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Carbon-Based Energy Systems > Advanced Combustion
The Sootless Diesel

Start Date: September 2011
Status: Completed
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Investigators
Christopher F. Edwards, Department of Mechanical Engineering, Stanford University

Objective
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.

Background

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).


Figure 1: Images of 3 fuels during the initial 1-2 milliseconds (ms) of Diesel-style combustion. Luminosity from hot soot is clearly evident with Diesel fuel (left column), almost absent with ethanol (center) and completely absent with methanol (right). This suggests that highly oxygenated fuels such as methanol may be optimal for sootless Diesel combustion. 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

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.

Approach

The project has three central tasks:

  • Task 1: Modeling. A model with simplified fluid mechanics but detailed chemical kinetics will be developed in order to explore the in-plume re-forming process. Reaction mechanisms for the basic fuels are already available, and some include basic soot-precursor mechanisms (e.g., the n-heptane fuel model).
  • Task 2: Extreme Compression Experiments. This task will provide experimental data that are sufficient – in combination with the modeling work – to support a basic understanding of the fuel transformation process.
  • Task 3: Fabrication of a Flexible Gas Injector. This task involves the fabrication of a flexible gas injector based on a previous design concept. To date, experiments have been conducted with injectors that are designed to be used with viscous, liquid hydrocarbon fuels. Modifications will be made to match to the injection requirements for this research, including the use of low-viscosity liquids (alcohols) or gaseous fuels. Systematic research requires that the fuel injection requirement be handled in a sound way.
 

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