If you have read How Car Engines
Work and How Diesel
Engines Work, then you are familiar with the two types of
engines found in nearly every car and truck on the road today.
Both gasoline and diesel automotive engines are classified as
four-stroke reciprocating internal-combustion engines.
There is a third type of engine, known as a two-stroke
engine, that is commonly found in lower-power
applications. Some of the devices that might have a two-stroke
Lawn and garden equipment (chain
saws, leaf blowers, trimmers)
In this edition of HowStuffWorks,
you'll learn all about the two-stroke engine: how it works,
why it might be used and what makes it different from regular
car and diesel engines.
Two-stroke Basics This is what a two-stroke
engine looks like:
You find two-stroke engines in such devices as chain
saws and jet skis because two-stroke engines have three
important advantages over four-stroke engines:
Two-stroke engines do not have valves, which simplifies
their construction and lowers their weight.
Two-stroke engines fire once every revolution, while
four-stroke engines fire once every other revolution. This
gives two-stroke engines a significant power boost.
Two-stroke engines can work in any orientation, which
can be important in something like a chainsaw. A standard
four-stroke engine may have problems with oil flow unless it
is upright, and solving this problem can add complexity to
For a complete explanation of what horsepower is
and what it means to performance, check out How
advantages make two-stroke engines lighter, simpler and less
expensive to manufacture. Two-stroke engines also have the
potential to pack about twice the power into the same space
because there are twice as many power strokes per revolution.
The combination of light weight and twice the power gives
two-stroke engines a great power-to-weight ratio
compared to many four-stroke engine designs.
You don't normally see two-stroke engines in cars, however.
That's because two-stroke engines have a couple of significant
disadvantages that will make more sense once we look at how it
The Two-stroke Cycle The following animation
shows a two-stroke engine in action. You can compare this
animation to the animations in the car engine
engine articles to see the differences. The biggest
difference to notice when comparing figures is that the
spark-plug fires once every revolution in a two-stroke
This figure shows a typical cross flow design. You
can see that two-stroke engines are ingenious little devices
that overlap operations in order to reduce the part count.
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 shown
As the piston finally bottoms out, the intake port
is uncovered. The piston's 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, as shown here:
Note that in many two-stroke engines that use a cross-flow
design, the piston is shaped so that the incoming fuel mixture
doesn't simply flow right over the top of the piston and out
the exhaust port.
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 exhaust strokes.
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
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. Now that you understand the two-stroke cycle you can
see why. 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 piston's 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 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. So why do cars and trucks
engines? There are four main reasons:
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
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 but can be seen in the following figure:
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 around any
two-stroke boat motor. The leaking hydrocarbons from the
fresh fuel combined with the leaking oil is a real mess for
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 marine and lawn-care
For more information on two-stroke engines and related
topics, check out the links on the next page.