What’s the Difference Between the Blow-by gas and Exhaust Gas?

Many of you have heard of "blow-by" or "blow-by gas" or words that come up when you're interested in engine tuning or crankcase decompression, so you've probably heard of it before.

But what is blow-by gas, after all? Most of us don't actually understand what's going on when it comes to something a little more in-depth, such as what does it do? Where did it come from and where it goes? 

If it's just blow-by gas, it's not that difficult.

WebiQ is here to bring you a little happiness and motorcycle knowledge. Trying to successfully handle blow-by gas and make the best use of the gas suddenly becomes difficult, but before you do, you need to understand the blow-by gas itself.

Gas flow in the upper part of the piston

First of all, a 4-stroke engine works on the following principle.

The details are much more complex elements, but roughly this is how the engine runs in the process. The gas on the upper side (combustion chamber side) of the piston goes in and out of the combustion chamber through this process, and it burns and expands along the way.

Gas flow at the bottom of the piston

The upper side of the piston is directly involved in engine power, so various commentaries have been roaring, but little attention has been paid to the gases on the lower side of the piston, which produces nothing in particular. It's safe to say that there was virtually no literature that explained that area at least 30 years ago.

When crankcase decompression started to attract attention about 20 years ago, it started to attract attention little by little, but since there are a lot of people who are against crankcase decompression, it seems that it is not yet out of the realm of the occult.

Putting the crankcase depressurization aside, the first and most important thing to remember is that there is also gas on the underside of the piston.

The underside of the piston is the inside of the engine itself, which is splashed with engine oil, but the inside of the engine is not a vacuum, nor is it all filled with oil. Except for the oil, the rest of it is filled with air.

But unlike the intake path above the pistons, the inside of the engine is basically a sealed space, so there is no gas coming out or going in from the outside (i.e. no gas flow).

However, there are some things that aren't basic, so I'll explain the non-basic things in the next section.

The state of gas inside the engine

A normal motorcycle engine is made up of the engine itself and the transmission in one piece. (On some foreign motorcycles and scooters, the transmission may be a separate unit or the transmission and engine may be separated and in a separate room.)

The inside of the engine is lubricated with oil, but it's full of parts that are spinning at high-speed Lube oil (i.e., engine oil) drops all over the place! It's flying around so violently that some of it are drifting violently through the engine in a fine mist. An engine oil storm is blowing around in the engine. You can ask them to imagine that.

If you open the oil filler cap when the engine is running, you can feel the storm of engine oil spewing out, but I wouldn't recommend it because the floor and your face will be covered in oil in an instant.

The inside of the engine is actually connected to the outside air.

When the engine is stopped, it cools down and becomes the temperature of the outside of the engine, and when the engine is started, it becomes hotter around 100 degrees. Whether the cooling system is air or water-cooled, the hot oil is circulating and the other side of the piston is heated by a flame of several hundred degrees Celsius in the combustion chamber, so it is natural that the oil will be hotter than the outside temperature.

Now, gases have the property of expanding when heated and contracting when cooled (Boyle-Charles' law). The same is true for the gases inside the engine (the air that the oil storm is blowing through). Volume changes. This volume change must be accommodated.

As the piston moves up and down, the internal space of the engine increases in volume and becomes negative pressure (positive pressure in the combustion chamber above the piston), and as the piston moves down, the internal space of the engine decreases in volume and becomes positive pressure (negative pressure in the combustion chamber above the piston).

In a typical four-cylinder engine, theoretically, there is no pressure change because when one piston goes up, another piston goes down, but in practice, the inertia of the air causes a pressure change because the air has to move between each piston.

In order to absorb such changes in the volume and pressure of the gas in the engine holes in the engine to expel inflated air and respond to pressure changes is open and is connected to the air cleaner box by a hose. It looks as if the gas inside the engine is breathing in and out of the engine. Crankcase Breather or simply breathe, I say. The hoses from the breather connect to the air cleaner box, but the air cleaner box is connected to the outside air for intake, and as a result, the inside of the engine is connected to the outside air through the hoses.

