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So here we have a four-stroke engine
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and here we have a two-stroke engine. If we observe
the anatomy of these two engines upon first glance
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they seem very similar. They both have a crank case
and a crankshaft together with a connecting rod
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as well as a wrist pin and a piston
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they also both have a cylinder
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and they both operate by converting
the reciprocation of the piston
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into the rotation of the crankshaft, which then turns
the gears of a transmission and ultimately
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the wheels of the vehicle. But beyond these core
similarities the four-stroke and the two-stroke
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start to fundamentally diverge resulting in two
very different approaches to internal combustion
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And what I want to do today is give you the
only video you need to watch to get a very
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solid understanding of how a four-stroke and a
two-stroke engine work. We'll also be exploring
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the differences, benefits and drawbacks of each
engine type, and this will then answer the question
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of why the four-stroke ultimately prevailed over
the two-stroke in most applications despite being
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larger, heavier, more expensive and more complex.
So let's get started with the four-stroke,
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Why is it called a four-stroke? It's because the
four-stroke engine needs four strokes to complete
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one combustion cycle. Every time the piston moves
from top to bottom or vice versa that's one stroke
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One stroke of the piston equals 180 degrees of
crankshaft rotation. Now the four strokes are:
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intake, compression, combustion and exhaust. During
the intake stroke the piston travels from top to bottom
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Which are also known as top dead center
and bottom dead center. As the piston does this
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it creates an empty space or vacuum inside the
cylinder. This newly created void is essentially
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a brief absence of air and because we have an
absence of air we also have an absence of air pressure
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In other words, we have low air pressure
inside the cylinder and atmospheric air pressure
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outside the cylinder. This air pressure difference
cannot continue to exist and air naturally seeks
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to equalize pressure everywhere. And so air
together with fuel from the outside rushes
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into the cylinder and fills it with a fresh air
fuel mixture. By the time the piston reaches BDC
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all the air and fuel that can hope to
get in have done so and the piston now starts to move upward
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As it does that it forces the air fuel
mixture into an ever smaller space. In other words,
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it's compressing the air fuel mixture which is
why this stroke is called the compression stroke
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Just before the piston reaches top dead center the
spark plug fires and ignites the air fuel mixture
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which finds itself between the two electrodes of
the plug. Although combustion inside an engine is often
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described as a bang or explosion that isn't what's
actually happening. An explosion is detonation
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which is a rapid uncontrolled process. In contrast
to this combustion is deflagration which is a much
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slower, more even and controlled process. Combustion
spreads out evenly outwards from the spark plug through heat transfer
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The small portion of air-fuel mixture initially ignited by the spark plug
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heats up and ignites the next layer of the air -fuel mixture. This layer then ignites the next layer
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and the process continues until all of the air
fuel mixture is burned. As the combustion flame front
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spreads it rapidly raises the temperature
and pressure inside the cylinder.
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Because the cylinder is sealed this pressure has nowhere
else to go so it ends up pushing the piston
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down the cylinder with great strength. This is our
combustion stroke and of the four strokes this is
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the only one that actually generates power,
and it does so by converting the energy released
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by combustion into motion of the piston which then
turns the crankshaft and ultimately the wheels
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By the time the piston reaches bottom dead center
again all the air fuel mixture has been burned
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and we now have exhaust gas or the remains of
combustion inside the cylinder
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As the piston moves up once again the exhaust gases leave
the cylinder and exit through the exhaust piping,
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catalytic converters and mufflers out into the
atmosphere. Now if you know a bit about engines
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then you've probably noticed how my explanation of
the four-stroke combustion cycle fails to answer
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some very important questions. And these are: how do
we allow air and fuel to get in during intake?
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How do we keep air and fuel as well as combustion
energy from escaping during compression and combustion?
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And how do we allow exhaust gases out?
