Engines

The StarRotor engine uses the Brayton cycle (see Figure 1), the same thermodynamic cycle employed by jet engines. In the compressor, ambient air(1) is compressed. The typical discharge pressure is 6 atm, but other pressures can be used (higher pressures increase power and lower pressures decrease power). The compressed gas(2) is then preheated in the recuperator. After the recuperator the compressed gas(3) goes into a combustor where the volume is further increased by burning fuel. Although many types of combustors could be used, we are considering a ceramic tubular type because it produces very little pollution.

After combustion the hot, high-pressure gases(4) are expanded in the expander to atmospheric pressure, thereby doing work. A portion of the work (approximately one-third) is re-invested back into running the compressor. The remaining work is sent to a rotating shaft where it propels the automobile, makes electricity, or any other useful function. The exhaust gases(5) from the expander are still quite hot so some of the waste heat is recovered in the recuperator by preheating the compressed air. Doing this allows less fuel to be used because some of the energy (heat) is transferred back into the compressed air. After the recuperator, the gas(6) is released to the atmosphere. The compressor and expander are gerotors, a positive-displacement device that can process the large volumes of gas required by Brayton cycle engines.

To create an even more efficient engine, atomized liquid water can be sprayed into the compressor inlet to keep the compressor cool. Keeping the compressor cooler allows compression to be more efficient thus the engine will be more efficient.

Please view the compressor and expander web pages for further information on how the engine works. Also click the link for answers to some frequently asked questions.

Benefits:

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.

It should produce very low pollution.
Advanced combustor technology reduces pollution, including unburned hydrocarbons, carbon monoxide, and nitrogen oxides.

It has multi-fuel capability.
Any liquid or gaseous fuel can be burned, including gasoline, kerosene, jet fuel, diesel, alcohol, methane, hydrogen, and even vegetable oil.

It should be inexpensive to mass produce.
The parts count of the engine is about 10% of a conventional automobile engine, and the majority of parts do not require complex machining.

There should be no vibrations.
All moving components are in pure rotation; there are no oscillating components therefore it is in balance.

It should be quiet.
Because the gas is fully expanded, there is low exhaust noise.

The engine is expected to have a long life and low maintenance.
The compressor and expander of the StarRotor engine have a slight clearance between the rotors, resulting in no friction or wear. Also, it should require very infrequent oil changes, perhaps every 100,000 miles. Because it has very few moving parts, it is expected to be very reliable and require very little maintenance.

The engine should be smaller than conventional internal combustion engines.
The StarRotor engine volume and mass are about half that of a conventional internal combustion engine. A 130-hp engine will occupy approximately 2 cubic feet.

It should have a high turn-down ratio.
The engine is efficient over a wide range of speeds and torques.

The StarRotor engine should be easily scalable.
Designs from 50 W to 50 MW are possible. (Please visit this DARPA White Paper for added details.)

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