You can understand a two-stroke engine by watching each part of the cycle. Start with the point where
the spark plug fires. Fuel and air in the cylinder have been compressed, and when the spark plug fires
the mixture ignites. The resulting explosion drives the piston downward. Note that as the piston moves
downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom
of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust
gases out of cylinder.
As the piston finally bottoms out, the intake port is uncovered. The pistons movement has pressurized
the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and
filling the cylinder with a fresh charge of fuel.
The Compression Stroke
Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the
compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the
crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor.
Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat
the cycle. It's called a two-stoke engine because there is a compression stroke and then a combustion
stroke. In a four-stroke engine, there are separate intake, compression, combustion and
You can see that the piston is really doing three different things in a two-stroke engine:
* On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel
mixture and capturing the energy released by the ignition of the fuel.
* On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in
air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so
that air/fuel is forced into the combustion chamber.
* Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and
exhaust ports drilled into the side of the cylinder wall.
It's really pretty neat to see the piston doing so many different things! That's what makes two-stroke
engines so simple and lightweight.
If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in
with the gasoline. In a four-stroke engine, the crankcase is completely separate from the combustion
chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings
on either end of the pistons connecting rod and the cylinder wall. In a two-stroke engine, on the other
hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it
can't hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod
and cylinder walls. If you forget to mix in the oil, the engine isn't going to last very long!
Disadvantages of the Two-stroke
** Ski-doo has since made some remarkable advancements to the efficiency of the 2 stroke engine. In some situations meeting or surpassing the efficiency specs of the majority of the 4 stroke models available this 10/11 season.
At the time of this writing, Yamahas 4 strokes remain at the top of the efficiency chart.
You can now see that two-stroke engines have two important advantages over four-stroke engines:
They are simpler and lighter, and they produce about twice as much power. However:
* Two-stroke engines don't last nearly as long as four-stroke engines. The lack of a dedicated
lubrication system means that the parts of a two-stroke engine wear a lot faster.
* Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You would burn
about a gallon of oil every 1,000 miles if you used a two-stroke engine in a car.
* Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon.
* Two-stroke engines produce a lot of pollution -- so much, in fact, that it is likely that you won't see
them around too much longer.
The pollution comes from two sources. The first is the combustion of the oil. The oil makes all
two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge
clouds of oily smoke. The second reason is less obvious, each time a new charge of air/fuel is
loaded into the combustion chamber, part of it leaks out through the exhaust port. That's why you
see a sheen of oil in the water around any two-stroke boat motor. The leaking hydrocarbons from
the fresh fuel combined with the leaking oil is a real mess for the environment.
These disadvantages mean that two-stroke engines are used only in applications where the motor
is not used very often and a fantastic power-to-weight ratio is important.
In the meantime, manufacturers have been working to shrink and lighten four-stroke engines, and
you can see that research coming to market in a variety of new snowmobile models.