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In recent years, the world has faced a large number of problems owing to increase in pollution. The rise in the temperature of earth's surface due to increase in the volume of green house gases in the atmosphere has lead to melting of polar ice caps and rise in the ocean levels. A high percentage of these green house gases come from vehicular exhausts. There has also been an increase in health hazards due to growth in vehicular emissions. Apart from all this, due to augmented usage, the sources of non-renewable energy have started depleting rapidly. For all these reasons, governments all over the world have started their search for a renewable source of fuel which can be used to run the vehicles efficiently and lower the environmental impact. We will now try to analyze the pros and cons of some vehicles run with alternate fuels, which are in the design and testing phase presently. These vehicles run on alternate fuel sources which help us obtain an emission free output. We shall study each of these engines and try to arrive at a conclusion as to which is the best option.

The first one we shall analyze is an electric vehicle. In this type of vehicle, propulsion is achieved by obtaining power from an electric motor. This motor, in turn receives its power from a number of rechargeable batteries. The main advantage of electric vehicle is that they can achieve 90% conversion efficiency and have greater control over the entire range of speed. These do not require gear boxes as in case of usual gasoline engines and can convert kinetic energy back into electricity, which supplies torque required for the braking system of the vehicle. However, the energy obtained from electric engines is less than that from the heat engine or gasoline engine. The batteries contain unstable chemicals which are hazardous to the environment. These are highly prone to contamination and need regular recycling. Operating large electric vehicles requires a continuous supply of energy from a power grid and large infrastructure to maintain it. Hence, it is cost effective only for small vehicles and cars. General Motors EV1 is one such car. In case of situations where higher power output is required, electric engines are coupled with other engines like diesel engines to obtain greater power.
Secondly, we will analyze vehicles run on natural gas. Natural gas vehicles (NGVs) are of two types: a) those which run on compressed natural gas (methane), and b) those which run on liquefied natural gas (propane). The main advantage of NGVs is that the cost of operation of these vehicles is very less compared to other vehicles. Compressed natural gas mostly used consists of methane, which undergoes nearly complete combustion releasing less carbon dioxide and providing more energy per unit quantity of the fuel. This helps to keep the environment clean. Arrangement can be made to refuel these vehicles from home through the existing natural gas lines. However, when natural gas is used as vehicle fuel, large quantities of fuel will be required. The main problem with natural gas is its storage and transportation. To hold natural gas in the gaseous form, high pressure vessels are required. Transportation of compressed natural gas is mainly done through pipelines, which is not feasible through oceans. Natural gas, when liquefied becomes very expensive and also highly unstable. This is usually transported by oil tankers. Hydrogen Sulphide (H2S), the main contaminant must be removed from natural gas before use. Honda Civic CGX is one car running on natural gas.
The third type of vehicle we will discuss is the vehicle run on hydrogen fuel. This type of vehicles are run by a) the energy generated from direct combustion of hydrogen, or b) by electric motors which are run with electricity obtained by converting hydrogen by means of fuel cells. The main advantage of this type of vehicles is that oxygen from atmosphere is used for the combustion of hydrogen and steam is given out through the exhaust. So virtually these vehicles are pollution free. However, the cost of hydrogen is very high and the efficiency of vehicles is very less. Another problem is with designing hydrogen tanks for these cars. The tanks are required to be extremely hard to hold liquid hydrogen at high pressure, which is maintained at a temperature of 260*C below zero. Another method of storing hydrogen is to store it inside a metal like magnesium. The hydrogen vehicles have not been very popular due to scare of hydrogen gas explosion. An additional problem is that the number of hydrogen fuel stations needs to be increased significantly. Mazda RX8 is a duel fuel engine running on hydrogen and gasoline.

The fourth type of vehicle is one using fuel cells to generate energy. In this case also, hydrogen is used in the preparation of fuel cells. Hydrogen and hydrogen containing compounds like methane are sent in as reactants. A continuous supply of fuel and oxygen from an external source will be the main requirement. The molecules of hydrogen and oxygen react at the electrodes to form water and current flows through the external circuit. The advantage of fuel cells is their high efficiency (nearly 50%) in converting chemical energy to electrical energy. These vehicles are emission free which makes them environment friendly. The problem also associated with these vehicles is the storage of hydrogen which is to be continuously supplied to the fuel cells. Hydrogen has to be maintained at high pressure and low temperature. Increase in the temperature of the cell can cause destruction of cell due to thermal loading. The reactant and oxygen must be sent in a uniform ratio to maintain the efficiency of the cell. Some examples of this type of vehicles are Mercedes-Benz F-Cell and Ford Focus FCV.

The main concern here is that the conventional sources of energy are going to run out sooner or later. Hence, it is of utmost importance to start researching on vehicles that can run on alternate sources of fuel. Hydrogen vehicles suit the present demands to reduce vehicular pollution but only more studies in this area can show which alternate fuel car is also an economically viable option.