Well the answer to all of this is the same thing
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and it's valves! The intake valve opens during
the intake stroke to allow air and fuel into the cylinder
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Both valves are closed during
the compression and the combustion stroke
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to prevent air and fuel, as well as combustion
energy from escaping the cylinder. And finally
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the exhaust valve opens during the exhaust stroke
to allow exhaust gases to escape the cylinder
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Now the valves are operated by the camshaft and
as you can see the camshaft has lobes on it
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As the lobe contacts the rocker arm the
rocker arm pushes onto the valve and opens it
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As the lobe releases the rocker arm the valve
spring ensures that the valve returns to its seat
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as soon as possible. The shape of the
lobe determines how much the valve opens and
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how long it remains open. The higher the lobe
the more the valve opens or the more left we have
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The broader the lobe the longer the valve
remains open and the more duration we have
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But for the engine to run well we must ensure that
the motion of the camshaft is synchronized to
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the motion of the crankshaft and the piston.
This is done by a cam belt or cam chain
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It connects the crankshaft with the camshaft to
ensure that the correct valve opens during the intake
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That both remain closed during compression and combustion. And that the correct valve opens during exhaust
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As you can see our chain is very
slack and this is because our display model
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doesn't have chain guides and tensioners which are
normally present to ensure proper operation of the chain
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As you can see the four-stroke engine needs
a lot of mechanical parts to get gases in and out
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of the cylinder. These parts are a source of weight
and complexity but also of friction and they
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actually have to steal some of the engine's power
to operate. As you can see I need some strength
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to overcome the resistance of the valve spring
and open the valve. This strength must come from
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somewhere when the engine is running. So the
valves actually steal a bit of the energy created
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by combustion to open themselves and allow the
next combustion to happen. Additionally, all of
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these mechanical parts are a source of potential
engine failure. If they aren't installed correctly
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and the timing of the engine isn't correct it can
result in the engine running poorly
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In extreme examples the timing can be so off that it results in
the piston contacting the valve on an interference engine
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An interference engine is one where the
valves and the piston occupy the same space but
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at different times. This can often enable a more
compact efficient and powerful engine but it also
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leads to engine failure in the event that the
chain or belt snaps. Now if we move over to the
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two-stroke we can see that it replaces all of this,
with just this. In the case of the four stroke this
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is the cylinder head together with the valve
cover. Whereas in the two-stroke we really have
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just a cylinder cover or cap. There are zero moving parts. No valves. No chains. No cams. No springs.
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And therefore less weight, less complexity, less cost
and less potential for failure
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So then how does the two-stroke get gases in and out of the
cylinder without valves? Well if there's one thing
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you should take away from this video it's this: The
four stroke only uses the area above the piston
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for the combustion cycle. Whereas the two-stroke
uses both the area above and below the piston
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In other words, the intake charge or air and
fuel mixture see both the area above and below
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the piston in a two-stroke. Whereas the intake
charge never gets below the piston in a four stroke
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Now let's observe the combustion cycle in a
two-stroke and we're starting with the combustion stroke itself
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and the piston at top dead center. So
combustion has just started. It's building pressure
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in the cylinder and pushing the piston down. While
at the same time we have fresh air fuel mixture
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below the piston and I'll explain how it got there
in just a moment. Now as the piston is going down
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it opens up the exhaust port which allows some
of the exhaust gas to start escaping from the cylinder
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and at the same time the downward motion
of the piston is also compressing the air fuel
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mixture below it and pushes it into the transfer
port which is still blocked off by the piston skirt
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As the Piston goes down some more it starts
to open up the transfer ports which then allows the
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compressed air fuel mixture from below the piston
to be transferred above the piston. The pressure in
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the cylinder has already decreased substantially
because much of the exhaust gas has been allowed
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to escape through the exhaust port. This means
that the compressed air fuel mixture coming out
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transfer port is actually at a higher pressure
than the remaining exhaust gases in the cylinder
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which means that as the air fuel mix enters
the cylinder it helps to push out the last of
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the remaining exhaust gases. But as you can see our
exhaust port is still open. It's not blocked off by
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the piston which means that the air fuel mixture
not only pushes the remaining exhaust gas out the cylinder
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The air fuel mixture itself is also free
to exit the cylinder through the exhaust port
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This brings us to the first drawback of the two-stroke.
