Pipe effiency

[Luke Davey]

What is the effiency of a pipe on a 2 stroke, in terms of scavenging the cylinder. What would the volume of the mixture in the cylinder after the ports are closed at room temp and pressure. Is the volume more than the volume of the cylinder.

 

[marcusgunby]

Now i am no expert but i have remembered a little info, where the volumetric efficiency of a good race engine can exceed 100% ie it can stuff more than the volume of air into the space it occupies.

 

[jmics19067]

This is what I believe to be true. On an average two stroke motorcycle engine the basics would be ; After top dead center above the piston rings, combustion, piston moving downwards below piston crankcase fuel air charge starting to compress---> exhaust port opens starting to let exhaust well exhaust----->intake ports open all of your compressed crankcase fuel air mixture is being pushed up thru the transfer ports also helping to push out exhaust-----> bottom dead center------>piston moving upwards closing intake ports on top of rings and pulling fresh fuel air mixture into crankcase below rings------> as the piston is closing off the exhaust port it is trying to push some more fuel air out of it and if you have a properly tuned expansion chamber for the rpm you are at, it will send a signal back from them funny shaped cones forcing the the fuel/ air mixture that escaped into the exhaust back into the cylinder right as the exhaust port is being covered by the piston. Which is how you get over 100% volumetric efficiency while "on the pipe" in the power range.--------> piston still moving upwards this is your "compression ratio" -------> spark plug ignites and the whole thing is started over again.

So if I am correct in my assumptions the pipe on a two stroke doesn't scavange the exhaust very efficiently. It doesn't really need to, the piston is pushing it all out plus the fresh intake charge. The big benefit is the cramming back in the escaped fresh air charge back into the cylinder at the right moment.

 

[Luke Davey]

Four strokes dont excede 100% cylinder volume of fresh charge unless they have forced induction (turbocharger, supercharger or air ram). So a two stroke is a far better alternative than a four stroke as a race engine?

 

[r_rider28]

A two stroke can not have 100% volumetric effeciencey. A 250cc engine that pulls in 250cc of volume (swept volume) looses over half of this volume during the transfer/scavenging phase, thus trapping less volume for the combustion cycle. Since volumetric efficiency is a measure of volume of displacement vs volume of mixture taken in for combustion it is not possible for a 2 stroke to have 100% volumetric efficiency. How much of this volume is pushed back into the cylinder by the expansion chamber I am not sure of but it is nowhere near the volume lost.

 

[jmics19067]

Four strokes dont excede 100% cylinder volume of fresh charge unless they have forced induction

a normally aspirated four stroke can achive over 100% volumetric efficiency but only when the valves closing shockwave travels the length of the pipe<intake runner and exhaust pipe> hits the end<carb or muffler> and travels back and is timed with the valve reopening. But this can only happen at the rpm that the engine was designed for. It is a very complex combination of piston/combustion chamber design, valve timing,open height, and overlap, and the inertia of the gases being moved.Something I can't begin to understand but I have read about it and I believe it is possible.

So a two stroke is a far better alternative than a four stroke as a race engine?

for a given displacement a well tuned two stroke should produce more power since it has a power stroke every revolution. That is why the yz 250 four stroke is a direct competitor against the 125 two strokes in AMA motocross. different races and sanctions have different rules so what actually works legally for you depends on the race your in. Hypothetically lets say your in some type of NASCAR type race where the rules state engine displacement, carb size and fuel tank size and the race is long enough recquiring pit stops. The two stroke could easily lose out in time refueling . A 2stroke can have a great advantage in say 1/8 mile sand drags where an engine to get up to rpm quickly is useful. the 2stroke has less recipricating wieght inside the engine to have to try and spin plus the power stroke every revolution. But if your riding on ice the slower buildup of power of the four stroke can be your advantage. Which is why Yamaha removes the counter balancers from their four strokes for AMA motocross but most guys who race in the woods add flywheel weight to thier 2 strokes.

 

[jmics19067]

Like I said this is what I believe to be true I have no real schooling in any science. My only references would be related to basicly bathroom reading "how to " books and constantly tinkering with something.That and trying to listen to people whom I can trust their information. And the accuracy of that is directly related to how well I can judge people :think:

How much of this volume is pushed back into the cylinder by the expansion chamber I am not sure of but it is nowhere near the volume lost.

This is something I am very much interested in myself because this would be directly related to whether or not you can achieve 100% volumetric efficiency. I will still swear out of basicly ignorance that you can achieve over 100% but only for the rpm you are tuned for.

please someone with a degree and years of tuning step in and set us straight!!!! Eric ? Rich?
Even though I dont know them at all but just by listening to them I would trust them with my first born when it comes to tuning a bike

 

[Sean Hilbert]

Luke,

There are many assumptions and subtleties baked into what is seemingly a pretty simple question on the surface. I’m going to start off with some assumptions, attempt some definitions, and then try to answer your questions.

