Interesting Idea

Bullockshaver

Consider the following problem see if you can come up with an answer . Man A leaves Cork to travel to dublin in a Ford mondeo he averages 60mph on his journey to dublin and uses a total of 23litres of fuel but man B leaves at the exact same time but he only averages 50mph so he arrives in dublin later than man A. By ignoring engine efficiency wind resistance etc exactly how much fuel will car B use on the journey

dgarrad

It depends on how much of the engine operation you ignore. Different vehicles get different distance per litre of fuel, but if you ignore that as well, then the answer should be 23 litres, as the distance travelled is the same for both cars. If you know any details about the engine than you'll have to include them. A quick google search quotes the mondeo as having a fuel efficiency of about 4000litres/100km/ton, so if you have the same details for man B's car then you can work it out differently.

(By the way, this could all be wrong, so try getting a second opinion! )

Civilian_Target

Seconded. Ignoring all the factors like wind resistance, and increased efficiency of lower speeds, then driver B will use exactly the same amount of fuel. I'd actually say the most important factors are the fuel consumption of the car, and the gear you drive in. Driving in 2nd gear to Cork would use a lot of fuel.

Bullockshaver

Yes of it course it should be 23 litres aswell. ignoring all resisting forces and consumptions etc. One more thing Say car A was a diesel car and car B was petrol.In travelling a journey if the two cars stay exactly together all the time at the same speed and the same time spent travelling the diesel car will use less fuel. So can i then say that Diesel contains more enrgy than petrol? Think about that one

Delphi91

Not necessarily.

Most cars produce their optimum fuel usage at 55mph. If you draw a curve of the rate at which fuel is being used against speed, its a curve which bottoms out at around 55mph. This is why open road driving is far more fuel efficient that town driving.

Now if the graph was symmetric, then it could be said that both cars would use the same amount of fuel (50 and 60 being equidistant from 55) - if we assume 55 to be the optimum speed.

So, it depends on the vehicle. If both vehicles were the same - with the only difference being their respective speeds - and if the optimum fuel usage was 55mph, then they would probably both use the same amount of fuel. If they have different types of fuel i.e. petrol and diesel, then it shouldn't make any difference as the original post says to ignore engine efficiency (diesel being more efficient than petrol)

Mike

Bullockshaver

You seem to be missing the point i dont mean to go into specific numbers like 55mph you speak of. I was just giving the example that it would take one of the cars longer i could have just as easily said 4000mph and carB is travelling 6000mph. Its just a theoretical note not meant to take fuel consumtion efficiency into account or any limiting factor/force. Simple fact is if you have two we'll call them engines which can burn diesel and petrol (for arguments sake)
in a journey of X miles if it is ran on petrol it would burn more petrol than diesel on the same journey

Capt'n Midnight

A/ By ignoring engine efficiency wind resistance etc exactly how much fuel will car B use on the journey

If you ignore all frictional losses and allow the car to go above 50/60 when freewheeling then the energy needed to on the journey is simply..

mgh + 1/2mVmax2

g=9.81ms-2
m=mass of car + driver + fuel (ok ignore the weight of burning fuel)
Vmax=50 or 60 mph - convert to ms-1
h=height of highest hill on the journey relative to starting point.
[actually if you limit speed to 50/60 then you would have to sum all the hills]

Bullockshaver wrote:

Yes of it course it should be 23 litres aswell. ignoring all resisting forces and consumptions etc. One more thing Say car A was a diesel car and car B was petrol.In travelling a journey if the two cars stay exactly together all the time at the same speed and the same time spent travelling the diesel car will use less fuel. So can i then say that Diesel contains more enrgy than petrol? Think about that one

Diesel burns hotter in and under higher pressure so you are way further up on the Carnaut cycle so diesel engines are more efficient than petrol for the same weight of fuel burnt - most petrol is cracked from diesel anyway so there is another few % inefficeincy there.

frictional losses - wind/tyres - are proportional to the Square of the speed
ie. 3600/2500 => 1.44 times the energy needed to move the car excluding acceleration and hills.

Similar sort of thing but much harder
http://ortvay.elte.hu/1999/e/problems.html

2.
We shoot straight up with a good-quality slingshot. The projectile is a solid steel bullet, 1/4 inch (6.35 mm) in diameter. The projectile hits the ground nearby in 12.5 seconds. What was the initial velocity of the bullet as it left the slingshot? Do not neglect air resistance! Shooting the same bullet with the same initial velocity at an optimal angle, what is the maximum possible distance for the projectile? What is the optimal angle for this record-distance shot?