The design of the engine means that some fresh
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unburnt fuel gets dumped out the exhaust. This of
course is the definition of inefficiency but there
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are ways around it and we will cover them in this
video. So the piston is now at bottom dead center
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and we have an air-fuel mixture above the piston
as the piston starts going up. Now in addition to
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pushing up and compressing the air fuel mixture,
the upward motion of the piston is also drawing
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more fresh air and fuel into the cylinder. As the
piston rises from bottom to top it creates a vacuum
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below the piston. Just like in the four stroke
vacuum is the absence of air which cannot
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continue to exist and so the air-fuel mixture
rushes into the cylinder through the intake
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port and occupies the area below the piston. When
the piston reaches top dead center again we have
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compressed air and fuel above the piston ready to
be ignited and fresh air and fuel below the piston
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ready to be pushed into the cylinder when the
piston descends again and opens up the transfer port
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The two-stroke combustion cycle and design
also explains why the two-stroke piston is much
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longer than the four-stroke piston. It must be long
enough to keep both the transfer and the exhaust
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port blocked off as it travels and approaches top
dead center. If the piston were shorter exhaust
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gases could enter the area below the piston but
more importantly the fresh intake charge could
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also escape out the exhaust port with the piston
at top dead center. As you can see each time the
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piston is at top dead center a combustion event
occurs. This also explains the name "two-stroke"
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The engine only needs two piston strokes to
complete its combustion cycle. In other words,
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each 360 degrees or one full engine revolution results
in a combustion event. Whereas in a four-stroke
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engine combustion occurs only every other time
the piston reaches top dead center
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Combustion occurs only every 720 degrees of engine rotation.
This means that the two-stroke produces twice as
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many combustion events or power pulses for the
same RPM which means that, at least in theory,
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the two-stroke engine can make twice as much power for
the same displacement compared to a four-stroke engine
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The other important thing to note is that
the strokes are very clearly defined and separated
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in a four-stroke engine. Each completed stroke of
the piston marks the beginning of one and the end
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of another stroke of the combustion cycle. But
the two-stroke engine sort of lumps the strokes together
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They overlap and occur simultaneously.
The two-stroke is actually multi-tasking which
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enables it to squeeze more action into the same
time frame. But as with everything there is a price to be paid
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My explanation the two-stroke
combustion cycle probably raised some questions
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like for example what is this big gaping hole in
the exhaust port? Or how come the downward motion
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of the piston doesn't just push the airflow
mixture back out through the intake?
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Well, no worries we're going to answer these questions
one by one and their answers will shed light
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on the price that the two-stroke pays for its
increased combustion frequency
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Let's start with how we prevent the air and fuel from going
back out the intake when the piston goes down
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To do that many engines use this. And this is a
reed valve which is placed into the intake like so
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A reed valve is essentially a one-way valve. It
allows gases to enter, but it prevents them from exiting
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When the piston moves up and creates a
vacuum inside the crankcase it also creates a low
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pressure zone in inside the crankcase, while a high
pressure zone remains outside in the atmosphere
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This pressure difference forces the reed valve
blades or petals open. Vacuum inside the crankcase
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means that there is very little pressure acting
behind the petals but there's atmospheric pressure
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acting on the outer side of the petals forcing
them open. When the air fuel mixture enters the
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crankcase the pressure soon equalizes. As the
piston starts going down and compressing the
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air fuel mixture pressure actually becomes greater
behind the petals on the crankcase side forcing
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them shut, The design of the reed valve is such
that the petals can only open in one direction
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and so crankcase pressure closes the petals
and keeps the compressed air fuel mixture from
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escaping back into the intake. Now the elephant
in the room that we have to address next is lubrication
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If you have ever owned an engine in
any kind of vehicle or appliance then you probably
00:15:11
know that lubrication is key for engines. Without
the protective film of oil metal to metal contact
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occurs and quickly leads to catastrophic failure.
As you may know in a four-stroke engine everything
00:15:22
under the rings of the Piston is constantly
lubricated by engine oil. This engine oil is also
00:15:28
usually circulated, filtered and pressurized by an
oil pump to ensure that the film of oil between
00:15:34
metal surfaces is always strong enough to
resist all the forces which are trying to break it apart
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So the elephant in the room is the area
under the piston inside a two-stroke engine
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If this area constantly sees air and fuel then how do
we keep it lubricated? Obviously getting the same
00:15:52
quantity of oil here as in a four-stroke and keeping
it circulated is impossible. It would end up in the
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combustion chamber and probably prevent combustion
from ever occurring. We also know that fuel is a
00:16:03
solvent making things even more difficult for
the rotating assembly of the two-stroke
00:16:09
So then how do we prevent metal to metal contact in
the two-stroke? Well the answer is a compromise
00:16:14
Instead of only getting air and fuel under the
piston we get air, fuel and oil in there
00:16:21
But we keep the quantity of oil low enough to prevent
it from impeding combustion but high enough to
00:16:27
provide some lubrication to the moving parts of
the engine. There are two possible ways to get the
00:16:31
lubricating oil or two-stroke oil into the engine.
