The Only Video You'll Ever Need to Watch to Know how 4 Stroke and 2 Stroke Engines Work and Differ
Summary
TLDRThis video script offers an in-depth comparison between four-stroke and two-stroke engines, highlighting their distinct mechanisms and operational differences. It explores the four-stroke cycle's need for valves and its complexity versus the two-stroke's simplicity, achieved through the use of transfer and exhaust ports. The script delves into the two-stroke's higher power output per revolution and its total loss lubrication system, which contributes to its shorter lifespan and higher emissions. It also touches on the potential of direct fuel injection to improve two-stroke efficiency and emissions, providing a comprehensive look at the engines' benefits, drawbacks, and future possibilities.
Takeaways
- 🔧 Both four-stroke and two-stroke engines share basic components like a crankcase, crankshaft, connecting rod, wrist pin, piston, and cylinder, but they differ fundamentally in their operation.
- 🔄 The four-stroke engine completes one combustion cycle through four distinct strokes: intake, compression, combustion, and exhaust, each corresponding to a 180-degree crankshaft rotation.
- 💨 In a four-stroke, valves are crucial for controlling the intake of air-fuel mixture, compression, and exhaust, and are operated by the camshaft synchronized with the crankshaft via a timing belt or chain.
- 🏎 The two-stroke engine, lacking valves, achieves gas exchange using the space above and below the piston, with the transfer and exhaust ports managed by the piston's movement.
- ⏱️ Two-stroke engines are characterized by their high power-to-weight ratio and simplicity due to fewer moving parts, but they also have lower efficiency and higher emissions.
- 🛠️ Lubrication in two-stroke engines is a compromise; a mix of air, fuel, and oil is used, leading to the total loss lubrication system where oil is burned and must be replenished.
- 🔥 The combustion process in engines is a controlled deflagration rather than an uncontrolled explosion, spreading heat and igniting the air-fuel mixture layer by layer.
- 🚫 Environmental regulations have played a significant role in the decline of two-stroke engines due to their inefficiency and higher emissions compared to four-stroke engines.
- 🛠️ The two-stroke engine's lifespan is typically shorter than that of a four-stroke engine, with rebuilds being more frequent but less complex and less costly.
- 🌐 Direct fuel injection holds potential for two-stroke engines, improving efficiency and emissions by introducing fuel only when compression begins, but this technology is still in developmental stages.
Q & A
What is the primary difference between a four-stroke and a two-stroke engine?
-The primary difference is that a four-stroke engine completes one combustion cycle in four distinct strokes: intake, compression, combustion, and exhaust. In contrast, a two-stroke engine completes its cycle in only two strokes, with the intake and exhaust processes occurring simultaneously with the compression and combustion strokes.
How does the four-stroke engine manage the intake and exhaust processes?
-The four-stroke engine uses valves, specifically the intake and exhaust valves, which are operated by the camshaft. The intake valve opens during the intake stroke to allow air and fuel into the cylinder, and the exhaust valve opens during the exhaust stroke to let exhaust gases out.
What role does the camshaft play in a four-stroke engine?
-The camshaft in a four-stroke engine is responsible for operating the valves. It has lobes that, when rotated, push against the rocker arms to open the valves at the correct times in the engine cycle.
Why are four-stroke engines generally larger and more complex than two-stroke engines?
-Four-stroke engines are larger and more complex due to the additional components required for the four separate strokes, such as the valves, camshaft, and associated timing mechanisms. These parts add weight, complexity, and potential for friction and engine failure.
How does a two-stroke engine overcome the need for valves?
-A two-stroke engine overcomes the need for valves by using the movement of the piston to control the intake and exhaust. The design uses the area above and below the piston for the combustion cycle, with ports in the cylinder that open and close as the piston moves.
What is the significance of the reed valve in a two-stroke engine?
-The reed valve in a two-stroke engine acts as a one-way check valve in the intake system. It allows the air-fuel mixture to enter the crankcase when the piston moves upward but prevents it from escaping back out when the piston moves downward.
How does a two-stroke engine deal with the issue of lubrication?
