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The purpose of any engine is to transform the input energy, either fuel and/or electrical energy, into mechanical work. The three common types of engines are the internal combustion engines, electric engines and hybrid engines. An internal combustion engine burns fuel in the presence of air and the fuel-air mixture produces a substantial amount of heat energy. The heat engine then converts to useful work with the assistance of a working fluid. In the electric engine, it transfers electric energy to mechanical energy using a motor. The hybrid engine on the other hand incorporates the basic ideas of both the electric and internal combustion engine. In order to effectively do this, a number of engine designs have been innovated and designed, each aiming at attaining a variety of factors such as fuel economy, range, endurance, reduced pollution and henceforth. This is all with the aim of improving the overall efficiency of the engine within reasonable standards. However, when developing the different types of engines, it will be imperative to take into account various factors such as cost, efficiency and/or the environment.

History of Engines
The development of internal combustion engines dates back to 1860 when J.J.E. Lenoir developed an engine for commercial use. This type of engine used coal gas and air to produce power. Further developments by Nicolaus A. Otto, Rodulf Diesel, Felix Wankel among others over the years, came up with design models that ensured an increase in efficiency. Internal combustion engines are classified into sub-groups depending on factors such as Thermodynamic cycles, working cycles, cylinder arrangements, fuel used, ignition, speed, number of cylinders and field of application among other classifications. However, they work at the same principle by employing pistons and a combustion chamber to convert chemical energy to thermal energy in the combustion chamber, which the pistons to useful mechanical energy. All internal combustion engines rely on the three critical elements, that is: fuel, air, and ignition.

Internal combustion engine
They are called heat engines because they convert thermal energy into mechanical work

Intake stroke
The engine works by drawing fuel from the fuel tanks into the combustion chamber after starting the engine, this stage is the intake stroke. The crankshaft pulls the piston downwards and creates a vacuum, which sucks in the fuel-air mixture into the combustion chamber. The mixture during this time is at atmospheric temperature and pressure.

Compression stroke
This stage involves compressing the fuel-air mixture. The piston reduces the volume of the mixture while at the same time increasing its temperature and pressure. After attaining the right temperature and pressure, the spark ignites the mixture, which burns to release heat energy.

Power Stroke
According to the law of the ideal gas, the increase in temperature results in an increase of pressure. This pressure forces the piston downwards, thereby, initiating the power stroke. During this stage, the gas is working on the piston. The next stroke is the exhaust stroke.

Exhaust Stroke
As the piston is almost down, the exhaust valve opens and the remaining exhaust gases are expended to make room for entry of fresh fuel-air mixture. The crankshaft converts the reciprocating movement to rotary motion, which is then connected to an axle, which drives the vehicle. The rotary motion also provides power to drive the propeller of a piston aircraft.

Electrical Engine
The medium transformed to mechanical energy is not fuel but it rather transforms electrical energy into mechanical energy. It usually involves the rotation of coils of wire, powered by an electric current, which creates a magnetic field .Rather than using fuel, the engine uses rechargeable batteries. The batteries provide power for the controller, which then gives power to the electric motor. The electric motor is the heart of the electric engine. The basic design of the motor includes a stator, which is the stationary part, and the rotating part, which is the rotor. The stator is made of an easily magnetized and demagnetized material such as soft iron or steel. The poles of the stator are wound up in the end with insulated wire. The numbers of turns decide the amount of strength that the electromagnet will possess. The principle of the electric motor works by placing a permanent magnet in the center of the stators and due to the constant attraction and repulsion by the stators, the permanent magnet will continue rotating once given a spin. This is because the ends of the stators are constantly changing their poles and since like poles repel and unlike poles attract, the permanent magnet will be constantly rotating to reach the opposing pole which is constantly shifting.The polarity of poles is reversed 120 times a second by supplying a 60 cycle alternating current .Most of the electric engines operate on alternating current, which implies that the current reverses at regular intervals reaching two peaks in its operating cycle.

Hybrid Engine
As its name implies, the hybrid engine incorporates the advantages of an engine using gasoline and that using electric power. The electric engine has a shorter driving range and less power when compared to a gasoline engine. On the other hand, a gasoline-powered engine is not environmentally friendly and is costly to operate. By having two power sources, the engine increases fuel economy, power, efficiency, and range. However, one of its obvious down points is that it becomes complex to design and is expensive. A good example of a hybrid engine is that of a submarine. The submarine uses a diesel engine when cruising in the sea but adapts an electric motor when it cruising below the surface of the sea. Porsche initiated the hybrid concept in 1897. One type of an engine that employs the hybrid principle is the Hybrid Inverted Controlled Motor and Retarder (HIMR). The engine, which takes the shape of a diesel engine, is fitted with a motor. The motor acts in tandem with the diesel engine. The motor derives it power from the output shaft. Driving the vehicle at low speeds ensures implies that there is less output therefore less exhaust is emitted. The car will also store energy for breaking which will be used for the next starting sequence.This type of engine is a regenerative braking according to U.S. Department of Energy. The other two types of hybrid engines according to the Department of Energy are electric motor drive and the automatic start/shutoff. The latter engine shuts the engine automatically when it stops and restarts it when necessary, which reduces the waste of energy when the vehicle stops. The electric motor drive engine on the other hand provides extra power for the engine in certain conditions, which require increased power such as accelerating. This engine is also efficient for low-speed driving conditions where the efficiency of internal combustion engines is less.This allows for the design of a car with a smaller engine. The hybrid system has the added advantage of a huge reduction on carbon dioxide emission, noise as well as having a better fuel economy

Conclusion
The issue of fuel economy and the environment have in the recent times attracted considerable interests, and manufacturers are now more keen to provide engines that offer the best efficiency, cost conscious and yet environmentally friendly. In examining the internal combustion engine and the electric engine, it is evident that the electric engine is more environmentally friendly and it greatly saves on cost. However, it has a shorter driving range and produces less power when compared to the internal combustion engine. On the other hand, even though the internal combustion engine has a superior range and is powerful, it is not environmentally conscious and is at the same time costly to operate due to high fuel costs.The minimum acceptable standard for emissions is 3.4g/mi of carbon dioxide according to the low emissions vehicle standard. This means that the internal combustion engine produce a significant amount of pollutants because of the fuel they burn. With the technological innovations characteristic in the modern world, engineers have innovated a hybrid engine, which takes the advantages of the two engines. However, the hybrid engine is costly and offers complexions in designing.