A question for the knowledgeable

[vespa]

If diesel is getting old does it lose some of it's cetane value? I deliberately let my tank run right down and filled up with fresh diesel and the car seems to have become more lively. It may be a daft question but I am aware petrol degrades with age.

[niall campbell]

How old & how long is car sitting ??

by letting the tank get very low, you are dragging all the rubbish out of the tank & clogging filter.

Also you may cause condesation in the tank by water forming with variable temperatures, again clogging filter; which can be drained in some filters; but it freezes in winter.

[Johnb80]

How old & how long is car sitting ??

by letting the tank get very low, you are dragging all the rubbish out of the tank & clogging filter.


Interested to hear your theory on this old wives tale.... how exactly does this happen?

Regards - J

[vespa]

When I said "low" I did not mean that low. I am aware that the octane in petrol diminishes after a period of time. The question is does cetane, the diesel equivalent, do the same?

[Teflon]

I claim no expertise, but here's what I think I know....

Cetane is actually hexadecane and it looks like if there is any of it in diesel then it's longish molocule means that it isn't going to evaporate very easily. The cetane number is used to indicate the compression combustability of a hydrocarbon, in the same way that octane number is used for spark ignition. Petrol itself isn't 100% octane, it's just that the name stuck over the years.

Diesel fuel is mainly stuff from the same family as parrafin (olefins) but at the heavier, thicker end. There's also a fair amount of so-called "aromatics", which is a fancy way of saying stuff with a ring-shaped molocule.

Although in theory petrol degrades by evaporation of the most volatile components over a long period, in practice the only place for it to go is up the very narrow vent pipe to the tank so it's not really going to happen. Ditto for diesel. There must be something volatile in it otherwise you woouldn't be able to smell it at the pump, but if it has nowhere to go it won't disappear.

In both cases, if you leave it long enough any crud and moisture will settle to the bottom.

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[niall campbell]

Interested to hear your theory on this old wives tale.... how exactly does this happen?

Regards - J
As anyone with farm or machine experience will tell you (diggers,360 etc) , you always fill your diesel tank at the end of the working day. Usually the tank is close to the engine warming the diesel. While it cools down, condesation will form in the tank. The less diesel you have in the tank, means more condesation.
So fill tank right up & you have very little condesation in tank.
if you look at tractors, they may well have glass bowls at bottom of 1 or 2 diesel filters. As water is heavier than diesel , it sinks to bottom of filter. The glass allows the operator to visually check if any water has coplected and needs drained.
In the car, this is exactly the same; with the drain off valve still there.
In winter this water will freeze.
Even if the tank is well away from the engine, the diesel will still be a different temperature and being in contact with the metal will cause condesation.
If it was an old wives tale, why have drains on filters ??

[Johnb80]

If it was an old wives tale, why have drains on filters ??

In your other post you actually said "by letting the tank get very low, you're dragging all of the rubbish out of the tank and clogging the filter" is this that I'm referring to. It's an often quoted thing that if you really think about it is wrong. Where does your fuel pump feed from? the top of the tank or the bottom? Unless the rubbish was floating on the surface this simply cant happen.

As far as condensation goes, there's little in the way of airflow to condense, I agree it would collect at the bottom of the tank and that's a good reason to keep it full especially in wintertime.

J

[niall campbell]

Then what is the point of a filter ? Why is it not the same size of a petrol car?

The metal tank will form condesation just standing still, no need for airflow; unless its insulated.

[Johnb80]

Then what is the point of a filter ? Why is it not the same size of a petrol car?

The metal tank will form condesation just standing still, no need for airflow; unless its insulated.

There is a finite amount of air in the tank which carries a finite amount of moisture, that's the point re airflow (inside the tank which is where the water comes from).

On the subject of sucking all of the rubbish from the bottom of the tank, where does pump suck the fuel from?

I'm not having a go and it is something Ive read many times which I feel is blatantly wrong and hence my discussion on this interesting topic.

