1. Field of the Invention
This invention pertains to the combustion of diesel or other liquid fuels in a small combustor, and more particularly to the providing of an external heat source for a Stirling engine.
2. Description of the Prior Art
Interest in small burners on the order of 2500 BTU per hour or less introduces a significant problem with the preparation of liquid fuels for combustion. Most liquid fuel burners prepare the fuel for combustion by pressure or air atomizing the fuel to produce small droplets thereby maximizing the surface area of the fuel exposed to the air. The mixing of the fuel and the air in this manner, enhanced by aerodynamic mixing, allows in most cases for efficient and clean combustion.
At extremely low flows required for small burners, pressure atomizing orifices are extremely small, on the order of 0.0005 inches. This is not a practical size when considering manufacture and the potential for the orifice plugging with foreign material. Additionally, pressure atomization at the normally high (50 to 100 psi) pressures adds a significant parasitic power requirement to the system. Alternative techniques to atomize the fuel, such as ultrasonic nozzles and spinning disc atomizers, are available. These, however, have their own unique problems and, with motors, bearings, crystals and power supplies, tend to be complicated, expensive and heavy.
Generally speaking, the simplest, and therefore lightest and least costly, approach to expose a large surface area of a small quantity of fuel to the combustion air is a wick, such as is used in lanterns and space heaters. The wick also requires the minimum fuel pressure of any system, nearly eliminating the parasitic power losses associated with pumping the fuel. However, this type of burner, as currently constructed, has many problems in the server environment of the Stirling engine. Preheated air at about 1000 degrees Fahrenheit is used to feed the burner. Preheating is necessary in a Stirling engine to recover exhaust heat to conserve system energy. This high temperature would result in the consumption of a conventional wick in the presence of a flame. The high temperatures also insure that some portion of the wick will be in a critical temperature range of 200 to 300 degrees Fahrenheit where elements of liquid fuels, particularly diesel, form a lacquer-like substance that plugs the wick, preventing fuel from passing to the wick.
An additional problem is the need for control of the rate of fuel flow to the burner to control the power output of the Stirling engine. Normal wicking action itself is an uncontrolled process. Variation in heat or light output in conventional applications is usually controlled by exposing more or less of the wick surface to the incoming air, allowing a variable amount of the fuel to be consumed. The amount of exposure is usually determined by visual means by the user. This approach, however, is not practical in a Stirling engine application.
Moreover, soot and smoke are major concerns of diesel combustion during start-up, operation, and shut-down.