Are these Single point 'cooker-ring' mixers more snake-oil?

[LPGC]

All this has got me wondering what it would take to make LPG work with a lager venturi. After all, carbs manage larger and have no real problems pulling liquid fuel in from an obviously lower vacuum signal. Is it down to the additional circuits carbs have?

It's an interesting question

Probably as carbs have slow running jets and accelerator pumps. My experience with a too large mixer was that I could get a sensible stepper opening at idle but it would go lean, or at least the stepper would have to open much more, when the throttle was open. No matter how much I adjusted the vaporiser could I get the same opening at out of idle to idle. The main bias on the vaporiser sets how much suck is needed for any particular stepper opening while the idle bleed just dribbles a constant (small) amount of gas in all the time. So you could get it to run at idle with a huge mixer (or just a bit of pipe hanging in the inlet) by opening the idle bleed but that would have next to no affect as soon as you open the throttle so it will go lean.

We can kind of emulate a carb idle jet by using LPG mixer system reducer idle bypass but some carbs have more than just an idle and a main jet. If we tried to make the same kind of setup as a more complicated carb for an LPG mixer system we might need a separate reducer for each jet but for a carb any draw from one jet doesn't necessarily affect the flow through other jets if jets are fed from a float bowl that's kept at the same level regardless of total flow from the bowl. Maybe other aspects include the different densities of LPG vapour and liquid petrol? For sure for a given flow of air you need far less volume of liquid petrol due to it being denser than vapour LPG. I also wonder - if we have this slight vacuum area that draws fuel from a carb jet outputting liquid petrol or outputting vapour LPG, will the effect on that slight vacuum area be different between outputting liquid petrol or vapour LPG? Can imagine that if we stick a small volume of petrol in that area it won't effect the vacuum much but if we stick a far greater volume of vapour LPG into that vacuum it is likely to reduce the vacuum. If I'm thinking along the right lines we'd need a stronger vacuum signal for the vapour LPG than we would for the liquid petrol because sticking the LPG vapour in reduces the vacuum more than sticking the petrol in.

Regards carb accelerator pumps, I know in theory if we suddenly open the throttle there's a moment when although total airflow increases immediately speed of airflow may momentarily drop, the air itself takes time to accelerate, which to either a carb or LPG mixer system could mean momentary lean mixture... Yet the lack of accelerator pump doesn't seem to effect LPG mixer systems to anywhere near the extent it would affect a conventional petrol carb. Momentary drop in velocity aside (because that should affect both carbs and mixer systems) I wonder if denser petrol takes more time to be accelerated to the same speed as airflow than vapour LPG (and if we suddenly have more air entering cylinders but fuel hasn't caught up yet we have lean mixture until fuel does catch up). If that's true then an ideal LPG mixer system has the inherent advantage over an ideal petrol carb of having far less need (perhaps no need) for an accelerator pump. But that would be for ideal systems and in practice it still wouldn't be so simple because the LPG mixer system reducer diaphragm has mass that will take time to respond to any change in vacuum signal, versus petrol being denser than vapour LPG may take more time to accelerate through it's jets than vapour LPG (which is likely to already be flowing at higher velocity from reducer through mixer into airflow than liquid petrol through carb jets into airflow in any case).

[Pinger]

We can kind of emulate a carb idle jet by using LPG mixer system reducer idle bypass but some carbs have more than just an idle and a main jet. If we tried to make the same kind of setup as a more complicated carb for an LPG mixer system we might need a separate reducer for each jet but for a carb any draw from one jet doesn't necessarily affect the flow through other jets if jets are fed from a float bowl that's kept at the same level regardless of total flow from the bowl. Maybe other aspects include the different densities of LPG vapour and liquid petrol? For sure for a given flow of air you need far less volume of liquid petrol due to it being denser than vapour LPG. I also wonder - if we have this slight vacuum area that draws fuel from a carb jet outputting liquid petrol or outputting vapour LPG, will the effect on that slight vacuum area be different between outputting liquid petrol or vapour LPG? Can imagine that if we stick a small volume of petrol in that area it won't effect the vacuum much but if we stick a far greater volume of vapour LPG into that vacuum it well reduce the vacuum... if I'm thinking along the right lines we'd need a stronger vacuum signal for the LPG than we would for the petrol because sticking the LPG in does in itself reduce the vacuum. Regards carb accelerator pumps, I know in theory if we suddenly open the throttle there's a moment when although total airflow increases immediately speed of airflow may momentarily drop, which to either a carb or LPG mixer system could mean momentary lean mixture... Yet the lack of accelerator pump doesn't seem to effect LPG mixer systems to anywhere near the extent it would affect a conventional petrol carb. Momentary drop in velocity aside (because that should affect both carbs and mixer systems) I wonder if denser petrol takes more time to be accelerated to the same speed as airflow than vapour LPG. If that's true then an ideal LPG mixer system has the inherent advantage over an ideal petrol carb of not needing an accelerator pump, but in practice it still wouldn't be so simple because the LPG mixer system reducer diaphragm has mass that will take time to respond to any change in vacuum signal, versus petrol being denser than vapour LPG may take more time to accelerate through it's jets than vapour LPG.

