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Old 09-08-2011, 03:40 AM   #11 (permalink)
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Quote:
Originally Posted by jamesqf View Post
Thanks! That works better.

Quote:
Originally Posted by jamesqf View Post
So your exhaust pipe between the exhaust valves and the turbo becomes a pressure vessel. What does the increased back pressure do to the engine?
It extends the engine working gas expansion ratio from 13 to 1 to closer to 14/15 to 1. The turbine has become another part of the engine cycle connected by the exhaust-pipe pressure vessel. In fact, the turbine replaces the muffler, reducing back pressure from that source.

My expectation is:
  • engine off hybrid mode - no effect
  • normal cruise mode - use of water injection to improve mass flow and kinetic energy to turbine proportional to available engine gas flow
  • high power, acceleration and climbing - water injection off as the turbine spins on just the exhaust gas
So let's go back to what we know of this vehicle, the thermodynamic model:

In normal cruise modes, 30-65 mph, the engine power needed is 6.7-29.5 kW. But the gas flow is too low to fully load the turbine unless we use water injection to increase the mass-flow rate.

At peak power, accelerating, climbing or ~100 mph, the 83 kW gas flow is high enough to load the turbine but exceeds the ability of the traction battery to absorb the 27.5-47.6 kW potentially generated. So the power draw is limited to the generator limit, the turbine is unloaded and exhaust pressure reduced.

Near as I can tell, the turbine exhaust ratio is roughly 2:1(*). So instead of 1 bar back pressure, a completely perfect, no loss of energy since the crank case is at 1 bar to push out the exhaust gas. This means the piston now has to do more work, exactly 2 bar cylinder pressure minus 1 bar crank case resistance . . . 1 bar extra pressure on the piston during the exhaust stroke. So let's do the math:
  • 75 mm - bore -> 0.0044 square meters
  • 101.3 kPa - 2:1 bar pressure difference -> 447.5 newtons
  • 84.7 mm - stroke -> 0.0847 meter
  • Joules to expel gas with 2:1 cylinder-to-case pressure -> 37.9 J
  • Max rpm, 4,500 rpm -> 75 revs/sec
  • 1/4th rpm is exhaust stroke -> 243.7 W at full power, fully loaded 2:1 turbine, overhead on engine
So to get potentially 27.9-47.6 kW at full power, we've increased the engine load by 243.7 W. Of course we're going to limit the turbo-alternator to 20,000 W and in real life, there where will be other losses. At lower speeds, the water injection will due to lower rpm, ~2,250 rpm, cost ~122 W to generate 2,220 to 3,840 W. I can live with these trade-offs.

The boundary of the engine has expanded as it now includes the turbo-alternator. There will be an effect on the piston engine but the engine boundary is larger than it was before. So we are gaining mechanical power that in past was lost in the excessive pressure drop when the valves opened. The turbo-alternator has become a less-bad muffler.

Bob Wilson

* - I am not sure what the turbo pressure ratio will be for any given power output. The turbo charger vendors are not terribly forthcoming with the turbine maps but one can easily choose whatever ratio one thinks is practical and use it as a multiplier to calculate the back pressure impact.

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Last edited by bwilson4web; 09-08-2011 at 06:39 AM..
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Old 09-15-2011, 02:35 PM   #12 (permalink)
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You could go with a smaller turbo that will spin at lower load and use an electric exhaust cut out to bypass the turbo when pressure or generated power gets to high.
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Old 09-15-2011, 05:30 PM   #13 (permalink)
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bwilson4web,

I thought I would inform you that your turbine maps are actually compressor maps. Finding good turbine maps is like finding hens teeth.

A turbine's power output also depends on the square root of the gas temperature (in Kelvin) flowing into it. So wrap your boiler section with insulation tape and warm the water in your water tank with an engine coolant loop.

Keep in mind that turbos spin very very fast. This is going to give you trouble mechanically and electrically. The turbine and compressor wheels are balanced as a unit operating one by itself or just separating the two is enough to warrant rebalancing if you expect a long service life. Keeping a home made rotor together is going to be a challenge. Balancing it is going to be even tougher. On the electrical side the inductive reactance of the coils will work against you at some point. This will progressively limit your output at higher rpms.

Your "oversized" turbine might be better suited to this project to keep the rpms down
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Old 09-15-2011, 07:21 PM   #14 (permalink)
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Quote:
Originally Posted by Phantom View Post
You could go with a smaller turbo that will spin at lower load and use an electric exhaust cut out to bypass the turbo when pressure or generated power gets to high.
I found a nice motorcycle turbocharger which I may go with after testing with the current one. The one I'm looking at is just under $300.

Bob Wilson
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Old 09-15-2011, 07:53 PM   #15 (permalink)
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Quote:
Originally Posted by ConnClark View Post
I thought I would inform you that your turbine maps are actually compressor
maps. Finding good turbine maps is like finding hens teeth.
. . . Keep in mind that turbos spin very very fast.
That was something the compressor map provided as well as an efficiency map of pressure ratios and speeds. The maps are not perfect but they are better than 'cursing the darkness.' <grins>

I have enough materials and resources to work through a lot of the problems and get some initial metrics. I have yet to find any prior art or a decent model to work from . . . which is reason enough to do the experiment.

Bob Wilson
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Old 09-19-2011, 04:16 PM   #16 (permalink)
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found something that may interest you

Rotary Steam Engines: Page 9
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Old 09-19-2011, 08:46 PM   #17 (permalink)
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Quote:
Originally Posted by ConnClark View Post
found something that may interest you

Rotary Steam Engines: Page 9
Thanks,

That link led to a number of interesting articles after the same goal but achieved by other means. Please understand I don't fault the earlier efforts for I believe we are all after the same goal. But the absence of their adoption suggests . . . I want to try another approach.

I have absolutely no idea if the approach I've proposed will work but I am curious to find out.

Bob Wilson
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Old 10-03-2011, 05:31 PM   #18 (permalink)
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bwilson4web,

I thought you might want to look at these links

Infinity Turbine - Organic Rankine Cycle Model ITxr Kit

or this

ITmini Experimenters Kit

You can't bolt this onto the exhaust but you can create a closed system loop system using refrigerant. You might also be able to tap heat from the radiator.

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