Pressure gauge for balancing

[Guest Nigelstephens]

Thanks for your contribution. These are my thoughts so far.

 

You are right about the difficulties of damping the pressure changes between cylinders. The customary way is to use a small air jet to reduce the amount of air passing through and to even out the rate of pressure change in the pipe going to the sensor. I need to do some tests on this to find the size of hole required.

 

For the sensor I was thinking in terms of a low cost pietzo that uses a strain gauge as part of wheatsone bridge. This would simply connect to both cylinders via tube then a take-off adaptor with jet hole and fix to the manifold at the usual position.

 

The software that interprets the sensor signal could average out the readings by talking an average of 10 readings to get a result. Display this then do the next reading. This would take say 0.5 seconds per display update. The display could be an LCD with the top line giving an indicator of balance and the lower line giving the pressure value.

 

I agree that testing will show whether it works or not and I will do this early before possibly waisting time.

[mike wilson]

If you are taking multiple readings (over, say, 1/10 of a second?) there will be variation even with damping and, depending on where in the cycle the readings hit, the average may not be a meaningful figure and would almost certainly not marry up with numbers taken from the other cylinder at the same time. This (matching cylinder timings) seems to be the biggest problem, to me. Is there some way of continuous measurement of a highly damped output using the components you propose?

[Guest Nigelstephens]

At 1200 rpm each cylinder is going to fire 5 times in 0.5 seconds. I think this would give sufficient averaging over 10 samples in tis time. What do you think?

 

The damping would need to be sufficient but with high damping the pulses could almost be compete be aliminated up to the point where the change in pressure is not sufficient to give a useful rate of change for indication.

[Guest ratchethack]

Now you've got me thinkin', and that's never a safe thing. . . .

 

I've just gotta ask.

 

Seems to me that getting the manifold vacuum differential between cylinders in balance is more critical at idle than at large throttle openings.

 

Here's my grand rationale:

 

At idle, tiny differences in throttle opening make quite large differences in RELATIVE manifold vacuum between right and left, because at idle, manifold vacuum is at its highest -- or as Pete would prefer to state it, low pressure is at its lowest. It's very sensitive to throttle imbalances. If the balance is even a tiny bit off, you get a relatively dramatic imbalance in cylinder filling volumes and this results in considerable vibration at idle and off-idle conditions. Hence idle bleed screws to fine-tune things.

 

At large throttle openings, manifold vacuum drops -- that is, relative manifold pressure rises. Therefore, cylinder filling differences are not nearly as significant by volume .

 

If this makes any sense, but you b'lieve I'm wrong, somebody pls. set me straight??

 

TIA

[mike wilson]

At 1200 rpm each cylinder is going to fire 5 times in 0.5 seconds. I think this would give sufficient averaging over 10 samples in tis time. What do you think?

 

The damping would need to be sufficient but with high damping the pulses could almost be compete be aliminated up to the point where the change in pressure is not sufficient to give a useful rate of change for indication.

 

I really haven't a clue 8-) The pressure variations in that sequence will be huge. Digital sampling may not be the best option. But I am sure that, if you are sampling both cylinders at the same times, there will be a differential (or there is _likely_ to be a differential) due to the cylinder timings being offset. This will give you a false reading. I believe analogue sampling with proper damping will cause less problems unless you can find some way of timing the sampling for each cylinder, so that they are being treated identically.

 

Maybe you could sample and discard all samples except the one with the highest reading?

[mike wilson]

Now you've got me thinkin', and that's never a safe thing. . . .

 

I've just gotta ask.

 

Seems to me that getting the manifold vacuum differential between cylinders in balance is more critical at idle than at large throttle openings.

 

Here's my grand rationale:

 

At idle, tiny differences in throttle opening make quite large differences in RELATIVE manifold vacuum between right and left, because at idle, manifold vacuum is at its highest -- or as Pete would prefer to state it, low pressure is at its lowest. It's very sensitive to throttle imbalances. If the balance is even a tiny bit off, you get a relatively dramatic imbalance in cylinder filling volumes and this results in considerable vibration at idle and off-idle conditions. Hence idle bleed screws to fine-tune things.

 

At large throttle openings, manifold vacuum drops -- that is, relative manifold pressure rises. Therefore, cylinder filling differences are not nearly as significant by volume .

 

If this makes any sense, but you b'lieve I'm wrong, somebody pls. set me straight??

 

TIA

 

I am not sure "vibration at idle and off-idle conditions" is correct. Balancing involves working at both tickover and throttle open conditions. Just because one is perfect does not mean that the other is. If you have perfect tickover synchronisation but a broken linkage between carbs/injectors, you will get very poor running off idle, to say the least.

 

Whilst the rider perception of roughness may be more acute at tickover, it is also possible that poor synchronisation at open throttle settings is causing equal or greater problems due to the higher power being developed. Maybe this is one of those areas where people with two degrees in maths might care to take a few weeks to come up with a theorem, whilst the rest of us enjoy ourselves? 8-)

[Guest Nigelstephens]

I don't see that digital sampling can be a problem if it is averaged. What we're interested in is the left to right bias of vacuum in the manifold. The actual number does not matter other than its more left/right and the degree of how far. Taking several readings over some time period (0.5 seconds say) will then cover several revolutions. The average of which should give a result for balancing.

 

That is unless we are sampling say 8 sucks on the left and only 7 on the right then the left side will show more negative pressure. But this would not happen the same for each averaging period it would be random and tend to give a range of average values. But this itself could be further smoothed by averaging at the expense of slower response of the instrument.

 

Adjustment of balance at larger throttle openings is not practical nor does it apply as it does at small openings. If you take the inverse of fully closed that is fully open butterflies there is no way to adjust in this situation because the butterflies are fully open. The differences in gas flow are then down to the resistance in the two systems. The difference in resistance are then down to tappet adjustment, bore and piston condition and manufacturing tolerances etc.... The vacuum pressure is substantially lower but could still be measured depending on the inlet restriction.

 

However, i can see that as the throttle gets closer to the idle position the balance becomes more useful. This is because adjustments are achievable and they have an effect.

 

I can see a portable gauge providing extra help where the balancing at 2500 rpm on the linkage and at tickover on the bleeds does not give a good adjustment at higher rpm, say 4000. I suppose the reason why 2500-3000 rpm is chosen to balance the linkage is that this will be roughly right for most throttle positions.

With the bypass circuit giving a fine tune for tick over. However, I know that on my Sport the throttle linkage is worn and the ball joint slop affects the sync of the two sides. Apart from replacing it, a solution might be to monitor balance when riding and use this information to help make a decision about the best place in the rev range to perform the linkage adjustment to give the best overall. Also one can make a tweak at traffic lights if reqired.

[mike wilson]

I agree with your general principles but there are some "dark side" forces at work here. Airflow in induction and exhaust tracts was one of the arcane arts that people either had a feeling for or lucked into the right figures, until computer modelling came to the rescue. Even then, it's not cut and dried.

 

To me, the two main things are to (i) make sure that the the samples are taken at the same timing of the induction on each cylinder. For that, you are going to have to use some sort of revolution sensor. There is already at least one in existence on EFI engines but you may not be able to use it for electronic reasons or reasons below. And (ii) on each cylinder, the samples need to be taken at a time that makes sense aerodynamically. No good sensing when the air is stationary, or nearly so. Which it is quite often. Ideally, the sensing should be done at peak air speed. That will vary through the intake cycle and at the specific location of the sensor. The existing bleed holes work fine for analogue comparative devices. There is no guarantee that they will do the same for a sampling device. You may have to take the samples there at timings that appear at odds with what you think should be going on in the inlet tract. To do that you may need to create a new timing sensor, which has to be properly attached and timed to the engine. Or you may have to place the sensor somewhere else.

 

A thought on the sensor - you might be able to use one that senses high flow (using whatever parameter) and only takes measurements at that time.

[Guest Nigelstephens]

Taking max values my have some merit. Thanks for that.

 

 

p.s. Were not that far apart. It would be great to meet up when the weather gets warmer!

[mike wilson]

Taking max values my have some merit. Thanks for that.

p.s. Were not that far apart. It would be great to meet up when the weather gets warmer!

 

Especially if we can get both bike on the road at the same time 8-)))

 

I'm just behind the southbound services at Washington.

[JuhaV]

Hello Nigel,

 

I found your pressure gauge project very interesting and was bondering same type of instrument when I had a Sport Corsa. However, nowadays when driving a Honda VTR SP1 the need for balancing throttle bodies seems to have almost completely vanished

 

A few years ago I developed my mental design for the pressure gauge up to the following stage. I try to shortly describe it here in case it would be helpfull to you.

 

The basic idea was to use a differential pressure sensor that would be capable of measuring pressure difference between two inputs. There are such sensors available but the pressure range at least for the economical ones is rather limited. Now, as you know if you plug the inputs directly to the left and right inlet manifolds, the pressure pulses are much too strong. However, if you would plug a pressure line into the inlet manifold, then add a restrictor into the line followed by a somewhat larger volume chamber, the pressure changes in that chamber due to the restrictor are damped down. If the size of the restrictor jet is made smaller at some point the pressure in the chamber stabilizes (becomes averaged) very effectively. The downside of this is of course the fact that the response time for the chamber pressure to follow the inlet manifold pressure increases.

 

In practise I found that very good restrictor jets can be manufactured from medical syringe needles that are available in different gauges. Just go to the pharmacy and buy a set of those needles in different sizes. Using two identical ones provides two restrictors that are identical to a very high degree. The chamber after the restrictor needs only be about a few cubic centimeters in volume. The ratio of the chamber volume compared to the aperture size of the restrictor jet defines the damping effect. I simply used plastic fuel filters to provide those necessary chamber volumes (the transparent ones that you plug into the fuel lines). A line (silicon tubing) from the inlet manifold was plugged into the filter inlet so that the syringe needle was first pushed into that same filter inlet remaining thus inside that silicon tubing.

 

Now considerer having each inlet manifold equipped with a pressure line, restrictor in that line and following with a stabilizing (averaging) chamber. Then plug the inlets of a differential pressure sensor into these chambers. What kind of signal would you see on the sensor's electrical output ? I suppose (not tested this in practise) that it would be slightly pulsing signal showing the left-right variation in the restrictor damped inlet pressure pulses. If you now add electrical low-pass filtering (= averaging) to that signal, there you are with the wanted pressure difference !

 

I suppose that the system could be damped & averaged down to the degree that it only needs to have a response time of a few seconds. That is fast enough for the system to be usable for adjustments.

 

What do you think ?

 

br, JuhaV

[Guest Nigelstephens]

JuhaV thats interesting and a possible solution. Not sure about the needles though. The air chanber (plenum) would average out the pulses too, but it is more complicated than I had invisaged.

 

I think I would like to try a few experiments when I have finished re-assembling by gearbox. I would like to do any damping via the electronics. But this depends on how the sensor performs. This is a big unknown until I actually try it.