John

[Teflon]

I think you two are at risk of a violent agreement!

Water is not going to condense out in anything like any significant volume unless you consistently drive the car between a monsoon climate and the Arctic. In the first case you would have a lot of moisture in the air and in the second you would have temperatures cold enough to condense it. It's all about dewpoint - if the air isn't hot enough to hold a lot of water (like, say, in winter) then it can't have a dewpoint high enough for vast amounts to condense out when the air (or tank temperature) drops.

True, over a long period of time, any moisture at all, whether from condensation or contamation will settle to the bottom. I can well imagine that farm machinery which might be filled up out in the rain rather than under the floodlit canopy at your local Esso would have some option to drain any water settling out. They also use a lot more fuel than your average car, so the risk is compounded over time. If you extend this logic to cars, the longer you have the car the greater the chance of small amounts of water settling in the tank adding up to be above than the level of the pump suction.

The reason for a bigger filter on diesel engines is simple. The stuff is considerably more viscous (especially when cold) than petrol, so it needs a larger filter surface area to ensure adequate flow in the worst of conditions. A diesel fitter can probably tell you if the next bit is right - I think the pressure at the filter is higher than for petrol engines and if so, a larger filter area is going to be better over a long time duration between services.


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[niall campbell]

There has to be air "intake" to the tank from filler cap, otherwise the tank would in theory be crushed sucking the fuel out.

We both know that.

It may be a myth as you suggest, or be the truth as I suggest; but allowing the fuel tank to be low will allow the fuel to slosh around excessively and lift the sediment from bottom of tank.

It is entirely up to you what you believe. Theory is a great thing, practice is another. Changing filters & not running low in fuel is my regime

[Johnb80]

There has to be air "intake" to the tank from filler cap, otherwise the tank would in theory be crushed sucking the fuel out.

We both know that.

It may be a myth as you suggest, or be the truth as I suggest; but allowing the fuel tank to be low will allow the fuel to slosh around excessively and lift the sediment from bottom of tank.

It is entirely up to you what you believe. Theory is a great thing, practice is another. Changing filters & not running low in fuel is my regime

The vent pipe does allow fresh air into the tank and the potential for moisture but it's very limited the amount of water the air can carry, it could of course over time build up to a significant amount.

So far as the fuel sloshing around stirring up and sediment etc, more turbulence will result by filling the tank unless of course you just top it up all the time when you've used a gallon or so.

The filters on diesels are bigger due to: The viscosity of diesel when it's cold, the finer tolerances of filtration required and to have space for the water trap.

Regards - John

[zollaf]

people leave canal boat fuel tanks empty over winter and often come back to a tank full of black death. this is also the reason you always fill an aircraft tank up before parking it up for the night. most modern cars however have plastic tanks so water isn't so much of a problem. forget the physics of how much water the air carries, it just happens.

[Johnb80]

people leave canal boat fuel tanks empty over winter and often come back to a tank full of black death. this is also the reason you always fill an aircraft tank up before parking it up for the night. most modern cars however have plastic tanks so water isn't so much of a problem. forget the physics of how much water the air carries, it just happens.

The physics are important, the only way normally for water to get into the tank is via the air so it is very relevant. I have never known aircraft to be filled up at night and yes I do have a Private Pilots License. Commercial aircraft are also only fuelled when the weather, payload and other factors are know and very rarely full.

John

[zollaf]

i only know about the plane thing from the RAF . maybe the forces and the civvies do things differently, i don't know ??

[Johnb80]

i only know about the plane thing from the RAF . maybe the forces and the civvies do things differently, i don't know ??
Probably ready to scramble at no notice in the RAF.

