MadSci Network: Engineering
Query:

Re: How big a piston an how hot a spark to convert a car to run on steam???

Date: Mon Dec 14 11:07:02 1998
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 912976791.Eg
Message:


Greetings:

The study of how engines work is a branch of science called thermodynamics. 
Engines that run on gasoline, methanol and diesel fuel are called internal 
combustion engines (ICE) and they use the rapid explosion and expansion of 
a compressed fuel air mixture in an enclosed cylinder to drive a piston 
downward during the power stroke. Typical ICEs use a 4 stroke process: 1) 
draw in the fuel air mixture into the cylinder on the pistonís down stroke 
2) compress the fuel air mixture in the cylinder on the pistonís up stroke 
3) ignite the fuel air mixture in the cylinder with a spark driving the 
piston down in a power stroke and 4) expel the exhausted fuel in the 
cylinder during an up stroke of the piston. 

Steam engines use an external fuel combustion process to convert liquid 
water to high pressure steam in a boiler. The high pressure steam is then 
transferred into the cylinder to drive the piston down during the power 
stroke. Typical steam engines use a 2 stroke process: 1) High pressure 
steam enters a cylinder and drives the piston down in a power stroke,2) the 
low pressure cooled steam and condensed water vapor are expelled out of the 
cylinder on the pistonís upstroke. Converting an ICE engine to run on steam 
would require a new cam shaft arrangement to open the valves in a 2 stroke 
process. This probably would not be to difficult to accomplish. However, a 
steam generating boiler and heater would also have to be added to the 
engine. Also, because steam engines run at a lower pressure difference and 
temperature  than ICEs they use larger diameter pistons for the same 
ammount of drive power. 

Your question suggests that it might be possible to convert water vapor to 
steam in the cylinder. This would require a flash boiler concept which 
might be similar to a flash water heater which only heats water when a hot 
water faucet is turned on. An atomizer can be used to convert liquid water 
to water vapor; however, delivering enough energy to convert water vapor 
in a cylinder head to steam, about 33 times per second, for a 2000 
revolution per second (RPM) engine, would be a difficult task. A spark plug 
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 spark would 
not heat a water vapor mixture fast enough to produce high pressure steam 
33 times per second because the heating occurs only along the path of the 
spark through the water vapor and the heat must spread by convection 
throughout the cylinder volume.

Two alternatives for flash heating water vapor to steam 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. 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. It turns out that 
water vapor has a molecular resonance at a frequency near 25, 000 Mega 
Hertz (abbreviated MHz - 25,000 million cycles per second) with a 
wavelength of 1.2 cm (0.5 inches). This means that water vapor is 
efficiently heated at this frequency because of a resonance vibration 
process between the hydrogen and oxygen atoms in a water molecule. 
Microwave energy at this wave length can be efficiently transmitted into a 
cylinder head through a quartz window in place of a spark plug and the 
entire volume would be heated just as a microwave oven heats an entire 
piece of food.

The major problem with microwave heating is achieving the microwave power 
necessary to flash heat water vapor to steam. It takes a typical 1000 watt 
microwave oven used in our kitchens several seconds to heat a grape sized 
sphere of water to steam. To accomplish this 33 times per second for each 
cylinder would require 100 kilowatts to a Megawatt or more of microwave 
power. There are large radar transmitters the size of an automobile that 
operate at 25,000 MHz at these power levels; however, the over all 
conversion efficiency from a wall plug to microwave energy is less than 
10%. This would make a very inefficient engine since most if not all of the 
engines output power would be required to generate enough electricity to 
power the microwave transmitter.

Best regards, Your Mad Scientist
Adrian Popa 



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