Quote:
Originally Posted by mpgmike
I've been rolling these thoughts through my mind for some time now. I keep coming back to the steam engine. Why did Watts choose steam over ambient air if the expansion properties of air exceed those of water vapor (steam)? Furthermore, why have many manufacturers continued to use water vapor in external combustion engines for hundreds of years since? If air has a greater expansion rate, why hasn't anybody figured that out??
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Gasoline and diesel engines do use air as the expansion medium. When gasoline powered cars became available the efficiency doubled over that of the current steam engines.
But it's not that steam is less efficient than air. Same for CO2 and other mediums. You can make up for the lost efficiency by means of a greater compression ratio and other methods. Even gasoline engines should be able to get better fuel efficiency with water injection, but only if the engine is designed for it. Just simply injecting water spray into the intake will usually lower engine efficiency. Increase the compression ratio and advance the ignition timing and water injection can make the engine more efficient beyond a level possible with just air.
I do believe that water is used in external combustion engines due to it's ability to keep temperatures down. If you add x amount of BTU's to water in a steam engine the steam may hit a few hundred degrees, whereas if it were air it would hit tens of thousands of degrees. This works in an internal combustion engine where the flame is right there and the heat wave is only temporary, but transferring tens of thousands of degrees to air through a heat exchanger would be extremely difficult.
Noble gasses are also touted as great working fluids that could substitute water, but are also expensive. Water is very abundant.
Hot air can explode when it reaches oiled parts, unlike steam.