Team-BHP - Why do pumps mix LPG with CNG occasionally?

I've been running my Swift VXi for around a year now on CNG and the cost benefits are great. Read the details here
Background: I have a 12KG tank which theoretically should take no more than 8KG of CNG. However, at times, all of the pumps where I frequent, the amount of gas filled is well over 9KG. I heard a taxi guy ask the pump attendant whether it is a 'mix today', to which the attendant usually nods as a yes.
After asking the taxi guy that mixing fuels can definitely not be good for the engine, to which he (and the other cabbies I've confirmed with) said that it 'cleans up the engine of the carbons in the injectors' and the pump owners/attendants never have any control over it since it comes from the company/CNG distributor. This definitely cannot be true since a couple of weeks ago, I had to get the injectors cleaned after running the car for over 8 months since I was experiencing the infamous RPM needle 'dance'.
Question for you folks is: Has any attendant (without hesitation) admitted that the fuel is a 'mix' when he overfills your CNG car and is it really good for the engine to clean up the accumulated carbon in the engine and injectors?

This has happened to me as well.
I filled up at a Pump in Bandra the other day, the guy started filling and shut off after filling around 3kg or 100 bucks.. When enquired why, he casually said its mixed. And started filling again. I was shocked when he said that the total was 11kgs. Now my tank does not take more than 8kg. and the indicator still showed the 1 green dot before the red one comes up. Also the car began feeling breathless after this fill and had to keep switching between Petrol and CNG all the time post that.

I asked around and most taxi guys confirmed that when its mixed, there seems to be an increase in the quantity off fuel dispensed. Any clarity on this would be appreciated. Thanks

Quote:

Originally Posted by Catalyst_delhi (Post 3081569)

This is the first time i have hear of this mixing happening , how can the system cope up with this. Well truly an eyeopener for me!

I'm also totally confused to hear this. How can a gaseous and liquid with different chemical composition fuel be mixed for combustion. Both are to be stored at different pressure levels too. Will try to get some info and post it here. Mean while if possible someone please shed some light on this.

Quote:

Originally Posted by chetanthaker (Post 3081466)

@CeeTee...
Use petrol man...
Petrol!!

Quote:

Originally Posted by sajusherief (Post 3081466)

Both CNG and LPG are gaseous. Though it is easier to Liquify LPG.
I presume after the pressure in the gas tank drops enough the LPG and CNG will start to mix and hence start giving problems.

Saw this cool link
http://drchemical.com.au/whats-the-d...nd-natural-gas

Quote:

Originally Posted by Sprucegoose (Post 3082136)

Both CNG and LPG are gaseous.

I know both are gaseous in state, sorry that i didn't explain properly what i meant. As you said LPG is easy to be liquefied under pressure whereas CNG need almost 10(or more) times the pressure you need to liquify LPG. Also, you need to maintain the same pressure to keep either of them in liquid state. If LPG cylinder is filled with CNG(I got no idea whether it's possible on first place) the pressure inside is not enough to keep the CNG in liquid state but gaseous and hence theoretically both will have to exist in 2 different states.

Edit: w00ps.

CNG is technically a dry gas. (Fluid above its critical temperature).
LPG is technically a saturated vapor which is compressed to achieve the liquid state.

How will the fuel dosing occur???
Pumps (petrol/LPG) can handle only liquid, vapor/gas in the liquid causes cavitation - which leads to erosion.
If there are no pumps, this means that the system utilizes the high pressure of CNG.
In such case, the LPG will will be entrained along as liquid droplet - which may again lead to erosion!

Quote:

Originally Posted by sajusherief (Post 3082173)

CNG is essentially methane CH4. We cannot liquify CH4 at room temperature simply because its critical temprature is -81 deg C.
Meaning at a temperature above -81 deg C say -80 ..-79 no matter how much pressure you use it will be in gaseous state. This is precisely CNG tanks are storing at 250 bar or so pressure to have decent amount of mass of fuel for driving needs.
Also if you liquify methane it is called LNG.(Liquified Natural Gas)
Hence also it is metered at the pumps in kgs not liters.
LPG is a mix of propane and butane which has its critical temperatures in positives, so it is stored in the tanks as liquid.Hence (Liquified Petroliun Gas)
Also it is metered at the pumps in liters.
If it is mixed, LPG will be liquid in the tank while CNG will be gas. Not sure how it will reach the engine and what mix. My guess is it will never gasify under the pressure existing in the CNG tank.:uncontrol

Been using CNG in Delhi for 10 years. I have never heard of LPG addition to CNG at any of the stations, the preventive maintenance of pumps require addition of some other cleansing agent once in a blue moon. The amount of CNG in your tank depends on two primary factors: the pump pressure (now limited to 200 bar against 250-280ish in good old times) and outside temperature (more CNG in winters compared to summers or at cool night when compared to summer afternoon).

Quote:

Originally Posted by sajusherief (Post 3082173)

From the first hand experience, CNG and LPG tanks are entirely different. While a car LPG tank(60ish litres) can be lifted by one person easily, the CNG one requires high efforts from two people. Speaks that the CNG cylinders are highly strengthened as compared to LPG ones.

It is a bit more complicated than that, AFAIK. You are talking about lean and rich gas. See here: http://www.hindustanpetroleum.com/en/UI/CNG.aspx#A3 http://www.energuide.be/en/questions...-on-my-bill/85

My hypothesis is that if the upstream separation plant is taking a planned shutdown/maintenance then you will get richer gas because the gas for CNG/customers needs to keep flowing nevertheless. That may account for the difference in calorific values and hence KGs filled (you may need to google up exactly how though).

norhog: In my limited understanding, the way to understand it is to think of a sponge. Even if you squeeze it tight, there's still some moisture left over. That is like lean gas :)

Quote:

Originally Posted by phamilyman (Post 3082626)

You are right sir, there is always some propane and butane components in CNG. Rich gas is where CH4 is about lower about 95% and rest is propane butane and some trace amounts of other gases. It is called rich because it has higher calorific value or high MMBTU (British Thermal Units). Lean gas is where CH4 is high, about 98% rest all else, and having lower MMBTU.:)

Never heard of any gas mixing out here in Ahmedabad. I wonder why would anyone do that though? The price of LPG is almost equivalent to the price of petrol, whereas CNG is atleast 10 Rs cheaper out here. Mixing LPG with CNG doesn't make sense, where the other way round it makes some financial sense for pump owners, but then again, I've never heard anyone practicing this method. Would it be the same thing as mixing ethanol with petrol? I don't know what kind of benefit or damage would it do in the long run.

The mix would be coming from the refinery stage itself where the raw gas may be rich, i.e less on methane and more of propane butane etc, or lean, vice versa. Also if the refineries at production stage were unable to separate out the heavier components for what ever reasons then CNG will have high content of PG.
It will be difficult to do mixing at dealer and distributor stage technically or as per Mr “”9thsphinx” rightly pointed out won’t be economically attractive.