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Two Stroke Engines – Examining Today’s Outboard Engines

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Examining Today’s Two Stroke Engines

In last month’s article we discussed the basic differences in how two and four stroke outboards operate and some of the common perceptions of each of these motors. This month we will attempt to dispel some of the misconceptions of two stroke engines and explain the technology behind these modern outboards. Next month we will take a look at “Dispelling the Myths of the Modern Four Stroke Outboard Motor

When looking at the vast array of outboard options on the market today, some potential outboard purchasers have dismissed modern two stroke motors due to some common misconceptions. To do so would be a mistake – the two-stroke of today is vastly different from those of years ago.

Common misconceptions for modern two stroke outboards include that they are loud, rough-running at idle, smoky and inefficient at low RPMs. These perceived disadvantages are in many cases no longer relevant. Gone are the days of smoky carbureted two-strokes where oil is pre-mixed with the fuel in the fuel tank. Even the later development of VRO type oil injection, where a pump adds oil to the fuel, is also gone. Because of current technology, the days on the water that were once ruined by an oil warning buzzer going off are also a thing of the past. Technology has answered all of these issues. Technology and design advancement have in fact allowed two strokes to be a viable and smart choice for many boaters looking for clean, powerful and efficient new power.

Editor’s Note: Find out more on Top Tips to Make your Boat FUEL EFFICIENT

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There are a few manufacturers producing modern two-stroke motors that have each answered many of these misconceptions with their own technology and have in turn created new two-strokes that are quiet, clean-burning, smooth and very fuel efficient. Motors by major brands such as Evinrude, Mercury Marine and Tohatsu are currently being manufactured for the US market in two-stroke models. The common factor between all of them is that the carburetor has been replaced by different forms of direct fuel-injection. Direct injection, or DI, has become the game changer in two-stroke motors. This has resulted in a cleaner, more fuel-efficient operation that meets today’s tighter EPA emissions guidelines. By injecting pressurized fuel directly into the combustion chamber of each cylinder, the precise amount of fuel ignites easily and is burned efficiently in the combustion chamber, with virtually no wasted fuel and cleaner exhaust. With direct injection there is no polluting un-combusted fuel leaving the engine. In fact, in today’s two-strokes there also is a complete burn of oil, resulting in much cleaner emissions and better fuel and oil economy.

Utilizing its own form of direct injection, Evinrude is claiming the cleanest emissions of any new combustion engine outboard in today’s market. Oil and fuel never see each other in engines such as the E-TEC engine, as oil is only being used for lubrication purposes. As a result, the amount of oil required is much less than an older model two stroke. This is yet another myth-dispelling fact that is helping to make a two stroke motor a viable choice.

Some manufacturers like Evinrude have even taken direct injection further by stratifying the injection at lower RPMs to improve fuel efficiency. Because the fuel is directly injected into the combustion chamber at a precise point in time, the onboard computer can determine that a much smaller amount of fuel is needed at lower RPMs and delivers only the amount of fuel required to create enough energy to move the vessel at the lower speed.

The easiest way to understand this technology is to imagine the combustion chamber as a closed room. Stratified injection would be the equivalent of an aerosol can in front of a lighter (the spark plug). When the lighter ignites only the gas exiting the can ignites. This produces just enough energy to move the vessel at the slower speed. Now as the throttle is applied and the RPMs increase, the whole room is filled with gas and the lighter then ignites, creating a much larger ignition, filling the whole room (or combustion chamber). This technology results in incredibly efficient fuel usage at the lower RPMs and still only puts the exact amount of fuel at the optimum time no matter how much power is needed. As a result these two stroke engines can provide excellent fuel economy, surpassing four stroke engines at lower RPMs and still be more efficient at higher RPMs, dispelling myths of poor performance and poor emissions at low RPMs.

Another misconception two stroke motors have endured is that of being threatened by ever increasing EPA emissions requirements. According to top officials at Evinrude and other manufacturers, this is not a problem. Current offerings such as the Mercury OptiMax, Evinrude E-TEC and those from Tohatsu all surpass emissions expectations for 2013. As technology advances these motors have the capability of being improved further as requirements dictate and these companies are already working to meet future requirements. The fact is that if outboards of the future require such devices as catalytic converters, they would in fact be at an advantage, as even with these devices attached they would be lighter and smaller than most four-strokes with the same requirement.

Simply put, emissions restrictions will not eliminate the two-stroke outboard from the market. Because motors such as BRP’s have such clean burning emissions, two-strokes have been able to capture a large market share, with government agencies adhering to strict guidelines and also in locations with strict emissions standards that many four-strokes could not provide. BPR, Mercury and Tohatsu have had similar results not only in the US but also abroad, where even more stringent requirements exist.

Companies have such faith in the future of two-strokes that they are utilizing them exclusively in their outboard line and are even extending this new technology to other markets.

Another perceived disadvantage of two-stroke motors is that they are noisy. This is no longer relevant. Technology and soundproofing have helped reduce noise levels comparable to those of four-strokes. Some still are quieter than the two-stroke of the past while being noisier than a modern four-stroke, but by design. Mercury has created a quieter two-stroke in their OptiMax while still maintaining a throaty performance sound. Noise should no longer be a deciding factor in the choice of two vs. four strokes.

While many of the conceived disadvantages of the two stroke outboard have been answered, many of the advantages hold true today. The simple fact is that the physical engineering of the two stroke engines allows for less parts and as a result less weight and physical size, along with simpler, less expensive maintenance. Unlike the four-strokes, a two stroke engine does not need oil changes every 100 hours or every season. The design of the motor does not require all the parts or the maintenance of parts you would be required to have with a comparable four-stroke. Motors such as the E-TEC do not require a break-in period or 100-hour services and in fact have a 3-year or 300-hour service period. In the long run this can equate to substantial savings over a comparable four-stroke and can avoid post purchase realization of service requirements and costs. Some models of two stroke engines even have the ability to self-winterize with a simple procedure at the throttle, taking just a few seconds. Many would argue that the modern two stroke engines require less maintenance than older two stroke engines and less maintenance than current four-stroke motors.

Performance, responsiveness and lighter weight along with developments in technology such as direct injection and product specific enhancements have made the modern two stroke engines a viable and intelligent choice. This modern technology dispels the common perception of two stroke engines and has brought them to the forefront of technology and outboard selection. With best-in-class emissions, outstanding fuel economy, lower maintenance costs and high performance these engines deserve serious consideration.

Glenn Hayes contributes regularly to All at Sea SOUTHEAST. Look for Part III of his ‘Outboard Roundup’ in our May issue.


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  1. I couldn’t agree more.
    Its safe to say that technology has finally delivered on the promise of the 2-stroke. I’ll stick my neck out and say it is MORE efficient now than a 4-stroke, because much less of its energy is wasted driving machinery and extra weight.
    This development started about 30 years ago, and has reached a pinnacle only dreamed of back then.
    I knew the day they stopped burning oil in the combustion chamber and went to direct lubrication, well, that would be the tipping point.


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Glenn Hayes
Glenn Hayeshttp://www.HayesStudios.com
Glenn Hayes is a writer and photographer based out of west central Florida and has marine industry background spanning almost a quarter century. He can be reached through his web site www.HayesStudios.

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