Part of the issue in comparing emissions with these engines is that EPA and CARB emissions for small engines use a different standard than for autos. Autos are rated in units of grams per mile. Small engines are rated in Grams per KW output per hour. So, to convert units, multiply HP * 0.746 * the emissions rating to get the grams per hour, then divide by the miles ridden in an hour. (Assuming a fixed speed - which can vary depending on the gearing, engine size, drive, ...)
Further complicating the issue is that the EPA and CARB has different emissions limits for 2 stroke versus 4 stroke engines. 4-stroke emission limits are MUCH lower than 2-stroke limits... (In 2005, 2-stroke emission limits for hydrocarbons (unburnt fuel) and nitrous oxides were reduced from 72 grams per KW-Hour to 50 Grams per KW-H. In comparison, in 2007, 4 stroke limits were reduced to 8 grams per KW-H.)
Standard 2-stroke engines tend to pump a lot of unburned gas through the engine and out the exhaust, as the exhaust port opens and remains open while the inlet port opens. The exiting exhaust tends to 'pull' the fuel-air mix into the cylinder, but some of the fuel goes straight through... About 25 to 30 percent. This process, where the fuel bypasses the cylinder, is known as 'short-circuiting.'
You can add a catalytic converter to a two-stroke, to allow it to meet CARB emissions standards for two stroke engines. The fuel STILL short circuits the cylinder - it just gets burned inside the converter. Mitsubishi came up with a design wrinkle
http://www.mhi.co.jp/technology/review/pdf/e383/e383151.pdf that allows their TLE series engines to exceed CARB emissions standards by adding a second, air-only port. This port gets opened first and the portion of the incoming gas which 'short circuits' the cylinder is mostly air, instead of air-fuel; this reduces unburned fuel to something less than 5 percent of the total. Mitsubishi refers to this approach as a 'stratified scavenging' design. Stratified scavenging not only allows the engine to meet CARB 2-stroke emissions standards without a catalytic converter, it also increases fuel mileage, as the wasted fuel is reduced by over 80 percent.
If you assume a 2 HP, 2-stroke engine on a bike traveling at 30 MPH where the motor meets 2-stroke CARB emissions standards, you can calculate the equivalent EPA emissions as if for a car. In this case, the bike still emits 5 to 6 times the pollutants than a car does, which meets EPA highway emissions standards. (the main problem being the unburned hydrocarbons) Potentially, If you added a catalytic converter to the Mitsubishi design, you could probably get close to meeting EPA highway emissions limits.
4-stroke CARB Emissions standards are about 6 times more stringent (8 grams versus 50 grams emissions) than 2-stroke CARB standards, so, if you do the same calculations for a 4-stroke motor, (with the same assumptions) it would meet EPA highway standards.
Two stroke exhausts can be designed so that they are 'tuned' to a specific engine RPM, so that the pressure pulse from the exhaust is partially reflected back towards the engine, and it 'pushes' some of the unburnt fuel back into the cylinder just as the port closes. Although this helps, it only really 'works' at or near the pipe's design RPM, so in use, it only really helps part of the time.
The HT type 2 stroke engine puts out many times the emissions per cc of displacement than does the auto engine with emissions controls. This is in part due to the lack of emissions controls, and in part because the bike runs for at least twice as long per mile (at cruise.)
The CARBII/EPA emissions which these small engines must conform to are intended for weedeaters/generators, or other off road or non-highway applications.
CARBII emissions are in units of grams per KW of output per hour. For two stroke engines, the limit is 50 grams (unburnt HC + NOx emissions) per KW Hour. (4-stroke engines are only 8 grams per KW per Hour!) A HT engine is about 3.5 HP per Grubee. Assuming a 3.5 HP (2.6KW) engine that just meets CARBII emissions and travels at a constant 30 MPH:
2.6KW x 50 grams = 130 grams emissions per hour, for this motor.
You spread the 130 grams over 30 miles (1 hour at 30 MPH) and you
end up with 4.3 grams per mile emitted, for hydrocarbons and nitrous oxides.
130 / 30 = 4.3 gr@mile
2-cycle Carbon Monoxide emissions are allowed to be about 500 grams per KW per hour. Following the same calculations as above for CO yields 52 grams of carbon monoxide per mile.
2.6KW x 500 gr. = 1300 gr. emissions per hr.
then:
1300 / 30 = 43.3 gr@mile
Per the EPA:
The current Federal certification standards for
exhaust emissions from cars are 0.25 gram per mile HC, 0.4 gram per mile NOx, and 3.4 grams per mile CO.
So, the hypothetical bike meeting CARBII standards emits 6.7 times more HC/NOx emissions per mile than the car which just meets the EPA emissions limit. 4.3 / (.25+.4))
In addition, that bike generates about 12 times the carbon monoxide emissions per mile.
