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Old 09-02-2012, 05:58 PM   #1 (permalink)
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How best to drive a car with no DFCO?

My 328i touring has an LPG system fitted,

but its an old skool "single point" mixer system, feeding the LPG into the inlet tubing behind the MAF.

This then passes through the throttle body and into the inlet manifold.

It occurred to me today that the engine is essentially functioning like a carb fed engine whilst on LPG, and that my usual practice of using DFCO for approaching junctions etc would mean that the engine is just pulling in LPG but not getting the "work " out it?!

so am I better just using the brakes to slow at junctions, after coasting as far as possible?

your thoughts please

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Old 09-02-2012, 06:53 PM   #2 (permalink)
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Indeed it appears that it always meters propane to the engine while it is running. I think it would be best to coast in neutral and use the brakes. That way the engine will be turning at idle speeds and will ingest less air / fuel mix as opposed to a higher rpm deceleration in gear.
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Old 09-02-2012, 08:12 PM   #3 (permalink)
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Quote:
Originally Posted by amcpacer View Post
Indeed it appears that it always meters propane to the engine while it is running. I think it would be best to coast in neutral and use the brakes. That way the engine will be turning at idle speeds and will ingest less air / fuel mix as opposed to a higher rpm deceleration in gear.

Tank now filled, will try to only idle up and use brakes..see if I can get a couple of tanks worth of data..

Cheers for your input!
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Old 09-02-2012, 08:37 PM   #4 (permalink)
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Quote:
Originally Posted by amcpacer View Post
That way the engine will be turning at idle speeds and will ingest less air / fuel mix as opposed to a higher rpm deceleration in gear.
solid thought, but it may not be correct.

if you clutch it or otherwise put the shifter into a neutral position, you will be turning an idle RPM, but RPM is not the end-all when it comes to efficiency, you have to look at total fuel flow, which you can approximate from airflow.

OP, do you have any way of tracking MAF voltage/frequency? if so, i'd take a look at what happens while engine braking to speed down compared to coasting in neutral, whichever indicates a lower airflow shoud net you better fuel economy. not all vehicles respond the same, so you'll really have to test on your own vehicle to be certain.
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Old 09-03-2012, 08:40 AM   #5 (permalink)
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Quote:
Originally Posted by RobertISaar View Post
OP, do you have any way of tracking MAF voltage/frequency? if so, i'd take a look at what happens while engine braking to speed down compared to coasting in neutral, whichever indicates a lower airflow shoud net you better fuel economy. not all vehicles respond the same, so you'll really have to test on your own vehicle to be certain.
Hi, I only have the stock MPG meter as fitted to the car..

In both scenarios (in gear Decel AND at Idle) the gauge sows "infinite MPG"..

However, in gear Decel means high revs, and fuel being sucked in to the cylinders at a rate higher than if I was actally driving .

Because the petrol injectors are connected to an emulator- the CAR thinks that there is no fuel entering the engine..whereas, we know its being drawn in along with the air via the intake

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