| MadSci Network: Engineering |
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 piston’s down stroke 2) compress the fuel air mixture in the cylinder on the piston’s 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 piston’s 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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