-Two-stroke engines deal with lubrication by mixing a small amount of oil with the air-fuel mixture. This mixture is then burned during combustion, which is why two-stroke engines are known for their smoke and smell. This method is less efficient and reliable than the lubrication system in four-stroke engines.
What is the 'total loss' lubrication system mentioned in the script?
-The 'total loss' lubrication system refers to the method used in two-stroke engines where the lubricating oil is mixed with the fuel and burned during combustion. Unlike four-stroke engines, the oil is not circulated and reused; it is 'lost' with each combustion cycle and must be replenished.
Why do two-stroke engines have a lower compression ratio compared to four-stroke engines?
-Two-stroke engines have a lower compression ratio because the compression process does not begin until the piston closes off the exhaust port. Before this, the cylinder is not sealed, allowing air and fuel to escape through the open exhaust port, thus limiting the compression ratio.
What is the Yamaha Power Valve System (YPVS) mentioned in the script?
-The Yamaha Power Valve System (YPVS) is a mechanism in two-stroke engines that uses a rotating valve to change the size of the exhaust port. This system aims to even out the power band, resulting in smoother and more linear power delivery across different engine speeds.
How does direct fuel injection benefit two-stroke engines?
-Direct fuel injection in two-stroke engines allows fuel to be sprayed directly into the combustion chamber, improving efficiency and emissions by preventing fresh fuel from being lost out the exhaust port. It also helps with lubrication by removing the solvent properties of fuel from the area under the piston.
Outlines
🔍 Introduction to Four-Stroke and Two-Stroke Engines
The paragraph introduces the viewer to the similarities and differences between four-stroke and two-stroke engines. Both engine types share basic components such as a crankcase, crankshaft, connecting rod, wrist pin, piston, and cylinder. They operate by converting the linear motion of the piston into the rotational motion of the crankshaft, which powers the vehicle. However, beyond these similarities, the two engine types diverge significantly in their approach to internal combustion. The video aims to provide a comprehensive understanding of how each engine works, their respective benefits and drawbacks, and why the four-stroke engine has become predominant despite its larger size, weight, cost, and complexity.
🔧 How Four-Stroke Engines Operate
This section delves into the workings of a four-stroke engine, explaining that it requires four distinct strokes to complete one combustion cycle: intake, compression, combustion (power), and exhaust. The intake stroke draws in air and fuel as the piston moves down, creating a vacuum that fills with an air-fuel mixture. The compression stroke compresses this mixture, and the combustion stroke ignites it, converting the energy release into motion that powers the vehicle. Finally, the exhaust stroke expels the combustion byproducts. The paragraph also addresses common questions about how these engines manage the intake of air and fuel and the expulsion of exhaust gases, setting the stage for a deeper explanation of engine mechanics.
🚫 The Drawbacks of Two-Stroke Engines
The script discusses the inherent inefficiency of two-stroke engines due to their design, which allows some fresh, unburnt fuel to be expelled through the exhaust. This not only reduces efficiency but also contributes to environmental concerns. The two-stroke engine's operation is then described in detail, highlighting how it manages the intake of air and fuel and the expulsion of exhaust without the use of valves, relying instead on the movement of the piston and the opening and closing of ports in the cylinder. The unique challenges of two-stroke engines, such as the need for a longer piston to cover both transfer and exhaust ports, are also explained.
🛠 Lubrication and Lifespan of Two-Stroke Engines
This paragraph addresses the critical issue of lubrication in two-stroke engines, explaining the challenges of keeping the engine's internals lubricated without interfering with combustion. It contrasts the four-stroke engine's oiling system, which constantly lubricates the underside of the piston, with the two-stroke engine's compromise approach, which involves mixing a small amount of oil with the air-fuel mixture. The paragraph also discusses the implications of this lubrication method on the lifespan of two-stroke engines, which tend to have shorter lifespans than four-stroke engines due to less consistent lubrication and the need for more frequent rebuilds.