Yes, the other circuitry in a carb has a bearing. If we look at the idle and progression ports they are downstream from the throttle plate and get actual vacuum as opposed to generated vacuum from a venturi. Once beyond progression, venturi dominates but LPG mixer systems don't have that (progression) luxury - only as you mention, the forward feed idle bias.
Yes, the diameter of the delivery tube to a venturi affects flow of the delivered fluid/gas.
For certain the need for accelerator pumps in carbs is to compensate for the liquid fuel's inertia. By the time that fuel has gone through the engine, the main circuitry should be ready to continue the flow. That I think more than any stalling of airflow which is more likely in engines with high states of tune (unlike most engines converted to LPG - esp mixer systems where a slight loss of top end power can be tolerated - are of the more loafing power delivery sort) where the flow can reverse and if it makes it back through the venturi then forwards again - it will have collected three doses of fuel and the last thing needed is more from an accelerator pump. Noticeable with tuned 2 strokes (which lack accelerator pumps - where flow reversal can be prevalent at low rpm and large throttle openings, they really respond to progressive application of throttle. Chasing the rpm as I think of it.
So, mixer system gain response from low gas inertia, offset by the need for a large delivery tube (very probably the real reason for venturis smaller than for petrol) and are denied the sophisticated circuitry of carbs that may - but only may - help transition on tip-in which would possibly enable a larger mixer. A reasonable summary>
Referring to an earlier post of yours, my conclusion as to how to obtain better top end power with a larger mixer (combined area) would be to deploy two mixers progressively and fed by separate reducers. If the juice is worth the squeeze that is.

Good post LPGC (as ever). I enjoy these conversations - I learn a lot from them. You have no idea how many major mistakes you save me form committing!

[Gilbertd]

ah no stepper or any sort of controls on miy stuff chaps .

I suspect that is what is causing the problem. When running open loop the power valve will be set at a fixed opening but my experience with closed loop systems is that the stepper will open to give more fuel as you open the throttle and then close back down to the default as the revs rise. Much the same as the accelerator pump on a carb.

[LPGC]

the biggest problem i have found with them is that , if i have the mixture right at the high rpm full throttle area , and also on the low rpm , and the cruise areas , then , if i go full throttle at say , 2000 rpm , the mixture leans right out at 3500 , then richens up again at 4000 ,mix is ok then to 6500 .

i have found this with every one i have used , ,i wonder if the rings with the centre feed would help with this ..?

of course it could be down to the omvl r90 e reducer ?

ah no stepper or any sort of controls on miy stuff chaps .

I suspect that is what is causing the problem. When running open loop the power valve will be set at a fixed opening but my experience with closed loop systems is that the stepper will open to give more fuel as you open the throttle and then close back down to the default as the revs rise. Much the same as the accelerator pump on a carb.

Does Rob mean only during quick acceleration or also during constant load running? If a carb has a mechanical accelerator pump it only operates when the throttle is being moved...

[robertXX]

sorry chaps i should have explained better .

i put it in 4th gear at 2000 rpm .

then floor the pedal for max throttle and simply hold it there , until 6500 rpm .

the air fuel ration will sit at the normal 13.5:1 as i accelerate ,

the revs rise , and as i get to 3000 rpm the mixture leans off .to say 15-16:1 ,

as i get to 3500-4000 rpm , the mixture comes back to where i want it , and stays there till 6500 rpm .

if i set the power valve open to being richer than i want , then this dip occurs but not enough to lean it right off and feel flat .

i had assumed it was only my 2.0 litre 8 valve astra engine doing it , and was connected with the inlet harmonics , or the mixer ,a plastic insert omvl 42mm one . or the regulator, a r90e omvl .


