Partially premixed combustion (PPC) is the next step after HCCI, according to Lund University. Shown is a Scania diesel engine running on gasoline. (Lund University)
Using gasoline in modified truck diesel engines can result in more than 50% efficiency if the combustion process is done correctly, according to research conducted at Lund University in Sweden. By employing a partially premixed combustion (PPC) process, fuel consumption for gasoline engines could reportedly be cut in half. The engine has been developed to achieve the right amount of ignition delay—a delay between fuel injection and combustion. During this delay, the mixing that happens produces minimal amounts of soot and nitric oxide. The end result could be a new generation of engines that would not require catalytic converters, the university claims.
The Lund University engine currently has 57% indicated efficiency, which translates to roughly 50% efficiency on the output shaft of the engine. “A reasonably efficient engine today would be in the range of 40-42%. We’re hoping to achieve 60% with this type of PPC process,” said Bengt Johansson, Professor of Combustion Engines at Lund University, and a 20-plus-year member of SAE International and SAE Fellow since 2005.
(Go to https://www.youtube.com/watch?v=G1_WMgSjGXk&feature=youtu.be to see video of the concept engine.)
The PPC concept is a follow-up to the HCCI (homogeneous charge compression ignition) type of combustion Lund has worked with since 1996, Johansson explained to SAE Magazines.
“About 2006 we decided to take the next step moving from fully homogeneous charge to partially premixed. At first we worked with conventional diesel fuel in the diesel engine but had problems getting the ignition delay needed for mixing before combustion started. Diesel fuel is simply too reactive,” he shared.
Lund researchers started with gasoline and similar high octane fuels around 2008. “All of a sudden it was possible to operate with late enough injection to prevent excessive wall wetting and still get long enough ignition delay to mix before combustion,” Johansson said. “With late injection the emissions of HC and CO got all the way down to US10 levels without a catalyst as no fuel was trapped in crevices. With sufficient mixing before combustion, soot can be kept low and NOx can be handled with EGR (exhaust gas recirculation). The most important [aspect] is perhaps that it is possible to control burn rate with the fuel injection strategy. We can get fast enough burn with any combustion timing by adjusting the level of stratification using multiple injection.”
Johansson notes that many modern diesel engines are using PPC calibration at part load. “But as load goes up above 20-25%, the ignition delay is not sufficient with diesel fuel. With gasoline it is possible to go all the way up to full load, and fuel efficiency is best at higher loads,” he said.
Competence Center Combustion Processes (KCFP) at Lund University is working with Chalmers and KTH to make combustion engines more efficient. KCFP focuses on combustion processes between “conventional” HCCI and the Otto and Diesel concepts. The university says that PPC is “a very promising concept” that it will continue to pursue.