🏎️ Performance and Design Differences Between Four-Stroke and Two-Stroke Engines
The script compares the performance and design of a specific four-stroke engine from a Honda Spacy scooter and a two-stroke engine from a Yamaha TZR Belgarda sports bike. It highlights how the two-stroke engine, despite its smaller size, produces more horsepower but less torque than the four-stroke engine. The differences in compression ratios between the two engine types are explored, with the four-stroke engine achieving a higher compression ratio due to its ability to utilize the entire cylinder volume. The paragraph also touches on the flexibility of four-stroke engines, which can adjust their performance characteristics through changes to the camshaft, in contrast to the fixed nature of two-stroke engines.
💡 Future Prospects for Two-Stroke Engines
The final paragraph discusses the potential future for two-stroke engines, particularly in light of direct fuel injection technology. Direct injection could address some of the efficiency and emissions challenges associated with two-stroke engines by introducing fuel directly into the combustion chamber, reducing the amount of fuel lost through the exhaust. The paragraph outlines the benefits of this technology for two-stroke engines and acknowledges the ongoing development challenges. It concludes by reflecting on the distinct characteristics and trade-offs of both engine types, emphasizing the educational value of the video in understanding these differences.
Mindmap
Keywords
💡Four-stroke engine
💡Two-stroke engine
💡Crankshaft
💡Compression ratio
💡Valves
💡Direct injection
💡Lubrication
💡Camshaft
💡Piston
💡Emissions
💡Efficiency
Highlights
Four-stroke engines require four strokes to complete one combustion cycle, while two-stroke engines complete a cycle in just two strokes.
Both engine types convert the reciprocation of the piston into the rotation of the crankshaft.
The four-stroke engine's four strokes are intake, compression, combustion, and exhaust.
In a four-stroke, valves are used to control the entry and exit of gases, operated by the camshaft.
Two-stroke engines lack valves and use the area above and below the piston for the combustion cycle.
Two-stroke engines have fewer moving parts, leading to less weight, complexity, and potential for failure.
The two-stroke engine's design allows for twice as many combustion events for the same RPM compared to a four-stroke.
Two-stroke engines use a reed valve to prevent air and fuel from escaping back through the intake.
Lubrication in two-stroke engines is achieved by mixing oil with the air-fuel mixture.
Two-stroke engines have a shorter lifespan due to their total loss lubrication system.
Four-stroke engines can have higher compression ratios, improving efficiency and power.
Two-stroke engines use a power valve system to even out the power band and improve low RPM performance.
Direct injection in two-stroke engines can improve efficiency and emissions by introducing fuel only when compression begins.
The future of two-stroke engines may lie in the development of direct injection technology.
Four-stroke engines offer more flexibility and can adjust valve timing for improved performance across RPM ranges.
Transcripts
So here we have a four-stroke engine
and here we have a two-stroke engine. If we observe the anatomy of these two engines upon first glance
they seem very similar. They both have a crank case and a crankshaft together with a connecting rod
as well as a wrist pin and a piston
they also both have a cylinder
and they both operate by converting the reciprocation of the piston
into the rotation of the crankshaft, which then turns the gears of a transmission and ultimately
the wheels of the vehicle. But beyond these core similarities the four-stroke and the two-stroke
start to fundamentally diverge resulting in two very different approaches to internal combustion
And what I want to do today is give you the only video you need to watch to get a very
solid understanding of how a four-stroke and a two-stroke engine work. We'll also be exploring
the differences, benefits and drawbacks of each engine type, and this will then answer the question
of why the four-stroke ultimately prevailed over the two-stroke in most applications despite being
larger, heavier, more expensive and more complex. So let's get started with the four-stroke,
Why is it called a four-stroke? It's because the four-stroke engine needs four strokes to complete
one combustion cycle. Every time the piston moves from top to bottom or vice versa that's one stroke
One stroke of the piston equals 180 degrees of crankshaft rotation. Now the four strokes are:
intake, compression, combustion and exhaust. During the intake stroke the piston travels from top to bottom
Which are also known as top dead center and bottom dead center. As the piston does this
it creates an empty space or vacuum inside the cylinder. This newly created void is essentially
a brief absence of air and because we have an absence of air we also have an absence of air pressure
In other words, we have low air pressure inside the cylinder and atmospheric air pressure
outside the cylinder. This air pressure difference cannot continue to exist and air naturally seeks
to equalize pressure everywhere. And so air together with fuel from the outside rushes
into the cylinder and fills it with a fresh air fuel mixture. By the time the piston reaches BDC
all the air and fuel that can hope to get in have done so and the piston now starts to move upward
As it does that it forces the air fuel mixture into an ever smaller space. In other words,
it's compressing the air fuel mixture which is why this stroke is called the compression stroke
Just before the piston reaches top dead center the spark plug fires and ignites the air fuel mixture
which finds itself between the two electrodes of the plug. Although combustion inside an engine is often
described as a bang or explosion that isn't what's actually happening. An explosion is detonation
which is a rapid uncontrolled process. In contrast to this combustion is deflagration which is a much
slower, more even and controlled process. Combustion spreads out evenly outwards from the spark plug through heat transfer
The small portion of air-fuel mixture initially ignited by the spark plug
heats up and ignites the next layer of the air -fuel mixture. This layer then ignites the next layer
and the process continues until all of the air fuel mixture is burned. As the combustion flame front
spreads it rapidly raises the temperature and pressure inside the cylinder.
Because the cylinder is sealed this pressure has nowhere else to go so it ends up pushing the piston
down the cylinder with great strength. This is our combustion stroke and of the four strokes this is
the only one that actually generates power, and it does so by converting the energy released
by combustion into motion of the piston which then turns the crankshaft and ultimately the wheels
By the time the piston reaches bottom dead center again all the air fuel mixture has been burned
and we now have exhaust gas or the remains of combustion inside the cylinder
As the piston moves up once again the exhaust gases leave the cylinder and exit through the exhaust piping,
catalytic converters and mufflers out into the atmosphere. Now if you know a bit about engines
then you've probably noticed how my explanation of the four-stroke combustion cycle fails to answer
some very important questions. And these are: how do we allow air and fuel to get in during intake?
How do we keep air and fuel as well as combustion energy from escaping during compression and combustion?
And how do we allow exhaust gases out? Well the answer to all of this is the same thing
and it's valves! The intake valve opens during the intake stroke to allow air and fuel into the cylinder
Both valves are closed during the compression and the combustion stroke
to prevent air and fuel, as well as combustion energy from escaping the cylinder. And finally
the exhaust valve opens during the exhaust stroke to allow exhaust gases to escape the cylinder
Now the valves are operated by the camshaft and as you can see the camshaft has lobes on it
As the lobe contacts the rocker arm the rocker arm pushes onto the valve and opens it
As the lobe releases the rocker arm the valve spring ensures that the valve returns to its seat
as soon as possible. The shape of the lobe determines how much the valve opens and
how long it remains open. The higher the lobe the more the valve opens or the more left we have
The broader the lobe the longer the valve remains open and the more duration we have
But for the engine to run well we must ensure that the motion of the camshaft is synchronized to
the motion of the crankshaft and the piston. This is done by a cam belt or cam chain
It connects the crankshaft with the camshaft to ensure that the correct valve opens during the intake
That both remain closed during compression and combustion. And that the correct valve opens during exhaust
As you can see our chain is very slack and this is because our display model
doesn't have chain guides and tensioners which are normally present to ensure proper operation of the chain
As you can see the four-stroke engine needs a lot of mechanical parts to get gases in and out
of the cylinder. These parts are a source of weight and complexity but also of friction and they
actually have to steal some of the engine's power to operate. As you can see I need some strength
to overcome the resistance of the valve spring and open the valve. This strength must come from
somewhere when the engine is running. So the valves actually steal a bit of the energy created
by combustion to open themselves and allow the next combustion to happen. Additionally, all of
these mechanical parts are a source of potential engine failure. If they aren't installed correctly
and the timing of the engine isn't correct it can result in the engine running poorly
In extreme examples the timing can be so off that it results in the piston contacting the valve on an interference engine
An interference engine is one where the valves and the piston occupy the same space but
at different times. This can often enable a more compact efficient and powerful engine but it also
leads to engine failure in the event that the chain or belt snaps. Now if we move over to the
two-stroke we can see that it replaces all of this, with just this. In the case of the four stroke this
is the cylinder head together with the valve cover. Whereas in the two-stroke we really have
just a cylinder cover or cap. There are zero moving parts. No valves. No chains. No cams. No springs.
And therefore less weight, less complexity, less cost and less potential for failure
So then how does the two-stroke get gases in and out of the cylinder without valves? Well if there's one thing
you should take away from this video it's this: The four stroke only uses the area above the piston
for the combustion cycle. Whereas the two-stroke uses both the area above and below the piston
In other words, the intake charge or air and fuel mixture see both the area above and below
the piston in a two-stroke. Whereas the intake charge never gets below the piston in a four stroke
Now let's observe the combustion cycle in a two-stroke and we're starting with the combustion stroke itself
and the piston at top dead center. So combustion has just started. It's building pressure
in the cylinder and pushing the piston down. While at the same time we have fresh air fuel mixture
below the piston and I'll explain how it got there in just a moment. Now as the piston is going down
it opens up the exhaust port which allows some of the exhaust gas to start escaping from the cylinder
and at the same time the downward motion of the piston is also compressing the air fuel
mixture below it and pushes it into the transfer port which is still blocked off by the piston skirt
As the Piston goes down some more it starts to open up the transfer ports which then allows the
compressed air fuel mixture from below the piston to be transferred above the piston. The pressure in
the cylinder has already decreased substantially because much of the exhaust gas has been allowed
to escape through the exhaust port. This means that the compressed air fuel mixture coming out
transfer port is actually at a higher pressure than the remaining exhaust gases in the cylinder
which means that as the air fuel mix enters the cylinder it helps to push out the last of
the remaining exhaust gases. But as you can see our exhaust port is still open. It's not blocked off by
the piston which means that the air fuel mixture not only pushes the remaining exhaust gas out the cylinder
The air fuel mixture itself is also free to exit the cylinder through the exhaust port
This brings us to the first drawback of the two-stroke. The design of the engine means that some fresh
unburnt fuel gets dumped out the exhaust. This of course is the definition of inefficiency but there
are ways around it and we will cover them in this video. So the piston is now at bottom dead center
and we have an air-fuel mixture above the piston as the piston starts going up. Now in addition to
pushing up and compressing the air fuel mixture, the upward motion of the piston is also drawing
more fresh air and fuel into the cylinder. As the piston rises from bottom to top it creates a vacuum
below the piston. Just like in the four stroke vacuum is the absence of air which cannot
continue to exist and so the air-fuel mixture rushes into the cylinder through the intake
port and occupies the area below the piston. When the piston reaches top dead center again we have
compressed air and fuel above the piston ready to be ignited and fresh air and fuel below the piston
ready to be pushed into the cylinder when the piston descends again and opens up the transfer port
The two-stroke combustion cycle and design also explains why the two-stroke piston is much
longer than the four-stroke piston. It must be long enough to keep both the transfer and the exhaust
port blocked off as it travels and approaches top dead center. If the piston were shorter exhaust
gases could enter the area below the piston but more importantly the fresh intake charge could
also escape out the exhaust port with the piston at top dead center. As you can see each time the
piston is at top dead center a combustion event occurs. This also explains the name "two-stroke"
The engine only needs two piston strokes to complete its combustion cycle. In other words,
each 360 degrees or one full engine revolution results in a combustion event. Whereas in a four-stroke
engine combustion occurs only every other time the piston reaches top dead center
Combustion occurs only every 720 degrees of engine rotation. This means that the two-stroke produces twice as
many combustion events or power pulses for the same RPM which means that, at least in theory,
the two-stroke engine can make twice as much power for the same displacement compared to a four-stroke engine
The other important thing to note is that the strokes are very clearly defined and separated
in a four-stroke engine. Each completed stroke of the piston marks the beginning of one and the end
of another stroke of the combustion cycle. But the two-stroke engine sort of lumps the strokes together
They overlap and occur simultaneously. The two-stroke is actually multi-tasking which
enables it to squeeze more action into the same time frame. But as with everything there is a price to be paid
My explanation the two-stroke combustion cycle probably raised some questions
like for example what is this big gaping hole in the exhaust port? Or how come the downward motion
of the piston doesn't just push the airflow mixture back out through the intake?
Well, no worries we're going to answer these questions one by one and their answers will shed light
on the price that the two-stroke pays for its increased combustion frequency
Let's start with how we prevent the air and fuel from going back out the intake when the piston goes down
To do that many engines use this. And this is a reed valve which is placed into the intake like so
A reed valve is essentially a one-way valve. It allows gases to enter, but it prevents them from exiting
When the piston moves up and creates a vacuum inside the crankcase it also creates a low
pressure zone in inside the crankcase, while a high pressure zone remains outside in the atmosphere
This pressure difference forces the reed valve blades or petals open. Vacuum inside the crankcase
means that there is very little pressure acting behind the petals but there's atmospheric pressure
acting on the outer side of the petals forcing them open. When the air fuel mixture enters the
crankcase the pressure soon equalizes. As the piston starts going down and compressing the
air fuel mixture pressure actually becomes greater behind the petals on the crankcase side forcing
them shut, The design of the reed valve is such that the petals can only open in one direction
and so crankcase pressure closes the petals and keeps the compressed air fuel mixture from
escaping back into the intake. Now the elephant in the room that we have to address next is lubrication
If you have ever owned an engine in any kind of vehicle or appliance then you probably
know that lubrication is key for engines. Without the protective film of oil metal to metal contact
occurs and quickly leads to catastrophic failure. As you may know in a four-stroke engine everything
under the rings of the Piston is constantly lubricated by engine oil. This engine oil is also
usually circulated, filtered and pressurized by an oil pump to ensure that the film of oil between
metal surfaces is always strong enough to resist all the forces which are trying to break it apart
So the elephant in the room is the area under the piston inside a two-stroke engine
If this area constantly sees air and fuel then how do we keep it lubricated? Obviously getting the same
quantity of oil here as in a four-stroke and keeping it circulated is impossible. It would end up in the
combustion chamber and probably prevent combustion from ever occurring. We also know that fuel is a
solvent making things even more difficult for the rotating assembly of the two-stroke
So then how do we prevent metal to metal contact in the two-stroke? Well the answer is a compromise
Instead of only getting air and fuel under the piston we get air, fuel and oil in there
But we keep the quantity of oil low enough to prevent it from impeding combustion but high enough to
provide some lubrication to the moving parts of the engine. There are two possible ways to get the
lubricating oil or two-stroke oil into the engine. The older method is to pre-mix the oil with the
fuel that goes into the fuel tank. The second more modern method is to have a separate tank for the
two-stroke oil and have an injection pump which injects the oil into the engine. Usually into the
carburetor where it mixes with the fuel and air and ends up inside the engine. Now the ratio of air
to fuel inside the engine is usually somewhere around 13:1 and the ratio of two-stroke oil
to fuel is anywhere from 24:1 to 50:1. This gives you an idea of just how minimal the amount
of lubricating oil is inside the engine at any one time. Because it's mixed together with the air
and fuel the oil also inevitably ends up inside the combustion chamber where it gets burned
This is where the smoke and the smell of two-stroke engines comes from. Two strokes essentially burn
small amounts of oil all the time. This is also why the lubrication system of two-stroke engines is
known as a total loss lubrication system. The oil never gets replaced like in a four-stroke. Instead
it's lost and topped up as necessary. This means that the lubrication of the rotating assembly of a
two-stroke isn't nearly as consistent or reliable as in a four-stroke engine which ultimately leads
to a significantly shorter maximum potential lifespan for a two-stroke. No amount of maintenance
or lubrication quality can overcome this inherent design constraint and the average lifespan of a
two-stroke engine on a motorcycle is somewhere around 15-20.000 kilometers. Some may do more
than that but many won't make it even that far and will need their pistons replaced and bottom ends
rebuilt well before that. Racing two-stroke motorcycles often need rebuilds after only
15 to 20 hours of operation. In contrast to this four-stroke engines on motorcycles, even small ones
can make it to 50 000 kilometers. Some even get to 100.000 kilometers and beyond. Whereas engines
on cars and trucks regularly make it passt three hundred thousand kilometers. Some even get up to a million
The flip side of this is that although two-stroke engines require more frequent rebuilds
their rebuilds are usually much less expensive and far easier to perform. The other problem with
