Researchers at Sandia National Laboratories’ Combustion Research Facility are working on low-temperature gasoline combustion (LTGC) operating strategies for inexpensive, high-efficiency engines that will meet stringent air-quality standards. Sandia researchers Isaac Ekoto and Benjamin Wolk say the goal of the LTGC project is a compression-ignition engine that combusts dilute-charge mixtures.
“The use of dilute mixtures avoids high flame temperatures that can lead to nitrogen oxide (NOX) formation,” Ekoto says. “LTGC operation increases engine efficiency relative to conventional spark-ignited gasoline engines through reduced heat transfer and pumping losses, along with increased conversion of fuel chemical energy into usable work via higher compression ratios and mixture-specific heat ratios.”
The challenge is effective ignition control when an engine is idling or at other low-load operating conditions, where slow burn rates can cause frequent misfires.
One method to improve LTGC engine operation at low loads is to employ a negative valve overlap (NVO) strategy in which hot residual gasses from a combustion stay in the cylinder and get mixed with fresh fuel in air for the next cycle. The hot residual gasses can ignite the new fuel, creating sparkplug-free, auto-ignition.
The Sandia team hopes to enhance auto-ignition by injecting a small amount of fuel during the NVO period. The high temperatures reached during NVO thermally decompose the fuel into a more reactive reformate stream laden with hydrogen; carbon monoxide; and small hydrocarbons, such as methane, acetylene, and ethylene.
“Quantifying the concentrations of these species provides valuable insight into how the fuel chemical energy is distributed among the different classes of light hydrocarbons in the reformate,” Ekoto says.
Photoionization mass spectrometry (PIMS) tests performed at Lawrence Berkeley National Laboratory’s Advanced Light Source, used vacuum ultraviolet light to continuously ionize the sample gas. The team developed an algorithm that identified sample constituents, based on their photoionization efficiencies. By combining the PIMS and other measurements, researchers evaluated the auto-ignition reactivity of the reformates.
“The reformate generally sped up auto-ignition when blended with the unreformed parent fuel,” Ekoto says, and the team was able to identify a handful of chemical species responsible for the speedup.
The next step is improving computer models to better map how to use the insights.
“Although conventional engine combustion simulations can accurately model flow dynamics, they typically rely on overly simplified combustion models that do not represent the full range of chemical pathways,” Ekoto says.
The team is working with University of Minnesota researchers, who are using the Sandia measurements as benchmark datasets for stochastic reactor models.
Sandia National Laboratory