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

charlieheard

I've had the opportunity to spend more time at home recently and have been using the car to stop 5 minutes from school and walk in, as we're too far for them to walk all the way yet. I've found that these shorter journeys are terrible for fuel consumption. I've always known that driving a cold car uses more fuel, but I'd never realised how much till I found that mpg drops by almost a third on these journeys!

More cycling for us, methinks

charlieheard

mark88man wrote: »

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

I know this is a fuel-saving thread, but that figure doesn't include major costs like depreciation, insurance and servicing. That works out around 50p/mile for a medium car and average mileage, or around 4 times the cost of fuel alone. So that's £360 per mile to work. :eek:

The problem is, I don't have the figures for a car left idle, which still costs money for depreciation, etc. So it's not a true comparison, but it gives some idea of the cost. But then you could offset the cost against the hassle and time (and availability?) of using other methods such as public transport, scooter or cycling.

15 years ago, I used to cycle the 11 miles from Wimbledon to the City and back every day, and reckoned I saved at least £1000 a year, while keeping fairly fit in the process. Can work for some

Uncledave_3

I will be getting a car with cruise control and wondered if by setting the speed this will save fuel?
I am new to these posts so be nice please.....

Uncle Dave

mark55man

cruise control is better than trying to keep a steady speed by foot.

what this thread is saying is that by "pulse and glide" if you can be bothered you can achieve extra savings, but probably more important what matters is being gentle getting up to cruise speed and avoiding the brakes

once you start watching your mpg, you will get better

Uncledave_3

Thanks Mark vbmenu_register("postmenu_16003843", true); While I am getting old thai is the first time with cruise/c.
Uncle Dave

Photoshopguru

I wish I had read this thread before swapping my lovely 1.8 GTi for a 1.4 TDi.. The road tax and fuel on the GTi was killing me. Plus servicing wouldn't have been pretty when it came out of warranty.. But I miss the oomf.. hate being stuck behind lorries, tractors and wally trolleys (caravans).

But having said that, the TDi is frugal.. 60mpg.. I wonder what it will be like once I employ the techniques discussed here :-)

harryhound

Uncledave wrote: »

I will be getting a car with cruise control and wondered if by setting the speed this will save fuel?
I am new to these posts so be nice please.....

Uncle Dave

It helps you avoid getting speeding tickets from cameras on a dual carriageways in the early hours of the morning too.

wiggers

I've just watched Martin's video and have a few comments.

1. The Smart car is not a typical vehicle, it has a very small engine compared with its wind resistance, so a lot of energy is being used in making a hole in the air. Accelerating slowly and slowing gently means that the average speed between junctions around town will be lower than the 'normal' driving style. So the cube law of wind resistance means much less fuel is used. A more common family car, with a more powerful engine and a lower drag coefficient, will see much less saving from this technique.

2. Energy is used in changing the momentum of a car. It really doesn't make any difference if you accelerate from 0-60 in 5 secs or 50 secs. The difference in total fuel consumption in accelerating gently comes from the lower average speed over a fixed distance (mentioned above) and differences in specific consumption between makes of car and types of engine.

Mark

charlieheard

wiggers wrote: »

I've just watched Martin's video and have a few comments.

1. The Smart car is not a typical vehicle, it has a very small engine compared with its wind resistance, so a lot of energy is being used in making a hole in the air. Accelerating slowly and slowing gently means that the average speed between junctions around town will be lower than the 'normal' driving style. So the cube law of wind resistance means much less fuel is used. A more common family car, with a more powerful engine and a lower drag coefficient, will see much less saving from this technique.

2. Energy is used in changing the momentum of a car. It really doesn't make any difference if you accelerate from 0-60 in 5 secs or 50 secs. The difference in total fuel consumption in accelerating gently comes from the lower average speed over a fixed distance (mentioned above) and differences in specific consumption between makes of car and types of engine.

Mark

You are not wrong in your statements, but there are a whole set of factors which modify them hugely. An engine is not equally efficient at all rotational speeds, loads and throttle openings. Also, mechanical losses in the drivetrain make up most of the energy consumption around town: only at higher speeds does wind resistance come into play. In fact, petrol engines are less efficient at low revs and small throttle openings, which "should" make them less economical when trying this approach. If energy consumption were truly only dependent on average speed, the differences would be tiny in town and unmeasurable on longer journeys.

Your comments also downplay the use of brakes to slow down. Certainly, a lot of the savings come from the gentle braking. Using the engine to slow the car gently means large times of zero consumption, as brakes just change speed (and so fuel) into heat. Being aware of your surroundings and using the brakes as little as possible make huge savings...

So while what you say is correct in isolation, it doesn't cover the car (?and driver) as a whole system...

harryhound

As debated further back in this thread, the engine management system of modern cars turn off the fuel supply when the car is pushing the engine and not vice versa (so don't free wheel down hills, Cuban style).

Would I be right in thinking a hybrid car uses the need to slow down to recharge its batteries ?