Two-stroke engines on comeback trail? TORONTO STAR FILE PHOTO...

Two-stroke engines on comeback trail?
TORONTO STAR FILE PHOTO
The Saab 96 (that's a 1960 model, above) disappeared from North American showrooms 40 years ago. It, along with the Sonnett II, were the last two new cars powered by two-stroke engines.

Apr 28, 2011



Gerry Malloy
Special to the Star

It has been more than 40 years since the last new cars powered by two-stroke engines (the Saab 96 and Sonett II) disappeared from North American showrooms, although they lived on in the infamous East German Trabant until the 1990s.

Today, you?re most likely to find a two-stroke (or two-cycle as it is sometimes called) powering a chainsaw or lawn trimmer. Four-stroke engines have gradually taken over much of the two-stroke?s traditional territory, which included outboard motors, off-road motorcycles and snowmobiles.

But the two-stroke is far from dead. In fact, because of its simplicity and efficiency, it is being considered for use in automobiles once again.

Simplicity, and thus low cost, has always been the two-stroke?s strong suit. In its most basic form, it comprises just three moving parts: a piston, a connecting rod and a crankshaft.

If you?re familiar with how an internal-combustion engine works, you know that there are four steps necessary to produce each power stroke: exhaust, intake, compression, and combustion (the power stroke).

In a four-stroke engine, such as those used in most cars today, each of those steps occurs during its own distinct upward or downward stroke of the piston ? hence the name four-stroke. That means the crankshaft must rotate twice to achieve one power stroke ? not the most efficient of operations.

A two-stroke engine performs all four functions in just one rotation of the crankshaft ? two piston strokes, one up and one down ? hence, the name two-stroke.

It does so by incorporating the intake function into the beginning of the compression stroke and the exhaust function into the end of the power stroke.

Invention of the two-stroke cycle is attributed to a Scottish engineer named Dugald Clerk, who patented his design in 1881. Since then, its implementation has been accomplished in a multitude of different ways, particularly in terms of intake and exhaust configurations.

The simplest approach uses open intake and exhaust ports in the side of the cylinder. As the piston moves up at the beginning of the compression stroke, the air-fuel mixture enters the cylinder through the intake port; as the piston moves past that port it blocks it off and the mixture is compressed.

As the pressure of combustion forces the piston down, it moves past an open exhaust port through which the remaining gases can escape.

The most common form of two-stroke engine today employs a loop-scavenging system, whereby the incoming air-fuel mixture first goes into the crankcase, below the piston, which pressurizes the mixture during its downstroke, before forcing it into the cylinder when the intake port is open.

Eliminating two non-power-producing strokes means the two-stroke is more efficient than the four-stroke, by up to 50 per cent. And it can achieve a much higher power output for a given engine size.

Which prompts the question, why hasn?t it become the norm?

Unfortunately, the two-stroke also has some negative aspects, chief of which are high exhaust emissions ? in part because of the simplicity of its intake and exhaust systems, which limit the ability for complete combustion.

Because the crankcase is part of the intake system in many applications, it cannot be used as an oil sump, so oil must be mixed with the fuel to lubricate the engine?s internal parts. That oil does not fully combust, further increasing exhaust emissions.

In addition, two-strokes operate most efficiently with an unfettered exhaust system, which tends to make them noisier than four-strokes.

Those negatives are the primary reason there are no two-stroke cars today. But the potential efficiency gains two-stroke engines offer mean there is more work going on to overcome them today than ever before.

For example, Lotus Engineering, working in collaboration with Queen's University Belfast and Orbital Corp. of Australia, unveiled a two-stroke research engine it called Omnivore at the 2009 Geneva auto show. With direct injection, variable compression ratio and a host of other high-tech features, it is being developed with automotive application in mind.

In the United States, Michigan-based EcoMotors International is developing a unique opposed-piston, opposed-cylinder, diesel two-stroke that is said to offer twice the power-to-weight output of a conventional engine.

Peter Hofbauer, who was responsible for development of VW?s first diesel and narrow-angle VR6 engines, conceived the design and is the company?s chairman. Don Runkle, who held several senior engineering positions in a 30-plus-year career at GM, is its CEO.

In Kansas, Grail Engine Technologies is working on a two-stroke concept with many of the same features as the Lotus engine that is said to have received interest from Honda.

And in Massachusetts, Scuderi engine is well along in the development of what it calls a split-cycle engine that uses parallel cylinders ? one for intake and compression, one for power and exhaust ? to produce one firing stroke per revolution.

With all that smoke, surely there will be some fire on the horizon.


http://www.wheels.ca/article/795282