Improving energy efficiency of combustion engines

Diesel engines are synonymous with power. They fuel America’s most impressive machines, from earth movers to 18-wheelers to commercial shipping’s supertankers. When compared to gasoline combustion engines, diesels are built stronger, last longer, and are also more efficient. However, they fail to meet their potential because their designs are based on assumptions. Until recently, scientists could only infer how chemicals and fluids interacted inside the engine. Engines based on this gap of fundamental knowledge achieve less than optimal power and efficiency and produce excessive pollution.

Recently, however, the knowledge gap has been narrowing in large part through work at the Combustion Research Facility (CRF), a Department of Energy Office of Science collaborative research facility at Sandia National Laboratories in New Mexico. Led by CRF Director Bob Carling, researchers at CRF have made significant progress on what they call an optical engine, an internal combustion engine with observation capabilities that has been evolving for the past 30 years. The CRF’s scientific team, led by Dennis Siebers, has been analyzing the chemistry and fluid dynamics within engines, and more specifically within individual cylinders. Today’s version provides a new way of looking at processes that are important to engine efficiency and energy output, as it can directly observe and much more accurately measure the interactions of air and fuel and how exactly they mix and react to pressure and time variations at different places in the combustion process.

Previously, engineers thought that unburned fuel in a diesel engine cylinder found its way to the cylinder wall, cooled there and then emerged through the exhaust system as soot. Diesel engines emit black exhaust clouds and for years they were engineered with this phenomenon in mind. As it turns out, this assumption is not true. The CRF researchers developed and utilized a wide variety of laser-based diagnostic methods to unravel the complex processes of diesel combustion and discovered that soot forms because of incomplete mixing of fuel with air in the gas phase of the combustion cycle. This has large impacts on engine design and manufacturing. According to McIlroy, the discovery of these new processes has facilitated “transformational changes in the way people think of and design diesel engines.”

Engineers have started to develop new “lean-burn” engines, which use smaller, more strictly controlled quantities of fuel, and mix it more thoroughly with air to produce more complete chemical reactions. They waste less fuel, pull more energy out of each drop of diesel, and significantly reduce soot output from the engine. In light of today’s environmental and fuel supply concerns, “lean-burn” technology is an important step towards more eco-friendly combustion engines, and today’s progress will inevitably yield more advanced engines that are even cleaner and even more efficient.

The CRF’s research was pre-competitive work, and as such, the fundamental discoveries made by the CRF were communicated to the Department of Energy, other national labs, and to the private companies that helped to fund the project. The CRF will continue its effort for years to come, as there are still large gaps in our fundamental knowledge of combustion processes.