here's a fun site to visit that has nice graphics and presents an interesting concept;

http://www.starrotor.com/indexflash.htm

Here's everybody's assignment;

Aside from the fact that the actual star rotors device might not function as hoped, what problems will probably keep the entire concept from working as advertised?fiter

I'll put in my two cents in a couple of days.

The engine described in the drawing can't work. The pressure at the compressor discharge is equal to the pressure at the expander inlet. I'm guessing that the compressor and expander are on a common shaft, although its not apparent from the drawing.Also blow by on the expansion side I think would be high. How is this baby lubed? Expansion ratio #'s don't look too good. In the end I think an axial flow turbine does a much better job.

I think their thermodynamic calculations are correct -- I tried running them and got essentially what they claim.

But. . .Here are the "pinch points" I see in the design:

1. The engine uses "inlet fogging". A 155 horsepower engine would require 23 gallons of water an hour for fogging. Besides filling up with gas, you would have to fill up with water too.

2. The heat exchanger to preheat the air going to the combustion chamber will have to be rated at 13,750 BTU, and will have to handle high temperatures (1500 deg F). This is similar to a "recuperator" used on turbine engines. A heat exchanger of that type will probably cost at least a dollar a BTU/min -- hence you're looking at $13,750 for the heat exchanger.

If you want to put one of these heat exchangers together yourself, you will need about 4600 feet of 0.6 inch outer diameter stainless steel tubing.

3. The storage tank will be able to boost the engine output by 30% for about seven seconds.

4. I wonder just how efficient a gerotor gas expander will be. It's noteworthy that they have announced that they have a compressor working, but don't make mention of the expander portion of the engine.

5. I think they are being a little bit deceptive in their efficiency claims. A gas turbine is not an especially efficient engine; diesels are more efficient. Diesel's do have relatively good durability.

I am the president of StarRotor Corporation and have read with interest the comments in this forum.  Because it is important that engineering opinions be based upon facts, I would like to clear up some issues.

Issue 1: The engine cannot work because the pressure at the compressor outlet and expander inlet are the same.

The StarRotor engine is a Brayton cycle engine.  In all Brayton cycle engines, the compressor outlet pressure is nearly identical to the expander inlet.  In Otto cycle engines, the heat input causes the pressure to rise because the gas is in a confined volume.  In Brayton cycle engines, the gas is not confined to a fixed volume, so the heat input causes the gas volume to increase.  Net work is produced because more work is produced in the expander than is required by the compressor.

Issue 2: Blow-by

Blow-by is our biggest challenge.  We are addressing it using a variety of tricks such as conformal coatings, labrythian seals, and high speeds.  Our testing protocol is designed to verify that we can reduce leakage to acceptable amounts.  It should be noted that jet engines have blow-by past the blades, but their performance is acceptable.  Because our rotor components do not touch, we eliminate friction and wear, but replace it with some blow-by. Our calculations indicate that blow-by losses should be minor, but we have not proven that yet.

Issue 3: Lubrication

We lubricate our engine in regions that are not subjected to high temperatures.

Issue 4: Low expansion ratios

With a recuperated Brayton cycle, high expansion ratios are not necessary to obtain high efficiencies.

Issue 5: Expander is larger than compressor

As described in Issue 1, the Brayton cycle relies on the expansion of gas volume.  The expander is about 3 times larger than the compressor.

Issue 6: Efficiency at part throttle

Axial flow turbines have poor performance when throttled.  They are well know for being efficient only over a narrow speed range.  This is because the blades are designed for a particular speed and gas density... deviations from the design condition are not efficient.  This is a characteristic of all such "dynamic" devices.  In contrast, the StarRotor engine is a positive-displacement device.  It will operate efficiently over a much wider range of speeds.  Because it is inefficient, we do not plan to throttle the engine to vary power output.  Instead, we will vary the compression ratio of the engine, which is much more efficient because it eliminates the irreversibility of the throttle.

Issue 7: Combustion products

It is true that combustion products could potentially coat the expander and heat exchanger.  To eliminate this problem, we plan to use a tubular combustor.  With over 20 years of experiments by Stuart Churchill at the University of Pennsylvania, he has never detected unburned hydrocarbons exiting his tubular combustor.  All the fuel must pass through the flame front, so it is completely combusted.

Issue 8: Claimed efficiencies of 80%

We have never claimed engine efficiencies of 80%.  We do claim that the compressor efficiency could be 80%.  Solar Turbines claims that some of their compressors exceed 85% efficiency, so we do not think our claim is unwarranted.


Issue 9: Unreasonable efficiencies

We have done extensive modeling of our recuperated Brayton cycle engine.  Numerous engineers have checked our calculations, and have invested in our company.  Our high efficiency claims result because we keep the compressor nearly isothermal by spraying atomized liquid water into the compressor, keeping it cool and reducing the amount of parasitic shaft work it consumes.  Water spray is a well-known trick with Brayton cycle engines.  It is rarely used with axial blades because the liquid water can pit the blades due to high-speed impact.  Our speeds are much lower, so we think we can tolerate liquid water in our system.

Issue 10: Inertia

Likely for automotive applications, we will hybridize our engine with an electric motor.  This offers a number of benefits.  For example, the automobile will start immediately so the driver does not have to wait for the combustor to heat up.  Also, the engine can be very small so it will have a low inertia.  For truck applications, rapid start and low inertia are not as important.

Issue 11: Water consumption

At full power, the engine uses 2 gallons of water for every gallon of fuel.  However, automobiles are rarely operated at full power.  Cruising down the highway takes only about 15 hp. At this power output, we need a compression ratio of only about 1.5 (not 6).  At this low compression ratio, the compressor temperature does not increase significantly so water spray is not needed.  Likely, water spray will be used on stationary applications (e.g., distributed power), which are running at full compression.  Obviously, it will be easy to supply water to a stationary application to get the extra efficiency.

Issue 12: Heat exchanger cost

Likely, in early production, the heat exchanger will be expensive because it is will be produced in low production volumes.  However, when mass produced, there is no reason to believe they will be prohibitively expensive...they are made of folded sheet metal.  High exhaust temperatures will require ceramic heat exhangers, which will be more expensive.  These would be used only where high efficiency is really important.

Issue 13: Gas turbines are not efficient

It is true that gas turbines are not very efficient, particulary when no recuperator is employed.  However, with a recuperator and water spray into the compressor, the cycle approaches the Ericsson cycle, which has the same efficiency as the Carnot cycle (the highest nature allows for a heat engine).

Issue 14: High temperature of expander

We plan to make the expander of ceramic so it can tolerate high temperatures.  Because our speeds are much lower than axial turbines, ceramics can be used in our application without flying apart due to centrifugal forces.  Minimal cooling will be required because ceramics can withstand high temperatures.

Issue 15: Check valves

The StarRotor engine requires no check valves.  When the compressor and expander are sized properly, there is no need for them.

Issue 16: Uneven heating

Unlike the Wankel engine, which does have uneven heating and resulting distortion, the StarRotor rotors see an average temperature of the inlet and outlet.  They are constantly spinning which averages the temperature variations.

Final Point:

We are still in development and do not claim to have all the problems solved.  However, we have thought through the StarRotor engine very carefully and believe that we will be able to solve every problem that confronts us along the way.  We have an outstanding engineering team that has solved every problem so far.  We expect to continue that tradition.

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