October 12, 2008

Electric Propulsion Systems

Electric Propulsion Systems

Hybrids are planning to use electric propulsion systems which prove to be very efficient and also environmentally friendly. There may be various approaches and methods in order to accomplish movement using electric power but the main idea is still the same. These systems are already working and will further help develop alternate fuel.

What is Electric Propulsion?

Electric propulsion is the acceleration of gases in order to produce propulsive thrust through electric and magnetic body forces, electric body forces or electric heating. The electric propulsion system is usually incorporated in rocket science wherein it manages energy enough to produce a powerful thrust. An electric propulsion system is an alternative to nuclear propulsion system. The total thrust is less powerful compared to a nuclear rocket but still enough to produce effects.

According to a number of studies, any engine used as a primary source of such powerful propulsion must produce exhaust velocities of around 10 to 20 km/s. There are also storable chemical systems used in rockets with an exhaust velocity of around 5 km/s but overall is less efficient.

Propulsion systems that do not require energy through chemical reactions are still needed. There are electric propulsion thrusters able to produce exhaust velocities of around 10 to 20 km/s which increases payload and reduces propellant mass. The consequences however, are less powerful thrusters consuming larger quantities of power.

The 3 Categories of Electric Propulsion

Electro thermal propulsion is when the propellant is electrically heated then isentropically expanded through a C/D or convergent/divergent nozzle. The system works as electrical energy heats propellants that produce gases as a result. The gases are sent through the C/D nozzle creating thrust. Catalyzed hydrazine or another neutral gas is used in thrusters like arc jets and resistojets.

Arc jets can also be used to heat the propellants via an electrical arc discharge. The arc in the arc jet is a beam of electrons produced from the cathode tip then gathers at the anode. Since an arc jet has a cathode and an anode, a constrictor is also present which is a narrow pathway between the two charges.

Electrostatic thrusters are also called ion thrusters. These use an ionized propellant accelerated through electric fields applied directly like gridded ion thrusters and Hall thrusters. The technique of propulsion is also known as ion propulsion technique since ions are mainly used in the process. Electrostatic energy is used to produce propulsion.

The electrons from an atom are stripped off then converted to ions. The ions are accelerated by electrical forces to high temperature without needing thermal energy producing thrust. The atoms after losing electrons become positively charged.

Electromagnetic thrusters produce thrust using electric and magnetic forces that interact with charged plasmas like ions and electrons. An example of these is the magnetoplasmadynamic thruster or MPD. The system heats the propellant to a plasma state before being accelerated. A large current is passed by electromagnetic forces through gas in order to ionize the propellant. Plasma is the ionized propellant which is then accelerated by Lorentz force, an electromagnetic force producing thrust.

Effect on Fuel

Decoupling engine speed and power output from the propeller will provide the opportunity to improve propeller efficiency. Since electric forces and electromagnetic forces kick in for support, gasoline and diesel propulsion systems in vehicles will decrease the chances of wasting a huge part of power and energy.

The chances of engine overload are eliminated resulting to better fuel economy and better gas mileage. In addition, there is higher efficiency over longer distances and various speeds and loads.

A study conducted to check how much electric propulsion systems can help diesel and gasoline engines showed that at least 10% fuel savings is achieved by simply allowing the engine to move along with the load reducing inefficiencies due to low load with high speed. Larger propellers can also save as much as 7% of fuel compared to traditional models.

With the total load split between multiple generators, as much as 20% of fuel can be saved plus another 13% by matching the power produced by the engine to the power required by the propeller. A variable-speed generator will help accomplish this. Overall, 30% to 50% can be saved compared to a very efficient diesel-electric or gasoline-electric system.

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