Intake vacuum energy recovery

[serialk11r]

Originally Posted by Bicycle Bob

I think that the easiest way to test this is with an Eaton electric roots supercharger. It has about the right pressure differential range, but might want some high-vac oil. It could be rigged to go from idle vacuum right into boost if you wanted to, either producing or drawing power. However, doing that would make a variable compression ratio even more desirable.

A Roots blower is very inefficient in principle because it directly mixes atmospheric and high pressure air before compressing. In certain speed ranges the exhaust port geometry can be tuned to basically use the "wave drag" of the rotor to produce adiabatic compression. When you run it in reverse, this feature actually works against you.

Variable compression ratio is far more effective than controlling throttling losses past some point. At idle, bleeding off 80% of the air from the cylinder with an effective static compression ratio of 10:1, the effective compression ratio is only 2:1. If you could have a 25:1 compression ratio at idle (effectively 5:1 in this example), thermal efficiency would go WAY up.

[Bicycle Bob]

Originally Posted by serialk11r

A Roots blower is very inefficient in principle because it directly mixes atmospheric and high pressure air before compressing. In certain speed ranges the exhaust port geometry can be tuned to basically use the "wave drag" of the rotor to produce adiabatic compression. When you run it in reverse, this feature actually works against you.

Variable compression ratio is far more effective than controlling throttling losses past some point. At idle, bleeding off 80% of the air from the cylinder with an effective static compression ratio of 10:1, the effective compression ratio is only 2:1. If you could have a 25:1 compression ratio at idle (effectively 5:1 in this example), thermal efficiency would go WAY up.

Yes, the Roots is a bit of a blunt instrument, but it is available on the shelf. A purpose built unit might use two vane pumps in series to keep the seals in their preferred range. With those, you could vary the eccentricity to get a continuous adjustment of the vacuum at a constant speed.

I don't understand your second paragraph at all.

[serialk11r]

Originally Posted by Bicycle Bob

I don't understand your second paragraph at all.

Variable compression ratio is more effective for increasing low load efficiency than throttling loss reduction (well, they work in tandem, but without high compression and high combustion temperature you cannot get high efficiency).

[Bicycle Bob]

Originally Posted by serialk11r

Variable compression ratio is more effective for increasing low load efficiency than throttling loss reduction (well, they work in tandem, but without high compression and high combustion temperature you cannot get high efficiency).

Well, yes. I was still thinking of using the throttle to control power. The variable compression is to create the same charge density at ignition no matter what the charge density is at the beginning of compression.

[serialk11r]

A variable displacement vane pump is actually a great idea now that I thought about it a little more, if lubrication and sealing are not an issue, since you can have internal compression/expansion with it. I imagine lubrication and sealing are issues in practice, but this is an interesting idea!

It could be a supercharger/energy recovery device with instant response mechanical control all in one.

I found these:
http://hpwizard.com/vane-supercharger.html
https://en.wikipedia.org/wiki/Powerplus_supercharger

I feel like in this day and age, it should be possible to implement this with carbon fiber parts and DLC + state of the art dry lubricant like BAM. The vanes' sealing force against the wall can perhaps be controlled with a pneumatic pressure + spring combo (the way I'm imagining this is you need to counter the centrifugal forces as the speed increases, so you use very lightweight vanes, a helper spring to provide pressure at low speed, and then a vacuum pump fighting the springs to reduce pressure as it speeds up or something).

Since the mechanical design is really simple you should even be able to do it at home, assuming you can get PVD coating services.

All that said, I still prefer the simplicity of external EGR, but if you're already strapping a supercharger onto an engine, this feels like it could be a very fun but also feasible DIY project for the neo-steampunk enthusiast.

[serialk11r]

Sorry okay I'm thinking about it more, so the vane pump has a displacement, and a compression ratio. You need the displacement to always be positive or else the pump will attempt to pump air away from the engine, consuming power.

What needs to vary is the inlet location. For maximum compression, you want to let air in where the vanes are furthest from the center of the rotor, close off the inlet where maximum inter-vane volume occurs, and open the exhaust near minimum chamber volume. To make it into a turbine with maximum expansion ratio, you want the exhaust port open from maximum volume all the way until the volume is nearly zero, then open the inlet briefly.

This is not easy to accomplish mechanically, since you have to change the port shape :/ You can change it in discrete steps but not continuously, so if you want to flip it from compressor to turbine mode without moving the rotor or housing around you can have valves to swap flow between intake and exhaust ports then open up a bypass built into the housing surface to make the displacement negative. That creates some fabrication challenges.

[Bicycle Bob]

I wouldn't worry about approaching the maximum theoretical efficiency on the first prototype. As the OP notes, we have a pipe that is now wasting power with a restriction where there could be an air motor, so it's all free.
BTW, I think there is also great potential for this in refrigeration. Using an orifice to restrict flow heats the refrigerant through turbulence as well as wasting the mechanical energy.

[serialk11r]

Off on a tangent, found that an Auburn U professor made a vane pump engine:
https://contest.techbriefs.com/2017/...portation/8456

Anyways back to the air motor, if you have fixed ports, there's basically a tradeoff between how well it works as a supercharger and how well it works as an air motor. If you use it as an air motor with internal expansion, your engine will lose efficiency at high load because the air motor has been turned into a useless vacuum pump. If it's used as a compressor, you lose efficiency at low load because it's needlessly compressing air.

If it's used like a hydraulic pump with no internal compression at all, it's basically like a Roots blower, with the advantage that variable displacement is easier to achieve.

I worked out the efficiency before at some point, using a Roots blower as an air motor is something like 40% efficient before mechanical losses and leakage. That's a bit better than the <10% I worked out for ITBs.

[Bicycle Bob]

Originally Posted by serialk11r

Sorry okay I'm thinking about it more, so the vane pump has a displacement, and a compression ratio. You need the displacement to always be positive or else the pump will attempt to pump air away from the engine, consuming power.

What needs to vary is the inlet location. For maximum compression, you want to let air in where the vanes are furthest from the center of the rotor, close off the inlet where maximum inter-vane volume occurs, and open the exhaust near minimum chamber volume. To make it into a turbine with maximum expansion ratio, you want the exhaust port open from maximum volume all the way until the volume is nearly zero, then open the inlet briefly.

This is not easy to accomplish mechanically, since you have to change the port shape :/ You can change it in discrete steps but not continuously, so if you want to flip it from compressor to turbine mode without moving the rotor or housing around you can have valves to swap flow between intake and exhaust ports then open up a bypass built into the housing surface to make the displacement negative. That creates some fabrication challenges.

Thanks for the bump, chunmin 78. I just sketched out a vane pump that looks like it might do an elegant transition from pump to motor. Start with a regular round housing, and a round hub to house the sliding vanes. The ports can take up most of the compression and expansion arcs. Now, imagine this housing with the manifolds sliding between the side plates of the pump, which are carrying the axle of the hub with them. All you have to do is to move the center of rotation from side to side and you go from max compression, to neutral, to max expansion.

[freebeard]

I'm still trying figure chumnin 78 out. https://ecomodder.com/forum/search.php?searchid=4563821

21 post in 22 minutes as of this post. Very polite, inquiring but with an authoritarian agenda. My working supposition is an instance of GTP-3.