Transportation - Energy and the Environment - page 223

Although electric drive vehicles powered by batteries or fuel cells are still under development,
their vehicle fuel efficiencies are not as promising, being scarcely better than existing vehicles.
The battery-powered vehicle suffers from an inherent weight problem that limits its equivalent
vehicle fuel efficiency and vehicle range. The fuel cell vehicle has a lesser weight problem, but
its dependence upon hydrogen fuel, whether generated on board or at fuel suppliers, complicates
the vehicle technology. The thermodynamics of synthetic hydrogen production and utilization in
the fuel cell does not yet provide a significant fuel efficiency advantage over the conventional
utilization of fuel in vehicles to offset the economic and vehicle design advantages that improved
conventional vehicles promise, especially considering the infrastructure needs of a hydrogen fuel
economy. Furthermore, the very low emissions of these electric drive systems become a less
valuable offsetting benefit as the competing conventional vehicles become cleaner.
The technology for reducing exhaust emissions is well-developed, especially for the SI engine.
Improvements to the engine and catalytic converter could reduce emissions further, should it prove
necessary to go beyond the U.S. national Tier 2 standards. Improvements to CI engine emissions are
more difficult to achieve, and they may always have higher nitrogen oxide and particulate matter
emissions than their SI counterparts.

PROBLEMS

Problem 8.1
Table 8.2 lists the characteristics of a selection of 2000 model year conventional vehicles. The fuel
efficiency (km/L) in highway mode and vehicle mass (t) are listed for 12 vehicles. (a) Plot the
fuel consumption FC (L/km), the inverse of the fuel efficiency, as a function of vehicle mass (t).
Estimate or calculate by linear regression the value of the slope m , where FC m (mass) is the best

fit for a straight line through these points that passes through the origin. (b) Calculate the average
value of FC times the vehicle mass (t km/L) for these vehicles, together with its standard deviation.
(c) Discuss whether or not, and why, these figures support the analysis of Section 8.3.

=

Problem 8.2
Tables 8.3-8.5 list characteristics of battery-powered electric, hybrid electric, and fuel cell passenger
vehicles. (a) For each of these three vehicle types, calculate the average ratio of vehicle mass (t)
to highway fuel economy (km/L). (b) Rank order these vehicle types according to the value of this
ratio.

Problem 8.3
Table 8.2 lists the characteristics of a selection of 2000 model year conventional vehicles. For these
vehicles, plot the urban fuel efficiency as a function of the highway fuel efficiency. Calculate the
average value of the ratio urban/highway fuel efficiency, along with its standard deviation. Discuss
the significance of utilizing both urban and highway fuel efficiencies in comparing the overall
efficiencies of different vehicles.