GET STARTED:

The efficiency of a transportation system depends on the characteristics of the vehicle, of course, but also on the infrastructure: the way the vehicle is fueled, and the physics of its motion.

A vehicle uses energy chiefly in two ways: to accelerate/decelerate, and through friction. To the extent the infrastructure minimizes both, the vehicle’s energy efficiency improves.

Friction is easy. Steel wheels on steel rails offer the lowest starting and rolling friction, which is why rail is the most efficient of existing transport systems. At moderate speeds over long distances, it can move over 400 ton/miles per gallon of fuel. That’s four times as efficient as the next most efficient transport mode: ships and barges can operate at about 100 ton/miles per gallon. Rail is also 15 times more efficient than trucks and buses — their rubber tires can roll at about 25 ton/miles per gallon at steady speed. Air resistance rises as the square of speed, and becomes a significant fuel hog over about 35 mph — which is why airlines move only about .8 ton/miles per gallon. Rail is 520 times more efficient.

Acceleration and braking are minimized by minimizing the number of stops and by keeping traffic flowing smoothly. Airplanes and ships at sea accelerate to cruise speed and then rarely have to slow or divert around other traffic. Traffic flow becomes an issue where it funnels to a limited resource — a runway or a port entry.

For trucks and cars, obviously, traffic is the chief reason for braking and acceleration. The limited-access highway system is the infrastructure we’ve developed to minimize speed changes. It works pretty well except when it doesn’t. Weather, accidents and rush hours can bring traffic to a halt. Nothing is less efficient than a fuel-burning engine at idle.

In an optimized road system, traffic stops are minimized by using traffic circles (roundabouts) in place of signals. Trucking companies would like dedicated lanes for heavy vehicles, so they could pull longer, heavier vehicles, separated from the turbulence of commuter traffic.

There’s another way to smooth traffic flow that requires pouring no concrete. “Networked GPS” (blog.brightcar.com/2008/04/networked-gps-navigation-dash-express.html) equips most vehicles with a navigation system that sends and receives location and speed data. Thus a driver can look at the dashboard map display and see if traffic is slow (yellow streets) or stopped (red streets) ahead, and divert to an alternate route showing green streets. The system simply optimizes use of existing resources (available right-of-way). The GPS display could also warn of weather or road work ahead, or show the price of fuel at each gas station along the route – or the location of electric charging stations.

Electric vehicles, with their large batteries, need a lot of infrastructure support. They need places to charge, at home, at curbside, in parking lots and garages. Utility companies and auto manufacturers are just now coming to grips with ways to standardize the charging interface.

At the same time, EVs can play an important role in helping smart grids operate efficiently (greencarcongress.com/v2g). Eventually, your EV will talk to the grid when it’s connected. If the battery is low, it will draw power from the grid to recharge, and you’ll be billed for this at retail electric rates. When the battery is fully charged but still connected to the grid, it’s available to feed power back as needed; the availability of stored power reduces the utility’s need to build and use expensive “peaking” power sources like natural gas turbine generators. You’ll be reimbursed for power you feed back. You’ll be able to program the EV to be fully charged when you expect to need it: before your morning and afternoon commutes, for instance.

One model for the charging or vehicle-to-grid (V2G) interface calls for an overhead charging plate in garages. The EV would have a pair of hinged stalks, like the trolley poles on an electric bus, to engage the charger contacts. An EV with trolley poles could draw power wherever overhead contacts can be safely installed.

Where hybrid-electric trucks and buses have access to dedicated lanes, they could use catenary trolley wires to draw full power for long uphills on electric power, or simply to recharge on the run over long distances. This isn’t rocket science: it’s 19th-century technology (en.wikipedia.org/wiki/Trolley_pole).

----------

About the author: Seth Masia is managing editor of SOLAR TODAY. Contact him at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

+ Comment on the "low-carbon cars" articles >

+ Order a copy of this issue: This e-mail address is being protected from spambots. You need JavaScript enabled to view it >

+ Get SOLAR TODAY in your mailbox >