Regards john

[Teflon]

people leave canal boat fuel tanks empty over winter and often come back to a tank full of black death. this is also the reason you always fill an aircraft tank up before parking it up for the night. most modern cars however have plastic tanks so water isn't so much of a problem. forget the physics of how much water the air carries, it just happens.
Material of manufacture is absolutely irrelevant. In fact, plastic tanks present a possibilty of volatiles in petrol leaching through. About 15 years ago motor manufacturers were hurridly looking at alternatives to plastic because the Californian emissions legislation was going to be tight enough to fail plastic tanks. They couldn't go back to Terne steel (lead coated) but in the end most went for an impermiable liner in the tank.

I can't imagine what the "black death" is, unless it's corrosion of the inside of a metal tank or un-drained crud. I'm betting canal boat tanks aren't as tightly sealed at the cap as car tanks, so the will get more moisture in. If it's a problem, leaving the tank empty is obviously the very worst way to go. Oil is a great way to protect metals from corrosion.

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[zollaf]

diesel bug, bacteria . turns the inside of your injector pump into a black sludge.nasty stuff.

[Johnb80]

diesel bug, bacteria . turns the inside of your injector pump into a black sludge.nasty stuff.

It's becoming worse now with the introduction of a % bio into normal diesel.

John

[zollaf]

more food for them. i have seen it a few times where people are running on veg oil and bulk buy it in 1000 litre ibc's. as you use it air gets drawn in, the oil absorbs the water from the air, since it is hydroscopic, then 3 months later you have a new life form in your tank. it also turn acidic and eats anything metal. expensive...

[Teflon]

diesel bug, bacteria . turns the inside of your injector pump into a black sludge.nasty stuff.
Now I've got it. Thanks.

I've had similar experiences with water/oil emulsions and inverts for lubrication. One day it all smells fine, the next day there's thick black scum on the top of the10,000 litre tank and it smells foul.

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[vespa]

A big thank you to all who have replied. I think I may have solved my sudden power surge (I Think being the operative words). I always use Millers ecomax at double strength and I mistakingly thought, in a hurry filling up, that the measure was 25 mls so put in two full measures. I have actually put in 4 times the amount by accident as the measure was 50 mls! How did I manage that? Well I have been doing a lot of gardening recently and various fertilizers and sprays have a 25 ml measure/5 litres. I must have got those measures in the now getting goosed grey matter, hence the overdose.

[Teflon]

My old man one destroyed an Anglia 105E engine with Redex - in the days when it was weapons grade instead of the stuff they sell now. He ran it out of juice so we stuck in a gallon from a tin oil can (remember those?). Once it was running, he went to put the can in the boot and spotted a tin of redex, so he poured the lot in.

As he drove off up the hill he gave it the beans and there was an enormous bang. When I took the head off there was a 1/2 inch hole in one of the pistons. The replacement engine cost £15 from a bone yard. Happy days.

[vespa]

My old man one destroyed an Anglia 105E engine with Redex - in the days when it was weapons grade instead of the stuff they sell now. He ran it out of juice so we stuck in a gallon from a tin oil can (remember those?). Once it was running, he went to put the can in the boot and spotted a tin of redex, so he poured the lot in.

As he drove off up the hill he gave it the beans and there was an enormous bang. When I took the head off there was a 1/2 inch hole in one of the pistons. The replacement engine cost £15 from a bone yard. Happy days.

The joys of Redex!. I used it regularly in all my carburetor cars. I used to pour it in neat via the air intakes from a large spoon. I remember doing it to one car and although the engine was hot the exhaust was still cold. I cleared the white smoke on the driveway and set off. As the exhaust heated up the unburnt redex started its white smoke effect and it lasted for several miles until all was burnt up. Very embarrassing! On another exhaust tale, I fitted a "new" front pipe and box on an MG, that my grandfather gave me. It had been stored at his shed in a large field. When I say shed it was brick built and enough room for 3 cars and his ferrets. After about 6 miles there was a smell of cooked meat and the car wasn't really running very well, so I returned home and took it off only to find that I had gassed and cooked a mouses nest complete with mice.