Rotary Screw Engine?

If I'm reading you right, I think Frank Whittle got there before you.

Great stuff John,

Thanks for the link.

Hi John,

No, I don't think this relates to a jet engine although I can see why you would think so. This would be a nearer relative of the Wankel engine I think, as the expanding gasses would be pushing the rotors around in a positive displacement mode. The jet engine and its near relative the turbo-prop must run at high speeds whereas a screw compressor (if we ignore the poor sealing of the meshing rotors) can ran as slow as you like.

In theory it would be possible to make just one pair of rotors do the job with a left spiral on one half of the rotors and a right spiral on the other halfs. I think however this would be less likely to run than a separate compressor and "motor".

Kind Regards

Chas

"need one non-return valve", a NRV, where exactly?

In a scenario of compressor to expander (via a explicit combustion chamber or built-in expander), the pressure has to be highest at compressor discharge. Then only the flow could take place through the expander and die out to atmospheric. So what is need for NRV?

Hi Yesyen,

I had envisaged that this would need an air receiver between the compressor and the combustion chamber. Thus when the inlet port and the combustion chamber coincide the compressed air would enter, however once combustion takes place then the pressure would blow back into the receiver without a NRV. The alternative would be to delay combustion until the inlet port and the chamber were no longer in coincidence which would be the same as having ignition occur when the piston was already on the down stroke.

I like your site, very good graphics.

Regards, Chas.

The one "none-return valve" is a big problem. How would you accomplish that? First you need to solve that.

Secondly, transferring from one "chamber" to a second chamber never really works well at all. To many losses.

Third, engine efficiency is the product of three efficiencies. A) the thermal efficiency strictly based on the compression ratio. Higher CR increases efficiency. B) Heat losses that are mainly a factor of the heated surface area relative to the volume of the combustion gases, the area should be a small as possible and C) the mechanical efficiency due to friction losses.

A) What maximum compression ration can you achieve? Can you get 10:1 or 12:1? B) the heat losses will be really really bad with such a proposal due the extreme large surface area in contact with the hot gases, you must calculate the total circumference of both screws, C) regarding friction you may have a winner as friction can be low. Unfortunately, the friction losses are the smallest losses in an ICE.

To show the effect of a large surface area is best shown by the Wankel engine which is easily 2 to 2.3 times as much as in a reciprocating engine, hence the low fuel efficiency due to heat losses. Your proposed screw pumps will be far far worse.

Most of all, the none-treturn valve will be the killer. How do you propose it to work?

Hi Floram,

Thanks for the tech info, it is the sort of thing I was looking for.

In answer to your points:-

1. The valve could be a simple flap type like that in a piston compressor or a valve opened and closed by a cam.

2. I don't understand why a two chamber system should be have more losses as turbochargers and blowers are effectively pre-compression chambers. Effectively this idea is pre-charging to full pressure.

3.

a. 10 bar (thus 10:1) is a normal pressure for a screw compressor

b. the heated area should be, I suppose, not the entire circumference of the rotor but only that of the helical combustion chamber. If the heated area in an ICE is so important would that not lead manufacturers to build motors with large single cylinders rather than multi cylinder configurations?

c. The friction issue is something I haven't considered as I have no way of assessing what they would be.

Regards, Chas.

To 1. Ok, a poppet valve will close off nicely lets the compressed air in. But how much? The flow has to be intermittent by this method.

This leads to point 2. If you compress 10:1 the volume is small and in order get it all into the combustion side there should be next to zero volume left in the compressor. In other words, you need likely more that 100:1 or higher depending on the space left in the compressor at "TDC". That is what I mean by the transfer.

To 3. Both, area and shape play a roll here. Both are very poor with regards to the flame front and burning all the fuel as well as the heat losses as I had said.

Yes, a single cylinder with a relative small ratio of surface area will have a lower fuel consumption. But many other factors come into play. Engine speed is low, hence low power density and a very heavy engine that eats up any lower fuel consumption in a mobile installation. Another point is smoothness of running or rather lack thereof. You would not want a garden tractor engine in your sports car, I suppose.

By no means do I want you to stop trying. But in the end you likely will find it not be very efficient. For me, getting the high pressure into the combustion side is the key problem. In the RPE the charged air is being compressed further in the cylinder, something you would not have in the proposed set up.

Regards

There was an old single cylinder tractor made, long ago, when I was small, which was apparently quite efficient despite fairly low compression ratio.

It was very big and heavy for it's output and the flywheel was unbelievably large.

Was funny to watch it run, because every time it fired it would blow a smoke ring out of the stack.

In answer to your question about single vs multi cylinder engines, as someone else said, power density, smoothness, weight and cost all go against the single cylinder.

That description sounds a little like a supercharger. The device has applications on piston-engined aircraft and some land vehicle engines.

By-and-large the supercharger has given way to the turbocharger in land vehicles of late.

check out this patent

http://www.google.com/patents/US5605124

Hi next30,

Interesting that such a patent actually already exists. Good you found it. It was issued Feb. 25 1997, filed Nov. 6 1995 that is 10 years before our discussions.

In my thinking I thought of the compressor screw and expander screw to be axially in line and right next to each other. The patent has them separate using a gear pump in between that functions as that non-return valve as proposed. A sketch at the time would have provided feed back as to how such a valve could be constructed and how it could function. We don't really know what Chas had in mind.

Anyway, we will see if it is any good by it appearing on the market. 17 years have gone by and there are only 3 years life left. Perhaps a manufacturer is waiting for its expiry and then goes to market? Just a thought.

Hi Floram,

I think you have misinterpreted his drawing. What he has there is a two stage compressor to boost the pressure before the expansion , and yes I was also envisioning a straight in line setup rather than a belt, chain or gear coupled system.

This little thing has been on my mind for > 30 years, since I was a young sales engineer on these compressors.

Thanks Next 30 for finding that patent. I wonder if the patent holder actually got one running?

In theory at least it should be vibration free (or virtually so) and quiet since the compression ration should be low.

Regards

Chas