When needed, the energy is taken from the source and consumed by one or more motors or engines. Sometimes there is an intermediate medium, such as the batteries of a diesel submarine.
[38]
Most motor vehicles have
internal combustion engines. They are fairly cheap, easy to maintain, reliable, safe and small. Since IC engines burn fuel, they have long ranges but pollute the environment. A related engine is the
external combustion engine. An example of this are steam engines. Aside from fuel, steam engines also need water, making them impractical for some purposes. Steam engines also need time to warm up, whereas IC engines can usually run right after being started, although this is not recommended in cold conditions. Steam engines burning coal release
sulfur into the air causing harmful
acid rain.
[39]
A modern scooter in Taiwan.
While intermittent internal combustion engines were once the primary means of aircraft propulsion, they have been largely superseded by continuous internal combustion engines:
gas turbines. Turbine engines are light and, particularly when used on aircraft, efficient.
[citation needed] On the other hand, they cost more and require careful maintenance. They also get damaged from ingesting foreign objects and produce a hot exhaust. Trains using turbines are called
gas turbine-electric locomotives. Examples of surface vehicles using turbines include
M1 Abrams,
MTT Turbine SUPERBIKE and the
Millennium.
Pulse jet engines are similar in many ways to turbojets, but have almost no moving parts. For this reason, they were very appealing to vehicle designers in the past, however their noise, heat and inefficiency has lend their abandonment. A historical example of a pulse jet in use was the
V-1 flying bomb. Pulse jets are still occasionally used in amateur experiments. With the advent of modern technology, the
pulse detonation engine has become practical and was successfully tested on a
Rutan VariEze. While the pulse detonation engine is much more efficient that the pulse jet and even turbine engines, it still suffers from extreme noise and vibration levels.
Ramjets also have few moving parts, but they only work at high speed meaning that their use is restricted to
tip jet helicopters and high speed aircraft such as the
Lockheed SR-71 Blackbird.
[40][41]
Rocket engines are primarily used on rockets, rocket sleds and experimental aircraft. Rocket engines are extremely powerful. The heaviest vehicle to ever leave the ground, the
Saturn V rocket, was powered by five
F-1 rocket engines generating a combined 180 million horsepower
[42] (134.2 gigawatt). Rocket engines also don't need to "push off" of anything, a fact that the
New York Times denied in error. Rocket engines can be particularly simple, sometimes consisting of nothing more than a catalyst, as in the case of a hydrogen peroxide rocket.
[43] This makes them an attractive option for vehicles such as jet packs. Despite their simplicity, rocket engines are often dangerous and susceptible to explosions. The fuel they run off may be flammable, poisonous, corrosive or cryogenic. They also suffer from poor efficiency. For these reasons, rocket engines are only used when absolutely necessary.
[citation needed]
Electric motors are used in
electric vehicles such as
electric bicycles, electric scooters, small boats, subways,
trains,
trolleybuses,
trams and
experimental aircraft. Electric motors can be very efficient, over 90% efficiency is common.
[44] Electric motors can also be built powerful, reliable, low-maintenance and of arbitrary size. Electric motors can deliver a range of speeds and torques without necessarily using a gearbox (although it may be more economic to use one). Electric motors are limited in their use chiefly by the difficulty of supplying electricity.
[citation needed]
Compressed gas motors have been used on some vehicles experimentally. They are simple, efficient, safe, cheap, reliable and operate in a variety of conditions. One of the difficulties encountered when using gas motors is the cooling effect of expanding gas. These engines are limited by how quickly they absorb heat from their surroundings.
[45] The cooling effect can, however, double as air conditioning. Compressed gas motors also lose effectiveness with falling gas pressure.
[citation needed]
Ion thrusters are used on some satellites and spacecraft. They are only effective in a vacuum, which limits their use to spaceborne vehicles. Ion thrusters run primarily off electricity but they also need a propellant such as
caesium or more recently
xenon.
[46][47] Ion thrusters can achieve extremely high speeds and use little propellant however they are power hungry too.
[48]
