Petrol Cost Cutting System Article Discussion Area

tomstickland

Martin's bit about the overflow is wrong. I've taken a few fuel systems apart and the excess fuel drains into the tank.

There are normally 4 pipes to a tank.
-main filler
-air return
-overflow from filler nozzle to tank
-air breather pipe from tank (very small diameter pipe from top of tank to outside world)

AndrewK_3

Thanks for that detail, Tom. I reckon we're going to need a diagram soon.. :-)

Am I right in thinking (from what you said) that:
On the pump:
-The main filler gets petrol from pump to tank
-The air return allows air / petrol fumes out of the tank to allow petrol in and prevent customer from being choked by 60 litres of fumes.
On the car:
-The overflow (don't quite understand how this works)
-Air breather pipe from tank (don't quite understand how this works either)

I reckon I've got some of these things confused - perhaps you can set me right?
Thanks!

dixie_dean_2

Cheers for putting me out of my ignorance Andrew. I know an Andrew Kirk and an Andrew Kerr. Either of them you?!

AndrewK_3

Hehe - no. I'm an Andrew Kember.

tomstickland

Am I right in thinking (from what you said) that:
On the pump:
-The main filler gets petrol from pump to tank
-The air return allows air / petrol fumes out of the tank to allow petrol in and prevent customer from being choked by 60 litres of fumes.
On the car:
-The overflow (don't quite understand how this works)
-Air breather pipe from tank (don't quite understand how this works either)

I was just talking about the car.
-petrol in
-air out
-overflow from filler neck to tank
-vent pipe from tank to air

praven

AndrewK wrote:

On a related note, praven says,
One possibility, given that praven is using a quick mental calculation based on the total bill, is that the "tank's full clunk" happens at different fill-levels for different pumps.

This could be the case, although I don't fill up with the same pump at each station every time so it would have to be specific to all the pumps at a station (which could be true).

So, if I were to start keeping records, would I have to fill up until I could see the diesel at a certain level in the filler pipe to get an accurate measurement of how much has gone in? I only fill up once every 2-3 weeks so it's going to take me a while to get some decent statistics. Should I make notes of all the journeys I make to get a better idea of what's going on? Also, how accurate do the pumps have to be?

Paul

AndrewK_3

@Tom - thanks for clarifying that.
@Paul - Hmmm. This is difficult. I was going to say that it doesn't matter how much fuel you put in, as long as you make a note of the number of litres, but that's not the case here, as you rightly point out.

If the idea here is to find out the difference between two brands of petrol, you could measure mpg from one brand for a few months, then switch to using the other brand. Calculate the mpg (or miles per litre, actually) by zeroing your trip meter when you fill up, and make a note of the number of litres you fill up with. If you use the same pump (!) every time, then at subsequent fill-ups, you can say, "I travelled x miles using z litres of fuel."

If you do this over a number of sequential fill-ups with the same brand of fuel, then variations in the pump stop-point (i.e. using different pumps) will not matter, because if you get less fuel, you will drive less miles on it. You can get an average fuel consumption figure, then swap to the other brand for a few months.

The problem with this approach is that, certainly in my case, life gets in the way. My driving habits and journey types will vary over the course of a few months. The weather changes too, which affects road conditions and (according to some on the forum) tank capacities and fuel efficiency. In conclusion, I'm not sure that you could get any meaningful or repeatable results without controlling your driving habits quite strictly.

All this assumes that the pumps are completely accurate, as I haven't got any other way of measuring fuel volume. No idea how accurate they are, but as they are used to sell specific volumes of stuff to us, I imagine that there are standards of accuracy that need to be adhered to. Perhaps the Weights and Measures Act 1985 covers this?

Bob63

A quick plea from a motorcyclist to diesel drivers here - Don't overfill your tank!

It is a fact that spilt diesel kills motorcyclists. It can also kill their pillion passengers, pedestrians and potentially other motorists involved in the accidents that are caused by two wheelers losing control on patches of spilt diesel.

How do diesel spills occur? Two ways - the most obvious is filling up to the neck of the filler cap and then continually click-click-clicking the pump to get the most in the tank. On the first bend or roundabout it all goes sloshing out over the road. You lose money somebody loses their life. The second way is through expansion. In the summer the diesel comes out of the underground tanks at the petrol station quite cold - much colder than the temperature at ground level. I'm sure you are aware of how hot a car gets when left in the sun. Well the diesel in the car gets hot as well and as it heats it expands. If your tank is full or almost full then the expanding diesel has nowhere to go but through the overflow and onto the road. People think diesel spills are a winter problem because they see the tell-tale rainbow patterns on the road, but believe me they are just as much of a problem in the summer.

If you want more info read http://www.killspills.org.uk

Don't overfill your tank. You are wasting money and you are killing me and my friends!

Mike

dixie_dean_2

At what speed is it a good idea to get into 5th gear for fuel economy? My instructor told me it's not really worth bothering with until about 50. Someone said on this forum between 2 and 3000 revs is good so is 5th gear best as long as the revs are ok? Any idea what speed would be best in an old escort? Also, is there a significant loss of efficiency doing 70-80mph as opposed to the 55mph that has been recommended?

Thanks.

ps - check out the stats on dixie dean: unbelievable.

tomstickland

Fuel consumption approx scales with speed squared. So doing 75 will use (75/55)^2 more fuel.

Most non sports model cars are geared and engine mapped so that 4th or 5th gear at 55mph gives best efficiency.

Remember though that it's accelerating that uses the most energy, so being able to maintain a constant speed without multiple stops and starts will give the best result. Hence why traffic lights are rubbish.