1. An attempt is made to discover how much fuel is saved by turning off your car engine when stationary. After some thought, the comparison is between restarting the engine and having it idling while stopped. So what is to be discovered is the idle time equivalent to a restart. A 1500cc Toyota engine is left at idle for a whole 90 minutes and fuel was used between 925and 955 ml of fuel in that time. That same amount of fuel is then used to start the engine many times and this number is counted. This turned out to be in the region of 780±30 times.
a) Turn 940±15ml for 90 minutes into millilitres per second.
b) Show that the amount of fuel used in a single start is about 1.2 millilitres and find the range of values that fit the data given.
c) Convert the two results into the time equivalent to one restart of the engine and show this is between 5 and 10 seconds. Use the variability given to suggest a sensible range for this length of time. Make a conclusion about cars that can turn themselves off when stationary.
1. Taking the middle values, 940ml in 90 minutes is 0.174ml/sec and 940ml for 780 restarts is 1.205ml. Dividing one by the other suggests 6.921 seconds. The 0.174 is ±.00285 and the 1.2 sec lies between 1.142 and 1.273, always using 4sf, which between 7.436 and 6.456 seconds, all of which round to about seven seconds. Here's a video of this explanation, which concludes with a test comparing economy between two otherwise identical cars, one with start/stop. The start/stop car gave better fuel economy by between 4.7 and 8.7% on city driving.
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