Are these Engines too Dirty?

Discussion in 'Laws, Legislation & Emissions' started by 2stroketech, Mar 7, 2011.

  1. 2stroketech

    2stroketech New Member

    Besides the whole global weather changy debate. What about how dirty these engines are or are not? Look I could care less about the debate, but I do like to keep my back yard clean and I like to save money. I feel that by saving money, I will have less of an impact on my surroundings. Now I also feel that there are some choices that are better than others.

    So what is the truth about these little engines? 2-strokes, 4 strokes, electric or compressed air. What are the pros, cons, cleanest, dirtiest and most efficient?

    Please try to keep the politics out of it.

  2. loquin

    loquin Active Member

    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.
    Last edited: Mar 8, 2011
  3. rogernelson

    rogernelson New Member

  4. machiasmort

    machiasmort Active Member

    As a fellow enthusiast and rider, you can plainly see the cost per mile in relation to fuel.

    It’s all about what is convenient to argue from a Political expediency point of view!

    What does common sense tell you? Is better for your children? That choice should still be yours to make but isn’t in the “HOME OF THE BRAVE”, anymore!

    Ask one of the Moderators to join the Combustion Chamber and I’ll tell you more!
  5. machiasmort

    machiasmort Active Member

    Lou, I did the best I could to understand what you were saying. Going by your calcs, would it be dirtier to ride my bike 30 miles or would I emit more with my car (going 30 miles)?
  6. loquin

    loquin Active Member

    It depends... If you're running a two stroke engine that meets CARBII emissions standards, compared to a car that meets EPA standards, then the bike would be 'dirtier.' By about 6 times. A 4-stroke meeting CARBII standards would be about the same as the car. (Because 4-stroke engines have a CARBII upper allowed emissions limit that is about 1/6th that of 2-stroke engines.

    Note that carbon dioxide emissions are proportional to the amount of fuel burned. So, if you get 150 mpg, and the car gets 30 mpg, the bike emits 1/5th the CO2 emissions of the car.