The hoses that connect the inside of the engine to the air cleaner box are called breather hose. Also known as blow-by hose! It's out at last! Blow-by!

A piston separates the combustion chamber from the inside of the engine.

As previously explained, the upper side of the piston is the combustion chamber and the lower side of the piston is the inside of the engine. In other words, it's just compartmentalized by pistons.

These pistons are moving up and down inside the cylinder, so of course, there are gaps.
If there's no gap (a perfect fit), the piston can't move up and down.

The gap is sealed with a piston ring and engine oil film to prevent the engine oil from leaking into the combustion chamber and to prevent the mixture from entering the engine from the combustion chamber.

If this was perfect, there would have been nothing wrong with it. A small amount of clearance is absolutely necessary for the piston to move. Even the piston ring is not a complete ring, it has one or more broken parts. You can't eliminate the gap by any means.

What happens when there is a gap?

As mentioned above, there is always a gap between the cylinders for the piston to move up and down smoothly. It's a very narrow gap, but at any rate, the upper side of the piston is where the pressure rises sharply due to the combustion explosion of the mixture.

It's that pressure that creates so much power. The combustion gas blows through the small space next to the piston. This gas that is blown out is called blow-by gas.

However, the air-cooled engine has a very large gap (so large that you can see the light leaking from the gap when you hold it up to the light), and quite a lot of combustion gas is released into the engine. It goes in.

In addition to the combustion gases, the unburned gas (unburned mixture) remaining in the combustion chamber is pushed into the engine at the same time. One of the causes of engine oil degradation can be the impurities in this blow-by gas, so you can imagine how it is leaking out into the engine.

The blow-by gas that keeps coming into the engine

Now, this blow-by gas is constantly leaking into the engine as long as the engine is running. What happens if more and more blow-by gas comes into the engine as it is?

If the inside of the engine is completely enclosed and isolated from the outside world, the pressure inside the engine will increase rapidly due to the pressure of the gas flowing into it. Eventually, the part that is most vulnerable to pressure (usually an oil seal or gasket somewhere) will be blown out, and the engine oil spewing catastrophe. It will be a
Worst case scenario, the engine blows.

Let the blow-by gas out!

We don't want the engine to break, so we have to do something to get rid of the blow-by gas that keeps coming in.

If only I could get out of somewhere successfully... and then there it is! That's the breather hose connected to the crankcase breather that just appeared!

There is a hose that absorbs the pressure change inside the engine, so if you drain the blow-by gas from this hose, the case is closed. Yes!

The fact is that the blow-by gas is much more than the volume change caused by temperature change and pressure change caused by the up and down movement of the piston. That is why the breather hose is also called a blow-by hose. Depending on the motorcycle model, there are some engines that have two venting hoses, because one hose is not enough.

A long time ago it was popular to blow-by-the-air

The blow-by hoses (breather hoses) are used to vent the blow-by gas that has blown through the engine, but to make it easier to get rid of it, the less discharge resistance the better! This is why it was common in the old days to open the blow-by hose to the atmosphere. However, the unburned gas in the blow-by gas (Harmful) to the atmosphere is not good for the environment, so opening the air to the public has a history of being banned.

However, the racing world didn't have such emissions regulations, so until recently, it was open to the atmosphere. And it was popular until the '80s to mimic that and convert blow-by's to open air in production cars for public roads (although it was illegal). The perception was that it was cool and racer-like to extend the blow-by hose to around the side of the license plate and show a glimpse of the hose outlet.

It's a stainless steel mesh oil hose (sort of) for some reason, even though there's not a lot of pressure on it, and it's stylish to decorate the hose outlet with aluminum parts for oil line fittings (sort of)! Bringing it next to the number with a plastic hose is lame! It was such a trend. In hindsight, it was an enigmatic modification that looked stupid and was quite stinky to drive behind because of the leaking blow-by gas.

Since it is widely known that opening up the air for blow-by gas has no effect, and since more and more healthy users don't like the fact that their motorcycles are crappy around them, such modifications have been completely downplayed.

