Global Climate & Energy Project
Advanced CombustionThe Sootless Diesel
Start Date: September 2011
Christopher F. Edwards, Department of Mechanical Engineering, Stanford University
The goal of this project is to develop sootless Diesel engines by re-forming the fuel as it is injected. Achieving this goal will require the development of a high-temperature, non-dilute, direct-injection combustion strategy that can be applied to a variety of transportation fuels (including alcohols and natural gas). If stoichiometric combustion can be achieved, the use of a three-way catalyst in the (sootless) exhaust system will provide an inexpensive, reliable and simple approach to controlling oxides of nitrogen emissions. The result will be an efficient, high-load, clean engine.
The Diesel combustion engine, which uses the high temperature from compressed gas for ignition, is the preferred choice for sustained, high-load operating conditions. Unfortunately, the approaches suitable for emissions abatement at light and moderate load – staged combustion using high levels of exhaust gas – are not suitable for use at high load.
If the temperature in the fuel plume is sufficiently high, and if the atomic ratios (O:C and H:C) can be managed correctly, it may be possible to perform kinetically limited fuel re-forming so that the combusted fuel jet contains no soot – only carbon monoxide, hydrogen gas and hydrocarbon fragments. It is the chemistry within the fuel plume, primarily during the first 1-2 milliseconds (ms) after start of injection, that determines whether the engine will produce soot (Figure 1).
The key to achieving sootless Diesel is a combustion system that uses direct injection, has a very high effective compression ratio, utilizes direct auto-ignition, and avoids soot. The peak temperature to be expected in such a strategy is in excess of 3000 K. This temperature is necessary to support rapid chemical reaction and is the key to this research effort (Figure 2).
Figure 2: The experimental component of the research will be performed in a free-piston, extreme-compression apparatus (shown above) equipped with extractive gas and in-situ soot measurement capabilities.
The project has three central tasks: