The Only Video You'll Ever Need to Watch to Know how 4 Stroke and 2 Stroke Engines Work and Differ

Summary

This 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

00:00

🔍 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.

05:02

🔧 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.

10:02

🚫 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.

15:06

🛠 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.

20:07

🏎️ 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.

25:08

💡 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.

Keywords

💡Four-stroke engine

A four-stroke engine is an internal combustion engine that completes one combustion cycle through four distinct strokes: intake, compression, combustion, and exhaust. Each stroke corresponds to a different phase of the piston's movement within the cylinder. The video script explains that this type of engine is larger, heavier, and more complex than its two-stroke counterpart, yet it has become the standard in most applications due to its efficiency and reliability. The script uses the four-stroke engine as a point of comparison to highlight the differences and complexities involved in its operation.

💡Two-stroke engine

A two-stroke engine is another type of internal combustion engine that completes its combustion cycle in only two strokes of the piston: the power stroke and the scavenging stroke. This engine design allows for a more compact and lightweight construction, but it comes with its own set of trade-offs in terms of efficiency and emissions. The video script uses the two-stroke engine to illustrate a different approach to internal combustion, emphasizing its historical prevalence and the reasons behind its decline in favor of the four-stroke engine.

💡Crankshaft

The crankshaft is a central component in both four-stroke and two-stroke engines, converting the linear motion of the piston into rotational motion. This rotation is essential for powering the vehicle's transmission and ultimately driving the wheels. The script mentions the crankshaft as a common element in both engine types, highlighting its role in the conversion of energy from combustion into mechanical work.

💡Compression ratio

The compression ratio in an engine is the measure of the ratio between the cylinder's total volume and the volume of the combustion chamber when the piston is at the top dead center. A higher compression ratio means the air-fuel mixture is compressed more, which can lead to more efficient combustion. The script explains that the four-stroke engine typically has a higher compression ratio than the two-stroke, contributing to its greater efficiency and power.

💡Valves

Valves in an engine regulate the intake of air and fuel, as well as the exhaust of combustion gases. The script describes how the four-stroke engine uses valves, operated by a camshaft, to control these processes, which is a complex but efficient method. In contrast, the two-stroke engine lacks valves and uses ports and reed valves for similar functions, which simplifies the design but introduces other challenges.

💡Direct injection

Direct injection refers to the process of injecting fuel directly into the combustion chamber, which can improve efficiency and reduce emissions. The script mentions that direct injection could potentially revolutionize two-stroke engines by addressing some of their traditional inefficiencies and allowing for more precise fuel delivery.

💡Lubrication

Lubrication is crucial for reducing friction between moving parts in an engine, which helps prevent wear and prolong engine life. The script discusses the different lubrication systems in four-stroke and two-stroke engines, noting that the latter's total loss lubrication system, where oil is burned along with fuel, leads to shorter engine lifespans and higher emissions.

💡Camshaft

The camshaft is a part of the engine that controls the opening and closing of the valves in a four-stroke engine. The script explains how the camshaft's lobes interact with the valves to regulate the engine's intake and exhaust processes, and how its timing is synchronized with the crankshaft to ensure proper valve operation.

💡Piston

The piston is a critical component in both engine types, moving within the cylinder to facilitate the combustion process. The script describes the role of the piston in creating vacuum for intake, compressing the air-fuel mixture, and being pushed by the expanding gases during combustion. It also contrasts the longer pistons in two-stroke engines, which are necessary for sealing the transfer and exhaust ports.

💡Emissions

Emissions refer to the pollutants released into the atmosphere by an engine. The script discusses how the two-stroke engine's design, which burns a small amount of oil along with fuel, results in higher emissions compared to the four-stroke engine. This has been a significant factor in the decline of two-stroke engines in many applications.

💡Efficiency

Efficiency in an engine is a measure of how effectively it converts fuel energy into mechanical work. The script explores the efficiency of both four-stroke and two-stroke engines, noting that while two-stroke engines can be simpler and lighter, they often suffer from lower efficiency due to their design, which can lead to unburnt fuel being expelled and higher overall fuel consumption.

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