Assumptions:

The main assumption here is that we are talking about wide-open throttle operation. The purpose of a throttle is to force the engine to operate at less than 100% efficiency, so it doesn’t make much sense to talk in terms of volumetric efficiency when running at part throttle (even though that’s where your engine operates 99% of the time).

Also, since you didn’t specify an engine speed, I’ll assume that you speaking in terms of the engine’s peak power RPM. Note that Volumetric Efficiency is a strong function of RPM.

Definitions:

It’s not really proper to speak in terms of “Volumetric Efficiency” for a two-stroke engine because there is so much more going on during the gas exchange process. In a four-stroke, the intake and exhaust processes are mostly decoupled – a separate stroke of the piston is reserved for each operation. In a two-stroke, on the other hand, the intake and exhaust processes are tightly coupled, and this makes the measurement of efficiency more complex. Because the intake air/fuel stream coming through the transfer ports must displace the spent gasses in the cylinder, there is a large opportunity for these two sets of gasses to mix – something that you don’t want to happen (when trying to achieve maximum power anyway – there are emissions benefits, but that’s a different topic). Because of this mixing opportunity, an added measure of efficiency must be introduced to keep track of how much the fresh charge is diluted by the remaining exhaust gases from the previous cycle. The terminology that the two-stroke community has settled on is Scavenging Efficiency. In words, Scavenging Efficiency is the volume of fresh charge that is in the cylinder with respect to the overall cylinder volume. Trapping Efficiency, the term that is most closely related to Volumetric Efficiency, is the volume of trapped gasses in the cylinder with respect to overall cylinder volume. Note that these trapped gasses are a mixture of fresh charge and combusted gasses from the previous cycle.

So....what's the answer?

All good two-stroke engine designs attempt to maximize both Trapping Efficiency and Scavenging Efficiency. The right combination of crankcase compression ratio, port geometry, port timing, and exhaust tuning can easily create a ‘supercharge’ effect in a two-stroke engine (i.e. the pressure in the cylinder at the time of exhaust port closing is higher than atmospheric pressure). This supercharging, however, only relates to Trapping Efficiency. Worst case, the porting configuration could encourage a lot of mixing between the fresh charge and the combusted gasses resulting in low Scavenging Efficiency and low peak power (even though the engine was supercharged by four-stroke standards). That is why all high performance two-stroke engine designs ‘flush’ a lot more fresh charge through the cylinder than what it can actually hold (this flushing process, by the way, is what gives two-strokes high hydrocarbon emissions compared to the four-stroke). The goal, for peak performance, is to flush the cylinder so that almost no exhaust gas remains, and to have a nice lump of fresh charge waiting just outside the exhaust port near to when the piston closes the port off. If the exhaust system is properly designed, a reversion wave (emanating from the converging cone of the pipe) will meet up with that fresh charge and push it back through the exhaust port just before the port closes – thus supercharging the engine. In this way, both Trapping Efficiency and Scavenging Efficiency are maximized.

Hope that helped…


Sean

 

[Brianc]

No normally aspirated engine can acheive 100% volumetric efficiency. The gasses in the pipe of a 2 stroke cannot reverse direction to ram mixture back in. The key is proper scavenging which allows the intake charge to flow in unrestricted and it is the ram charging effect on the piston compression stroke that allows the the engine to acheive the higher volumetric efficiencies.

 

[125 rider]

Originally posted by Brianc
The gasses in the pipe of a 2 stroke cannot reverse direction to ram mixture back in.

Sure they can. A properly timed expansion chamber at the right rpm will have negative waves sucking out burnt gasses and a returning positive wave for stuffing back in unburnt gasses right before the piston closes the exhuast port.

 

[Hogwylde]

Originally posted by r_rider28
A two stroke can not have 100% volumetric effeciencey................................Since volumetric efficiency is a measure of volume of displacement vs volume of mixture taken in for combustion it is not possible for a 2 stroke to have 100% volumetric efficiency. How much of this volume is pushed back into the cylinder by the expansion chamber I am not sure of but it is nowhere near the volume lost.

uuuummmmmm.........WRONG!!!!!
Read this>>How Two-stroke Expansion Chanbers Work, and Why You Should Care...... www.motorcycle.com/mo/mcnuts/em-pipes.html

Here is a quote from the article "Simply put, it's because two-stroke exhaust systems use pressure waves emanating from the combustion chamber to effectively supercharge your cylinder"