'Petrol efficiency experiment; an increase of 20%' blog discussion

charlieheard

scape wrote: »

I don't believe this will save fuel - in fact, quite the opposite. Considerably more fuel is used when accelarating up to a speed than decelerating with your foot completely off the gas. So on-gas, off-gas, on-gas, off-gass (even subtly) will use more fuel than gently hoding the gas at a point which keeps the car at a certain speed. On a side note, this style of driving may also be uncomfortable for passengers.

Actually, that method is recognised as the best way to save fuel, though not necessarily the safest. It works because you only "use" the engine at full throttle - that reduces pumping losses caused by the throttle butterfly. In between the engine uses little or no fuel. It's used in economy marathons, where they get thousands of mpg by switching of the engines totally and coasting for ages. It's also the method used by economy nuts, in addition to pumping up their tyres and slipstreaming lorries a few feet behind their bumper. They claim to get over 100mpg doing it this way, but my nerves would be in tatters...

As I said, not the safest. Following someone whose speed is constantly changing is a nightmare, and is very likely to cause someone to try to pass unnecessarily, just to get away from the perceived idiot in front of them. And pumping up tyres reduces their ability to grip, making the car unsafer. As for slipstreaming trucks...:rolleyes:

scape wrote: »

not really! Maybe less people are caning it down the motorway because travelling at 90+mph compared to 70mph uses vastly more fuel than you would expect. Come to think of it - you may be right. I've been driving for 5 years now - when I first started, fuel was 72.9p/l - so maybe thats the reason why.

I've noticed fewer people speeding past as I try to keep to a constant 70mph - wish I had one of those adaptive cruise controls to help. I do notice people keen to pass me, who then slow down again so I repass them a few miles later. :rolleyes: One difference is that I rarely use motorways or dual carriageways during business hours nowadays.

And I remember petrol breaking through £2 a gallon (45p a litre) soon after I'd bought my first car (which I've still got )

alanfp

kaya wrote: »

you cannnot accuratley judge fuel efficiency by volume

But we buy it by volume, so this is the correct measurement to use when talking about saving money.

What you could do, of course, is to always buy fuel on very cold days to maximise the weight (or rather, mass) of fuel that you get for your litre, but as fuel is held in underground tanks, the temperature (and hence dencsity) won't actually vary very much.

Lostprophet

I upgraded my car a few weeks ago, from a Renault Laguna to a BMW 320cd Sport.
I do a lot of miles (950 a week) so was interested to see the difference between petrol and diesel.

In the Renault, I was getting 4 journeys to a tank, so give or take 400 miles.
In the BMW, I was getting 5 journeys. Not too bad, but also not amazing.

So when I filled up on the way home last Tuesday night, I decided to do an "economy run". Each time, I set the cruise control at 70MPH and made sure never to rev over 2,000RPM wherever I could help it.

I ended up getting 6 full journeys, plus the 30 miles from the filling station to my house on the Tuesday, AND a journey into Cardiff and back again on the Saturday from where I live (maybe another 40/50 miles round trip?) so I was pretty pleased!!

Today I realised the tyres were massively underpressurised, so I've corrected that and I'm hoping now that my MPG figures will start going up even more.

Great on the wallet, but a little disappointing because it's such a nice car to drive fast!

drewwa

Interesting thread...

I tried a 'how many miles can i get out of my car' with my W-reg Audi A4 1.9 TDI.

Rules

1. Aircon off
2. 56 mph top speed (sat nav measured, not speedo)
3. No higher than 2000 rpm.

Results:

770 Miles from 55 litres of fuel.

I usually get 600 miles or thereabouts.

You can see a day by day acount on my blog if you're interested.

Cheers,

Drew.

drewwa

shawad wrote: »

Regarding the debate on 56MPH earlier in this thread. This figure is quoted everywhere as it converts to (near enough) 90KPH, which is the speed limit in most of europe for outside built up areas. That's the only reason it's used, not wind resistance, not because it's a magic number for the efficiency of internal combustion engines.

