As shown in this cross-section of the Sandia DISI engine, the central location of the fuel injector (f) and the design of the piston bowl (b) help stratify the fuel near the spark plug (e). Also shown are the piston (a), the piston-bowl window (c), which allows a wide-angle view from below into the combustion chamber, and the pent-roof window (d), which allows a side view into the chamber.
At Sandia’s Alternative Fuels Direct-Injection Spark Ignition (DISI) Engine Lab, Combustion Research Facility researchers Magnus Sjöberg and Wei Zeng, together with Dave Reuss at the University of Michigan, are developing the fundamental understanding needed by industry to design and optimize engines for alternative fuels. The current focus is on flex-fuel engines that can run on pure gasoline or gasoline/ethanol blends.
This team is examining a spray-guided, stratified-charge combustion system—a spark-ignition (SI) lean-burn technology that achieves high thermal efficiency at low-to-medium loads by utilizing overall dilute combustion and minimal intake-air throttling. Typically, the flame speeds of ultra-lean mixtures are too low to allow stable SI operation. However, sufficiently high burn rates can be ensured by injecting the fuel late during the compression stroke and concentrating the fuel near the spark plug. The research team is examining ways to capitalize on the different fuel properties to increase engine efficiency and performance, while achieving clean combustion. For this, a single-cylinder research engine is used for both performance testing and in-cylinder optical diagnostics.
Current research on stratified-charge engine combustion is focused on developing a broad understanding of in-cylinder processes, with emphasis on cyclic variability and fuel-property effects. With a solid science base, the engine industry can better decide what combinations of fuels specifications and combustion technologies provide the most efficient engine, while best utilizing available alternative fuels and meeting stringent emissions standards.
Read the full article at the CRF website.
Read their article, “High-speed imaging of spray-guided DISI engine combustion with near-TDC injection of E85 for ultra-low NO and soot,” in Proceedings of the Combustion Institute.
Read the abstract for their article, “NOx-reduction by injection-timing retard in a stratified-charge DISI engine using gasoline and E85,” in SAE International Journal of Fuels and Lubricants.