Strokes Go Up, Pollution Goes Down

Coast Guard Boat

So, after a couple of seasons with my new four-stroke 3.5hp engine, (had to drop down to 3.5hp or face putting 80 pounds on the transom where formerly 40 pounds of 5hp had ridden), the engine just wasn’t as friendly to an all-out-door environment as the old two-stroke was. Happily, my mechanic was able to address — and explain —what the changes from two-stroke to four-stroke means. This column is about that.

What is a Stroke?
The basics of a two-stroke engine versus four-stroke engine has to do with what is known as “Thermodynamic” cycle. Basically, in a two-stroke engine, the beginning of a compression stroke and the end of the combustion stroke also occurs at the same time as the intake and exhaust functions. In a four-stroke, each of these functions takes place within its own stroke.

The return of the engine to its original position after going through these strokes is the “Thermodynamic” cycle — two strokes for the old model and four strokes for the newly legislated, current state of the art.

So, What’s the Problem?
A decade ago, the two-stroke motor, found on 75 percent of all boats and personal watercraft (jet skis), generated 1.1 billion pounds of hydrocarbon emissions each year. This was the annual equivalent of spilling as much oil and fuel into US waterways as the Exxon Valdez 15 times over.

These high emissions were attributed to the design inefficiency of the two-stroke motor, which had remained essentially unchanged since World War II. What made it so inefficient?

Largely, it was this — about a quarter of the fuel and oil, mixed directly into the furl, went unburned, and thus was emitted directly into the water and air. Imagine having the gas dock operator selling you three gallons of gasoline and charging for four gallons, putting the pollution aside.

The EPA estimated that one hour of operation by a 70-horsepower, two-stroke motor emitted the same amount of hydrocarbon pollution as driving from New York to Los Angeles in a modern automobile — and back.

Four-stroke engines emit 97 percent less pollution than conventional two-strokes. Why? Simple: four-stroke outboards use the same combustion process used in automotive engines.

This means that, unlike the two-stroke engine, four-stroke engines never have an exhaust and intake valve open at the same time. This keeps any unburned fuel from being ejected from the engine.

Also, four-stroke engines don’t require additional lubricating oil to be blended in with the gasoline to operate and aren’t part of the exhaust, unburned or otherwise.

Like your car engine, the lubricating oil is a separate system within the engine complex, stored in the crankcase, unmixed with the gasoline. Also, they burn hotter, (read: more efficiently) and thus are more efficient at gas consumption. Manufacturers quote as much a 25 percent better fuel mileage.

Of course, slowing down would help even more — tests show that you burn over 50 percent more fuel at WOT than at mid-ranges. See what the effect on your speed over the water is if you just throttle back.

OK, So What’s the Problem Now?
Well, they are heavy. When I went to replace my five-horsepower engine, I found that the four-stroke replacement weighed 80 pounds — twice what the two-stroke engine weighed that it was to replace.

When I contemplated trying to get that engine in and out of an eight-ft dinghy, floating dock-side, I said, “Ah, that’s not going to happen without me and/or the engine ending up in the drink.

What’s the next size down?” Three and a half hps in a four-stroke weigh about what five hps do in a two-stroke. A car engine, not a weed wacker!

But the two-stroke guys haven’t missed some this dialogue. The latest technology response is direct fuel injection. The required fuel mixture is injected directly into the cylinder after the piston passes the exhaust port. This prevents any unburned fuel from being prematurely forced out of the engine.

DFI retains the advantages of a two-stroke engine’s efficient power cycle and lighter weight, and greatly lowers pollution levels normally generated by two-strokes. In short, they’re not your father’s two-strokes any more.

All this can’t be bad for boaters…

Many thanks to Roy Bartel of Remsenburg Marina for his staunch support of US Coast Guard Forces…

BTW, if you are interested in being part of USCG Forces, email me at [email protected] or go direct to the D1SR Human Resources department — the folks there are in charge of new members matters and can help you “get in this thing . . .”

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