however , i got the same on the 1.6 16v astra engine , with a ally spun mixer from tinley tech , 36mm i think it was , and the same regulator ,also this has a long runner inlet so that could be doing it , however, i then made my own inlet , with shorter length, bigger diameter runners , and it still did it .





regards
robert

[Pinger]

sorry chaps i should have explained better .

i put it in 4th gear at 2000 rpm .

then floor the pedal for max throttle and simply hold it there , until 6500 rpm .

the air fuel ration will sit at the normal 13.5:1 as i accelerate ,

the revs rise , and as i get to 3000 rpm the mixture leans off .to say 15-16:1 ,

as i get to 3500-4000 rpm , the mixture comes back to where i want it , and stays there till 6500 rpm .

if i set the power valve open to being richer than i want , then this dip occurs but not enough to lean it right off and feel flat .

i had assumed it was only my 2.0 litre 8 valve astra engine doing it , and was connected with the inlet harmonics , or the mixer ,a plastic insert omvl 42mm one . or the regulator, a r90e omvl .


however , i got the same on the 1.6 16v astra engine , with a ally spun mixer from tinley tech , 36mm i think it was , and the same regulator ,also this has a long runner inlet so that could be doing it , however, i then made my own inlet , with shorter length, bigger diameter runners , and it still did it .





regards
robert

I'll throw the following into the mix for discussion as a possible explanation of the above.
Up to 3000 rpm there is zero turbulence in the mixer as the gas speed through it is below the speed at which it would begin. Thus, the vacuum signal to the mixer is generated solely by the venturi in accordance with Bernoulli's Theorem. But that signal isn't quite enough at the power valve setting you have so it goes weak. We might expect this to continue as the rpm rises to the 3500 - 4000 rpm range, but turbulence in the mixer will start to intrude and as it is a restriction there is vacuum generated in the same way that placing a hand over the air intake (where it draws from the atmosphere) could be created. The vacuum signal generated by the venturi will be as before but now there is additional vacuum to add to it and that is sufficient to draw LPG at the power valve setting in use hence the mixture can richen to a value the engine wants.
To characterise the mixer's behaviour then, we'd say it is operating purely on Bernoulli's principal up to 3000 rpm and a mixture of Bernoulli's and restriction created vacuum above 3000 rpm. Plausible?

[LPGC]

To characterise the mixer's behaviour then, we'd say it is operating purely on Bernoulli's principal up to 3000 rpm and a mixture of Bernoulli's and restriction created vacuum above 3000 rpm. Plausible?

I see potential for a setup to get gradually leaner with increasing airflow, then there to come a point when the mixer venturi becomes more restrictive to incoming air which presents as partial vacuum between mixer and throttle body. The combination of the two could give the symptoms described of close to correct mixture at lower loads, leaned off at mid loads and richer again at high loads. Any vacuum at the mixer gas output points will pull on the reducer diaphragm and cause it to output gas, regardless of whether the vacuum is caused by the venturi effect / shield effect or by partial exposure to engine vacuum because of the restrictive size of the mixer venturi. In some ways a restrictive mixer has same effect (richening mixture) as a very dirty air filter on a carb engine.

Difficult to put the theory to the test if we can't get a mixer system to run properly using a mixer that doesn't place some restriction on airflow anyway. In my last post I nearly added we've recently been talking 2 subjects, your 'can we fit a bigger mixer' and Rob's rich lean rich mixture but the 2 subjects are related.

[Pinger]

I see potential for a setup to get gradually leaner with increasing airflow, then there to come a point when the mixer venturi becomes more restrictive to incoming air which presents as partial vacuum between mixer and throttle body. The combination of the two could give the symptoms described of close to correct mixture at lower loads, leaned off at mid loads and richer again at high loads. Any vacuum at the mixer gas output points will pull on the reducer diaphragm and cause it to output gas, regardless of whether the vacuum is caused by the venturi effect / shield effect or by partial exposure to engine vacuum because of the restrictive size of the mixer venturi. In some ways a restrictive mixer has same effect (richening mixture) as a very dirty air filter on a carb engine.

Thing is though, the mixture shouldn't lean with increasing airflow as by Bernoulli's Theorem the pressure drop increases with the square of the speed of the airflow. Where do you see the potential for it go awry?

Difficult to put the theory to the test if we can't get a mixer system to run properly using a mixer that doesn't place some restriction on airflow anyway. In my last post I nearly added we've recently been talking 2 subjects, your 'can we fit a bigger mixer' and Rob's rich lean rich mixture but the 2 subjects are related.

