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Protecting Alaska's Cook Inlet watershed and the life it sustains since 1995.
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2-Stroke vs 4-Stroke

Cook Inletkeeper has been conducting outreach and education regarding 2-stroke boat engine motors and hydrocarbon pollution in the Cook Inlet watershed.

What's the difference?

2-stroke engine diagram
2-stroke engine diagram
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.


Watch this video to learn more about how engines work.

Burn rate for five types of boat engines (source: Popular Mechanics)
Burn rate for five types of boat engines (source: Popular Mechanics)
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 to the right 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 1/6th of 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.

Water Quality in our Waterbodies

In response to repeated high levels of hydrocarbons observed during 2000-2004 water quality sampling, the ADEC declared the lower 19 miles of the Kenai River as “Impaired” in 2006. From 2005-2007, the Kenai Watershed Forum (KWF), in collaboration with the Kenaitze Indian Tribe, implemented a two-stroke buy-back program to encourage boaters to upgrade their engines and remove ageing carbureted engines from the river. Despite boater participation in the buy-back program, continued water quality exceedances required the state to take actions to eliminate the source of the pollution. In March 2008, the Alaska Department of Natural Resources Division of Parks and Recreation banned the use of two-stroke engines, with the exception of direct fuel injected (DFI) engines, within the Kenai River Special Management Area. Water quality testing results from the summers of 2008 and 2009 indicated a greater than 70% reduction in total aromatic hydrocarbons (TAH) concentration as compared to the 2007 data. While water flow and volume was greater in 2009, research showed that the reduction in hydrocarbons was most likely due to the motor restriction regulations enacted in 2008.

Water quality studies done on the Little Susitna River from 2007-2011 found that about 40% of the motors on the river were older 2-stroke engines, ranging from 10-60% depending on the day. Engine horsepower on the Little Su ranges widely, from 25 to 225 hp, averaging 65 hp across all engine types. Big Lake 2009 & 2013 water quality studies showed that approximately 13% of boats used carbureted 2-stroke engines. Though that may not seem like a lot of boats, 2-stroke engines contribute approximately half of the petroleum hydrocarbons (oil and gas) in the lake. Boaters typically use higher horsepower engines on Big Lake, averaging about 90 hp across both engine types.

Personal Watercraft (PWC) or Jet Skis

Brown bear on jet ski near King Salmon, AK
Brown bear on jet ski near King Salmon, AK
So what about Jet Skis? Using personal watercraft (PWC) is another option that boaters have to enjoy the lakes in the Susitna Valley. Big Lake is a popular place to ride Jet Skis, where they account for nearly 25% of all watercraft, and the banks of the lake are often lined with colorful PWC. Jet Skis are propelled as the engine-powered impeller sucks in water from beneath the watercraft and shoots a high-powered jet of water behind it. Most, if not all, of the newer sit-down PWC are equipped with highly efficient, fuel injected 4-stroke engines. However, many boaters in the Valley still operate older stand-up 2-stroke PWCs that are not as fuel efficient and may leak fuel and oil into the water. Below are some tips to keep our waters clean and prevent fuel and oil spills, regardless of which kind of PWC you own.
  • Fuel and change your oil in your PWC on land, and not on the banks of the lake. This ensures drops or spills won’t contaminate the water.
  • Carry oil absorbs in your cargo compartment in case you do have a spill or leak.
  • If your PWC has a bilge or cargo area that could have oily water in it, empty it away from the boat launch and over gravel or grass.
  • Watch this video to learn more about engines in PWCs (filmed by SBT manufacturer and supplier of PWCs in Clearwater, FL).