The older method is to pre-mix the oil with the
00:16:38
fuel that goes into the fuel tank. The second more
modern method is to have a separate tank for the
00:16:43
two-stroke oil and have an injection pump which
injects the oil into the engine. Usually into the
00:16:49
carburetor where it mixes with the fuel and air
and ends up inside the engine. Now the ratio of air
00:16:54
to fuel inside the engine is usually somewhere
around 13:1 and the ratio of two-stroke oil
00:17:00
to fuel is anywhere from 24:1 to 50:1. This
gives you an idea of just how minimal the amount
00:17:08
of lubricating oil is inside the engine at any
one time. Because it's mixed together with the air
00:17:13
and fuel the oil also inevitably ends up inside
the combustion chamber where it gets burned
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This is where the smoke and the smell of two-stroke
engines comes from. Two strokes essentially burn
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small amounts of oil all the time. This is also why
the lubrication system of two-stroke engines is
00:17:31
known as a total loss lubrication system. The oil
never gets replaced like in a four-stroke. Instead
00:17:38
it's lost and topped up as necessary. This means
that the lubrication of the rotating assembly of a
00:17:44
two-stroke isn't nearly as consistent or reliable
as in a four-stroke engine which ultimately leads
00:17:50
to a significantly shorter maximum potential
lifespan for a two-stroke. No amount of maintenance
00:17:56
or lubrication quality can overcome this inherent
design constraint and the average lifespan of a
00:18:03
two-stroke engine on a motorcycle is somewhere
around 15-20.000 kilometers. Some may do more
00:18:09
than that but many won't make it even that far and
will need their pistons replaced and bottom ends
00:18:14
rebuilt well before that. Racing two-stroke
motorcycles often need rebuilds after only
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15 to 20 hours of operation. In contrast to this
four-stroke engines on motorcycles, even small ones
00:18:26
can make it to 50 000 kilometers. Some even get
to 100.000 kilometers and beyond. Whereas engines
00:18:33
on cars and trucks regularly make it passt three
hundred thousand kilometers. Some even get up to a million
00:18:40
The flip side of this is that although
two-stroke engines require more frequent rebuilds
00:18:44
their rebuilds are usually much less expensive
and far easier to perform. The other problem with
00:18:50
the total loss lubrication system is that burning
oil is obviously very bad for emissions which is
00:18:55
the key reason why many legislations around
the world have outlawed the manufacture of
00:19:00
street legal two-stroke vehicles and only allow
off-road and competition two-stroke vehicles to
00:19:07
be made and sold. The difference in lubrication
systems between the two engines is also embodied
00:19:12
and evident in the piston rings. The four-stroke
has three rings whereas the two-stroke only two
00:19:18
The third set of rings in a four-stroke are
oil control rings and they prevent oil from
00:19:23
getting into the combustion chamber Oil control
rings are obviously unnecessary in a two-stroke
00:19:28
because the oil gets into the combustion chamber
anyway, with or without them. But there's a benefit
00:19:33
for the two-stroke here because less rings means
less friction and less power losses due to this friction
00:19:39
But the piston rings don't just differ in
number they're free-floating or allowed to rotate
00:19:44
in the four stroke, whereas in the two-stroke they
are pinned in place using small pegs
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Now these pegs are missing from this particular piston but
you can still see the holes where the little pegs
00:19:55
or locating pins go. Their purpose is to prevent
two-stroke piston rings from rotating. This is
00:20:00
done to prevent the piston ring ends or gaps from
snagging onto the exhaust or the transfer ports
00:20:06
which will lead to engine failure. Instead the
Rings are fixed to ensure that the gaps always
00:20:12
travel over the area between the transfer ports
to eliminate the possibility of getting snagged
00:20:19
by the ports. The free floating ring system of the
four-stroke is actually superior because allowing
00:20:24
the rings to rotate can help ensure more even
wear of the rings and a longer lasting engine
00:20:29
The lack of consistent lubrication is also why
two-stroke engines often use ball bearings and or
00:20:35
roller bearings as the rod and crank bearings. Ball
bearings are more expensive and require more space
00:20:41
than plain bearings but they're far more resilient
to poor lubrication environments. On the other
00:20:46
hand most four strokes can use plain bearings
which are very small and inexpensive and which
00:20:51
together with a good film of oil offer excellent
load bearing capacity and higher shock resistance
00:20:57
But our example four stroke engine actually uses
ball bearings too. The reason is that this is a
00:21:02
very compact and inexpensive engine which has a
very small oil pump that isn't able to provide
00:21:07
the kind of lubrication that oil pumps on larger
more advanced four-strokes can. Another area where
00:21:13
two strokes are limited is the compression ratio.
