Pressure sensors.

The Original Tom · Post by **The Original Tom** » Fri Aug 03, 2007 9:37 am

I'm looking into the possibility of switching off my LPG supply on over-run, to further reduce fuel costs. The way I plan to do it is to use a MAP style sensor, and incorporate it into a circuit capable of switching the relay on the vaporiser.
I was wondering about this switch:
http://rswww.com product no 407-237

My main concern is: will a maximum pressure of 1.034 bar be enough? Theoretically (on a non-boosted engine such as mine) the manifold pressure will never exceed about 0.9x bar will it?
The minimum pressure of 0.124 bar should be low enough even coming off max revs with a closed throttle surely?

Any other input people have is greatly appreciated.

Tom.

Post by **katanaman** » Fri Aug 03, 2007 9:49 am

Why not just put a micro switch on the throttle stop like the older mechanical injection cars use?

The Original Tom · Post by **The Original Tom** » Fri Aug 03, 2007 10:20 am

If I do that then won't the fuel also cut off when I sit at idle?
It would be hard to regulate the flow any more than on or off for the gas.

kiwicar · Post by **kiwicar** » Fri Aug 03, 2007 11:38 am

hi Tom
surley, if you cut the supply to the vaporiser on over run the Vap will empty, the general plan to stop gas flowing to the mixer... however when you open the throttle again you will have an empty vaporiser and mixer plomming, which will take time to fill, to describe the result as throttle lag would be a bit of an understatment!

I think it will make changing up and down the box a chore to say the least and will probably result in having to open the throttle further when wanting to accelerate from over run, therefor using more Gas. this plan works on an EFI car because the fuel is always present at the injectors, just the firing pulse is removed.
BL did a similar thing on metros with the fuel suply into the carb float bowl, in this case to reduce hydrocarbons on overrun, made a right dog a compleet hound to drive as the float level was never constant.
Mike

ChrisJC · Post by **ChrisJC** » Fri Aug 03, 2007 12:08 pm

I would use an overrun cutoff switch from a flapper EFi.

Chris.

stevieturbo · Post by **stevieturbo** » Fri Aug 03, 2007 12:53 pm

lots of n/a csrs have a map sensor....so any "1 bar " sensor should work just fine

1 bar being atmospheric pressure.

The Original Tom · Post by **The Original Tom** » Fri Aug 03, 2007 1:52 pm

It's on SU carbs if that makes a difference?

Kiwicar - I understand what you're saying, but with LPG the vaporiser fill sso quickly that there's no noticable lag (to me) between switching the switch and getting power, it is, to all intents, instant, due to the gas being at about 100 bar so filling quite fast

.
Also the vap will be hot and the gas will vaporise instnatly too.
I hadn't thought about the gap in-between gears though - I'd imagine that on an auto this problem wouldn't manifest, but on a manual gearbox it certainly would.
I'll let you know if I dig up any more info.

Post by **katanaman** » Fri Aug 03, 2007 9:03 pm

Sorry I should have added you use the module as well. It picks up on RPM and when the throttle is closed.

I cant think of a way to use a MAP sensor without it involving a processor and software to accurately control when fuel is off and on. It could probably be done by analogue but either way its not going to be straight forward unless you can dig up a circuit.

ChrisJC · Post by **ChrisJC** » Mon Aug 06, 2007 9:06 am

I still say get an overrun cutoff valve from a flapper EFi. It goes open-circuit when the inlet vacuum goes over about 20inHg. It's designed for exactly what you want, just hook into the power feed for the solenoid on the vapouriser.

Chris.