Diesel Engines Burn Leaner and Cleaner

Oct. 7, 1999
CIDI engine technology builds the ultimate diesel beast.

Sherri Singer
Associate Editor

The Partnership for a New Generation of Vehicles (PNGV) is a cooperative research and development program between the federal government and the U.S. Council for Automotive Research. The members include DaimlerChrysler Corp., Ford Motor Co., and General Motors Corp. A goal of the program is to develop technologies for a new generation of vehicles able to go 80 miles on a gallon of fuel, three times the range of comparable 1994 family sedans such as Ford Taurus, Chrysler Concorde, and Chevrolet Lumina. Year 1994 benchmarks when the program was established. In addition, the next-generation vehicles must be comparable to existing models in most other ways, as well as meet or exceed federal safety and emissions requirements.

One technology being considered for future automobiles is the compression-ignition direct-injection (CIDI) engine. These engines incorporate several features that let them meet the requirements levied by the PNGV. For example, there are four valves for each cylinder. An electronically controlled fuel-injection system supplies fuel to each injector from a high-pressure source by way of a common rail. On the aluminum block sits a variable-geometry turbocharger and exhaust-gas aftertreatment for nitrous oxide and soot emissions. CIDI engines are slated for use in hybrid-electric vehicles.

These features make an interesting comparison to those of diesels in production today. Currently, most diesel engines have two valves per cylinder. Fuel injection is via rotary pump and turbochargers have a fixed geometry. There are oxidation catalysts in the exhaust, and pneumatic actuators on the cast-iron structures. But, the difference between CIDI and traditional diesel engines is nominal. In CIDI engines, the higher heat resulting from the increased pressure in the chamber makes the mixture ignite, thus, compression-ignition. Direct-injection injects fuel directly into the entire combustion chamber as compared to an indirect-injection diesel, which distributes the fuel first to a prechamber and then spreads into the main chamber. Compression-ignition and diesel are essentially synonymous terms. Diesel is a process where the heat of compression is used to light the charge. So, in terms of combustion process, CIDI differs from traditional diesels only in that it is a specific form of diesel engine.

A low-sulfur or alternative fuel will power the CIDI engine. One fuel being considered is dimethyl ether (DME). DME is made from natural gas and is highly combustible in CIDI engines. It produces almost no soot upon combustion and does not increase other emissions such as nitrous oxide. Another possibility is to reformulate current diesel fuel. Sulfur levels in ordinary diesel fuel will contaminate many aftertreatment devices for nitrous oxide and soot emissions, making it necessary to tweak the recipe.

Overcoming challenges
Challenges facing mass production of CIDI engines include reducing nitrous oxide and soot emissions, engine weight, and costs. As far as emissions are concerned, research shows the hot air within the cylinder completely vaporizes fuel by the time it has traveled 1 in. from the injector. Also, combustion occurs in the vaporized fuel and produces small soot particles whose size and concentration increase as they move further down the combustion chamber. New studies show how the engine’s intake conditions can be designed to prevent liquid fuel from hitting cylinder walls. These results help design fuel injectors now in production.

Advanced catalytic converters and exhaust gas recirculation (EGR) cut emission levels further. EGR mixes exhaust gases with intake air to increase the heat of the charge, reducing nitrous-oxide emissions.

Two catalyst technologies for the CIDI engine are currently being developed. The first consists of catalyst materials supported on a ceramic block. It reduces nitrous oxide to nitrogen by reacting with hydrocarbons under low fuel-air ratio operating conditions. Hydrocarbon levels in the engine exhaust are low, so additional hydrocarbons must be added upstream of the catalyst to fuel the reaction. This lowers fuel economy by a few percent.

These catalysts have removed 30 to 35% of nitrous oxide in initial tests. Hydrous metal-oxide-supported powder catalysts have converted up to 70% of nitrous oxide in lab experiments at temperatures as low as 170°C. Hydrous metal-oxide-supported catalysts on alumina-cordierite cores show up to 60% reductions at temperatures of approximately 200°C.

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