2 stroke turbo - how it work

Oct 3, 2020
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Hi, are some days I try to understand well how work a turbo on a 2 stroke engine. Some people in this forum have try to explain me that and I thank all them. But I need more explanation about it. Because I think it is a very interesting argument and need to be study in deep.

So for what I've heard turbo provide a "balance" between intake and exhaust pressure and the fuel mixture isn't blow trough the exhaust port by the turbo boost because turbo do the necessary backpressure to contain that. Exaust pressure need to be some psi over the intake pressure.

It's all correct??
someone can confirm it or explain the working principles better??
 
Oct 9, 2009
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I won't go in tremendous depth on this, but I will discuss key ideas. Engines generate power based on the amount of mass flow passing through the motor. Mass flow only occurs from a point of high pressure to a point of lower pressure. Call them intake manifold absolute pressure (IMAP), cylinder pressure, and exhaust manifold absolute pressure (EMAP). If the pressure is equal at both points flow does not occur and power cannot be generated.

A turbo works based on the prior principle. Keeping it simple, boost increases IMAP and power is gained so long as the increase in EMAP, also called drive pressure, does not increase in proportion to boost. When the change in boost equals the change in drive pressure, no more power is made, but you see more boost on the gauge. Therefore, a turbo will increase the volumetric efficiency of an engine and allow it to make more power so long as the change in boost is greater than the change in drive pressure.

Finally, sparing many details, 2 strokes operate using pressure differentials between the intake and cylinder (on the intake stroke) and the cylinder and exhaust pipe (on the exhaust stroke) to generate mass flow. 2 strokes differ from 4 strokes because a piston cannot push exhaust from the cylinder when back pressure is high. Back pressure kills mass flow on a two stroke because it reduces the pressure differential. Compared to a 4 stroke, 2 strokes flow much more air per cc of displacement. That is why two strokes use such large turbos...they flow more air! But, unlike 4strokes, which capture all of the air mass passing through the cylinder, a 2 stroke takes a sample of the air passing through it. That means some portion of air and fuel mass is wasted into the pipe when it mixes with exhaust gases. Etec motors reduce fuel loss via direct injection after exhaust port closure.

There is more to this, but that is all I will say at this point.
 
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Oct 3, 2020
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Thanks for your answer! Do you think reeds or rotary valve are mandatory to turbocharge a 2 stroke or is also possible do it on a piston port engine?
 
Oct 3, 2020
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Anyway I'm not completely agree with you because if the exhaust pressure is lower than the intake pressure ALL the fuel mixture is blow out! So no power gain.
 

volcano buster

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Nov 26, 2007
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If you take a clear syringe and point it down into some fluid and pull up 100 cc's of fluid according to the mark on the body of the syringe, then lift it up and turn the tip up, you will not have 100 cc's of fluid. You would typically have a percentage of that plus some air. This "percentage" is the volumetric efficiency of the vacuum device. Your pistons are also vacuum devices with a volumetric efficiency value. There are differences between 2 and 4 stroke engines but for simplicity I'll lump them together. When a piston is allowed to draw in an air/fuel charge it is not able to fill the combustion chamber with 100% of the available space with the proper air/fuel mixture, (4 stroke car engines have been noted to be about 85%). Forced induction systems improve this volumetric efficiency of cylinder filling by pushing the mixture in. Thus with more boost you have the more power the engine can make.
 
Oct 9, 2009
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Anyway I'm not completely agree with you because if the exhaust pressure is lower than the intake pressure ALL the fuel mixture is blow out! So no power gain.
You missed the point. The pressure differential across the motor happens in two steps, which I described. 1. Intake to cylinder (fill)... ignition/expansion... 2. cylinder to pipe (exhaust). Ignition creates the differential in cylinder pressure prior to the exhaust port opening. Charge is drawn from the cylinder into the pipe, creating vaccum in the cylinder based on port velocity. At that point, IMAP is higher than cylinder pressure/vacuum so air is drawn into the cylinder until the intake ports close. Then, pressure wave refraction in the pipe pushes mixed charge from the pipe back into the cylinder until the exhaust port closes. I simplified my explanation because IMAP/EMAP is the easiest way for non technical people to think about it. That conveys what is needed for you unless you plan on designing a motor, and it is informative to people other than engineers. I am not going to get into the details of thermodynamics and fluid mechanics more than I just did. That is how a 2 stroke motor works. You have the skill to google it. Yes, I am leaving things out. I said that. But, unless you are capable of doing Diff Eq or Partial Differential Calculus, that is where I stop.

