As can be seen from the graph below, this cycle occurs at low and medium loads during conventional two-stroke combustion.
Plot of cylinder pressure vs. time for conventional two-stroke at light and slightly reduced loads. The vertical peaks are firing cycles, and the area between peaks are misfires.
By igniting the entire mixture without the use of a spark, the EXP-2 is able to burn all of the fuel and oil in the cylinder in every cycle, eliminating the misfiring cycle described above. By eliminating misfiring, the motor actually burns much more of the fuel that it consumes; this reduces the amount of unburned fuel and oil released into the atmosphere, which greatly decreases hydrocarbon emissions. The graph of cylinder pressure below illustrates the resulting constant pressure, which indicates that the fuel is burning on each cycle.
Plot of cylinder pressure vs. time for EXP-2 two-stroke at light load. The smoothness of the graph indicates continuous regular combustion with no misfiring.
Honda’s riding team of Chuck Miller, Paul Ostbo and Greg Bringle finished 7th overall and first in class in the Baja 1000.
As a test for this technology, Honda built a 400cc single-cylinder bike for off-road and desert endurance racing. The 400cc single design was chosen because it has a large combustion chamber and a high piston speed, making for difficult burn characteristics; if the EXP-2 system works for this configuration, it will work for smaller piston engines. Fuel injection was also used for ease of setup and fuel measurement, although the system was designed to work with carbureted systems as well. The race results were very good even though the bike was not designed to win races, but to test new technology.
When the dust settled, the EXP-2 had earned 5th overall and 1st in both the under 500cc and Experimental classes at the Granda-Dakar rally; 1st in the two-stroke class and 8th overall in the Nevada Rally last year, and 7th overall motorcycle at the Baja 1000.
Check out a riding impression by our tech editor
Compared to Honda’s current NXR780 four-stroke twin rally race bike, the EXP-2 has very similar performance with several advantages. While the single cylinder EXP-2 produces 54hp to the big NXR’s 71, they both make 58 lb-ft of torque, but the EXP-2 is 118 pounds lighter, giving it a slightly better power-to-weight ratio. What all this boils down to is that the EXP-2 has about the same real-world performance as the 780, but with substantially better fuel economy and lower emissions. By increasing the mileage of the bike, it can be raced carrying less fuel, which improves handling and decreases rider fatigue. The problem of unburnt fuel escaping with the exhaust has yet to be solved, but for all you two-stroke die-hards out there: stay tuned, this is the start of something we have all been waiting for.