the total loss lubrication system is that burning oil is obviously very bad for emissions which is
the key reason why many legislations around the world have outlawed the manufacture of
street legal two-stroke vehicles and only allow off-road and competition two-stroke vehicles to
be made and sold. The difference in lubrication systems between the two engines is also embodied
and evident in the piston rings. The four-stroke has three rings whereas the two-stroke only two
The third set of rings in a four-stroke are oil control rings and they prevent oil from
getting into the combustion chamber Oil control rings are obviously unnecessary in a two-stroke
because the oil gets into the combustion chamber anyway, with or without them. But there's a benefit
for the two-stroke here because less rings means less friction and less power losses due to this friction
But the piston rings don't just differ in number they're free-floating or allowed to rotate
in the four stroke, whereas in the two-stroke they are pinned in place using small pegs
Now these pegs are missing from this particular piston but you can still see the holes where the little pegs
or locating pins go. Their purpose is to prevent two-stroke piston rings from rotating. This is
done to prevent the piston ring ends or gaps from snagging onto the exhaust or the transfer ports
which will lead to engine failure. Instead the Rings are fixed to ensure that the gaps always
travel over the area between the transfer ports to eliminate the possibility of getting snagged
by the ports. The free floating ring system of the four-stroke is actually superior because allowing
the rings to rotate can help ensure more even wear of the rings and a longer lasting engine
The lack of consistent lubrication is also why two-stroke engines often use ball bearings and or
roller bearings as the rod and crank bearings. Ball bearings are more expensive and require more space
than plain bearings but they're far more resilient to poor lubrication environments. On the other
hand most four strokes can use plain bearings which are very small and inexpensive and which
together with a good film of oil offer excellent load bearing capacity and higher shock resistance
But our example four stroke engine actually uses ball bearings too. The reason is that this is a
very compact and inexpensive engine which has a very small oil pump that isn't able to provide
the kind of lubrication that oil pumps on larger more advanced four-strokes can. Another area where
two strokes are limited is the compression ratio. The compression ratio of the engine is the ratio
between the largest and the smallest volume of the cylinder. Or the ratio between cylinder
volumes when the piston is at bottom and top dead center. The higher this ratio the more we compress
the air fuel mixture leading to the air and fuel molecules being packed closer together which then
improves combustion speed and strength leading to higher efficiency and power. Now both of our
demonstration engines are actually single cylinder 125 CC motorcycle engines. Our four stroke is from
a 90s Honda Spacy scooter. It makes 10 horsepower and 10 newton meters of torque, and has a bore of
52.4 millimeters and a stroke of 57.8 millimeters. Our 2 stroke engine is from a Yamaha TZR Belgarda
sports bike which is also from the 90s. It makes an impressive 28 horsepower and a less impressive
16 newton meters of torque and has a bore of 56.4 and a stroke of 50 millimeters. One of the reasons
why the four-stroke engine looks very underpowered is because space constraints in a scooter lead to
restrictive intake and exhaust designs. The other reason is that the scooter engine is focused on
economy and user-friendly power, whereas the sport bike engine is focused on building peak
power which it manages to build and hold over only a very narrow part of the top of the power band
Although both engines have similar bore and stroke specs they have very different compression ratios
The four-stroke engine has a compression ratio of 9.5:1 whereas the two-stroke engine
manages only 5.9:1. Why is this the case? Well the reason is very simple and it's because
the four-stroke utilizes the entire volume of the cylinder. Compression begins when the
piston is at bottom dead center and ends when the piston reaches top dead center. Things are
different in the two-stroke. Compression does not begin at bottom dead center. Instead it only
begins when the piston closes off the exhaust port. Before the exhaust port is closed off the cylinder
isn't sealed and air and fuel can't be compressed. Instead they're free to escape through the exhaust port
This is why in a two-stroke the compression ratio or the ratio between the largest and the
smallest cylinder volume is the ratio between this and this volume. Obviously THIS is much smaller
than THIS and leads to a much lower compression ratio which ultimately limits efficiency and
performance in a two-stroke engine. The final difference is that the four stroke is inherently
more flexible thanks to its more complex, bulkier design and the key factor behind this lies in the camshaft