If you want to put in the time, you can look up how...

1. reed valves work
2. pipe design changes the dynamics of pressure wave magnitude and wave speed
3. Exhaust valves change port velocities and velocity behaves according to bernoulli's equation in fluid mechanics

If you make it that far, I am happy to answer more questions.
 
Oct 9, 2009
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To be clear, I never said IMAP is greater than EMAP. I said the change in IMAP is greater than the change in EMAP. That implies they start from a base pressure. I leave that out to simplify. Thermal expansion created during ignition creates the differential from cylinder to the pipe. The remainder of the process is explained above.

Good luck!
 
Oct 3, 2020
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To be clear, I never said IMAP is greater than EMAP. I said the change in IMAP is greater than the change in EMAP. That implies they start from a base pressure. I leave that out to simplify. Thermal expansion created during ignition creates the differential from cylinder to the pipe. The remainder of the process is explained above.

Good luck!
Thanks for your explanations and for taking an interest in this thread.
Understand what you mean in mass flow etc.

Anyway my mainly question is what prevent the over scavenging in a turbo 2 stroke application?

In a n/a application the backwave in the pipe do it itself but in a turbo application the fuel mixture is Pressurized, so the only pipe back wave can't contain the the fuel mixture blow out trough the exaust port by the turbo boost.

So were is the key of this system to do more power and not blow out un burned fuel?
 
Oct 9, 2009
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I gave you all the pieces. Think about it. To generate boost requires drive pressure. Where is drive pressure located? It is in the pipe. That means the whole system is pressurized. The more efficient the system, the more boost is generated per unit of drive pressure. Boost is the change in IMAP and drive pressure is the change in EMAP. The changes in these pressures do not stop how the motor functions unless drive pressure exceeds boost, then the motor starts to choke flow, meaning more exhaust gets trapped in the cylinder during refraction and air mass cannot pass through the motor. Think about that.

Next, Is it bad to waste a greater amount of clean mass flow (air and fuel) in the pipe? Not necessarily. Assuming the same mass is swept into the cylinder as is factory, excess charge wasted in the pipe produces a cleaner charge mixture that gets swept into the cylinder. Thus, more power can be generated because the scavenged portion contains less exhaust. This is something to think about. I am leaving out changes in wave speed caused by changes in fluid density. In the end, this is a small effect on power increase. I explain this only so you understand the effect. If back pressure goes up, flow out goes down and more exhaust as a percentage of swept mass goes in. To be clear, exhaust is used up, meaning it cannot be used to generate power. Scavaging is driven by the mixture in the pipe (clean charge and exhaust) and the mass pushed back in by the strength of the pressure wave (the big effect).

Next holding constant the swept mixture, if all we do is change the density of the mixture going in the motor via boost, then the sled will make more power simply because there is more air mass in the same swept volume. This is the primary contributor to more power with a turbo.

I explain these concepts individually because it allows you to understand the individual components. The system is dynamic and a change in each effect impacts the other. That is all I am going to contribute. Hopefully, you understand. Happy sledding.
 
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Oct 3, 2020
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I gave you all the pieces. Think about it. To generate boost requires drive pressure. Where is drive pressure located? It is in the pipe. That means the whole system is pressurized. The more efficient the system, the more boost is generated per unit of drive pressure. Boost is the change in IMAP and drive pressure is the change in EMAP. The changes in these pressures do not stop how the motor functions unless drive pressure exceeds boost, then the motor starts to choke flow, meaning more exhaust gets trapped in the cylinder during refraction. Think about that.