The compression ratio of the engine is the ratio
00:21:18
between the largest and the smallest volume
of the cylinder. Or the ratio between cylinder
00:21:22
volumes when the piston is at bottom and top dead
center. The higher this ratio the more we compress
00:21:28
the air fuel mixture leading to the air and fuel
molecules being packed closer together which then
00:21:34
improves combustion speed and strength leading
to higher efficiency and power. Now both of our
00:21:39
demonstration engines are actually single cylinder
125 CC motorcycle engines. Our four stroke is from
00:21:46
a 90s Honda Spacy scooter. It makes 10 horsepower
and 10 newton meters of torque, and has a bore of
00:21:52
52.4 millimeters and a stroke of 57.8 millimeters.
Our 2 stroke engine is from a Yamaha TZR Belgarda
00:22:00
sports bike which is also from the 90s. It makes
an impressive 28 horsepower and a less impressive
00:22:06
16 newton meters of torque and has a bore of 56.4
and a stroke of 50 millimeters. One of the reasons
00:22:12
why the four-stroke engine looks very underpowered
is because space constraints in a scooter lead to
00:22:18
restrictive intake and exhaust designs. The other
reason is that the scooter engine is focused on
00:22:23
economy and user-friendly power, whereas the
sport bike engine is focused on building peak
00:22:28
power which it manages to build and hold over
only a very narrow part of the top of the power band
00:22:34
Although both engines have similar bore and
stroke specs they have very different compression ratios
00:22:40
The four-stroke engine has a compression
ratio of 9.5:1 whereas the two-stroke engine
00:22:45
manages only 5.9:1. Why is this the case? Well
the reason is very simple and it's because
00:22:52
the four-stroke utilizes the entire volume of
the cylinder. Compression begins when the
00:22:57
piston is at bottom dead center and ends when
the piston reaches top dead center. Things are
00:23:03
different in the two-stroke. Compression does
not begin at bottom dead center. Instead it only
00:23:08
begins when the piston closes off the exhaust port.
Before the exhaust port is closed off the cylinder
00:23:15
isn't sealed and air and fuel can't be compressed.
Instead they're free to escape through the exhaust port
00:23:21
This is why in a two-stroke the compression
ratio or the ratio between the largest and the
00:23:27
smallest cylinder volume is the ratio between this
and this volume. Obviously THIS is much smaller
00:23:34
than THIS and leads to a much lower compression
ratio which ultimately limits efficiency and
00:23:39
performance in a two-stroke engine. The final
difference is that the four stroke is inherently
00:23:43
more flexible thanks to its more complex, bulkier
design and the key factor behind this lies in the camshaft
00:23:50
A camshaft is essentially the mechanical
brain of the engine and changing the camshaft can
00:23:55
completely transform the character and performance
of the engine. But you don't even need to change
00:24:01
the camshaft, you can simply shift its angle
in relation to the piston which is known as
00:24:06
advancing or retarding the camshaft to impact
engine power and torque. And this can be done
00:24:12
while the engine is still running. Most modern
engines on cars incorporate variable valve timing (VVT)
00:24:17
systems which advance and retard camshafts as
required to produce good responsiveness and
00:24:23
torque at low RPM and good power at high RPM. Many
systems even pack multiple lobe profiles into a
00:24:30
single camshaft and can switch between them during
engine operation giving us low valve lift
00:24:36
at low RPM and high valve lift at high RPM which
further improves responsiveness power and torque
00:24:42
throughout the rev range. In contrast to this the
two-stroke cannot change the position of its ports
00:24:48
They are a fixed part of the engine's construction.