If what we are discussing is correct, it should be identifiable by reducing manifold pressure as the turbulence in the mixer increases. In the case of the car in question, if it has OBD2 and MAP is a monitored parameter, it should show on any of the phone apps that can report OBD2 data. Or a plain old vacuum gauge.

[LPGC]

Thing is though, the mixture shouldn't lean with increasing airflow as by Bernoulli's Theorem the pressure drop increases with the square of the speed of the airflow. Where do you see the potential for it go awry?

If what we are discussing is correct, it should be identifiable by reducing manifold pressure as the turbulence in the mixer increases. In the case of the car in question, if it has OBD2 and MAP is a monitored parameter, it should show on any of the phone apps that can report OBD2 data. Or a plain old vacuum gauge.

Once installed with our chosen reducer, mixer/venturi, etc our mixer system setups allow 3 ways of adjusting response/mixture. There's the inline adjuster (or stepper motor if a closed loop system), reducer sensitivity adjustment and the reducer idle bypass. At very low loads (and especially if we dialled in some idle bypass) the inline adjuster makes less difference to mixture than reducer sensitivity, so there is that interplay between reducer sensitivity and inline adjuster. If we install a system and find we don't need to set the reducer sensitivity to anything like maximum sensitivity it begs the question could we have fitted a bigger mixer/venturi which would have had less restriction to airflow so would have affected top end engine power less than the smaller mixer/venturi. If you do enough playing around with mixer systems on enough different models of vehicles and try enough different mixers with various venturis you find that some reducers (even of supposed same model) can have different maximum sensitivity and different mixture linearity (for same mixer/venturi) than others, with some reducers you can get away with a bigger mixer venturi than with others, on some setups it can be difficult to get rich enough idle mixture or top end power or both, you can sometimes trade mixture linearity for better flowing mixers that restrict engine power less than a smaller worse flowing mixer.

There's two aspects to the point you make in your second paragraph I quoted above. One aspect is that if the mixer reduces airflow into the engine the manifold pressure will be lower, I agree with that, the same also implies that some of the restriction is caused by the mixer, so like I said there would likely be almost the same lower pressure at the gas output of the mixer so the reducer would see that extra signal and respond with extra gas thus richenening the mixture. The other aspect is that if the car has a map sensor it reads manifold pressure not the pressure between the mixer and throttle body. It would be difficult to get an accurate reading of pressure between the mixer and throttle body even if we fitted a sensor somewhere in there, if we install it pointing towards incoming air it might give a higher reading than if we point it away from incoming air etc. It would be difficult to accurately compare manifold pressure to pressure between mixer and throttle body, there obviously can't be more air flowing through the manifold to cylinders than flows through the mixer but some of the other things we've been talking about could affect pressure readings even if they could both be measured accurately enough anyway.

[Pinger]

The other aspect is that if the car has a map sensor it reads manifold pressure not the pressure between the mixer and throttle body.

MAP would suffice though for this test wouldn't it? As long as the throttle is wide open - or creating less of a restriction than the mixer - the mixer will set the level of depression in the manifold and that's what we are looking to measure.
At part throttle when airflow is low, the mixer wont yet be intrusive to flow and therefor none of the turbulence created vacuum we are seeking to quantify is present and no need to try and measure it.
I've probably overlooked something as I'm prone to do, but I think the above is valid.

[LPGC]

The other aspect is that if the car has a map sensor it reads manifold pressure not the pressure between the mixer and throttle body.

MAP would suffice though for this test wouldn't it? As long as the throttle is wide open - or creating less of a restriction than the mixer - the mixer will set the level of depression in the manifold and that's what we are looking to measure.
At part throttle when airflow is low, the mixer wont yet be intrusive to flow and therefor none of the turbulence created vacuum we are seeking to quantify is present and no need to try and measure it.
I've probably overlooked something as I'm prone to do, but I think the above is valid.

The accuracy (or lack of) of the vehicle map sensor might be surprising.

Two ways of looking at whether the mixer is obtrusive at part throttle... Up to a point (of mixer obtrusiveness) we could open the throttle further to get more airflow, then at that point the mixer wouldn't be restricting airflow or power from the driver's point of view. But you mean outright obtrusive, at some point in the airflow the mixer has got to cause a depression in pressure or it wouldn't suck and gas out of the reducer, then the question is does the depression or aspects like turbulence cause an obstruction and I think if the mixer throat/venturi is smaller than the throttle body (or at least small enough for the mixer to do it's job properly - job being to cause gas to be sucked out of the reducer) in most/all cases it will cause an obstruction.

