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Re: Advice please!

Posted: Tue Oct 27, 2020 8:30 pm
by Pinger
Pretty sure I enquired about that one and the ID was around that - and no larger.
260hp requires 33% more air than for 195hp. The duct I'm fitting my mixer into is 90mm ID and the throttle body is 72mm.

I need/want to better understand the gas dynamics that were present today. Clearly, my assumption that the air would recover (ie reduce in velocity and regain its pressure) after passing through the first mixer and before presenting at the second was wrong. What I created was an overall vacuum either side of the downstream mixer in addition to that created by its venturi effect - possibly greater than it's venturi effect (in essence, it was the equivalent of a good old fashioned choke). I have to ascertain if that prevails over the entire operating range and if it's even desirable. There's very possibly an element of what LPGC has eluded to re one mixer being hungrier than the other. If so, it was given ample opportunity today to make its presence known as each mixer was being fed directly from the reducer without any shared plumbing. I just need to rethink after what I've learned today and work out what next. Gas flow is a mechanical issue - governed by physics. A mechanical solution is required. I already have one in mind and it's simpler that dual mixers.

Re: Advice please!

Posted: Tue Oct 27, 2020 10:40 pm
by Gilbertd
I take it that's 72mm ID? If 72mm OD then the Range Rover mixer I've got would slot straight on.

I'm afraid mechanics to me means bits that go up and down or round and round. Few people understand electrics as they can't see it and I'm afraid gas flow is much the same for me, I know what it does but wouldn't have a clue how to start working it out.

Re: Advice please!

Posted: Wed Oct 28, 2020 9:56 am
by Pinger
Gilbertd wrote:
Tue Oct 27, 2020 10:40 pm
I take it that's 72mm ID? If 72mm OD then the Range Rover mixer I've got would slot straight on.
It wouldn't physically fit as the cowling that fits above is much larger (to accommodate the duct for the IAC valve) and would be so close to the butterfly as to create mixture distribution problems. The Vortec has its throttle body where a (single down draught) carb normally is with a V8 but offset to the front.
The duct the mixer sits in is 90mm ID.


Gilbertd wrote:
Tue Oct 27, 2020 10:40 pm
I'm afraid mechanics to me means bits that go up and down or round and round. Few people understand electrics as they can't see it and I'm afraid gas flow is much the same for me, I know what it does but wouldn't have a clue how to start working it out.
I'm beginning to gain a better understanding of what I encountered yesterday (and can expand on it later if there's interest).

I do have a question though that relates to the issue with an oversized mixer.

In essence, it is exactly the same problem as faced by carbed engines when the venturi (or choke) is oversized in pursuit of maximum power over all else. The engine will idle from its idle circuit but when the throttle is opened the idle circuit can no longer cope and there isn't sufficient air velocity in the venturi to create enough signal to draw fuel. The solution? Dip the clutch, bring the revs up enough to get the air moving and slip the clutch to keep it going. Neither viable or desirable with LPG and an auto box.
But - and I've not had the courage or wit at the time to try it - if an LPG motor attains high enough rpm to get past its immediate out of idle (tip in?) phase, will the increased air speed then allow the ('oversized') mixer to function?

Re: Advice please!

Posted: Wed Oct 28, 2020 10:08 am
by LPGC
Pinger wrote:
Tue Oct 27, 2020 8:30 pm
If so, it was given ample opportunity today to make its presence known as each mixer was being fed directly from the reducer without any shared plumbing. I just need to rethink after what I've learned today and work out what next. Gas flow is a mechanical issue - governed by physics. A mechanical solution is required. I already have one in mind and it's simpler that dual mixers.
If you feed 2 mixers from 1 reducer there's always shared plumbing, the plumbing meets at the reducer, the loop I mentioned merely becomes longer and reaches all the way back to the reducer instead of only reaching as far as the T in the pipe between mixers.

It is a physics question but a very complicated one, we can use rules of thumb but to accurately work things out such as pressures at certain points in the system / air intake through a mixer might be something a vehicle manufacturer might use a super-computer for these days and then results may need to be confirmed/proven in practice.

By all means if I was having problems like you are I might try a restriction further upstream than the point of gas entry (I think at one point I said that mixers spaced further away from the throttle body, or at least on a carb, can help airflow and signal) but I'd try to steer away from using multiple mixers plumbed to reducer(s) in series because I see potential for this further complicating things.

This model of engine has been successfully converted using a single conventional mixer mounted on the throttle body in the past.

Re: Advice please!

Posted: Wed Oct 28, 2020 11:26 am
by Pinger
LPGC wrote:
Wed Oct 28, 2020 10:08 am
If you feed 2 mixers from 1 reducer there's always shared plumbing, the plumbing meets at the reducer, the loop I mentioned merely becomes longer and reaches all the way back to the reducer instead of only reaching as far as the T in the pipe between mixers.

It is a physics question but a very complicated one, we can use rules of thumb but to accurately work things out such as pressures at certain points in the system / air intake through a mixer might be something a vehicle manufacturer might use a super-computer for these days and then results may need to be confirmed/proven in practice.
Agreed - it's a ball ache I don't need.
LPGC wrote:
Wed Oct 28, 2020 10:08 am
By all means if I was having problems like you are I might try a restriction further upstream than the point of gas entry (I think at one point I said that mixers spaced further away from the throttle body, or at least on a carb, can help airflow and signal) but I'd try to steer away from using multiple mixers plumbed to reducer(s) in series because I see potential for this further complicating things.

This model of engine has been successfully converted using a single conventional mixer mounted on the throttle body in the past.
But, what yesterday did show was that a restriction upstream allows me to open up the mixer downstream (for more airflow). And said restriction is what I'd proposed as an upstream mixer. So looking to pursue that. To proceed though, it's an answer to this question:
if an LPG motor attains high enough rpm to get past its immediate out of idle (tip in?) phase, will the increased air speed then allow the ('oversized') mixer to function?
that I'm after. Any ideas? Has anyone pushed through the 'lean' barrier and found out? - and survived to tell the tale!

Re: Advice please!

Posted: Wed Oct 28, 2020 3:13 pm
by LPGC
A restriction upstream may cause partial vacuum between the restriction and mixer on the throttle body, in the right circumstances this could mean you don't need a mixer on the throttle body at all - My baked bean can 'mixer' simply had the top removed from the can to fit onto the throttle body, a hole made in the other end and a gas feed entering the side. It worked very well but I expect it would have been more limiting to max airflow than a proper mixer due to causing more turbulence.

I've seen setups that idled too lean but ran with correct rich mixture off idle. I've said before (might have been another thread) that I like to see setups have stepper motor slightly more open at idle than at off idle, it kind of shows that the venturi size (and hence restriction to intake air) limit airflow as little as possible (couldn't fit a bigger venturi or it couldn't be made to idle with rich enough mixture). Could argue that reducer idle bypass's are there to allow for this condition.

For a given size outside fitting and given size venturi 'slimline' (low profile / short) mixers are known to give less linear signal than 'full size' mixers. Again probably best to avoid trying to understand all the physics in detail (I don't claim to understand them lol) but I believe air flow will usually be fastest through the centre of a hole and if a hole is big enough a small airflow through it won't create any signal at all to a pipe connected to the side? Otherwise (and if signal was proportional to airflow regardless of venturi size) we could fit any size mixer we wanted and we'd always fit a big enough mixer as to create zero restriction.

Every venturi size will have a max airflow ability and as limits are approached will create more restriction to airflow, of course whether this restriction is enough to impact on the engine's breathing ability depends on the size of the venturi. It will also have a signal versus airflow slope that will probably start flat (no signal) for very low (relative to venturi size) airflows and probably not be fully linear versus airflow all the way through that slope. The reducer will also have a signal versus gas flow slope that will start off flat (no gas flow) for very low signal and will peak at the reducer's max flow ability given enough signal, this slope also won't be totally linear. If we plotted the 2 response slopes on a graph and added them together we'd have a 3rd slope which would show us gas flow for given airflow, high points in the 3rd slope might be rich mixture, low points in the 3rd slope might be lean mixture... Where either of the earlier 2 slopes bottomed out (lack of signal or signal response) / topped out (max signal response, max flow from the reducer) the 3rd slope would too and the engine would run with incorrect mixture at that point. Probably not even quite as simple as that though... E.g. a 5 litre engine might use 100cfm of air at full throttle at low rpm, also use 100cfm at part throttle at medium rpm, also use 100cfm at lower throttle at high rpm but in each case the mixer and signal might be effected by other dynamics besides just total airflow - assuming the mixer causes some restriction to incoming air, those 3 conditions with same airflow might see the mixer and signal be effected by different partial exposure to varying manifold vacuum.

