Is There A Stroker In Your Future?
IS THERE A STROKER IN YOUR FUTURE?
IN 1993, HONDA QUIETLY announced that it had found a way to cut two-stroke emissions and fuel consumption, yet retain the usual virtues of lightness, simplicity and low cost. The bike built to showcase this Activated-Radical Combustion technology was the EXP-2, which finished fifth in its Granada-Dakar Rally debut.
Honda’s idea is not the widely acclaimed direct injection, as popularized by the Australian Orbital engine. Instead, it is a chemical method of initiating combustion; the only unusual part is a movable exhaust gate that retains hot exhaust gas within the cylinder. The heat in this gas breaks down fuel molecules, forming active fragments called radicals. During compression, these radicals create a cascade of further breakdown, culminating in actual hot ignition. If you’ve seen a two-stroke engine with an air leak “run-on” after its ignition has been shut off, you’ve seen the uncontrolled phenomenon.
Such combustion has existed for many years, but Honda has learned to control it. The exhaust gate controls combustion timing by adjusting the amount of exhaust retained as engine load and rpm vary. Although the gate in the fuel-injected EXP-2 is computercontrolled, less-sophisticated production engines will operate with carburetors and, probably, mechanical-pneumatic gate control.
The EXP-2 has a full-time spark ignition, but combustion is radical-initiated from about 5 percent load up to 50-60 percent load. Below 5 percent, too little exhaust heat is generated to achieve auto-ignition, and above 60 percent, the cooling effect of the large incoming charge is too great. The engine’s idle is irregular like a classic two-stroke’s, but as the throttle opens slightly, it becomes as smooth as a fourstroke and pulls from low revs without misfire across an incredibly wide powerband.
The payoff is that the region of radical-initiated combustion exactly covers the region of maximum two-stroke exhaust emissions. At 0-30 percent throttle, a conventional twostroke fires irregularly because there is little fresh charge and a lot of exhaust in the cylinder. It may take several misfiring revolutions to accumulate enough mixture in the cylinder to fire. This produces the twostroke’s irregular “ring-ding” sound-and its heavy discharge of unburned hydrocarbons from the misfiring cycles.
But AR-Combustion doesn’t depend on happenstance conditions between a plug’s electrodes; it depends only on average conditions in the cylinder. As the piston rises and active radicals are created by controlled exhaust-gas heat, the charge auto-ignites at many points in the cylinder-every time. Without misfire, emissions and fuel consumption are drastically reduced. In fact, the EXP-2 proved to be more economical than its four-stroke competitors.
At a recent press seminar held at its Torrance, California, headquarters, we were told why Honda is pursuing this technology: Asia’s developing countries need cheap transportation, but excessive pollution has forced them to impose emissions standards. Honda believes it can supply the needed vehicles by using twostroke engines operating on AR-Combustion.
In preparation for this seminar, the director of the Asaka R&D Center, Mr. Matsuda, was twice sent to the U.S. That’s serious. Production is almost certain to begin in some markets soon.
But why tell the story here? Because Honda believes that this technology can enable other kinds of vehicles, such as snowmobiles and watercraft, to meet forthcoming U.S. emissions regulations. The message is clear: Licensing Honda’s technology is a cost-effective, simple way for two-stroke engine producers to meet these new standards. The alternatives-developing four-strokes or direct-injection twostrokes-are arguably more complicated and expensive.
Could two-stroke streetbikes have a place in our future? Does AR-Combustion have future applications in fourstrokes? We are listening.
Kevin Cameron