Next, Is it bad to waste a greater amount of clean mass flow (air and fuel) in the pipe? Not necessarily. Assuming the same mass is swept into the cylinder as is factory, excess charge wasted in the pipe produces a cleaner charge mixture that gets swept into the cylinder. Thus, more power can be generated because the scavenged portion contains less exhaust. This is something to think about. I am leaving out changes in wave speed caused by changes in fluid density. In the end, this is a small effect on power increase. I explain this only so you understand the effect. If back pressure goes up, flow out goes down and more exhaust as a percentage of swept mass goes in. To be clear, exhaust is used up, meaning it cannot be used to generate power. Scavaging is driven by the mixture in the pipe (clean charge and exhaust) and the mass pushed back in by the strength of the pressure wave (the big effect).

Next holding constant the swept mixture, if all we do is change the density of the mixture going in the motor via boost, then the sled will make more power simply because there is more air mass in the same swept volume. This is the primary contributor to more power with a turbo.

I explain these concepts individually because it allows you to understand the individual components. The system is dynamic and a change in each effect impacts the other. That is all I am going to contribute. Hopefully, you understand. Happy sledding.
Clare! Thanks again! For your patience also.

Anyway many people here said drive pressure need to be about 2-3 psi more than the boost pressure in a turbo 2 stroke application . So you don't agree with them?
 
Oct 3, 2020
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Don't misunderstand me I completely agree with you on what have you explain me because it's logical.

but don't understand why the majority of turbo 2 stroke guys say "pipe pressure should be 2-3 psi over the intake pressure on a boosted 2 stroke otherwise it spits out the exhaust the fuel charge"
 

dansvan

Active member
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Apr 14, 2011
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2 strokes do need some back pressure (pipe pressure) to maximize their power. Look at the size of the actual exhaust port and compare it to the outlet size (stinger) of the pipe. Its always smaller. Having run a 0-10 PSI gauge on countless pipe/can combos I can assure you they all make and use back pressure/pipe pressure. Lower elevations/higher air density use larger openings and less pressure. Long wide open power runs use less back pressure due to heat build up. As elevations go up the pipe/can openings get smaller to make up for the lack of air density. That's why many companies made high altitude cans and pipes that were different than the low altitude versions. Many racers made adjustable bleeds on their pipes to control this pressure. Some used different sized washers on the outlets to control it. Some made cable activated bleeds. All to control and tune the pipe pressure. From my own experience based on a skidoo 800 Ptek for example, testing 5 different cans in one test session actually driving the sled and observing the pressure gauge, for my combination the high altitude higher back pressure cans (8psi for example) killed power and promoted detonation. Running the straight aaen pipe with no can was about 2psi. adding the Aaen can bumped it to about 3-3.5 psi.

Now throw in a turbo into the equation. The compressor side provides the pressure on the intake side, and the turbine side provides the back pressure (restriction) to make the system function properly. It adds more variables, too tight of an AR ratio and you get faster spool up due to increased restriction of the exhaust being forced around the turbine wheel driving it harder but at the expense of choking the motor at higher rpm and power levels. Run a larger AR and get less back pressure in the pipe but slower spool time. Its all a balance.

There does not need to be a vacuum in the pipe for a 2 stroke to work. Even a pipe with say 10psi of back pressure isn't going to prevent the exploding air fuel charge from coming out of the cylinder when its the only place the rapidly expanding explosion can go.

Running a boost pressure gauge right next to a pipe pressure gauge, will show the pipe several psi higher than boost pressure. Why do they double up pipe retaining springs? Why do pipes designed for normal N/A use crack welds and split seems? Because they are seeing much higher pressures than they were originally designed for, and its higher than the boost pressure on the inlet side.

I mentioned in the other thread also that using the pipe pressure to control the rising rate fuel pressure regulator on a carb'd turbo system is desirable because it puts the increased fuel pressure ahead of the demand curve. The pressure in the pipe in a turbo application will always rise faster than the intake side because the increase in exhaust pressure is what drives the increase in intake pressure.
 
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