75MPH figures are also quoted since that's 120KPH and that used to be the limit in most European countries for motorways.

That may be the case, but there is a reason why 56mph is the magic efficiency speed....

Reciprocating piston engines are most efficient at the point at which they generate the most torque.

You'll find that in any given car, 56mph in top gear probably equates very closely to the rpm torque peak. It will vary a bit depending on the car.

(E.g. My Audi A4 1.9 TDI generates peak torque of 260lbft at 1950 rpm. This equates to 60mph on the speedo, or 57 on the satnav)

So, 56mph is not a magic number, but it's a good 'average' for the most economical speed across a variety of cars.

If you are able to find the rpm at which your car's engine develops its peak torque value from the manual or online, then drive up to this rpm value in top gear according to the tacho, whatever speed you are doing at that point is your own personal 'magic speed efficiency number'.

Cheers,

Drew.

charlieheard

drewwa wrote: »

That may be the case, but there is a reason why 56mph is the magic efficiency speed....

Reciprocating piston engines are most efficient at the point at which they generate the most torque.

That is ONLY true when the engine is working at full throttle. Unless you can run your car at full throttle at 56mph, the torque peak is meaningless. At lower throttle openings, the torque peak could move considerably, especially when you add on all the extras like losses in the gearbox, alternator, water pump, power steering and air conditioning pump.

There is no single "magic number" for all cars. Driving at 60mph instead of 56mph might cause a minute increase in consumption due to the 20% increase in power needed to overcome wind resistance, but it might also allow the engine to work more efficiently and cause a drop in consumption. It depends on your car. However, driving at 70mph or 80mph will have an effect, due to the rapid increase in air resistance. Every 10mph increase at these speeds adds approximately 50% to the power required to overcome wind resistance. (Power required is proportional to the cube of the speed)

But it's driving style that has as great an effect. A constant speed is much more efficient than continual acceleration and braking, and the more significant the changes in speed, the worse the consumption. You can drive an efficient car inefficiently, or vice versa.

pault123

drewwa wrote: »

Interesting thread...

I tried a 'how many miles can i get out of my car' with my W-reg Audi A4 1.9 TDI.

Rules

1. Aircon off
2. 56 mph top speed (sat nav measured, not speedo)
3. No higher than 2000 rpm.

Results:

770 Miles from 55 litres of fuel.

I usually get 600 miles or thereabouts.

You can see a day by day acount on my blog if you're interested.

Cheers,

Drew.

Some good research Drew, what bhp 1.9 tdi have you got?

I find after sticking to a constant 60 on motorway jounrneys its not hard to get to the magic 60mpg average. Still impresses me how frugel a 1.9 torquey engine can be :money:

John_Pierpoint

charlieheard wrote: »

(Power required is proportional to the cube of the speed)

E=MVsquared ?

John_Kennet

I have been driving more smoothly for several months now (last 12 tank fill ups) and it has increased my fuel efficiency by exactly 10%. This has been without increasing journey times or driving notably slower on motorways.

My mileage has risen from 7.5 miles per litre to 8.2 miles per litre in a Vauxhall Vectra 1.8. As I fill up using a company fuel card I have accurate mileage and fuel figures for over a year so I can be very confident of this figure.

I was unaware of Martin's campaign when I started doing this, I had been inspired by the free web book "without hot air" about the figures behind our future energy choices.

I wonder how many folk stop to think that each mile they drive to work costs them almost £100 per year in wages just for the fuel. (225 work days a year times 1 mile each way at 12p a mile = 225*2*0.12 = £54 - then adjust for tax and National insurance = £91 for a higher rate tax payer)

mark55man

£100 per mile - what a good (bad) way to look at it

I have lapsed a little from my extreme frugality earlier, but despite the cheaper petrol now, am still regiularly exceeding 400 miles in my 1.8 zafira, when previously it was only 370