All these small engines emit far less carbon dioxide than a car does, as CO2 emissions are proportional to fuel usage.
Part of the issue in comparing emissions with these engines is that EPA 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. (Assume 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.'
Now, 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 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.
That being said, if you assume a 2 HP, 2-stroke engine on a bike travelling 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 emit 5 to 6 times the pollutants than a car does, which meets EPS 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.
Now, 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...
Here's a
link to the calcs I did.
Now, 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.
