http://physics.colorado.edu/phys3070Clicker questionClicker questionClicker question 2Clicker question 2Engine efficiencyFederal mandatesMaximum engine efficiencyTransmission efficiencyTire efficiencyAir frictionTotal energy to travel distance XOak Ridge StudyThe 55 mph speed limitEnergy storage devicesJudah Levine, 3070L29 1http://physics.colorado.edu/phys3070http://physics.colorado.edu/phys3070Judah LevineJudah LevineJILA SJILA [email protected]@colorado.edu303 492303 492--7785 (27785 (2--7785)7785)M,F 11M,F 11--Noon, W 1Noon, W 1--3 pm3 pmalso by appointmentalso by appointmentJudah Levine, 3070L29 2Clicker questionClicker questionA car accelerates from 0 to 60 miles per hourwith constant acceleration. In the absence of all friction, during the acceleration:a: the applied force increases linearly with the speedb: the power supplied by the engine increases with the speedc: the applied force and the power both increase with speedd: the applied force is largest at lower speedse: the applied force is largest at higher speedsJudah Levine, 3070L29 3Clicker questionClicker questionA car accelerates from 0 to 60 miles per hourwith constant acceleration. In the absence of all friction, during the acceleration:a: the applied force increases linearly with the speedb: the power supplied by the engine increases with the speedc: the applied force and the power both increase with speedd: the applied force is largest at lower speeds e: the applied force is largest at higher speedsJudah Levine, 3070L29 4Clicker question 2Clicker question 2If a raindrop falls with constant velocity, then it mustbe true that during its fall:a: the weight of the drop is constant and air friction is not importantb: the weight of the drop is greater than the force of air frictionc: the weight of the drop is equal to the force of air frictiond: the force of air friction is much larger than the weight of the drope: none of theseJudah Levine, 3070L29 5Clicker question 2Clicker question 2If a raindrop falls with constant velocity, then it mustbe true that during its fall:a: the weight of the drop is constant and air friction is not importantb: the weight of the drop is greater than the force of air frictionc: the weight of the drop is equal to the force of air frictiond: the force of air friction is much larger than the weight of the drope: none of theseJudah Levine, 3070L29 6Engine efficiencyEngine efficiency2000 rpm3000 rpmEnginestallsIncompleteintake and exhaustefficiency→Optimum operatingpointEngine speed (rpm) →Judah Levine, 3070L29 7Federal mandatesFederal mandatesCorporate Average Fuel Economy Act (1975):Sales-weighted average of engine efficiency forcars must increase from 13.8 miles/gallon in 1975 to 27.5 miles/gallon in 1988SUVs and “light trucks” NOT included Typical SUV15-20 mpgJudah Levine, 3070L29 8Maximum engine efficiencyMaximum engine efficiencyEnergy needed to travel distance Xat vehicle speed v:vxPtPoptimumoptimum×=×That is why a bicycle is so much more efficient than walkingJudah Levine, 3070L29 9Transmission efficiencyTransmission efficiencyFrictional forces in bearings arealmost independent of speed andmass of car.“Classical” frictional force4-wheel drive much poorer and degrades with speedJudah Levine, 3070L29 10Tire efficiencyTire efficiencyproportional to mass and speedFlexingJudah Levine, 3070L29 11Air frictionAir friction2cvFair=Depends on shape ofObject:Square “box” worstForce of air friction3cvvFPairfrictionair=×=−Judah Levine, 3070L29 12Total energy to travel distance XTotal energy to travel distance XEngine Wheels and bearingsAir frictiontotal55 mphEnergy required→Vehicle speed →Judah Levine, 3070L29 13Oak Ridge StudyOak Ridge Study24%Judah Levine, 3070L29 14The 55 mph speed limitThe 55 mph speed limitQQProposed by President Nixon in 1973Proposed by President Nixon in 1973QQImplemented by Federal government Implemented by Federal government under threat of losing highway fundsunder threat of losing highway fundsQQPrior speed limits ranged from 80 to no Prior speed limits ranged from 80 to no limit (limit (““reasonable and properreasonable and proper””))QQRepealed in 1994/1995Repealed in 1994/1995QQStates raise speed limits to 65/75States raise speed limits to 65/75––8585ththpercentile rulepercentile rule––Energy efficiency takes another hitEnergy efficiency takes another hitJudah Levine, 3070L29 15Energy storage devicesEnergy storage devicesQQBatteries recharged from electricity gridBatteries recharged from electricity grid––Power/weight ratio not favorablePower/weight ratio not favorable••ZincZinc--air air ““airair””cells cells ••energy density about 5energy density about 5××leadlead--acid batteriesacid batteries––Maximum range about 200 milesMaximum range about 200 miles••Post office delivery vansPost office delivery vans––Pushes the emissions problem somewhere Pushes the emissions problem somewhere elseelseQQFlywheelFlywheel––No serious testsNo serious
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