Thread: Diesel fuel

IMO the vaporisation is not neglectable at all cos the fuel returns hot in the tank and if it's warm outside too i'd say that the consumption can increase a bit, also diesel is quite volatile though, not as petrol but more than water... i wont be surprised at all if the consumption will get back to normal after a proper cap is refitted

Originally Posted by sierrafery

IMO the vaporisation is not neglectable at all cos the fuel returns hot in the tank and if it's warm outside too i'd say that the consumption can increase a bit, also diesel is quite volatile though, not as petrol but more than water... i wont be surprised at all if the consumption will get back to normal after a proper cap is refitted

It does not return very hot at all, comparatively speaking. If this was the case in areas where it is very hot this would be common and it is not. I lived in Newman for 2 years and never had any issues with fuel evaporation diesel / petrol in cars, motorbikes or jerry cans and if it was going to happen this would be the place with 50deg c plus days.
As said it will happen to an extent but only very minimal.
The flash point for diesel in Australia is around 61.5 deg c (varies from 52-96 around the world depending on composition). The flash point for Petrol is around -45 deg c. This clearly shows why diesel are harder to start as they need a lot more heat.

I dont want to argue but around 70*C as the fuel returns on a Td5 seems hot enough to me, i presumed that on others is somewhere around there too, and by descrition the filler cap is there "to prevent escape of the fuel vapour"(as in the book) that's what made me think twice about the vaporisation effect

Not arguing at all, it is all good discussion, but 75deg C is not excessively hot for a diesel return line. Remember the low flash point for diesel is 65deg c, why diesel is harder to ignite and takes 65 deg c as a minimum with flame to ignite. So effectively would take at least this temp to really even start minimal evaporation. In Newman for example a car parked on a driveway could easily get over 80 deg c internal temp with ambient temps in the shade up to 56 deg c. Often the car is not in the shade but in the direct sunlight so the temp does increase even more. In these temps you can visibly see petrol evaporating in a haze. Diesel not so much.
Run a little experiment, make a couple of containers, 2 with complete open tops and maybe something to scale of a fuel tank (capacity versus opening diameter of fuel cap - scale) and fill with x a mount of diesel and petrol and conduct some evaporation tests. A good way to see. Something simple like 500ml fruit containers would suffice, measure and mark, checking daily. The petrol will evaporate no question but diesel will be much slower.
I am certainly not saying that there is not some evaporation but it would be minimal. 1.5 l per 100kms is a bit excessive with just the cap off. That equates to around an evaporation rate of 6-9 litres over this period. That is a fair amount of diesel to evaporate from a tank. The fuel is not always at 75 deg c and as the poster said was sitting for 2 weeks, so the diesel would be mostly ambient temp. I have personally driven without a cap for 600kms with no noticeable difference to fuel economy. The fuel cap is to assist pressurization of the fuel system and still has a vent so vapour can vent to atmosphere. So by this any evaporation occurring will still occur to some extent.
I do carry a spare generic cap now however more for the following reasons and these are generally why vehicles have a fuel cap.
Pressurization of fuel system to assist fuel to flow better from the tank.
Stop fumes emitting to atmosphere uncontrolled - carcinogen risk to people, fire risk, explosive risk, nauseating risk.
Protect fuel supply contamination.
In steel tanks prevent corrosion.
Prevent from bacteria etc.
Cheers
Craig

Originally Posted by sierrafery

I dont want to argue but around 70*C as the fuel returns on a Td5 seems hot enough to me, i presumed that on others is somewhere around there too, and by descrition the filler cap is there "to prevent escape of the fuel vapour"(as in the book) that's what made me think twice about the vaporisation effect

Originally Posted by p38arover

I remember many years ago when my father suggested an engineering change to the Falcon (Dad was a mechanical engineer), Ford wrote advising the cost was too high to implement mid-model year - 5 cents per vehicle.

And the cost of the Cost Accountant's advise? $5,000.