Emissions of smog-forming compounds has been greatly improved by use of advanced catalytic converter technology, careful electronic control of combustion, and other technologies. Still, the vast majority of cars on the road today are far behind what is possible with this technology.

Efficiency at converting fuel to forward motion is quite low. Greenhouse gas emissions are still a major problem.

However, despite recent improvements, such as VW's TDI system, diesel is still far worse in terms of smog-forming emissions than gasoline.

Diesel is most often used not because of clean operation, but because its efficiency can make diesel vehicles cheaper to operate, and because of favorable torque characteristics of diesel engines.

Battery electric cars also offer the advantage of zero tailpipe emissions (since there is no tailpipe). However, there has been much debate as to how clean these cars actually are. The reason for this debate centers around the fact that it really depends on where the car is used, and whether it is charged during peak or off-peak electricity use hours. In the best case, if the electricity were to come from sources such as wind or solar power, battery electrics are truly zero emission vehicles. In many areas, battery electrics are ultimately powered by hydro-electric, nuclear, natural gas, oil or coal sources.

In many areas, electricity is primarily generated from sources such as hydro electric or nuclear, which continuously produce about the same amount of electricity. In these cases, if an electric car is charged during off-peak hours, it may just be using surplus energy that would otherwise have gone to waste. However, if the same car is charged during peak electricity use hours, or even if a lot of battery electric cars begin charging during off-peak hours, that extra demand is supplied by sources such as natural gas, coal or oil.

The end result is that a battery electric car may be significantly cleaner or significantly less clean than a gasoline powered car.

Battery electric cars offer the convenience of "refueling" at home. Battery electric cars can be built to achieve amazing acceleration, but are often limited in order to extend range.

Even with very lightweight bodies and excellent aerodynamics, current battery technology limits the range of the average electric vehicle to about 100 miles (160 km). More than anything else, this is what has limited the public acceptance of these cars.

Such a car would offer the advantage of battery-electric propulsion over relatively short distances (in those regions where the electrical power generation is such that there is a real advantage to battery electrics). At the same time, the presence of the internal combustion engine would allow the car to travel greater distances when needed.

The disadvantage of this approach is that such a car requires a lot of batteries and a large electric motor capable of powering the car alone. That, together with the gasoline engine, would make for a car that is both expensive and heavy. This weight would, in turn, hurt the car's overall efficiency.

Such a car could be designed as a serial hybrid, where the internal combustion engine is connected to a generator, and only an electric motor directly powers the wheels. It could also be designed as a parallel hybrid, where both the internal combustion engine and electric motor directly drive the wheels some or all of the time. It could also be some other variation or combination of serial or parallel design.

However in the case of a charge-sustaining hybrid, the only external source of energy is fuel for the internal combustion engine.

Since charge-sustaining hybrids seldom drive far (if at all) on electric power alone, this allow the electric motor and batteries to be relatively small, making the vehicle more affordable, and improving efficiency by reducing weight.

Also, since the gasoline engine doesn't necessarily have to propel the car alone, it too can be made smaller and more efficient. A typical gasoline-only car has an engine that is much large than necessary for maintaining a cruising speed, in order to provide lots of extra power for acceleration. In a hybrid car, the gasoline engine can instead be optimized for steady-state cruising. The resulting engine might provide relatively poor acceleration along, but coupled with the electric motor, the car can achieve the performance of a gasoline-only car that has a significantly larger engine.

While different from one another, both the Insight and Prius are somewhere between a parallel and series hybrid.

Both the Insight and Prius use gasoline for the internal combustion engine portion of the hybrid powertrain. However, it would also be possible to build a diesel-electric or natural gas-electric hybrid, with the relative advantages and disadvantages of diesel or natural gas propulsion.

There are at least two forms of fuel cell electric cars that are envisioned: 1.) hydrogen fuel cells that are supplied by an on-board tank of hydrogen, and 2.) fuel cells that have on board reformers to extract hydrogen from gasoline or natural gas.

The basic hydrogen fuel cell generates electricity using hydrogen in a process that doesn't produce any exhaust worse that water vapor. This electricity then powers an electric motor.

There are a few problems with using hydrogen as a fuel source, though. First of all, its relatively low energy density means that even with a large tank, a hydrogen fuel cell powered car would have a fairly limited range - perhaps a couple hundred miles. This means that efficiency in the form of lightweight body technologies and aerodynamics would be very important for such a car.

Another problem with using hydrogen is where that hydrogen comes from. In truth, fuel cell technology is really just another means of storing energy. This energy (the hydrogen) would most likely either come from reforming fossil fuels, or from using electricity to isolate hydrogen from the air. In the former case, this would require a new refueling network, and still wouldn't get us away from using fossil fuels. The later case would potentially allow for generating that hydrogen using electricity at home, getting around the infrastructure problems. It would still leave the same issue as battery-electric cars, being that the ultimate cleanliness of this technology depends on how that electricity is generated.

Disadvantages of this form of fuel cell car include the extra expense, bulk and complexity of having the reformer on-board, and the fact that the car is still dependant of fossil fuels.