If we had an engine that could idle very efficiently (little airflow to idle) but could generate a lot of power for it's size (could term this a wide dynamic range) we'd need the mixer venturi big enough as not to restrict airflow for the high bhp ability but that would be a big mixer venturi related to airflow for the efficient idle. In this scenario we might have a lean mixture at idle (because at idle we've fallen into the flat area on the response slopes) but correct mixture when the engine is producing more power (because we're into a more linear area on the overall response slope).

Re: Advice please!

Posted: Wed Oct 28, 2020 5:53 pm
by Pinger
Pressure drop through a venturi is proportional to the square of the fluid velocity passing through it. The air requirements of an engine at WOT are pretty much linear with rising rpm - and air velocity increases are linear also.
Seems to me that the problem with larger mixers is that between idle (where the bleed can be utilised) and high rpm where the mixer may generate sufficient signal given its proportional to the square of the air speed - there's a hole to fall into just out of idle where (for me at least) lurks the backfire risk (and general unresponsiveness anyway). Carbs on petrol face the same and deployed all sorts of trickery to overcome it.

Given the mere placing of that 45mm ID mixer upstream of my current mixer permitted freeing up its area, I'm looking to deploy an upstream venturi to get through the just out of idle 'hole' while minimising the turbulence. The pressure drop of an upstream restriction (even though its ID is bigger than the feed mixer) obviously presents as a pressure drop at the mixer. But it will I think in its effect be more linear (not related to velocity squared) than the core of a venturi. The pressure drop is a consequence of that process - the friction having robbed the air of internal energy.
How restrictive this is re top end power remains to be seen but if at higher airflows the mixer steps up, then the upstream restriction can possibly be stood down. Did you ever notice that the choke plates on twin choke Webers fitted to some Fords were not symmetrical either side of their pivot shaft but could be pulled open when airflow requirements demanded?

Reducer sensitivity is a vague concept to me though. I read somewhere that it can be considered to be the equivalent of petrol height in a carb's float chamber (the higher that level, the more easily the fuel can be drawn). Is that a reasonable analogy?

Re: Advice please!

Posted: Wed Oct 28, 2020 9:50 pm
by Gilbertd
Explained nicely here http://www.diy-lpg.co.uk/articles/files ... -loop.html but what the sensitivity adjustment does is adjust the gas pressure. Pressure is always marginally below atmospheric so has to be sucked in and the sensitivity adjustment adjusts how much for a given amount of 'suck'.

Re: Advice please!

Posted: Thu Oct 29, 2020 10:53 am
by Pinger
The 'pressure head' in a carb's float chamber is a decent analogy then.

My reducer is delivering at above atmospheric pressure though (and that is with the idle bleed closed). TT say this is OK. I guess the pressure at the outlets drops pretty quickly once out of idle.

Re: Advice please!

Posted: Fri Oct 30, 2020 4:04 pm
by Pinger
Tried an upstream restrictor (45mm ID) with 7 of the mixer holes and it was chronically lean. At no load 3000rpm it looks like it should work (actuator at 1600. Drive it and the actuator goes fully open, it's first box green lean and that's just in the second TPS box.
Back to 7 blocked, 4 open holes on the mixer and I think that's it - for now at least.

Re: Advice please!

Posted: Thu Nov 12, 2020 11:39 pm
by Bobby B.
Hello,
Sorry for your troubles. I have 2012 Caprice PPV 6.0 V-8 which was built by Holden in Australia which I purchased last year. It had a LPG system added to it and was a alternative fuels demo car and has only 20K miles on the odometer. The system is an eight port vapor injection type. This car runs like a champ and when put on the Dynometer it made 377HP for a 22HP gain over the 355HP on gasoline mode. I just want to let you know that there are V-8 running on propane with awesome results.

Good Luck & Best regards,
Bobby B.

Re: Advice please!

Posted: Fri Nov 13, 2020 9:18 am
by Pinger
Bobby B. wrote:
Thu Nov 12, 2020 11:39 pm
Hello,
Sorry for your troubles. I have 2012 Caprice PPV 6.0 V-8 which was built by Holden in Australia which I purchased last year. It had a LPG system added to it and was a alternative fuels demo car and has only 20K miles on the odometer. The system is an eight port vapor injection type. This car runs like a champ and when put on the Dynometer it made 377HP for a 22HP gain over the 355HP on gasoline mode. I just want to let you know that there are V-8 running on propane with awesome results.

Good Luck & Best regards,
Bobby B.
I'm stubbornly sticking with my simple single point system precisely because it is simple!
The more I read of the injection systems the more I appreciate how well they work. 377hp is pretty impressive. If I can get the 260hp my truck left the factory with I'll settle for that.