There is another aspect - volumetric efficiency. The engine is pumping fuel and air mixture, with correct mixture on LPG from a mixer around 6% of the manifold volume is LPG not air, with correct mixture on petrol if the petrol remains in liquid form until it gets into engine cylinders the percentage of liquid petrol in the manifold is far lower than 6%. Face value that would mean the engine might only make say 95% of the same torque/power on LPG as on petrol but that is before we factor in the difference in calorific values of LPG versus petrol. But even if we measure the same manifold pressure on LPG as on petrol there's still the potential for the mixer to be causing a 5% obstruction to airflow, in that case we'd be reading the same manifold pressure only because what the mixer takes from incoming airflow it adds to the mix in terms of LPG vapour.

[Pinger]

But you mean outright obtrusive, at some point in the airflow the mixer has got to cause a depression in pressure or it wouldn't suck and gas out of the reducer, then the question is does the depression or aspects like turbulence cause an obstruction and I think if the mixer throat/venturi is smaller than the throttle body (or at least small enough for the mixer to do it's job properly - job being to cause gas to be sucked out of the reducer) in most/all cases it will cause an obstruction.

The depression created in the venturi doesn't necessarily have to cause a restriction as the engine sees it. If the speed of the air flow is below the critical value at which it becomes turbulent, there will be no losses, only a trading of velocity and pressure while the air maintains its energy level. After gaining speed and lowering in pressure as it passes through the venturi, once out of the venturi, given a decent flow path the air will slow again and regain its former pressure.
Only when the airflow is fast enough through the venturi to create turbulence do irrecoverable losses occur ie, post venturi, the air cannot return to its earlier values of velocity and pressure (as energy has been squandered to heat and noise due to the turbulence) This, when the pressure doesn't recover and thus reduces charge density is the definition of 'throttling' - all the way to 'choked flow'.

I just did a quick calc on the Astra 2.0l at 4000 rpm with a 42mm mixer and the air speed through it would be 48m/s. Ricardo stated that peak volumetric efficiency occurred when gas speed in the inlet port was in the region of 37 to 49m/s. The Astra's gas speed between 3500 and 4000 rpm is 42 to 48m/s - a pretty close correlation.

edit PS. Same calc for the 1600 and 36mm mixer give gas speeds of 46 to 52m/s at 3500 to 4000 rpm. Again, in agreement with Ricardo's contention that 37-49m/s is as fast as the gas flow can be before turbulence arrives and intrudes on volumetric efficiency.

[LPGC]

It was you who wondered if we could use a larger mixer venturi... Are you saying with your setup there wouldn't be an advantage to using a larger venturi? Just curious, I realise your answer might be no and we're just into this as a thought experiment.

I'm not sure exactly what Ricardo references are, was this in context of flow through LPG system mixers, throttle bodies, just general physics, etc? We know for sure that if we fit an inlet manifold with port runners that are too wide it can negatively effect engine breathing / volumetric efficiency by pushing the rpm at which the manifold would help make most torque too high in the rpm range, similar to having intake ports in the head that are too big etc.

If we're just talking LPG mixers or throttle bodies, it stands to sense that up to a point in flow a certain diameter/size won't have any detrimental effect but if we're on the borderline for flow of the diameter a smaller diameter will start to have detrimental effect on flow. It is usually possible to get some basic level of understanding by thinking about what happens at (impractical) extremes of what we're thinking about... I picture a railway tunnel blocked off at one end with only an LPG mixer as vent, then think about what's likely to happen if there's a windspeed of only less than 1mph towards the wide open entrance to the tunnel, will the mixer be restrictive only when airflow through it reaches 50m/s or will it also be restrictive when flow through it is slower?

[Pinger]

It was you who wondered if we could use a larger mixer venturi... Are you saying with your setup there wouldn't be an advantage to using a larger venturi? Just curious, I realise your answer might be no and we're just into this as a thought experiment.

My SBC would benefit I think with a bigger mixer for top end power - but the tip-in would be too problematic.

I'm not sure exactly what Ricardo references are, was this in context of flow through LPG system mixers, throttle bodies, just general physics, etc? We know for sure that if we fit an inlet manifold with port runners that are too wide it can negatively effect engine breathing / volumetric efficiency by pushing the rpm at which the manifold would help make most torque too high in the rpm range, similar to having intake ports in the head that are too big etc.

