MadSci Network: Engineering

Re: Use laser(s) to flash heat water in a small tube(s) to power a car?

Date: Mon Aug 23 10:13:16 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 934995412.Eg


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  
THERMODYNAMICS. Generally, the greater the difference between the high and 
low temperatures, the greater will be the useful power output and 
efficiency of the engine.

Engines that run on gasoline, methanol and diesel fuels are called internal 
combustion engines (ICE) and they use the rapid explosion, heating and 
expansion of a compressed fuel air mixture in an enclosed cylinder to drive 
a piston downward during the power stroke. Currently internal combustion 
engines have a number of advantages over external combustion engines with 
the fuel being part of the heating process being a major advantage. 

Steam engines are external combustion engines (ECE) and they use an 
external fuel combustion process to convert liquid water to high-pressure, 
high-temperature steam in a boiler. The high-pressure, high temperature 
steam is then transferred into the cylinder to drive the piston during the 
power stroke. However, a steam generating boiler and heater also have to be 
added to the vehicle to run an ECE engine. Because steam engines run at 
lower pressure and temperature  differences than ICEs, they use larger 
diameter pistons to produce the same amount of drive power and are less 

Your question suggests that it might be possible to use a flash boiler 
concept for an ECE. An atomizer can be used to convert liquid water to 
water vapor before heating; however, delivering enough energy to convert 
the water vapor in a cylinder to super heated steam, about 33 times per 
second, for a 2000 revolution per minute (RPM) engine, would be a difficult 
task. A spark plug in an ICE is designed to ignite an explosion which then 
heats the fuel air mixture in a few milliseconds as the explosion fills the 
cylinder head. A flash heater would have to heat a water vapor mixture 
fast enough to produce high pressure steam 33 times per second and 
the heat must spread by convection rather than explosion throughout the 
cylinder volume.

Two alternatives for flash heating water vapor to steam in an ECE come to 
mind. These techniques have been studied to possibly replace spark plugs in 
future ICE engines. One is microwave heating similar to microwave ovens and 
the second is laser heating of the fuel air mixture as your question 
suggests. ICEs with laser ignition have been demonstrated using a laser 
beam focused inside the cylinder head through a quartz window in place of 
the spark plug. A focussed, pulsed laser can easily reach a temperature to 
ignite a fuel air mixture; however, here again the ignition of the complete 
fuel air mixture requires an explosion to fill the entire cylinder head. 

Microwave heating is probably a more viable concept than laser heating for 
steam engines. It turns out that water vapor has a molecular resonance at a 
microwave frequency near 25,000 Megahertz (abbreviated MHz - 25,000 million 
cycles per second) with a wavelength of 1.2 cm (0.5 inches). This means 
that water vapor is more efficiently heated at this frequency because a 
resonant vibration process occurs between the hydrogen and oxygen atoms in  
water molecules.
Microwave energy at the resonance wave length can be efficiently 
transmitted directly into a cylinder head through a quartz window in place 
of a spark plug and the entire volume would be flash heated just as a 
microwave oven heats an entire cup of liquid. Using the cylinder as the 
flash heating container might actually make this more of an ICE process.

The major problem with both microwave and laser flash heating is in 
achieving the ammount of radiation power necessary to flash heat water 
vapor to steam. It takes a typical 1000 watt microwave oven used in our 
kitchens many minutes to heat a cup of water to steam. To accomplish this 
33 times per second for a large cylinder would require 100 kilowatts to a 
Megawatts (million watts) or more of microwave or laser power. Today, there 
are large microwave and laser transmitters the size of an automobile that 
operate at these power levels; however, the over all conversion efficiency 
from a battery to microwave or laser energy for these systems is far less 
than 30%. 

Laser diodes with power conversion efficiencies greater than 50% are being 
developed, however they currently generate only a few watts of power. 
Recently our laboratory was able to generate a record 1000 watts of 
continuous laser power using a large number of laser diodes to pump (excite 
resonances) in a pencil sized YAG laser crystal; however, the over all 
efficiency from battery power to light power for this laser is less than 
30%. Also, we have to get rid of 70% of the total input power (3300 watts) 
as waste heat. This waste heat might be used to preheat the water vapor 
before it is injected into the cylinder to improve the over all efficiency 
of a steam engine.

Molecular resonance effects have been used in MASERS (microwave amplifiers) 
and LASERS (light amplifiers) since the 1950s and 60s. More recently 
magnetic resonance imaging (MRI) and other quantum resonance effects have 
been used at very low power levels for medical applications. Using 
molecular and atomic resonances for heating at high power levels has been 
studied for thermonuclear fusion power generating applications at the 
national laboratories using huge building sized lasers; however, this 
technology has not yet reached break even operation were the power required 
for operation is less than the power generated by the reaction.

In conclusion, microwave or laser systems are still to large and 
inefficient to quickly produce large amounts of steam. However, when 
considering the environment it is useful to calculate the over all 
efficiency of the total process to operate a motor vehicle, including the 
power required to obtain and deliver the fuel. Also, we must consider the 
waste products resulting from the operations. Using all these 
considerations and new molecular and atomic resonance techniques, we may be 
able to make water/steam or hydrogen/oxygen fueled vehicles a viable 
technology in the future.
Best regards, Your Mad Scientist
Adrian Popa 

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