The Star Rotor motor 2

[SBBlue]

Okay, 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?

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

 

[mackerm]

I agree that the Star Rotor has obvious advantages over scroll compressors including the placement of ports. Which got me to wondering: why are scrolls preferred to gerotors for gas compression? They're mechanically and conceptually simpler.

The best I could figure is that there is more gas leakage between the inner gerotor's "point" and the outer gerotor's curved face than between the two curved faces in a scroll compressor. Similar to blowing air through a short constriction vs. a long tube of the same diameter.

One other class of compressors I forgot about is "claw compressors."

http://www.rietschlepumps.com/products/operating.asp

These are similar to Roots blowers, but the lobes actually pinch off and compress volumes of gas internally.

 

[mackerm]

Sorry. I'll try again for claw compressors.

http://www.rietschlepumps.com/products/operating.asp

http://www.rietschlepumps.com/products/operating.asp

 

[StarRotorMan]

Mackerm,

I learn something every day. This is the first I've heard of a claw compressor.

The efficiency, capacity, and pressure ratio is low for our application, but is surely is interesting.

StarRotorMan

 

[StarRotorMan]

SSBlue,

33% ain't bad for a part-load engine. The 40-50% efficiency is for a wide-open diesel. A part-load diesel engine is about 20% efficient.

If we find that the part-load StarRotor engine is insufficiently efficient, then we can always hybridize it for autmotive applications. We are considering doing that anyway because the heat-up time for the StarRotor engine may be inconveniently long for the average driver.

StarRotorMan

 

[GraviMan]

Been thinking a little more (I know - dangerous habit). Are there any thermodynamic packages available to simulate this and other engine cycles? Haven't really done any calcs since uni, other than a couple of Excel spreadsheets...

What I am wondering is can the compressor and exhaust heat-ex be combined? I am thinking that if the compressor was broken down into stages (hypothetically), and the exhaust heated up the inlet air in stages that would be more efficient. This is a fantastic concept, and I can't help but want "have a play".

Will you be optimising the design for cost or mass? You have already thought out a number of applications, so I guess this will determine design.

Mart

 

[SBBlue]

StarRotorMan wrote:

"33% ain't bad for a part-load engine. The 40-50% efficiency is for a wide-open diesel. A part-load diesel engine is about 20% efficient."

Could I ask you for a reference on that?

The figures I am familiar with are considerably higher than that. For example, let me cite a paper from the Diesel Engine Research Center at the University of Wisconsin entitled "The Influence of Boost Pressure on Emissions and Fuel Consumption of a Heavy-Duty Single-Cylinder Direct Injection Diesel Engine" (SAE Technical Paper 1999-01-0840)

The single cylinder engine they use is essentially one-sixth of a Caterpillar 3406 diesel engine -- a very common but little bit dated engine that develops about 500 hp. The various operating conditions that were analyzed were those adapted by the EPA for engine emission testing; the so-called "six mode FTP" conditions. They are as follows: 1) Idle -- 700 rpm, 0% load, 2) 821 RPM, 25% load, 3) 993 rpm, 75% load, 4) 1672 rpm, 95% load, 5) 1737 rpm, 57% load, and 6) 1789 rpm, 20% load.

Neglecting the efficieny at mode #1, which by definition has to be zero, the efficiencies reported were as follows:

Mode 2: 25% load, 35.63%

Mode 3: 75% load, 38.09%

Mode 4: 87% load, 33.8%

Mode 5: 57% load, 32.11%

Mode 6: 20% load, 29.38%

Note that these were "baseline" conditions before any attempts were made to maximize efficiencies by varying boost pressures.

I also might point out that if you just wanted to do a thermodynamic analysis on the diesel engine cycle, assuming an adiabatic engine without friction (which we assumed to get an efficiency of 33% for the StarRotor system), the engine efficiency will be quite impressive -- somewhere around 60%.

The numbers I cite are actual, measured engine efficiencies on a commercially avaiable engine -- at both full load and part load conditions. They are considerably better than the numbers that you claim for diesel engine efficiencies at part load -- 20%.

So who I am to believe?

 

[StarRotorMan]

Graviman,

It is more efficient to keep the compression as cool as possible because compression work is proportional to inlet temperature.

We do not design our engines using canned thermodynamic packages. I have a software package that gives the thermodynamic properties of gases as a function of temperature and pressure. It has proven useful when doing engine designs.

Regarding optimizing for mass or cost, it depends upon the application. For the military or aircraft, mass is the key.

StarRotorMan

 

[StarRotorMan]

SBBlue,

The StarRotor website shows data for the efficiency of a diesel engine as a function of load. You can find it at the following address:

http://www.starrotor.com/3-slides/3-3-03.htm.

This slide was pulled from the web; the original source is cited at the bottom.

When I cited the 20% efficiency in my earlier message, I was assuming the engine was operating at 10% load, which is suitable for cruising down the highway.

You describe data for a 500-hp diesel engine, which is a fairly large engine. Also, you were describing part-load data for 20% and above.

To make valid comparisons, it is necessary to define the engine size, and what is meant by "part load." I was assuming a smaller engine suitable for an automobile and defining "part load" at 10% of full load, which would be suitable for cruising down the highway.

You are concerned that the StarRotor engine may not be efficient at part-load conditions. There are a number of strategies for using a StarRotor engine under part-load conditions:

Stategy 1: Hybridize

In this stategy, a smaller StarRotor engine would be used. When it is operating, it will operate at closer to wide-open conditions. Peaking would come from the electrical system. This strategy is used with conventional internal combustion engines as well.

Stategy 2: Run constant speed, adjust compression ratio

In this strategy, a 130-hp engine would operate at a 6:1 compression ratio. To get 15 hp for cruising, the compression ratio would be reduced to about 1.5:1.

Stategy 3: Reduce speed, adjust compression ratio

If both the engine speed and the compression ratio are reduced, then it is not necessary to reduce the compression ratio as much. For example, if the engine speed is reduced to 25% of maximum, the compression ratio only needs to be dropped to about 3:1 to get 15 hp.

We are still early in our development, so we have not selected the final stategy.

I hope this discussion is helpful and clarifies some of the issues.

StarRotorMan