Low temperature combustion is a combustion concept that bears simultaneous reductions in pollutant emissions as well as fuel consumption. Homogeneous Charge Compression Ignition is perhaps one of the oldest forms of low temperature combustion. In this method, a homogeneous blend of fuel and air is compressed until it auto-ignites. Homogeneous Charge Compression Ignition pairs the high efficiencies of conventional diesel compression ignition combustion with the homogeneous low soot attributes of typical spark ignition combustion.
Through higher levels of dilution with air or residuals, engine-out NOx emissions are kept low. Reference to these factors, Homogeneous Charge Compression Ignition has exhibited near zero NOx and soot emissions paired with efficiencies similar to conventional diesel combustion. Unfortunately, this approach is only achievable over a narrow, part-load operating range owing to lack of direct control over the start heat release rate. To provide control over heat release, an in-depth understanding of the fundamental combustion mechanism is needed.
A number of alternative low temperature combustion modes, which provide control over the start and rate of heat release, depend on a direct fuel injection event to initiate a stratification of equivalence ratio and therefore, mixture reactivity. This approach can be effective, but unfortunately, the fuel-air mixture inhomogeneities pose a risk of higher particulate matter as well as NOx emissions.
Instead of trying to use forced fuel-air mixture stratification to control the heat release rates in low temperature combustion, Benjamin Lawler at Stony Brook University in collaboration with Derek Splitter, James Szybist, and Brian Kaul at Oak Ridge National Laboratory proposed a new combustion mode that controls the amount of thermal stratification in low temperature combustion. The method, termed as Thermally Stratified Compression Ignition, implements direct water injection to control the temperature distribution and mean temperature in the cylinder. Therefore, this approach offers control over the start and rate of heat release in low temperature combustion. Their work is published in Applied Energy. “While other researchers are unanimously pursuing approaches that stratify the equivalence ratio in the cylinder using direct fuel injection, we propose an alternative approach that intentionally stratifies the temperature distribution in the cylinder.” Benjamin Lawler said on Thermally Stratified Compression Ignition.
The authors adopted their proposed advanced combustion mode, Thermally Stratified Compression Ignition. The method used direct injection of water to control the start as well as rate of heat release in low temperature combustion. The main aim of the study was to better understand the impact of water injection on low temperature combustion, with particularity on the comparison between operation with and without water injection. There was also a focus on the effects of water injection amount and timing. At the end, the authors determined the load limits with and without water injections.
The research team observed that adding water retards combustion phasing owing to the latent heat of vaporization of water, which subsequently cooled the mixture. The amount of phasing retard was proportional to the amount of water injected. For this reason, water injection could be used for cycle-to-cycle control of the start of heat release in low temperature combustion.
Direct water injection in the range of 20-70 degrees before top dead center reduced the rate of heat release by cooling the mixture in the regions targeted by the spray. This forcibly amplified the level of thermal stratification in the cylinder. Direct water injection could therefore be used for cycle-to-cycle control over the rate of heat release in low temperature combustion. Reference to this, the high load limit was improved from 3.6 bar for Homogeneous Charge Compression Ignition to about 8.4 bar Indicated Mean Effective Pressure in Thermally Stratified Compression Ignition.
“By taking a fundamentally different approach to control over the heat release process in low temperature combustion, Thermally Stratified Compression Ignition is able to significantly expand the operable load range of low temperature combustion and enable a clean, high efficiency, combustion mode over the full operating range. Furthermore, Thermally Stratified Compression Ignition and other low temperature combustion modes are fuel independent, meaning that they can be paired with sustainable biofuels or electrofuels to produce a completely carbon-neutral, efficient, and clean transportation and power generation solution.”, Lawler said.
The outcomes of their study present the potential of water injection to allow for cycle-to-cycle control over the start and rate of heat release in low temperature combustion, therefore, resolving the major limitation of pure Homogeneous Charge Compression Ignition combustion.
Benjamin Lawler, Derek Splitter, James Szybist, and Brian Kaul. Thermally Stratified Compression Ignition: A new advanced low temperature combustion mode with load flexibility. Applied Energy, volume 189 (2017), pages 122–132.Go To Applied Energy