2-Stroke vs 4-Stroke
Boaters throughout Alaska utilize a variety of ways to fish, recreate, and access remote lakes, rivers, and coastline in the summer. One of the most common ways is by boat, whether it be with a 2-stroke or 4-stroke motor. Older two-stroke engines rely upon either mixed fuel, which combines a two-cycle oil with gasoline into one fuel tank, or the engine has a two cycle oil reservoir that allows the oil to be mixed at the carburetor or injector before burning. A four stroke engine requires only unleaded fuel for power; oil is added only for lubrication into a crankcase, similar to automobiles. Motor manufacturer and technology can have a marked impact on fuel efficiency.
In order to understand just how much oil and gas may be entering our lakes and rivers, it is important to consider the type and horsepower of the engine in use, as well as its inefficiency. The amount of hydrocarbons contributed by any one 2-stroke is variable based on motor manufacturer, age, maintenance, and operation. Fuel burn rate and discharge can increase with motor horsepower and operation. The chart above shows the burn rate of five different types of 115 hp engines.
To calculate the number of gallons each motor would discharge, burn rate is multiplied by the percent inefficiency. At an inefficiency of 4% (average value used to represent a 4-stroke motor), at idle the Yamaha would discharge 1/50th of a gallon, at cruising speed 1/6thof a gallon, and wide open throttle 1/3rd of a gallon of gasoline in an hour. At an inefficiency of 27% (value used to represent a carbureted 2-stroke motor) discharge at idle would be 1/7th of a gallon, at cruising speed 1 gallon, and wide open throttle 2.5 gallons of gasoline per hour.
Increasing motor efficiency will result in lower total aromatic hydrocarbons (TAH) discharge per boat. This is why having more 4-stroke motors is desirable from a water quality standpoint.
Another way to look at it is that Big Lake could accommodate more 4-stroke motors and still meet water quality criteria than if there were as many carbureted 2-strokes on the lake.