1) You are responsible for selecting automobiles that will be used in your company’s fleet and have been asked to determine the energy consumption of the described vehicles for each of the driving processes described below. The vehicles have the following combined wind drag and rolling friction force during average operation conditions. The relationship for this force is
Ff = (0.214m + 2.63)
where, m=vehicle mass and Ff is the combined wind and rolling friction forces in N. Note the numerical parameter have units associated with them.
One automobile is an all-wheel drive hybrid vehicle with a mass of 1450 Kg and it is powered by a storage battery- motor combination that provides electricity to four wheel mounted motors/generators. It does not have plug in capabilities at this time. The batteries in the hybrid configuration are charged using an internal combustion engine-electrical generator combination at the end of the driving cycle or by regenerative braking. The battery in this problem has sufficient energy storage to accomplish the given driving cycle and it is used to keep account of the energy used in each driving process. For the purpose of this problem the battery is initially fully charged. Determine the change in the chemical energy of the battery for each described process. The efficiency of the battery charging process is 90% when it is charged from the engine. The efficiency of the regenerative braking mechanism is 85% and is defined as the energy provided to the battery divided by the braking energy needed to reduce the speed of the automobile. The battery charging process is included in the regenerative braking efficiency. The battery discharge efficiency is 90%. The battery efficiencies are defined as the energy provided by the battery divided by the change in the chemical energy of the battery during the discharge and the reciprocal of this parameter for the charging phase. The hybrid regeneration braking system can operate down to a velocity of 10 Km/hr when the standard friction brakes are applied. All processes can be considered to be isothermal The efficiency of the engine-generator system is 30%.
The second automobile has a mass of 1200 Kg, has the same all wheel motor drive configuration and is powered by a conventional internal combustion engine with a generator that has an efficiency of 41%. In this configuration there is no regenerative braking mechanism.
The different driving cycles are composed of the following individual activities.
• The vehicle accelerates from a stop to a speed of 35 km/hr at constant eleavation. This acceleration takes place in 18 seconds at a constant accelerate rate of 0.54 m/s2.
• The vehicle stops from a velocity of 35 km/hr at a constant deceleration rate of 0.59 m/s2 during regenerative braking and of 0.65 m/s2 during frictional braking. This process is also at constant eleavation.
• A hill climb with an elevation change of 125 m at an angle of 9 degrees and a constant velocity of 35 km/hr.
• Calculate the power from the battery required during the hill climb described in part (c).
• A hill descent with an elevation change of 125 m at an angle of 3 degrees and a constant velocity of 35 km/hr.
• Energy consumed per km during a level driving distance.
4.1) Determine the chemical energy change in the battery or fuel consumed and the thermal energy (heat) added to the environment (heat flow) for a), b), c), e) and f) of the above driving processes.
4.2) Using the above results and the given information, consider two driving cycles that your company uses. The first is composed of 51 start and stops, 5 hill climbs and descents and a level, constant velocity component of 21 km. The second driving cycle is composed of two start stops, one hill climb and descent and 210 km of level, constant velocity driving. The first driving cycle is used twice as often as the second.
4.3) Based on your calculations, write a recommendation for purchasing the type of vehicles.. The company plans to purchase four vehicles. The heating value of gasoline is 32.0 MJ/L. The cost of gasoline is $0.75/L. The recommendation is to the Chief Financial Officer of the company who has an MBA and a public relations background.
To do this problem it will help if you draw your system and identifying the energy property changes and the energy interactions at the boundaries. In some cases, you may need a sub system analysis to identify the energy interaction, especially the heat flows. Also, there is no external energy input until the end of the driving cycle for the hybrid vehicle, all the driving processes are supplied by the battery. For the non-hybrid vehicle, there is an external energy input from the fuel supplied to the engine for driving process.