Ricardo is discussing gas speed through an inlet port and how it sets the limit of 4T breathing and thus piston speed.

If we're just talking LPG mixers or throttle bodies, it stands to sense that up to a point in flow a certain diameter/size won't have any detrimental effect but if we're on the borderline for flow of the diameter a smaller diameter will start to have detrimental effect on flow. It is usually possible to get some basic level of understanding by thinking about what happens at (impractical) extremes of what we're thinking about... I picture a railway tunnel blocked off at one end with only an LPG mixer as vent, then think about what's likely to happen if there's a windspeed of only less than 1mph towards the wide open entrance to the tunnel, will the mixer be restrictive only when airflow through it reaches 50m/s or will it also be restrictive when flow through it is slower?

There are a number od factors that influence the commencement of the critical point of turbulence and, choked flow. Find them in Reynold's Number.

[LPGC]

My SBC would benefit I think with a bigger mixer for top end power - but the tip-in would be too problematic.

Ricardo is discussing gas speed through an inlet port and how it sets the limit of 4T breathing and thus piston speed.

There are a number od factors that influence the commencement of the critical point of turbulence and, choked flow. Find them in Reynold's Number.

What mixer is on your SBC? There was a time when it was common to fit a mixer designed for a big Mercedes engine on American V8's but the mixer was restrictive for the bigger American V8's. Impco might be an improvement but is expensive.

Is the subject of mixer venturi size completely compatible with the subject of flow through an intake port into a cylinder with receding piston or are we assuming that the best speed for flow through an engine intake port is also the best speed for flow through a throttle body or mixer?

What conclusions do you draw from applying Reynolds number to the typical LPG mixer system setup that has curved shaped venturi of smaller diameter than the throttle body or carb it sits on? The only conclusion I draw is that if the engine is powerful enough to need to suck enough air for maximum power for a choke point to be realised the choke point is more likely going to be at the smallest diameter at the intake which will be the diameter of the mixer. I haven't taken shape or the mixer venturi into consideration but it doesn't seem I need to because it's going to be the choke point regardless of shape, only if the curved shape of the mixer is the ideal shape less ideal shapes would make for turbulence at lower intake air velocity?

[Pinger]

What mixer is on your SBC? There was a time when it was common to fit a mixer designed for a big Mercedes engine on American V8's but the mixer was restrictive for the bigger American V8's. Impco might be an improvement but is expensive.

My mixer is as below - but with some of the holes blocked off. I've no real intention to be changing it as it works well enough, but I have another potential application - which I'll come to.

IMG_4871.JPG (970.39 KiB) Viewed 3731 times

Is the subject of mixer venturi size completely compatible with the subject of flow through an intake port into a cylinder with receding piston or are we assuming that the best speed for flow through an engine intake port is also the best speed for flow through a throttle body or mixer?

It is hard to be exact. For example, Ricardo gives lower gas speeds for diesel engines compared to petrol. The reason I presume is that a diesel inlet port will be more restrictive due to its shape which is such to impart swirl to the incoming air in a way that petrol engines don't require. The dimensional element in Reynold's Number isn't always obvious. In the context of what we are looking at diameter is the required characteristic length. To answer your question then, the best speed through a mixer is very probably the same (or similar) to that in the port (valve throat to be specific) but will be reached at a lower rpm as the mixer flow area will be less than the inlet valve throat area.

What conclusions do you draw from applying Reynolds number to the typical LPG mixer system setup that has curved shaped venturi of smaller diameter than the throttle body or carb it sits on? The only conclusion I draw is that if the engine is powerful enough to need to suck enough air for maximum power for a choke point to be realised the choke point is more likely going to be at the smallest diameter at the intake which will be the diameter of the mixer. I haven't taken shape or the mixer venturi into consideration but it doesn't seem I need to because it's going to be the choke point regardless of shape, only if the curved shape of the mixer is the ideal shape less ideal shapes would make for turbulence at lower intake air velocity?

I don't apply Reynold's Number as such. The points you raise are valid. Within the Reynold's Number calculation there is for example consideration of the 'surface roughness of the conduit so the form of the conduit is relevant. As are flow co-efficients. The minute differences (IMO) between flow through a valve throat and a mixer would require CFD modelling to arrive at a more precise answer.

