|MadSci Network: Engineering|
There are many types of engines for doing work and they all have one thing
in common, they convert high temperature liquid, solid or gas materials
into low temperature liquid, solid or gas materials and extract power to do
work in the process. These processes are based in the laws of
A steam engine is a classic example of this process. Water is heated to
high temperatures and pressures (steam) in a boiler that is external to the
power producing mechanisms. The high pressure steam then drives a turbine
or pistons to turn wheels, gears, propellers etc. In this process the steam
is cooled and returns to the liquid water state. The steam engine is called
an external combustion engine (ECE) because the heating takes place
external to the power generation portion of the engine. While ECEs can be
very powerful, they tend to be large, heavy and not very efficient. That is
why steam powered automobiles were not very practical and internal
combustion engines were developed.
Internal combustion engines (ICE) have become the most popular type of
engine for vehicles because they combine the heat generating and cooling
process with the power generating mechanisms. This enables a much smaller,
lighter weight, more efficient engine then ECEs can provide. In ICEs the
fuel is exploded inside of the engine to rapidly produce high temperatures
at high pressures. This then drives the power generating mechanisms
(turbines and pistons) while the explosion byproducts, the exhaust gases,
There are a number of explosive liquid fuels on which an ICEs can run
besides gasoline, including diesel oil, heating oil, and methanol (a type
of alcohol). Fuels in the form of gasses are also used in ICEs including
propane, hydrogen, and carbon monoxide. The fuels that are stored in the
gas state generally require large, heavy, fuel tanks that can hold the
gasses at high pressure. However, propane gas and hydrogen gas fueled
vehicles are becoming more common, particularly for large vehicles such as
busses and trucks in urban areas where air pollution is a problem. The
exhaust gasses from exploding propane and hydrogen generate much less air
pollution than most liquid fueled ICEs.
It turns out that a for given volume of liquid, gasoline can generate more
heat and power in an explosion than the other common liquid fuels (more
miles per gallon); however, the exhaust by products do cause air pollution.
Liquid hydrogen could be a more efficient fuel, with nonpolluting steam and
water as the exhaust byproducts. Unfortunately to remain a liquid, hydrogen
must be stored in tanks at below its very cold boiling temperature (- 252
degrees C, -422 degrees F). Liquid hydrogen fuel is used in space rockets
but it is not very practical for use in automobiles.
The city of Riverside California has several hydrogen fueled busses in
service that use large fuel tanks filled with nickel hydride. Nickel
hydride has the unique ability to absorb several atmospheres of hydrogen
gas in a low pressure tank. (The chemical effect is similar to putting lots
of sugar into a glass of water without changing the volume of the
liquid).The busses still require a very large fuel tank the size of the bus
frame but they are about one quarter the size of a hydrogen pressure tank
with the same volume of fuel. I have not heard if there have been any car
crashes into the Riverside busses yet to see what would happen to the
released hydrogen gas.
You can find out lots of information about the types of questions that
you are asking on the "How Stuff Works" web site. Look under
Automotive Engines.The site has great animations of
different types of engines and fuels.
Best regards, Your Mad Scientist
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