The use of wet ethanol also provides the opportunity to save energy during the production of a biofuel. Using a blend of 80% ethanol and 20% water by mass, cycle-to-cycle control of the intake temperature over a range of greater than 50K is established. The effect of injector spray angle and wall temperature on the intake temperature range is also examined. Finally, a MATLAB thermodynamic engine cycle simulation is used to predict the exact fraction of fuel that evaporates in the air as a function of injection timing, showing that wet ethanol 80 is an ideal fuel candidate for this intake temperature control strategy.