DIESEL ENGINE FUMIGATION

(Courtesy of Algas Corporation, a Division of Precision General Inc.)

The Problem.

The Clean Air Act of 1990 addresses the pollution problem of exhaust smoke and particulate materials. It also addresses the requirements for reduced HC, CO, and NO_x emissions for all on-road vehicles. Until the passage of the 1990 law, smoke and particulates were not regulated. Prior Clean Air Acts made on-road vehicles in excess of 26,000 pounds GVW virtually exempt from the emissions control program.

Generally, diesel engines produce high levels of NO_x emissions and significant smoke and particulate material levels. Generated smoke may be classified as "Hot Smoke" or "Cold Smoke". Hot Smoke results from incomplete combustion during heavy engine loads. Hot smoke also results from an over-rich fuel mixture. Insufficient oxygen prevents the diesel fuel from completely oxidizing. Besides generating particulate pollution, hot smoke performs no useful work; it reduces net engine power and lowers vehicle mpg. Traditional engine adjustments do nothing to hot smoke production. Fumigation reduces most hot smoke production resulting in reduced pollution and increased vehicle mpg.

Smoke produced during light engine loads is called "cold smoke". Traditional engine adjustments (engine timing and advance), leaking injectors, and poor quality diesel fuel are the contributors to cold smoke. Cold smoke produces the same emission pollutant particulate matter. Fumigation does not abate cold smoke.

Mandated reductions have forced manufacturers to reexamine the diesel engine combustion process to discover answers. For four-stroke diesel engines, fumigation is a relatively low cost and easily adaptable solution.

What is Fumigation?

A fumigation system injects a gaseous or liquid fuel into the intake air stream of a compression ignited engine. This fuel burns and becomes a part-contributor to the power producing fuel. While alcohol and gasoline may be used, gaseous fumigation seems to exhibit the best overall power yields, performance, and emissions benefits. LPG (Liquified Petroleum Gas) or CNG (Compressed Natural Gas) is used. The fumigation process is divided into major fraction and minor fraction fumigation. When more than 50% gaseous injection is used, it is known as major fraction fumigation. Minor fraction fumigation is under 50% gaseous injection. One after-market fumigation component supplier, Algas Corporation, using 20 to 25% fumigation rate, reports excellent performance benefits and reduced target emissions. One test for a 1984 6.9 liter diesel engine in an F250 chassis showed a 64% reduction in NO_x and 45.2% reduction of particulates. CO was almost unchanged. However, total emissions of hydrocarbons in grams per mile increased .

Rudolph Diesel's 1901 patent mentions the diesel fumigation process. Not until the 1940's were there any commercial fumigation applications. Cooper-Bessemer introduced large, big-bore, stationary engines which ran on 95% natural gas and 5% diesel. Since then other manufacturers have used fumigation to boost power output. Fumigation has been successfully used in agricultural machines for many years. To date, Texas and Colorado have accepted fumigation as an alternative fuel.

Benefits

For nonstationary variable RPM engines, variable ratio fumigation is more effective than fixed rate fumigation. As a result, engine load and fumigation rates are matched. Reduced NO_x and particulate emissions are the most outstanding benefits. Minor fraction fumigation yields significant (up to 25%) gains in smoke-limited torque gains. There are other technical non-pollution related gains. Tests show reduced exhaust temperature, reduced crankcase solids, reduced injection service and longer acceptable crankcase drains. Diesel exhaust odor is also reduced.

How Does It Work?

The major engine modifications include:

-- addition of a venturi-draft tube in the air intake system

-- secondary fuel tank, safety controlss, and switches

-- acceleration-deceleration control vaalve

-- microcomputer control.

The venturi-draft physically introduces and mixes LPG or CNG with the air charge. Since diesel engines operate with near zero manifold vacuum, an air-venturi draft tube is necessary to create a signal proportionate to engine speed. During installation the baffles and the tube are calibrated for fumigation onset and maximum fumigation rate. The fuel mixer is added to the draft tube. This provides the point of introduction for the gaseous fuel. Mixer calibration is based on the maximum fumigation rate. Fuel regulators differ for CNG and LPG fumigation systems. Since CNG tank pressure may reach up to 3,600 psi a primary regulator is necessary. LPG fumigation systems do not require this component since tank pressures are less than 300 psi. A secondary regulator allows the LPG or CNG to expand to a gaseous state. This regulator provides a safety factor and locks out fuel flow in the event of a system failure.

The acceleration-deceleration control precisely times the fumigator fuel. If fumigation occurs prematurely on acceleration or is late in turning off on deceleration, fuel is wasted. Late fumigation on acceleration produces a lean, poorly combusted power cycle. Drivers notice a power lag on acceleration. This control instantly senses a change in engine load and responds by properly adjusting the fumigation rate for the required diesel-delivery rate. The control output signal results in the opening or closing of the mixer control valve. This valve controls the fumigation fuel flow rate.

Properly sized LPG/CNG fuel tanks are necessary to provide fumigation for the quantity of diesel carried. These tanks must meet state and federal safety requirements. In some locations, installation position (i.e., inside the frame) is also regulated. The Diesel Fuel Limit Switch prevents excess fuel delivery when the fumigation system is active. This switch prevents over fueling which wastes fuel and produces exhaust smoke. This switch allows 100% of diesel flow in the event that fumigation fuel ceases to flow.

The microcomputer controls the functions of each of these devices. It performs the logic function to ensure that the system operates within design parameters. It also can sense malfunctions allowing the engine to default into a standard diesel, nonfumigated engine.

REFERENCES:: Algas Carburetion DFC 400 Diesel Fumigation Systems, Algas Corporation, 1991.; McCray, James. Fumigation of Diesel Engines with Gaseous Fuels: A Practical Solution to Urban Air Pollution. Unpublished report by Power Research.

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