Low Temperature Combustion – A Thermodynamic Pathway to High Efficiency Engines

This article makes the argument that compression ignition combustion processes that are “flameless” with volumetric energy release, described as Low Temperature Combustion (LTC), are a thermodynamic pathway to maximizing the efficiency of reciprocating piston, internal combustion engines. The argument is built from the foundation of determining the maximum theoretical efficiency for an IC Engine and then identifying the irreversibilities associated with the various in-cylinder processes. Low Temperature Combustion minimizes the sum of the irreversibilities for work generation from incylinder processes. The underlying objective of Low Temperature Combustion is to keep in-cylinder temperatures low through volumetric energy release via auto-ignition of dilute air fuel mixtures, as opposed to flame propagation. Because LTC depends on autoignition, the methods used to achieve it are dependent on the auto-ignition characteristics of the fuel being used. Differences in physical and auto-ignition characteristics of fuels mandate different approaches for establishing and controlling LTC. It has become common to label the different approaches with a descriptive acronym, leading to a proliferation of terms such as HCCI, PCCI, CAI, etc. Because the energy release is volumetric it is a challenge to operate at low loads with good combustion stability, and at high loads without excessive rates of pressure rise. Good progress is being made addressing these challenges but doing so will put added burden on the gas exchange and control systems of the engine.