But what a two-stroke can do is change the size of
00:24:54
the exhaust port. And this finally brings us to the
big round hole in the exhaust port it's meant to
00:25:00
house THIS. And this is a part of a system called
YPVS which stands for Yamaha Power Valve System
00:25:07
Other manufacturers also developed their own
systems with their own acronyms but all of these
00:25:13
systems have the same goal and despite what their
name is saying the goal isn't to increase power
00:25:18
instead the goal is to even out the power band of
a two-stroke resulting in a smoother, more usable
00:25:24
and more linear power band and power delivery. Our
particular example is what's known as a guillotine
00:25:29
style power valve and it rotates to make the
exhaust port smaller at low RPM and
00:25:36
larger at high RPM. Now the power valve works together
with another signature feature of two-stroke
00:25:42
engines and that is the expansion chamber present
in the exhaust. Now the expansion chamber relies on
00:25:48
the principle of wave resonance which is similar
to what you experience when your voice creates an echo
00:25:53
When the initial acoustic wave of your voice
encounters an obstacle it gets reflected back as
00:25:59
a delayed weaker acoustic wave. The exhaust pulse
coming out from the engine is also essentially a
00:26:05
wave front and when it encounters the changing
diameter created by the expansion chamber it
00:26:10
reflects another wavefront back to the cylinder.
This wavefront can then be timed to push back
00:26:16
the fresh air fuel mixture that's trying to escape
through the exhaust port. Now at low RPM the piston
00:26:22
is moving slowly and this means that the reflected
wave from the expansion chamber can easily make it
00:26:27
in time to push the fresh air fuel mixture back
into the cylinder. This is why we don't need the
00:26:33
entirety of the exhaust port and can reduce its
size. The benefit of the reduced exhaust port size
00:26:39
is that during combustion we keep the cylinder
sealed away longer, which means that we can harness
00:26:45
more of the combustion's energy which results in
better responsiveness and torque at low RPM
00:26:52
As RPM increases the piston moves faster and faster and
so there's less and less time for the reflected
00:26:57
wave from the expansion chamber to make it back
to the cylinder and push the air and fuel back in
00:27:02
So we increase the size of the exhaust port to
buy more room and time for the reflected wave to
00:27:08
do it's thing. We do sacrifice the increased duration
of cylinder sealing but we're already at high RPM
00:27:14
so building power and torque really isn't an issue
at this point. So it seems that a reduced lifespan
00:27:20
together with high emissions and poor efficiency
spelled doom for the two-stroke. Well there's a
00:27:25
glimmer of hope in the future of two strokes and
it comes in the form of direct injection
00:27:30
Direct injection basically means that we're spraying
fuel directly into the combustion chamber via an injector
00:27:35
instead of bringing it in from the
intake port via a carburetor. Direct injection
00:27:41
also has benefits for four-stroke engines and many
modern four-stroke engines and cars feature direct injection
00:27:46
But the benefits of direct injection
are more significant in the case of two strokes
00:27:51
By spraying fuel directly into the chamber we
can start introducing the fuel only when the
00:27:57
exhaust port gets closed off by the piston and
compression actually begins. This prevents fresh
00:28:02
fuel from being dumped out the exhaust port which
obviously dramatically improves efficiency and emissions
00:28:07
It also removes the solvent properties
of fuel from under the piston which can help
00:28:12
improve the lubrication of the rotating assembly.
However there are some challenges to implementing
00:28:16
this technology and it's still in the development
stage and only time will tell if and when it will
00:28:22
become a mass production reality. And there you
have it. Two seemingly similar but actually very
00:28:27
different types of engines. Each with its own
benefits and drawbacks, which I hope this video
00:28:31
managed to explain in an understandable
and illustrative manner. As always thanks
00:28:36
a lot for watching I'll be seeing you soon
with more fun and useful stuff on the d4a channel
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