I'm looking to change the fuel filter on my girlfriends Corsa 1.0 and the depressurizing of the fuel system looks a bit tricky having had a look at the Haynes manual.
I did this job on my own Primera a couple of weeks ago and all I had to do was remove the fuel pump fuse and try to start the engine - this did the depressurizing.
Any advice for the corsa? The manual mentions fuel rails and such without any pictures to help me find where they are talking about!
|
|
I wouldn't bother, I have never found it to be a problem. As I see it, fuel is liquid, and liquid can't* be compressed, therefore the fuel won't expand when depressurised either. You will only get a small amount as any rubber pipes relax.
|
|
On the other hand it could be nasty. I thought I did all the right things on a Micra, but still ended up with the rear seat platform frighteningly full of fuel.
|
|
|
Liquid cant be compressed ?
I take it you dont have any science qualifications then ? PV/T ?
Oh yes it can. Not by anything as much as a gas admittedly.
Expansion isnt likely to be the issue though, the fuel could spray all over when a connector is undone.
|
|I take it you dont have any science qualifications then ? PV/T ?
Oh yes it can. Not by anything as much as a gas admittedly.||
HENCE THE ASTERISK NEXT TO CAN'T!
|
|
|
|
I would either pull the fuel pump fuse, or the fuel pump relay, and then try to start the car. There is usually some type of accumilator in the fuel system that maintains fuel system pressure to aid quick starting - it is this accumilator that you empty by depressurising the system.
For info, that liquid can't be compressed is a bit of a myth. Fluids have a property called bulk modulus of elasticity which relates their cahnge in volume to a change in pressure. It is just the liquids (and solids for that matter) are much much stiffer than gases.
In the design of aircraft hydraulic systems, the bulk modulus of the hydraulic fluid is an important design variable, it affect the "water hammer" force that you get when you shut a valve quickly against a rapid flow, and it affects the response times of dynamic systems. The compressibility of hydraulic fluid can't be ignored in many applications.
If liquids and solids couldn't be compressed, sound would not be able to travel underwater, and vibration would not pass through solids in the way that it does.
Number_Cruncher
|
>>For info, that liquid can't be compressed is a bit of a myth
I said that some time ago and Aprilia said similar thing,though not in so much detail,thanks for that
>>I would either pull the fuel pump fuse, or the fuel pump relay, and then try to start the car.
Best way of not getting soaked in fuel, less dangerous as well
--
Steve
|
|
Petrol compresses by a few percent - rubber pipes etc also expand a bit when under pressure and this gives a 'hydraulic accumulator' effect. IIRC this Corsa does not have a fuel accumulator as such, therefore the amount of fuel that will be ejected when you undo the union is small. You could pull the fuse and crank it, as has been said; if its been stood for a few hours then I would just wrap a rag around the union as I undo it - will be fine.
|
Thanks chaps; meanwhile, back at the question...
...you reckon I may be able to try the same technique as with the primera (remove the fuel pump fuse and attempt to start the car)? The manual mentioned a valve(?) on the fuel rail, but without any pictures of where this might be. I'm a novice at diy mechanics and so can't always recognise the parts that they are referring to - could anyone with a corsa take a quick snap of the bits I should be looking for and point them out?
|
|
Not sure what you're on about. The fuel filter is under the back of the car. Fuel pump fuse is nr. 26. Pull it out and crank the engine, then change filter (keep rag handy, it will leak).
|
|
|
>>The manual mentioned a valve(?) on the fuel rail, but without any pictures of where this might be. I'm a novice at diy mechanics
Not being funny,if car has been standing as long as your post, pressure will have gone..just as mentioned replace the filter using rag on joints to absorb the excess fuel,should be self priming after that without any problems,if you need that much advice you would be best advised to get someone that knows what they are doing.not so easy doing the job for you over cyberspace
--
Steve
|
Anyone got a link to the data on compressing liquids?
I've not found anything on the internet and I guess the pressure must be very high. As a mere physics student I was always taught that a liquid was incompressible.Presumably only at normally achievable pressures.
--
I wasna fu but just had plenty.
|
Hi GWS,
To answer your question indirectly, the speed of sound in water is approximately 1500 metres per second, and the density is 1000 kg/ cubic metre. If you look at the equation for the speed of a longitudinal wave, you should find that the modulus is easy to derive from the given data.
As a mere physics student, I'm sure that you'll have no problem!
;-)
Number_Cruncher
p.s. I have some bulk modulus data at home which I'll post when I get back from work
|
|
|
|
|
Don't know about Vauxhalls but on a Ford the de-pressurizing valve is like a tyre valve on the fuel rail that supplies the injectors.
|
Number Cruncher,
Thanks for reminding me that I was/am rubbish at maths.
My feeling is that any transient compression due to a wave passing through is not quite the same as hydrostatic bulk compression.The sonic wave is going to introduce expansion and contraction which will not tend to change the overall nett volume over the cycle.
I read somewhere that a huge amount of energy is needed to compress a liquid, but no details were given.
--
I wasna fu but just had plenty.
|
Compressibility of a substance is the inverse of its bulk modulus. i.e. more dense liquid is less compressible.
Bulk modulus of water (just looked it up) is about 0.5x10E9m2/N. At the bottom of the ocean, water is compressed by 2-3%. This is a bit of a 'red herring' though.....
In automotive fluid like brake fluid and fuel, most compressibility is actually due to entrapped air. Ordinary polyetheylene brake fluid has a few % trapped air and so compressed by a few %. That silicone rubbish (DOT5) they keep trying to sell us has much more entrapped air, so compresses much more and leads to a poor pedal feel. Don't forget that rubber hoses also expand and contract a bit.
|
|
|
For a few liquids, the bulk modulus is listed in archaic units and in units suitable for calculation below
Petrol 150,000 psi 1.03 GPa
Skydrol 340,000 psi 2.34 GPa
Water 310,000 psi 2.14 GPa
So, for petrol, a change in pressure of 10 MPa results in a 1% volume change. Bulk modulus is important at the pressures involved in common rail diesels.
The square root of bulk modulus over density does govern the longitudinal wave speed in fluids, just as quasi-longitudinal wave speeds in solids are governed by the square root of Young's modulus over density (~5 km/s in steel). Indeed, the sound speed in air, commonly written as square root of gamma * R * T can be recast as the square root of an 'elastic' constant divided by density.
However, Aprilia is quite right to point out that the expansion of rubber pipes, and the compression of entrapped bubbles is, in practice, responsible for most compliance in automotive fluid systems.
Number_Cruncher
|
|
|
|
|
|
|
|
|
