Hydraulic Hybrids and starRotor engine

Some other alternative car engine technology are the hydraulic hybrid and the jet engine like starRotor. Popular Science discusses these two technologies, the $25 million X prize for a 100mpg car and the previously mentioned Aptera 300mpg car

Hydraulic hybrids use hydraulic and mechanical components instead of electrical ones. A variable displacement pump replaces the motor/generator, and a hydraulic accumulator (which stores energy as highly compressed nitrogen gas) replaces the batteries. The hydraulic accumulator, which is essentially a pressure tank, is potentially cheaper and more durable than batteries. They can recover 80% of the energy used in braking (which is a lot better than the electric regen braking), which is great for vehicles with a lot of stops like UPS trucks, garbage trucks and buses.

The EPA estimates hydraulic hybrids could improve fuel economy up to 55 percent, versus 30 percent to 40 percent for a conventional gas-electric hybrid.
The EPA believes in some ways, hydraulic hybrids will be superior to gasoline-electric hybrids, such as the Toyota Prius and Ford Escape Hybrid. One big advantage: they cost less, meaning consumers will see a more immediate financial payoff. The cost of the hydraulic system — about $1,000 — would be paid back in nine months of driving, EPA officials say. The EPA is working on pilot projects with Ford Motor Co., the U.S. Army, Eaton Corp. and United Parcel Service to get the
technology into the real world as quickly as possible.

Researchers at Texas A&M University have invented the StarRotor engine, which has the potential to achieve more than 80 mpg, with low emissions approaching those of fuel cells, at a cost approaching that of a conventional internal combustion engine ($2000 to $5000).

The StarRotor engine uses the Brayton cycle, the same thermodynamic cycle employed by jet engines.

Being a close relative of jet engines, it has a very high power density, meaning it can pack a lot of power into a light-weight package. The StarRotor engine volume is about 20 to 75% of a conventional internal combustion engine, and its mass is about 20 to 40%. (Note: The wide range depends upon the desired efficiency.) The StarRotor engine has very few moving parts. The StarRotor engine can use any liquid or gaseous fuel, such as gasoline, diesel fuel, kerosene, alcohols, or methane. Premium high-octane fuels have no benefit to a StarRotor engine, so the least expensive fuel grades can be employed.

The StarRotor engine is projected to be very efficient (45-60%). By simply replacing conventional engines (15-20% efficiency) with a StarRotor engine, fuel economy will double or triple. For example, a conventional luxury car getting about 25 mpg on the highway would get about 75 mpg. A conventional economy car getting 40 mpg would get about 120 mpg.

Do they have a working StarRotor engine? Not yet. The key components of the engine, which are the compressor and expander, have been designed by StarRotor. They currently have a working compressor (which can also be used as a low-temperature expander). They are working on getting a high-temperature expander, recuperator, and combustor in place to complete, assemble, and test the engine.

The first application of a starRotor will be distributed electric power because the utility industry is willing to pay more per kilowatt than the automotive industry. Therefore, they can be profitable at small production rates. Then they can expand to go after the automotive industry, which they are targeting in 2010.

A DARPA white paper discusses scaling the StarRotor engine from 50 W to 50 MW.

Related info:
Hybrid Vehicles at wikipedia

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