Automobile engines have a Carnot efficiency of around 60% but are about 25% efficient in practice. As long as the internal combustion (Otto) engine continues to be used, large increases in efficiency are unlikely, but sizable incremental savings are still available. Alternatives include electric vehicles and fuel cell vehicles. The transportation sector is important because transportation accounts for 27% of total United States energy use. Most of the energy used for transportation is petroleum based. That means that importing countries depend on the exporting countries’ good faith and on their continued willingness to supply oil at a relatively cheap price. While the United States pays some of the highest prices for oil, both because domestic oil is more expensive than imported oil and because the average transportation cost is large, the price of gasoline at the pump is among the lowest in the world. This is mainly due to the difference in tax structures among countries.
In Europe, automobiles are generally smaller than in the United States because of the higher energy prices consumers pay for gasoline and diesel fuel (because of the high local taxes). Smaller cars use less material to manufacture and, because the weight is smaller, cost less to run. Many drivers in the United States are reluctant to purchase small cars because they are concerned about safety (this is apparently partly responsible for the boom in sport utility vehicle, or SUV, sales). Ironically, though people in SUVs are less likely to be killed or injured in a direct front, back, or side collision, the overall death rate is higher in SUVs than in smaller cars because of the large number of fatalities in rollover accidents, which have a relatively high probability of occurrence.
American cars are much lighter than in the 1970s, a direct result of the first energy crisis. A conventional suburban car gets a mileage around 30mi/gal. It is clear that the weight of most cars could be reduced further. A countervailing trend is for more and more SUVs and pickup trucks to be sold. These vehicles weigh more than normal cars and as a result get worse mileage. By 2000, fewer than half of all vehicles purchased for personal transportation in the United States were ordinary automobiles. A large SUV might get a mileage of just 17mi/gal, while a small pickup gets 25 mi/gal. The reason for the better car mileage is legislative—Congress adopted Corporate Average Fuel Economy (CAFE) standards after the first energy crisis to force manufacturers to raise mileage, and it worked. The rise in SUVs and pickups have pleased the manufacturers because these vehicles were not covered by the CAFE standards. In addition, the low average energy prices of the late 1990s lulled drivers into expecting cheap imported oil would continue to be available. Political pressure for increasing CAFE standards abated, and car mileage stagnated. As a result, the fleet average for American vehicles experienced a decrease in mileage. The lowest fleet average mileage was achieved about 1989. Even imported cars’ mileage has slipped. On the basis of history, only legislative mandate will be successful in increasing mileage in the absence of external threats such as giant leaps in world oil prices.
Before the 1970s, most American cars were rear – wheel drive; now most are front-wheel drive, which is more energy efficient. Most modern automobile engines are more efficient overhead cam engines. However, much more energy could be saved by continuing to pursue ways to decrease drag, and the installation of advanced automatic transmissions that are controlled electronically. Other measures, some partly introduced include improved fuel injection systems, improved tires, improved lubricants, and reduced engine friction. All these measures are good conservation measures because they come at no or low cost (low cost means short payback times).
California’s quest for less-polluting cars led to designation of quotas for lower-emission vehicles. Many ideas for automobiles that run on electricity or energy stored in high-tech epoxy-based flywheels have been proposed. The high hopes California had for so-called zero-emission vehicles were apparently unwarranted, and few total electric vehicles have been designed or produced. There have been difficulties with battery design (conventional lead – acid automobile batteries weigh quite a lot). The nickel-metal hydride battery is the apparent technology of choice for electric vehicles. In addition, battery-powered cars do not perform well at low ambient temperatures, so these cars are not common outside the southwest. Another strike against battery-powered cars is the lack of charging infrastructure. And such vehicles are not really zero emission, because gas, oil, or coal must be burned to produce the electricity used to recharge the batteries.
California’s quest has had the effect of boosting prospects for low-emission vehicles that are hybrid (combined gasoline engines and electric systems that work together). Hybrid features that save fuel include automatic shutdown of the engine at stops and regenerative braking (converting the work done in braking into energy stored in the battery). In hybrids, the gasoline engine runs only at or near peak efficiency, and excess energy recharges the batteries. Since the fuel is conventional and the electric system recharges in use, there is no need for a special infrastructure as is the case with pure electrics. Honda introduced the first commercial hybrid, the Insight, followed quickly by Toyota with the Prius. Honda has begun to sell Civic hybrid models.
Fuel cells, which are devices that combine fuel (ideally hydrogen) with oxygen from the air without combustion, are expected to supply automobiles with energy in the near future. If the hydrogen were produced by electrolysis using solar energy, such a vehicle really would involve zero emission (outside the water made from combining hydrogen and oxygen). Fuel cells have been used in the past for transportation, but usually liquid metal fuel cells in buses because of the relatively high temperature of operation (300 to 1200°C). The development of low – temperature proton-exchange membrane fuel cells have led the auto manufacturers to engineering development. These are typically 50% efficient at transforming fuel to energy, about twice as efficient as Otto engines. (Very small fuel cells are being used as an alternative to battery packs in some electronic devices.)
The fuel cell option suffers from a lack of an infrastructure to supply the hydrogen to vehicles and the problem of how to carry sufficient hydrogen. Hydrogen is a dilute gas, and it has a very much smaller amount of energy per volume than liquid fuels. Possible solutions to the storage problem range from mixing hydrogen into borides to pressured tanks to storage as hydrides in metal containers. However, many questions still surround the storage question. Alternatively, methanol (a liquid) can be chemically transformed to produce hydrogen, and this may be a more probable choice for the short term. The disadvantage is the presence of carbon dioxide in the exhaust stream.