I'D LIKE to begin this column with an apology. I'm sorry for ruining your day.
This story is about emissions. More specifically, it's about the surprising level of emissions spewing from on-road motorcycles and scooters. In California, such bikes make up 3.6% of registered vehicles and 1% of vehicle miles traveled, yet they account for 10% of passenger vehicles' smog-forming emissions in the state. In fact, the average motorbike is about 10 times more polluting per mile than a passenger car, light truck or SUV, according to a California Air Resources Board comparison of emissions-compliant vehicles.
For those of you who are wondering why I'm being such a killjoy, my reason is this: I've been hearing from an increasing number of readers who want to know if two-wheelers, which consume far less fuel, are also smog busters. Because scientific questions tend to come with complicated answers, I thought I'd do my best to explain what pollutants a gas-powered motorbike emits and why.
Motorcycles and scooters are, on average, about twice as fuel efficient as cars. Compact and lightweight, their internal-combustion engines do a better job of converting fuel into energy that makes the vehicle move. But extracting more energy from the fuel has a downside. It produces greater amounts of a smog-forming emission called oxides of nitrogen.
Oxides of nitrogen are one of three pollutants the Environmental Protection Agency and the Air Resources Board measure to see whether vehicles meet acceptable emissions levels and can be sold legally. Smog-forming hydrocarbons -- unburned compounds in fuel that escape through the tailpipe, fuel lines and gas tank -- are also measured, as is carbon monoxide. Carbon dioxide, a greenhouse gas, isn't measured by either agency, but motorcycles are generally better than other vehicles in this regard since they use less fuel per mile.
As with other passenger vehicles, there are technologies to offset motorcycle emissions, such as catalytic converters, but those technologies tend to be too big, too heavy or too hot to fit on a motorcycle and work as effectively as similar systems on larger, enclosed vehicles that have more space to accommodate them. That's why the EPA and the air board are more lenient on bikes than they are on other passenger vehicles.
"The emissions picture [for motorcycles] is fairly grim," said John Swanton of the Air Resources Board, "but we think it's fair for where motorcycles are today."
