Ah, the sounds of summer. Birds chirping, kids playing... and the unmistakable sounds of two-stroke outboard motors.

For decades two-stroke engines have been the standard motors used not only on boats, but also for snowmobiles, lawnmowers and any other contraptions that need lightweight powerful engines.

Unfortunately two-stroke engines have their drawbacks -- not only are they noisy, they are also major polluters.

According to one Environment Canada estimate, a 70-horsepower two-stroke outboard motor emits the same mass of hydrocarbon pollution in an hour as a new car does after driving 8,000 kilometres.

The problem is that anywhere from 30 to 50 percent of the fuel in a two-stroke engine passes through the combustion chamber unburned. In outboard motors, this unburned fuel is exhausted into the water, along with much of the oil that must be mixed with the fuel.

As there are about three million watercraft in Canada, the use of two-stroke engines contributes to a significant amount of pollution being dumped into the water and spewed into the air.

In two-stroke engines, the intake and exhaust cycles are combined into a single piston stroke, and a mixture of air and fuel blows the exhaust products out of the engine.

Four-stroke engines use four distinct piston strokes to control the fuel and exhaust cycles. As the piston shoves out the exhaust, less unburned fuel is emitted into the water and air.

U.S. Environmental Protection Agency (EPA) has been phasing in emission requirements for outboard engines and personal watercraft since 1998. California, which has the most stringent air quality regulations in North America, is moving even more quickly in this area. By 2006, only four-stroke engines will be able to meet the state emission regulations.

While these regulations affect only the sale of new engines, many jurisdictions have already banned powerful personal watercraft as they produce even more pollution than a standard outboard motor. The California Air Quality Board estimates that a personal watercraft used for seven hours will produce more smog-forming emissions than a 1998 passenger car driven for 100,000 miles.

Environment Canada's Environmental Technology Centre (ETC) has conducted tests on two-stroke outboard engines for a variety of compounds formed through the combustion process. The tests showed that two-stroke outboards produce 12 times as many of certain types of hydrocarbons as four-stroke engines, and five times as much oil and grease.

The ETC also compared exhaust emissions from a light-duty van, a 9.9-hp two-stroke outboard and a 9.9-hp four-stroke outboard. The two-stroke produced 50 percent more carbon monoxide than the four-stroke and nearly 60 times more than the van.

The two-stroke also emitted 15 times more unburned hydrocarbons than the four-stroke and nearly 125 times more than the van.

Canada also has plans for emission regulations in the future, and Environment Canada already has a voluntary agreement with the Canadian Marine Manufacturers Association to sell outboard engines and personal watercraft that comply with U.S. emissions standards.

While two-stroke technology is also advancing, mainly through the use of direct fuel injection, many companies have already recognizing four-strokes as the engines of the future. They cost about 15 to 20 percent more than a comparable two-stroke engine, but boat owners with four-stroke engines will save a significant amount of money on fuel.

And when the time comes to buy a new outboard motor, some boaters might make the decision on noise alone, happily forgoing the familiar roar of the two-stroke engine.