Free-Piston Engines: The Future?
While we wait for battery and hypercapacitor breakthroughs to allow us to go 100% electric and ditch internal combustion engines, series plug-in hybrids (electric motors turn the wheels and a gas generator kicks in when the batteries are empty) are our best bet to dramatically reduce transportation-related oil consumption and CO2 emissions.
The first few series plug-in hybrids (like the GM Volt and Fisker Karma) will use regular 4-cylinder gas engines as generators, but free-piston engines could potentially change all that somday: 1) They have very low friction, 2) only one moving part, 3) are about 50% efficient (about TWICE as good as gas engine and better than diesel), 4) and they generate electricity directly. Read on for more details.
How a Free-Piston Engine Works?
Invented in the 1920s, free-piston engines are finally starting to look like they could hit the big time. "Having a cheap and efficient way to generate electricity is becoming more important as automakers develop electric vehicles with onboard generators for recharging the battery pack and extending range."
Indeed, in a free-piston engine there is no mechanical connection between the piston and a crankshaft. The pistons aren't connected to anything! "Instead, [in the model looked at by researchers] two opposing pistons just shuttle back and forth inside a chamber. To generate electricity, the pistons could be equipped with rows of magnets that shuttle past metal coils to create an electrical current." This design also makes it easier to achieve homogeneous charge compression ignition (HCCI), which further increases efficient. We also assume that the whole package would be lighter than a traditional engine, helping extend all-electric range on plug-in hybrid cars. Very promising design.
Free-Piston Engines are Efficient and Flexible
Technology Review writes:
these engines could be 50 percent efficient at generating electricity--close to the efficiency of hydrogen fuel cells and much more efficient than conventional generators. Free-piston engines are efficient in part because they have fewer moving parts than conventional engines do. The engine configuration also makes it practical to tune the engine so that the fuel in a combustion chamber burns very quickly. Faster combustion allows the engine to get more work out of a given amount of fuel, improving efficiency. It can also improve emissions.
Indeed. the flexibility mean that a free-piston engine could optimize itself on the fly to burn different fuels: Gasoline, diesel, natural gas, hydrogen. It could be the ultimate flex-fuel engine!
These won't be on the market overnight, but many car makers are doing R&D; and trying to figure out how to overcome the downsides of free-piston engines (how to muffle the sound, how to better control burning cycles, etc). We'll have to wait and see.
Via Technology Review, Next Big Future
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