On Wed, 28 Jan 2009 12:47:52 -0000, Dave Baker wrote:
Rather than it get lost in the pointless drivel (dribble?) in the Petrol v
Diesel (Prius) squabble I'll start this new thread.
One of the many programs I've written over the years to help me in my
automotive related business is a spreadsheet which calculates the power and
fuel useage a car requires to travel at a given speed. It takes the weight,
rolling resistance, frontal area and drag coefficient, works out the wheel
bhp and flywheel bhp required at each speed from these and then applies
average BSFC (brake specific fuel consumption) data for both petrol and
diesel engines to calculate the fuel requirement and mpg. You can also enter
the price per litre of each fuel and it will calculate the pence per mile
fuel use.
You can also alter the speed in any of the calculation rows to a new value
and it'll tell you how much power you'll need to achieve that top speed and
what the fuel consumption would then be.
By playing with the car weight and drag data you can find out very quickly
what factors affect power requirement and fuel consumption. Basically weight
affects rolling resistance which is a big factor at low speed but not at
high speed and aerodynamic drag is the reverse. For good economy at low
speed you want a low weight vehicle and at high speed a low drag one.
Using it you can easily see what is needed to design a genuine 100 mpg car.
Low weight, low drag, efficient diesel engine. A 2000 lb car with low
rolling resistance tyres, 17 sq ft frontal area, 0.26 Cd drag (same as a
Prius) should do 110 mpg at 60 mph and 90 mpg at 70 mph with an efficient
diesel engine. For a one or two occupant vehicle this is not a difficult
concept to realise. The average hatchback in the 80s weighed less than that
and many cars such as the Mini have been much lighter.
This morning I've tweaked the program to be more user friendly and added
some guidelines for the data inputs for various cars. It's hosted here.
http://www.mediafire.com/?zk0mkoj41jx
It's an old Dos Borland Quattro format which is what I've used for donkey's
years for spreadsheet work cos I'm too set in my ways to use newfangled
windows spreadsheets but they should be able to recognise it without any
problem.
Play with the numbers and you'll see what your actual car ought to be giving
in mpg terms. If I enter the data for my Focus (3000 lbs, 0.013 RR, 22 sq
ft, 0.33 Cd) I get 38 mpg at 70 mph which is what it actually does. You can
then alter those numbers to see how easily we could all be driving small,
single user 100 mpg cars if the will were there or if fuel prices went up
enough.
We don't need hybrids, regenerative braking or any other unusual tactic (not
that I'm decrying them) to achieve this. Just common sense, efficient diesel
engines, low weight, size and drag.
In no more than 5 minutes I could sketch out a small streamlined car that
could carry two people and luggage and get them from London to Aberdeen on 5
gallons of fuel for 25 quid. My Focus needs three times that. If you want to
move house it won't be the ideal vehicle for you but as most cars only have
one person in them for 99% of the time it would do for most of us for
general use and commuting.
The trouble is that although I may only need to move myself for commuting,
I need to transport myself, the wife and kids at other times, trolley(s),
diy bits and pieces, shopping, bulky items, etc., so I'd end up having to
own two cars. Most people - even singles - sometimes carry a carful and so
cars generally need to be big enough for the reasonably common occurences.
SteveW