Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Lecture Notes

Transportation

Energy Use in Cars 1: Stop-and-go driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Question

• People recommend commuting by bicycle rather than by car to save energy.

• When a cyclist pushes the button to make a traffic light change, a bus full of people has to come to a stop and then has to burn extra fuel to get back up to speed.

• Is this extra fuel burned more than the fuel saved by commuting by bicycle than by car?

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Approach

• Determine the engine efficiency and mechanical energy to accelerate up to speed

•Use this to figure out how much fuel is needed to accelerate the bus.

•Compare this to the fuel saved by cycling instead of using a car.

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Determining Efficiency

The chemical energy from fuel goes to two main places:

•Mechanical Energy (to accelerate the bus, overcome air resistance and rolling resistance)

•Heat loss

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Determining Efficiency

%25InEnergy Fuel

OutEnergy Mechanicalengines gasolinefor Efficiency

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Method

1. Determine the mechanical energy to accelerate the bus to 50km/h

2. Next, use this in the efficiency formula to determine fuel energy and # of litres of fuel

3. Compare this to the amount of fuel the cyclist is saving by using a bicycle instead of a car.

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

1. Determining MechanicalEnergy to accelerate the bus

• All the kinetic energy (K.E) of the moving bus gets converted to heat energy in the brakes to slow the bus down

• When it stops, it must reaccelerate.

•Thus the mechanical energy to accelerate= kinetic energy

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

1. Determining MechanicalEnergy to accelerate the bus

2

2

1.. mvEK

First find the total mass of the bus + passengers:

(m/s) bus of speed

(kg) passengersbus of mass total

v

m

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

1. Determining MechanicalEnergy to accelerate the bus

kg 28,220

)passenger

kg 70)(passengers (120kg 19,820

m

m

First find the total mass of the bus + passengers:

a)

b)

c)

m/s 14km/h 50 v

J 0177.2)m/s 14)(kg 28,220(2

1.. 62 xEK

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

2. Determining Fuel Energy

Using an energy estimate of 37% for a bus with diesel engine:

Efficiency

OutEnergy MechanicalInEnergy Fuel

J 0149.737.0

J 0177.2InEnergy Fuel 6

6

xx

To determine how many litres of fuel, use:

litres of #

Joules of #diesel ofcontent Energy

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

2. Determining Fuel Energy

Energy content of diesel = 38.7x106 J/L,Therefore:

diesel ofcontent Energy

Joules of #litres of #

diesel of litres 0.19J/L 017.38

J 0149.7litres of #

6

6

x

x

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

3. Determining Fuel SavedBy Using a BicycleThe average gasoline car consumes 0.076 litres per km travelled. For a 10km long commute, you would save 0.76 litres of gasoline by using your bicycle.

If you made the bus stop once during the 10km ride, you would be costing 0.19litres to save 0.76 litres.

So it is still a good idea to ride you bicycle instead of using a car!

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Question

If stopping a bus costs 0.19 litres of fuel, and cars consume 0.076 litres/ km, how long would your commute need to be to save an amount of gasoline equivalent to stopping the bus once?

0.014 km0.26 km0.42 km1.0 km2.5 km

L/km 0.076

L 0.19km 2.5 E.

:Answer

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Discussion

•If your commute is less that 2.5 km, you save less than 0.19 litres of fuel . Thus, stopping the bus during your ride will cause a larger fuel consumption than you save.

•If you stop a bus, there will most likely be other vehicles on the road that you stop too. This would make us think that the amount of gas wasted is more than 0.19 litres.

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Discussion

•However, if you didn’t stop the bus, it would probably come to red light in a few stops anyway.

•Keeping this in mind, it is difficult to decide whether you save more fuel by cycling or by using a car.

•What are possible ideas to design efficient systems for transit and bicycles? Crossing lights? Designated cycling routes?

Stop-and-Go Driving

Physics and Astronomy Outreach Program at the University of British ColumbiaPhysics and Astronomy Outreach Program at the University of British Columbia

Bibliography

1. New Flyer. Hybrid Transit Solutions (online). http://www.newflyer.com/pix/Brochures/hybridbrochure.pdf [6 May 2010].

2. Energy Information Administration. Converting Energy Units 101(online). http://www.eia.doe.gov/basics/conversion_basics.html [21 August 2009].

3. Soimar M. The Challenge Of CVTs In Current Heavy-Duty Powertrains (online). Diesel Progress North American Edition. http://findarticles.com/p/articles/mi_m0FZX/is_4_66/ai_62371160/ [21 August 2009].

4. MacKay DJC. Sustainable Energy - Without the Hot Air (online). UIT Cambridge. p.31. http://www.inference.phy.cam.ac.uk/sustainable/book/tex/ps/1.112.pdf [21 August 2009].

Stop-and-Go Driving