Honda has been researching and developing the Activated Radical Combustion (ARC) two-stroke engine for several years. As a showcase of the engine's viability and durability in the most arduous conditions, Honda's R&D Asaka Center successfully raced the experimental EXP-2, 440-cm³, single-cylinder, off-road motorcycle in the gruelling Granada-Dakar Rally in 1995, the event that covered 8000 km of mostly desert terrains and in which the two-stroke had been thought to be at a disadvantage for its inherent gas-guzzling trait. The EXP-2 placed fifth overall in the motorcycle category, proving its worth in this most arduous test of machines and riders.

Honda is now readying a production dual-purpose (on- and off-road) motorcycle employing the ARC technology. With the ARC, Honda is applying a "third" combustion process to the piston-compression, internal combustion engine. The first and second piston-compression, internal combustion engine types are, of course, the spark-ignited gasoline (and alternative fuels) engine and the compression-ignition diesel.

Honda observes that the "burning" of gasoline in an engine is not a simple, procedural process; mix fuel with air, add ignition source, bang, and let the process take its effect. It is chemically more complex, with a large number of intermediate chemical reactions. In essence, asserts Honda, combustion begins with an initiating reaction that forms highly reactive intermediate molecules, or activated radicals, from the stable fuel and air of the incoming charge. Propagating reactions continue with various molecules reacting with these active radicals. These propagating reactions from both additional reactant molecules and more active radicals to continue the combustion process, which goes on as long as it unleashes enough energy to continue the chain.

The ARC phenomenon was observed by many owners of two-stroke-powered motorcycles, generators, and other products, whose engines would continue running after the electrical ignition was shut off. This auto-ignition was generally attributed to a pre-ignition caused by hot spots in the combustion chamber, and engineers' attention was focused on eliminating it. Honda pays due homage to the researchers who had earlier discovered the role of active radicals in the auto-ignition phenomenon: Yakov Zoldvitch in the 1930s, and more recently notable efforts by Toyota's Masaaki Noguchi and his team (The Toyota-Nippon-Soken combustion, SAE 790840), and Shigeru Ohnishi of the Nippon Clean Engine Research Institutes who had actually constructed a stationary engine for generators on the active radical principle in the late 1970s.

In 1992, Honda started a small research project, under the direction chief engineer Minoru Matsuda, to determine if this auto-ignition process could practically solve the irregular combustion problem for two-stroke engines. The team, led by Yoichi Ishibashi, made surprisingly rapid and fruitful progress. While Ohnishi had earlier broadened the engine's auto-ignition range by throttling the flow of fresh charge from the crankcase into the cylinder (usual two-stroke breathing practices), this was not sufficient for a mobile power plant.

Honda engineers determined that, for any given engine load, the most important variables controlling the occurrence of the auto-ignition process were the temperature of the residual gas and the pressure remaining in the cylinder when the exhaust port was closed. That last value, explains Honda, pressure at exhaust closing, or PEC, could be readily regulated by a very simple mechanical design, a movable valve to throttle flow at the exhaust port, and thus retain higher pressures in the cylinder.

For a vehicle engine, the ARC auto-ignition process could not be the sole source of ignition over the engine operating range. So in the Honda ARC application, spark ignition is still employed at very lowest load conditions, such as at idling, for which there was so little incoming charge that sufficient temperatures could not be maintained, and at high loads for which there was too little residual charge and too little heat from the charge. The ARC could operate from roughly 5% of peak load to 60%, but it is most efficient from about 6 to 22% load, almost exactly the range at which irregular combustion causes the most problems in a conventional spark-ignited two-stroke engine.

The EXP-2 experimental motorcycle's 402-cm³ capacity was chosen to compete in the Granada-Dakar rally against a horde of large-displacement four-stroke competitors. Honda took its own NX4780 Vee-twin four-stroke offroader, whose performance and fuel consumption characteristics were placed under scrutiny, whereupon the EXP-2's specifications were determined. The EXP-2 employed an exotic PGM-FI fuel injection system, hitherto reserved to Honda's Grand Prix racers. The fuel injection was chosen for the computer's ability to be readily brought in to control the exhaust valve operation, not because it was essential for fuel feed.

The production-prototype ARC engine is a liquid-cooled, single-cylinder 250-cm³ unit breathing from a conventional side-draft carburetor. The electronic control unit controls the stainless-steel ARC (exhaust port control) valve which is actuated by an electric servo motor, ignition change-over between spark-ignition and ARC-auto-ignition, and carburetor idling and slow-jet. The ECU is fed with various information including transmission gear position, engine rpm, coolant temperature, and throttle opening angle.

Honda claims the following improvements with the ARC: