What's the down-side to over-speccing a vaporiser/reducer?

[CNG]

What's the down-side to over-speccing a vaporiser/reducer.

Reducers/vaporisers seem to be specc'ed on output pressure and the likely power rating of the engine they're designed to be fitted to. Reading what i can on this, 'claimed outputs' seem to be optimistic, thus a 200HP is actually only good for 150HP. Thus apart from paying more for them, what's wrong with putting... say a 300HP Reducer/vaporiser on a 150HP vehicle, and have done?

[Brian_H]

The obvious one would be the cost, others could be increased size of the unit making it more difficult to install.

Not sure how sensitive a large reducer would be on a very small engine to manifold vacuum or if it would even be an issue.

[LPGC]

Not many down sides but as Bri has implied a massive reducer on a mixer system can be a different kettle of fish to a massive reducer on a sequential system. Probably wouldn't want to try fitting say an R90 on a very small engine'd generator, sensitivity is a key factor for mixer systems. But you're not really going to come up against many problems unless we're talking extremes, e.g. if a mixer system reducer is good for a 5L engine good results are also likely to be possible if the same reducer is fitted on a 1L engine, but you probably wouldn't want to fit the same reducer on a 100CC engine.

Even on a sequential system you can find that a reducer might have no load pressure a few points higher than very light load (such as idle) pressure, so a massively oversized reducer might see pressure at idle a few points above pressure at slightly higher engine load (the usual no load slight over-pressure extended to very light load over-pressure) where-as a smaller reducer might see pressure at light and medium loads closer to the same as idle pressure. But as much depends on make/model/design than power rating.

It's arguable whether an ideal sequential system reducer should be totally pressure stable over it's entire operating range (zero load to it's maximum bhp rating) or whether it would be better if it outputted lower pressure at idle and increasing pressure with increasing load. This is because one of the key challenges is getting injectors to accurately meter gas at both extremes of idle and flat out engine operation. At idle and particularly in some over-run conditions the injectors need to pulse for a short time, the short time cannot go below the minimum at which the injectors can accurately meter gas, but with the same pressure and nozzles they also have to be capable of flowing enough gas for flat out. Except that in reality pressure is often a bit higher at idle than at flat out and this exaggerates the demands on the injectors (and installer).

In practice there isn't a sequential system reducer (not made these days anyway) that is capable of outputting more pressure (relative to manifold pressure reference connection) with increasing manifold pressure... It's due to the design, they all have a diaphragm separating a vacuum chamber from a gas chamber, the diaphragm acts on a jet which opens/closes to meter fresh gas into the gas chamber, a spring pushing the diaphragm away from the vacuum toward gas pressure biases the output pressure. To make a reducer with rising pressure over rising engine manifold vacuum the vacuum side of the diaphragm would need to be wider than the gas side of the diaphragm.. so would need a reducer setup with completely separate gas chamber and vacuum chamber to allow the vacuum chamber to be bigger than the gas chamber but that would over-complicate the design, increase the size, and arguably a more pressure stable design could be made in less space by keeping with the conventional design.

The output pressure of most reducers is also slightly higher when cold than when warm. Like the falling pressure under heavier engine loads trait this trait is also a negative, because a colder reducer will output colder gas and cold gas is more dense than hot gas so really we'd want slightly less pressure from a cold reducer than from a hot reducer.

It would be possible to fit a stepper motor to the pressure adjuster screw (to adjust the spring bias) and I believe there has been a reducer with such setup in the past. Such stepper equipped reducer could be set to correct for falling pressure under load and higher pressure when cold... but the stepper probably couldn't respond quickly enough for some tip-in/out situations. It would be nice though to have a reducer that gave a couple of points lower pressure at idle and a couple points higher pressure under heavy load.

The power rating of most reducers isn't far from the on-paper spec... in the right conditions. The right conditions might include output pressure setting, a lot of reducers are capable of flowing more gas in total at higher pressure settings than at lower pressure settings. Then again, with some reducers regardless of where you set the pressure it doesn't take much engine load for pressure to fall to the more usual pressure that the reducer would be set at. E.g. Some reducers are usually set at 1.2 bar output pressure, you might be able to increase pressure to 1.8 bar but there is no point if it only takes a bit of engine load for pressure to fall to the 1.2 bar that the reducer would normally be set at anyway (in this case setting 1.8 bar could only be a negative because the engine wouldn't need 1.8 bar for idle and where you did want the 1.8 bar pressure would have fallen to 1.2 bar... it would be better to set pressure at 1.2 bar anyway).

[CNG]

Thanks all. If I have this right, aside from the installation-bulk, sequential is less fussy, pure cost considerations, and excepting some extremes, there isn’t too much to go wrong with over-specced vaporiser/reducers.

