The major advantage of electric vehicles is that the electricity can come from a number of primary sources, in particular renewable energy sources like wind, hydro, solar and geothermal, at least one of which is available almost everywhere in the world. In addition, electric motors are up to 90% efficient and mechanically simple, they have high torque, and can be so finely controlled that they eliminate the need for transmissions. Environmentally they are quiet and so clean that their direct emissions are nearly zero.
In some hybrids, further fuel savings are accomplished by turning off the engine when the vehicle is stopped, and restarting it when the brakes are released. This could be implemented in a conventional ICE vehicle, although not as easily or efficiently as running an electric motor for an instant start.
Whatever drivetrain setup is used, hybrid electric vehicles that cannot be plugged into an electric power source get their electric energy from two sources:
If kicking an addiction to (foreign) oil is an objective, your hybrid electric vehicle has to plug in and drop out.
The economy of a PHEV over a regular hybrid electric vehicle is demonstrated by the results of Google's RechargeIT driving experiment. In this experiment, the Ford Escape Hybrid got an average of 32.2 mpg for all trips, and the Toyota Prius Hybrid got an average of 48.4 mpg. Mileage for the plug-in hybrid versions of those two vehicles was much higher: 49.1 mpg for the Ford Escape Plug-in Hybrid and 93.5 mpg for the Toyota Prius Plug-in Hybrid.
But that's just the beginning of the potential of plug-in electric vehicles. PHEVs still require a liquid fuel engine, usually burning fossil fuel, to power the vehicle when the battery is low. Ultimately a PHEV should only be a transitional solution by so reducing its dependence on liquid fuel, especially from foreign petroleum sources, that it essentially becomes a full-time BEV. Consider the following.
The first problem involves the energy density or power to weight ratio in battery design. Heavy lead-acid batteries have been replaced by Nickel-Metal Hydride (NiMH) and Lithium-ion (Li-ion, like those in a laptop) which have much higher energy densities, and whose costs are rapidly falling. There is as much energy in 1 gallon of gasoline weighing 6.15 lbs as there is in a bank of lead-acid batteries weighing 1,979 to over 2800 lbs. However, state-of-the art Li-ion batteries with the same energy only weigh about 400 to 530 lbs, and prototype Li-ion nanowire batteries reduce that weight about 3.5 to 5 times. In addition, since electric motors are more efficient than internal combustion engines, the amount of energy in those batteries will get you down the road further than that gallon of gasoline. For example, the all-electric Tesla roadster has an Li-ion battery pack weighing about 990 lbs, hence an energy density of roughly 2 gallons of gasoline, with a mixed city/highway range of more than 200 miles. This is an energy use roughly equivalent to 100 mpg with a cost greatly cheaper than going that 100 miles on fossil fuel.
The second problem is slow recharging time. Since quick-charging batteries tends to harm them due to heat, you have to trickle-charge them. Generally this means that PEV batteries are most conveniently charged when they are parked at home during the night, fortunately an off-peak (thus cheaper) period for electricity.
Both of these problems can be ameliorated with an infrastructure of ubiquitous trickle-charging stations, for example in parking lots and garages. If battery packs could be charged wherever BEVs and PHEVs are parked, they could then be kept relatively small, staying fully charged with frequent short boosts instead of one long one.
A 2009 PHEV typically goes 40 to 50 miles on electricity alone with a fully charged battery pack. Since the average US trip is within 40 miles of the home, at least 1/2 of the round trip can be powered by electricity, but with enough charging stations everywhere that trip's range under electric power could be increased substantially. Since the average commute to work is about 25 miles in the US, the average round trip to work would require going a maximum of 10 miles on liquid fuel in a PHEV, unless the vehicle could be charged during the workday. If your workplace is within the range of your PEV's battery, and if it could be fully charged while you are working, for example under a solar carport with an electrical outlet, you would never have to burn liquid fuel (namely gasoline) to commute round trip to work.
Woolsey's 1000 mpg scenario is based on the potential of PHEVs, assuming today's scarcity of electric charging stations. At most, only a tiny liquid fuel engine should be required to recharge a PEV's batteries in an emergency, rather than to drive it down the road. (This would employ the more simple series rather than the parallel drivetrain configuration, which doesn't use the ICE to directly propel the vehicle, but just to run the generator.) With plug-in charging stations almost everywhere we could eliminate the need for an engine except for emergency backup charging, one similar to the generators in RVs to supply power when an electric outlet isn't available, something on the order of 2 to 6 horsepower.
Now we're talking something like 5000 mpg on gasoline. If this small engine burned, say, pure ethanol, the vehicle itself is off petroleum as a fuel entirely.
A lot of electricity is generated from non-renewable sources like coal, diesel and natural gas, so we're still using fossil fuels when we plug our PEVs into the electric grid. However, there are renewable energy sources everywhere on Earth, such as solar and wind, that can generate enough electricity to charge plug-in vehicles without using any fossil fuel whatsoever.
Provide hybrid electric vehicles with an on-board battery charger that can draw energy from an electric power outlet, whether supplied by the electric grid or by standalone renewable sources such as photovoltaic solar panels and wind generators. Further reduce dependence on fossil fuels with engines in PHEVs that run on flex fuels derived from biomass.