This copy and paste:
''Laminar flow: A Reynolds number between 0 and 2000 indicates laminar flow.
Unstable flow: A Reynolds number between 2000 and 3000 indicates unstable flow that is changing from laminar to turbulent.
Turbulent flow: A Reynolds number above 3000 indicates turbulent flow.''

is of use because it it shows the three flow regimes (and a numerical - if it can be calculated - value at which they will prevail). From Ricardo's gas speed figure of 49m/s being the point at which turbulence is starting to intrude we could assume that the regime is the second one unstable flow. If we consider Rob's 2.0l Astra engine it is in that region he experiences weaking mixture and as it enters the third regime - turbulent flow where the turbulence becomes intrusive, that is where the engine sees vacuum created by the turbulence and thus a greater draw on the mixer richening the mixture. If, we were in position to calculate accurately from Reynold's Number, we would be able to predict which of the three flow regimes would exist at any given parameters and predict the transition points. Understanding that there are these three flow regimes is the important part for more basic calculations. NB Bernoulli's Theorem is more useful at the level at which I operate.

My interest in this is driven to some extent by the possibility that I may want to use a mixer system on a 2-stroke. Not a conventional crankcase scavenged 2T but an externally scavenged one using a blower.The big difference here relative to 4T is that there is no pumping stroke and if the mixer was downstream of the blower the pressure drop across the mixer venturi created by the air flow through it is the only pressure drop possible because, as the flow regime moved from unstable to turbulent, there can be no additional turbulence created vacuum because there is no pump downstream - unlike the 4T.

[Pinger]

I have remembered a practical application of the onset of turbulence due to the gas speed reaching a critical value as per Reynold's Number. A book by Dalton explained a method whereby he could find that point in a flowing port using a very small ball on the end of something stiff enough to allow positioning but not so stiff as to deny movement - something akin to fuse wire IIRC. His technique is to establish flow through a port using vacuum and then introduce the ball on the wire into the port and move it around to explore the gas speeds. When the ball follows the air flow the flow is non-turbulent. When it is moved into a more restricted part of the port where the gas speed is already higher - and then made higher still by the presence of the ball - and the speed goes beyond the critical value and becomes turbulent, the ball starts flapping about physically showing the region of turbulent flow. Dalton would then open up the port in that region to lower the gas speed.
Dalton would investigate the entire port by that method and the finish point would be when either no turbulence was present or, with an increase in gas speed, turbulence could be detected everywhere - indicating that the port was flowing evenly.

[LPGC]

A lot to think about and a lot to reply to lol.

I don't think we're likely to improve the design or mixers by doing Reynolds number calculations or checking turbulence / experimenting with balls on the end of fuse wire...

I think, as already implied, the point of fuel entry on an LPG mixer is going to need more total suck than a petrol carb because we need to suck in more volume of vapour LPG than we do liquid petrol, putting that volume in does in itself reduce the suck at the point of the suck. Perhaps it would be possible to make an LPG mixer that didn't restrict airflow any more than the throttle body or carb it was mounted on, I picture just a tube same diameter or wider than the carb / tb diameter with holes around the inside for venturi effect to suck the gas vapour out of... Maybe this could give rich enough mixture under engine load conditions, but carbs need the idle jet for low load conditions because there isn't enough venturi effect at low airflows to suck fuel out of the main jet and it seems that same problem would be exaggerated with vapour LPG that is (ballpark) 180 times less dense than liquid petrol and for intents and purposes has similar stoch ratio. An LPG mixer doesn't have an idle jet, if we could solve the problem of too lean mixture for an LPG mixer at low airflows / loads then we should more easily be able to get around the need for an idle jet in a petrol carb. Perhaps a hybrid LPG injection / LPG mixer system would work with the mixer system doing most of the main jet fuelling and injection system doing the low load / idle jet function? I would expect pitfalls to include the timing/frequency of pulsed fuel delivery from a single injector would be critical so as not to give any cylinder richer/leaner fuelling than other cylinders but that could be overcome by increasing pulse frequency and fitting a very small nozzle to very small injector(s) or by having 1 injector per cylinder port injection. But if you're going to go this route you might just as well have complete LPG fuel injection instead of the hybrid mixer system.

Does the supercharged 2 stroke still have a reed valve type setup? I don't think I fully grasp the problem you're saying would be with a boost pressure referenced reducer and blow through system.. But you could make it a suck through system (with associated bang inside the supercharger risk I know)?

[Pinger]

A lot to think about and a lot to reply to lol.

I don't think we're likely to improve the design or mixers by doing Reynolds number calculations or checking turbulence / experimenting with balls on the end of fuse wire...

I think, as already implied, the point of fuel entry on an LPG mixer is going to need more total suck than a petrol carb because we need to suck in more volume of vapour LPG than we do liquid petrol, putting that volume in does in itself reduce the suck at the point of the suck. Perhaps it would be possible to make an LPG mixer that didn't restrict airflow any more than the throttle body or carb it was mounted on, I picture just a tube same diameter or wider than the carb / tb diameter with holes around the inside for venturi effect to suck the gas vapour out of... Maybe this could give rich enough mixture under engine load conditions, but carbs need the idle jet for low load conditions because there isn't enough venturi effect at low airflows to suck fuel out of the main jet and it seems that same problem would be exaggerated with vapour LPG that is (ballpark) 180 times less dense than liquid petrol and for intents and purposes has similar stoch ratio. An LPG mixer doesn't have an idle jet, if we could solve the problem of too lean mixture for an LPG mixer at low airflows / loads then we should more easily be able to get around the need for an idle jet in a petrol carb. Perhaps a hybrid LPG injection / LPG mixer system would work with the mixer system doing most of the main jet fuelling and injection system doing the low load / idle jet function? I would expect pitfalls to include the timing/frequency of pulsed fuel delivery from a single injector would be critical so as not to give any cylinder richer/leaner fuelling than other cylinders but that could be overcome by increasing pulse frequency and fitting a very small nozzle to very small injector(s) or by having 1 injector per cylinder port injection. But if you're going to go this route you might just as well have complete LPG fuel injection instead of the hybrid mixer system.

Agreed, by the time injectors are deployed, be as well let them do everything.

Does the supercharged 2 stroke still have a reed valve type setup? I don't think I fully grasp the problem you're saying would be with a boost pressure referenced reducer and blow through system.. But you could make it a suck through system (with associated bang inside the supercharger risk I know)?

Not supercharged - merely blower fed. A 2T (other than crankcase scavenged) cannot self aspirate. Think of it as an orifice engine with inlet and exhaust ports simultaneously open and a continual flow between the two through the cylinder. Thus configured a mixer placed downstream from a blower would never see vacuum (hence can be placed downstream from a throttle valve). Upstream as you say is a lot easier dealt with - but too dangerous if (and it is only an if) an intercooler was employed.
Only crankcase scavenged engines use reeds so no need for them in externally (blower) scavenged motors as the piston will close the transfer ports (which the blower directly feeds).
2T engines despite being simpler mechanically than 4T are a lot more complex in operation. Suck, squeeze, bang, blow - or wheeze, squeeze, please, wheeze - describes 4t. Try explaining a 2T in such simple terms!

[LPGC]

I know the basics of how 2 strokes work lol, like you say though the process is more difficult to grasp or explain than 4T, the exhaust and intake kind of happen simultaneously. Could have 2 x 2T engines, both the same size / bore / stroke, both from the same manufacturer, but the porting can make a massive difference to all of torque, bhp and fuel economy (in different measures). Dad used to have a theory about 2T boat outboard engines that an engine with the biggest capacity to bhp ratio (specific power output) would generally be more low rpm torquey and run smoother than one with a higher specific output. At the time you could buy a 70hp or 85hp engine, they'd be the same engine except for a seemingly minor porting difference but the 70 would usually be a 'nicer' engine with a flatter torque curve and use less fuel even at port throttle than the 85.

Know a bit about superchargers too, the old numbering system for Roots type blowers on diesel / loco engines etc.

I don't see much of a distinction between supercharger and blower, unless by blower you mean a setup that gives little boost pressure but delivers the volume of air to the intake port on the 2T? Just something to do away with the need for crank scavenging and maybe reed valves? Different on the diesel 2T's, in theory a blower/supercharger is more beneficial to a direct injection 2T diesel than to a 2T carb petrol? In fact a direct injection supercharged 2T diesel (like some loco engines) is in many ways easier to understand than a conventional 2T petrol.

If it's not compressing air an intercooler wouldn't be fitted anyway? Still is possible better blow through than draw through to prevent having explosive gas and air mixture inside the blower?

Where is the throttle on this 2T blowered petrol engine? I still don't completely follow why you couldn't use a mixer between the blower and the intake port with the reducer referenced to pressure just before the mixer. I could see that working perhaps with a need to lengthen the intake to limit backpressure pulses from the intake port going back/forth through the mixer.