At the extremes, we're more likely to get into trouble with mixer-systems than sequential. With sequential, provided we don't stray too far away from the design output pressures we're fine.

I was asking a general question. In my case I've a Tomasatto AT12 which was sold as capable of delivering 140HP - or if I believe other's claims 250HP - There were, before model changes two variants, seems mine is the lower spec?

My Volvo is no road-burner, thus in petrol form puts out 140Hp (105Kw). I took the capacity as wrote, it seems unless something else is wrong, I’ve made a purchase error. I'll cover what I've done in more detail in my Volvo conversion thread, but I suspect ithe Tomasatto AT12's output is a tad under 140HP. Well enough at 1/2 full, and above, yet the Nevo will decide on 'low pressure' log entries at over 4000rpm, this where the tank gets to roughly 1/4 full.

Quite how I can have low-pressure when even 1/4 full, there's 700psi available seems odd, it points to the reducer? Of course there might be injector-delivery issues too, but I’d have thought that’d give different alerts?

Larger reducers are £60-70ish, which in the greater scheme of things is damn all. Reading the above words, seems wise to remove any question over this, and shove a bigger capacity thing on and be done.

And there was I, worried about over-spec?

Tomasatto AT12 140HP.png (277.95 KiB) Viewed 3025 times

Tomasatto AT12 250HP.png (403.29 KiB) Viewed 3025 times

[Brian_H]

The low pressure on the Nevo I think will be the gas pressure post-reducer rather than whats in the tank. I don't know how the CNG side works gauge wise, but I'd suspect it doesn't pay any attention to it performance wise. On LPG it would be what its seeing in the hose where your combined gas/map sensor is. You should be able to see some freezeframe data for the error which will show you the various sensor readings at the time of the fault (I've done this on the app, but never on the pc software so not sure how you'd do it without dragging the laptop out to the car to look myself).

Note that its configurable in the software what pressure it will switch back at, the installation tab, followed by Basic will show you the right page, the bit your looking for is under switching to petrol (pink box) minimal pressure. I think it sets it by default to half the reference pressure you have set, but you can alter it further. It may be that you have it set a bit too high currently? The freezeframe data if you can get it might give more of a clue to go on if you can post it.

[LPGC]

Yes, you might have 700psi in the tank but the pressure referred to is the post reducer pressure (feed to the injectors).

If the reducer won't flow enough for the demands of the engine then when you put your foot down that pressure will fall away. Pressure is likely to be a bit lower under load anyway (though not by much) even with a reducer that's on top of the job. As Bri says check the error screen, though it's likely the pressure reading at the time it switched back to petrol is very close to the switch back to petrol pressure, so again a Bri says check what you've set the switch back to petrol pressure to.

[Brian_H]

Also not sure if its an issue with the CNG ones, but the LPG Tomasetto vapourisers can freeze up and fail. I remember Simon saying something about that when I was in his yard last time, he actually had one there with a chunk blown out of the rubber seal where it had failed. I've had one of the two I've got need a rebuild kit as the diaphram had gone hard in it, though it had done a good mileage at that point. Not had them fail and they aren't particually difficult to rebuild (certainly easier than the Necam ones are) and are relatively cheap so not all bad though. Obviously the CNG one is a different unit and may not be an issue there, Can't say on that having never had CNG.

[LPGC]

Could be different with the CNG range because they don't need as much heat, not as much of a drop in temp lowering the pressure of a gas compared to the effect of the liquid/vapour phase change of LPG.

Tomasetto used to make a range of LPG reducers that were for the most part based on much the same design, on some of the higher power rated models the main difference over smaller power rated models was the addition of a liquid (input) gas pre-heater. With some of the big power rated models gas output temperature was quick to drop when they were worked to maybe half of the supposed rating. The smaller models like Alaska and Arctic seemed to fair better and be more reliable than the bigger models but have still seen plenty problems with failed gaskets etc on them.

Since then it seems Tomasetto have increased their range and deleted some models. I haven't tried all the newest in the range or any of the CNG versions. Some of the high power older models are still made e.g. the Antartic rated at 365bhp but I wouldn't fit one on anything approaching 365bhp.

[CNG]

I'm sure I read before that Tomasetto are not favorites with the trade, only the usual suspects don't seem to offer CNG variants, or where they do, they're not imported to the UK. Like much of the supplies for CNG kit, my hand gets forced.

I'm due to call Dai (Classicswede) soon. I suspect he can set the Nevo up over the phone, whatever the issues, I'm sure he'll sort it. Thanks to you both. I'll look to bumping-up the switchover setting. I'll cover this in the conversion thread.