By Mike Milne

Four-stroke engines generate power using four strokes of each piston — fuel and air intake, compression of fuel mixture, combustion and exhaust — with valves allowing separate intake of fuel and expulsion of exhaust gases.

Two-stroke technology, with intake of fuel mixture plus compression on one stroke and combustion plus exhaust on the other, initially dominated the marine outboard engine field in the 20th century.

In spite of inherent light weight and high torque, early two-strokes also produced more emissions than four-strokes, because exhaust and fuel ports were open at the bottom of the power stroke, allowing some of the fuel and oil mix escape with the exhaust. But by the early 1980s, environmental regulators demanded lower emissions.

With lower exhaust emissions as a built-in advantage, four-stroke technology was adopted by most outboard manufacturers. Now, four-stroke marine outboards also boast other advantages:

  • Many four-strokes are so quiet, boaters often have to check for exhaust water flow at idle to see if the engine is running. Lower noise levels were an initial advantage over two-strokes and builders have worked hard to keep it. The new 2.1-L. 75- to 115-hp Mercury FourStroke outboards launched for 2015 have a sealed top cowl and idle relief muffler to make the engines even quieter.
  • Four-stroke outboard engine manufacturers have continued to reduce emissions through the addition of electronic fuel injection (EFI) and electronic engine control, in areas such as air induction, spark tuning and idle speed that also add fuel efficiency.
  • With research and development focused on four-stroke development, more advanced marine outboard technology has emerged. Mercury Marine signaled its commitment to four-strokes in 2004 when it launched the supercharged Verado lineup. Now available from 175 to 350 hp, Verado outboards are quiet and smooth, with low emissions, electro-hydraulic power steering and good fuel mileage. The newest version is a 400-hp Mercury Racing model.
  • With lubrication provided by oil in the crankcase, four-stroke outboards do not need oil in the fuel mix, unlike two-stroke engines. Initially a strong environmental advantage for four-strokes, that still provides convenience and peace of mind.
  • Light weight was initially a two-stroke advantage but four-stroke outboard manufacturers are working hard to reduce weight and gain competitive advantage. The Mercury 75- to 115-hp FourStrokes had a 20-lb weight advantage when launched in 2015 and the 150 FourStroke, introduced in 2012, cited a 19-pound advantage over competitors.

With the automotive industry solidly committed to four-stroke engine technology along with the bulk of the outboard manufacturers, four-stroke has an advantage in market and design momentum that is sure to bring even more new advantages and innovations. Increases in efficiency and reductions in weight will bring improved performance. That’s always good news for boaters.