There's no point in going to the trouble of being complicit in air pollution, so let's all stop converting to open air.

It's not like it's a performance improvement, so there is no point of bringing air pollution!

Where does the blown-by gas go?

Now, what happens to the blow-by gas that is safely drained from the inside of the engine via the breather and enters the air cleaner box?

Actually, this gas is sucked back into the engine's combustion chamber to be burned again, mixed with the normal intake air.

This system, in which the blow-by gas is re-burned without releasing it into the atmosphere, is called a "closed-loop system". If you have a modern motorcycle, it's always a closed-loop system.

Why is oil dripping from the blow-by hose?

By the way, if you open the air cleaner box, you may find that the area around the blow-by-bose connection inside is covered in oil. If you remove the hose, oil can drip from the inside.

The reason for this is because along with the blow-by gas, you'll also get a storm of oil inside the engine. There's an oil storm blowing inside the engine, but there's a fair amount of blow-by gas, so when the gas drains, the oil goes with it.

However, if the oil keeps coming out, the oil will be reduced and the inside of the air cleaner box will be tapped with oil. There is an oil separator (called a gas-liquid separator) in the middle of the box to prevent this from happening, but if the oil still can't be separated and ends up in the air cleaner, oil can spew out into the box. In most cases, too much engine oil is the cause.

And even if it doesn't go that far, the oil mist that is discharged with the blow-by gas is cooled on its way through the hose and oil condensation in the hose. You may do this. This is why oil is dripping from the hose.

Especially in the winter, when it's cold, condensation can easily condense and the condensation can emulsify with the moisture in the engine, making it look like mayonnaise stuffed inside the breather hose. As the mayonnaise cools and freezes, there is no way for the blow-by gas to escape, so you'll be in the same situation as I mentioned above, 'if the engine was sealed', and you'll be blowing out the oil seals and gaskets.

You might think that's extreme but there are two cases where the gasket has been blown out, so it is a great enemy to be alert even just around me.

Does a 2-stroke engine produce blow-by gas?

The 2-stroke engine does not have a hose to release the blow-by gas. Why is that?

If blow-by gas is defined as incomplete gas or an unburned mixture of gases that slip through the side of the pistons and into the crankcase due to combustion pressure, then even a 2-stroke engine would have blow-by gas, right? Wouldn't it seem that the 2-stroke engines that don't have oil rings on the piston rings or fewer rings to prevent blow-by gas are more likely to produce blow-by gas?

The correct answer is even a 2-stroke engine will have a blow-by.

So why don't blow-by hoses exist on 2-stroke engines? The reason for this is that, in the case of a two-stroke engine, the inside of the crankcase is an air intake passageway.

There are no closed loops and no heckling, and the inside of the crankcase is always full of unburned gas, to begin with. The structure of the engine is fundamentally different from that of a 4-stroke engine, in which the intake air mixture flows into the crankcase under the pistons, so the crankcase must be sealed.

Also, 2-stroke engines are full of holes in the sides of the cylinders, and the majority of those holes (all but those for the exhaust) are directly connected to the inside of the crankcase. I don't have time to worry about blow-by gas because it's destined to get into the crankcase through that hole in the crankcase in comparable quantities of unburned gas.

In summary, even in a 2-stroke engine, blow-by gas is generated, but the place where the blow-by gas escapes is originally the pathway for the unburned mixture, so the generated blow-by gas mixes with the mixture and is drawn back into the combustion chamber to be burned.  It goes like this.

However, having blow-by gas blowing through the side of the piston means that compression is leaking, which is far more of a problem in a performance-oriented 2-stroke engine.

Never plug the breather hose.

The role of the crankcase breather and the blow-by hose is to prevent air pollution by re-burning the unburned gas while releasing the blow-by gas, which is fatal to the engine, and the pressure inside the engine that is increased by the blow-by gas.

Blow-by hose may feel jammed, but cutting or blocking them can 100% lead to serious problems. So don't ever plug it up!