The effect of mass on fuel economy Abstract The hypothesis: “If an increase in mass will reduce the fuel economy of a small car then the amount of fuel used per kilometre driven will increase for a car carrying greater mass” was tested by measuring the fuel used by a Nissan Pulsar each week driving to and from work. The mass in the car was increased each week by the addition of bricks. The hypothesis was supported because it was shown that an increase in mass resulted in an increase in fuel use. Introduction. The price of oil keeps increasing with each new war and whenever a major cyclone hits oilfields. This has lead to an increase in the price of petrol. High petrol prices lead to higher costs for all products that must be transported from one place to another (Star News, 2006) Many cars are unnecessarily large. A family of four can fit comfortably in a medium sized car but many have larger, heavier cars. There seems to be a link between the size and mass of a car and its fuel economy. Fuel economy is a rate expressed as the amount of fuel the vehicle uses to travel a particular distance, usually measured in L/100km. Mass is not the only variable that can effect fuel economy. The age of the car, make and model, engine type, fuel type, coefficient of drag, level of maintenance (including lubrication, replacement of worn parts and tyre condition) can all affect fuel economy (Heap, 1998). By means of a simple investigation that will vary the mass of a car over several weeks of normal city driving (i.e. to and from the place of work) it is expected that there will be a direct link between fuel economy and mass. Aim: To determine the link between mass and fuel economy for a medium sized car. Hypothesis: If an increase in mass will reduce the fuel economy of a small car then the amount of fuel used per kilometer driven will increase for a car carrying greater mass. Materials: 1995 Nissan Pulsar Hatchback Standard octane unleaded petrol Bathroom scales House bricks Plastic shopping bag Dry sand
Fig. 1 The car in the foreground is the car used in the investigation, a 1995 Nissan Pulsar Hatchback (1995 Nissan Pulsar, 2006)
Method Week 1 1) The driver was massed using the bathroom scales and the mass was recorded. 2) The car was driven to the petrol station closest to the house and the tank was filled with standard octane unleaded petrol. 3) The odometer of the car was reset. 4) The car was driven normally, along the same route each day, to and from work for a working week (5 days), taking the same route every day. 5) The car was driven to the same petrol station as before and the tank was filled with standard octane unleaded petrol. 6) The number of litres put into the tank was recorded. 7) The reading on the odometer was recorded. Week 2 8) The driver was massed on the bathroom scales and the mass was recorded. 9) 50kg of bricks were weighed on the scales and put in the hatch of the car. 10) Any discrepancy in the mass of the driver was compensated for a. If their mass had decreased then an equivalent mass of dry sand was put in the plastic bag and placed in the hatch of the car b. If the driver’s mass increased then one or more bricks were removed the car and massed and the difference was made up with dry sand. 11) The steps from 2 to 7 in week 1 were repeated. Weeks 3 to 7 12) The steps in week 2 were repeated, adding an additional 50kg of bricks to the car each week until a total of 300kg were added in week 7. Variables: Independent Variable: Dependent Variable: Controlled variables:
Mass added to car. Amount of fuel used in a work week Mass of driver (with sand compensation), Route driven, Time of day driven, Car used, Starting amount of petrol each week (full tank).
Safety: The seatbelt was worn at all times in case of accident. All other road rules were complied with including speed to prevent accidents.
Diagram (map) of Route:
Home
Work
Fig 2 Map of route to work (West Melbourne, 2004) Legend: Route to and from work from home.
Results: Table showing increase of mass, and fuel economy calculated from fuel used and distance travelled. Week
Mass of Driver (Kg)
1 2 3 4 5 6 7
80 81.2 81 81 81 80.2 79.8
Sand and brick adjustment (Kg) 0 -1.2 -1 -1 -1 -0.2 +0.2
Mass of Bricks and sand in car (Kg) 0 50 100 150 200 250 300
Distance travelled (Km)
Fuel used (L)
Fuel Economy (L/100Km)
150 148 165 151 150 149 152
13.8 14.5 16.8 16.7 17.7 25.0 20.0
9.2 9.8 10.2 11.1 11.8 16.8 13.2
Fuel economy (L/100km) with increasing mass
Fuel economy (L/100km)
20
15
10
5
0 0
50
100
150
200
250
Mass added to car (kg)
Fig 3. The above graph includes a straight line of best fit for the data created using Excel
300
Discussion: In this investigation the independent variable was the mass added to the car each week, The dependant variable was the fuel economy in L/100km. The controlled variables were: the mass of driver (with sand compensation), route driven, time of day driven, car used, starting amount of petrol each week (full tank). Variables that could not be controlled were the weather, traffic conditions, and general wear on the car. The results show a clear connection between the mass of the car and the fuel economy. Generally, as the amount of mass in the car increased there was an increase in the amount of fuel used. It is possible to draw a straight line of best fit for the plotted points on the graph. There was only one point that did not fit with the rest of the data. In week 6 when 250kg was added to the car the fuel economy was 16.8 L/km which is higher than for the 300kg week. The unusual result for week 6 can be explained by referring to the traffic conditions during that week. For the whole week there were disruptive roadworks on the way to and from work. They caused the traffic to back up and the car was idling for at least 30 minutes each way each day. This compares to the ‘normal’ weeks when there was very little idling time (less than 5 minutes each way at traffic lights). Idling car engines use more fuel than engines that are running in gear (Zyerunkle, 2002). The design of the investigation was adequate for a preliminary study but could not really be used to generalise for all small cars. For this particular Nissan Pulsar it is definitely advisable to remove any excess mass before driving to minimise fuel use but the results for other cars might not be the same. It is suggested that the experiment be repeated with several different vehicles so that a more general conclusion can be made. It is inferred that the road works had a significant effect on fuel economy in week 6 so a repeat of the week 6 trial with 250kg on board would help to clarify the reliability of the results. The ideal situation would be to repeat the experiment on a racetrack to remove more uncontrollable variables and to do the testing all in one day to minimise the effect of different weather conditions. Conclusion: The hypothesis: “If an increase in mass will reduce the fuel economy of a small car then the amount of fuel used per kilometre driven will increase for a car carrying greater mass” was supported by the results which showed that the amount of fuel used in a week increased with increased mass. Excess mass should be removed from cars and smaller cars that are less massive will use less fuel than more massive cars.
Bibliography 1995 Nissan Pulsar [Image] (2006) Retrieved August 20, 2006, from http://www.goodcardeals.com/nissan/pulsar/black Heap, A. (1998) Everyday car maintenance. Johannesburg: Scrap books. Turner, P. (Executive Producer). (2006, August 18) Star News at Ten. [Television broadcast] London: Star Television West Melbourne [Image] (2004) Retrieved August 20, 2006 from http://www.mapsonline.com.au/vic/melbourne/west/size=10 Zyerunkle, W, (2002) How to save money and drive safely. Sydney: Revhead press.