A camshaft is essentially the mechanical brain of the engine and changing the camshaft can
completely transform the character and performance of the engine. But you don't even need to change
the camshaft, you can simply shift its angle in relation to the piston which is known as
advancing or retarding the camshaft to impact engine power and torque. And this can be done
while the engine is still running. Most modern engines on cars incorporate variable valve timing (VVT)
systems which advance and retard camshafts as required to produce good responsiveness and
torque at low RPM and good power at high RPM. Many systems even pack multiple lobe profiles into a
single camshaft and can switch between them during engine operation giving us low valve lift
at low RPM and high valve lift at high RPM which further improves responsiveness power and torque
throughout the rev range. In contrast to this the two-stroke cannot change the position of its ports
They are a fixed part of the engine's construction. But what a two-stroke can do is change the size of
the exhaust port. And this finally brings us to the big round hole in the exhaust port it's meant to
house THIS. And this is a part of a system called YPVS which stands for Yamaha Power Valve System
Other manufacturers also developed their own systems with their own acronyms but all of these
systems have the same goal and despite what their name is saying the goal isn't to increase power
instead the goal is to even out the power band of a two-stroke resulting in a smoother, more usable
and more linear power band and power delivery. Our particular example is what's known as a guillotine
style power valve and it rotates to make the exhaust port smaller at low RPM and
larger at high RPM. Now the power valve works together with another signature feature of two-stroke
engines and that is the expansion chamber present in the exhaust. Now the expansion chamber relies on
the principle of wave resonance which is similar to what you experience when your voice creates an echo
When the initial acoustic wave of your voice encounters an obstacle it gets reflected back as
a delayed weaker acoustic wave. The exhaust pulse coming out from the engine is also essentially a
wave front and when it encounters the changing diameter created by the expansion chamber it
reflects another wavefront back to the cylinder. This wavefront can then be timed to push back
the fresh air fuel mixture that's trying to escape through the exhaust port. Now at low RPM the piston
is moving slowly and this means that the reflected wave from the expansion chamber can easily make it
in time to push the fresh air fuel mixture back into the cylinder. This is why we don't need the
entirety of the exhaust port and can reduce its size. The benefit of the reduced exhaust port size
is that during combustion we keep the cylinder sealed away longer, which means that we can harness
more of the combustion's energy which results in better responsiveness and torque at low RPM
As RPM increases the piston moves faster and faster and so there's less and less time for the reflected
wave from the expansion chamber to make it back to the cylinder and push the air and fuel back in
So we increase the size of the exhaust port to buy more room and time for the reflected wave to
do it's thing. We do sacrifice the increased duration of cylinder sealing but we're already at high RPM
so building power and torque really isn't an issue at this point. So it seems that a reduced lifespan
together with high emissions and poor efficiency spelled doom for the two-stroke. Well there's a
glimmer of hope in the future of two strokes and it comes in the form of direct injection
Direct injection basically means that we're spraying fuel directly into the combustion chamber via an injector
instead of bringing it in from the intake port via a carburetor. Direct injection
also has benefits for four-stroke engines and many modern four-stroke engines and cars feature direct injection
But the benefits of direct injection are more significant in the case of two strokes
By spraying fuel directly into the chamber we can start introducing the fuel only when the
exhaust port gets closed off by the piston and compression actually begins. This prevents fresh
fuel from being dumped out the exhaust port which obviously dramatically improves efficiency and emissions
It also removes the solvent properties of fuel from under the piston which can help
improve the lubrication of the rotating assembly. However there are some challenges to implementing
this technology and it's still in the development stage and only time will tell if and when it will
become a mass production reality. And there you have it. Two seemingly similar but actually very
different types of engines. Each with its own benefits and drawbacks, which I hope this video
managed to explain in an understandable and illustrative manner. As always thanks
a lot for watching I'll be seeing you soon with more fun and useful stuff on the d4a channel
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