Two-stroke engines run cleaner than ever

Outdoor power products like gas-powered chainsaws, weed trimmers, leaf blowers and water pumps really do run more cleanly and efficiently these days, and an incidental side-by-side comparison I stumbled upon this past summer proved it to me.

I first noticed something interesting minutes after firing up a new gas-powered weed trimmer I'd bought to supplement the 10-year-old model we've been using at our house. Old faithful still runs as good as new, I just wanted a second machine now that a couple of my kids are old enough to help me wrestle our big country lawn into shape. What I wasn't expecting was any obvious difference in exhaust gas cleanliness and fuel efficiency between old and new machines. But, as it turned out, the difference was unmistakable.

Burning the same gas-and-oil mixture from the same gas can, the exhaust from the new weed trimmer was odourless and invisible while the old model still puffed out its wisps of pungent blue smoke, just like it always had. And this difference is especially striking when you understand how dead-simple 'two-stroke' engines are. What could engineers possibly do to make them better?

The type of power plant in your car is called a four-stroke motor. This means it uses a complicated series of valves to operate, along with a reservoir of oil (that you need to change from time to time) to keep everything lubed. Two-stroke engines, on the other hand, (like those in my pair of weed trimmers), have none of the same valves. They're also lubricated by oil that's pre-mixed with gasoline. This makes two-stroke engines very simple, cheaper to build, lighter in weight and easier to maintain than four-strokes. On the downside, these two-stroke advantages have traditionally come with lower fuel economy and much dirtier exhaust emissions. In fact, when government emissions regulators first took steps to reduce small engine pollution about a decade ago, pundits predicted that the two-stroke engine would be forced into extinction because it was impossible to make them meet new air-quality standards. And while that prediction has proven true for some kinds of two-stroke motors, it hasn't eliminated all of them. Some have evolved to deliver remarkably low emissions and fuel consumption, as I learned first-hand back in August.

When our household water well pump broke this summer, we relied on a two-stroke gas-powered pump for several days to draw water from Lake Huron while I gathered hardware for the repair. That little pump used less than 300 ml of gas to fill a 1000 litre tank on the back of my pick-up truck, and it pumped all this water in less than 15 minutes. Amazing.

To answer the how-they-do-it question, I called Brad Lacroix, technical manager with ECHO Canada. And to understand his answers, you need a little technical background.

Almost all internal combustion engines use two parts called a 'piston and cylinder' to capture the energy released by a burning mixture of gasoline and air. And where these parts used to be made by factory equipment that drilled and ground the metal to shape, the ECHO two-stroke cylinders are actually cut with a kind of laser beam. This creates much smoother internal surfaces that seal better, last longer and do a better job preventing unburned gases from escaping into the atmosphere. The microscopic bumps and valleys on these laser-cut surfaces range from 5 to 10 microns in size. To put that into perspective, a typical human hair is a whopping 30 microns in diameter. We're talking very, very smooth and precise parts, here.

Another efficiency-boosting feature of the new two-stroke engines is the use of a feature called a 'squish band'. This is a raised area of internal castings that forces the gas-and-air mixture into the middle part of the piston only. When all combustion occurs here (as opposed to some happening around the perimeter of the piston) much cleaner and more complete combustion occurs. Add to this other refinements like variable spark timing, piston rings that dissipate 60% of all engine heat, and internal intake ports that prevent unburned gases from leaving the engine prematurely, and you enjoy some very serious efficiency gains.

If all of these technical details seem too complicated to bother with, don't worry. The bottom line is simple: today's top two-stroke outdoor power equipment runs so much more cleanly and efficiently than ever before, and it does it for reasons that would have seemed impossible just 20 years ago. And if this doesn't offer some hope for how technical progress can help us live both comfortably and responsibly at the same time, I don't know what does.