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Old 09-04-2011, 10:54 PM   #1 (permalink)
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A project progress report

BACKGROUND

Traditional engine thermodynamics report that 1/3 of the fuel combustion energy is delivered as mechanical, 1/3 the energy goes out the exhaust, and 1/3 in engine cooling. In the past, there have been several efforts to recover some of the exhaust energy such as a German effort to use a closed steam engine and several attempts at thermocouple generators. None has resulted in a practical automotive product but it is so tempting.

What we are describing is called a 'topping cycle' which has been used in power plants and large cargo ships for decades but not yet found in cars.

THERMAL ANALYSIS

The first step is to understand the energy source:

Part of the fuel chemical energy is released in the catalytic converter where the last of the hydrocarbons are oxidized with the nitrogen oxides. But the bulk of the energy comes from the hot exhaust gas that glows the exhaust manifold a dull red. This is the hot-side of the engine.

What we propose to do is inject water just behind the catalytic converter to increase the pressure and kinetic energy of the exhaust stream to turn a turbine-alternator. An open-loop, steam engine, the exhaust heat is converted to kinetic energy captured by the turbo-alternator where the exhaust temperature is over 100C to avoid condensation in the turbine.

The maximum Carnot efficiency is a function of the hot and cold temperatures and in this case could range from 33-57%. In real life, we won't achieve this efficiency but it represents the maximum. But this is a fraction of the assumed, 1/3 heat available in the exhaust.

We know the Prius power required as a function of speed so taking 1/3d as the starting point we can estimate the best case, power that might be achieved which ranges from a low of 2.2kW at 30 mph to up to 16.9 kW at 65 mph. Since our plan is to dump the extra power into the Prius traction battery and its power output is limited to 20 kW, 20 kW is the maximum practical power output of the turbo alternator.

Back-pressure loss vs turbo-alternator generation

There is a risk that the increased back-pressure losses would subtract as much power from the engine as is generated by the turbo-alternator. However, the turbine spins so fast that given equal pressure, the distance covered more than makes up for any loss of piston power on the exhaust stroke:

Or at least that is my understanding. Fortunately, we have further information from the turbine pressure-mass charts.

Turbine performance is mapped using the mass-flow versus the pressure ratio:

Sad to say, the total exhaust mass-flow including injected water is unlikely to reach mass-flow rates for normal turbine operation. Worse, my first turbine, a T04E-50, is sadly oversized for this experiment. Still, it may provide enough energy to test other aspects of the proposed design.

The T3-40 comes closer but we may have to look at much smaller turbines, perhaps those found in radio controlled, model aircraft. Fortunately, the Rankine cycle may allow less refractory turbine materials to be used.

TURBINE EXPERIMENTS

Several years ago, I bought an Ebay turbo charger to learn about them:

I could not find credible sources about turbo-charger engineering and mechanics. But I could buy one, take it apart and gain insights.

So one of the first steps is to understand the dimensions:

Yes, it would have been easier if mechanical drawings and technical specifications were available. But lacking them, buy one and 'blueprint' it.

So what I need is a high-speed, 16-20 kW generator, about 3" inches (76 mm) diameter and 1.5" (38 mm) in length. This is a challenge.

ALTERNATOR EXPERIMENTS

My first thought was to go with a high-speed, homopolar generator:

My thinking was the mechanically simple rotor would be easier to design and build than one built with a composite, magnetic rotor and external stators. So I bought a 3" diameter with 1" hole magnet to test to find out if this was practical:


According to the magnet vendor, the surface Gauss is 13,600 so using standard homopolar formula, the model looked like:

Sad to say, the expected voltages were too low and currents much too high for practical electrical circuits. But that is theory, how does the real part work?

So I decided to test the magnet at a slower speed to see what sort of voltages were generated in the nickel plating that is as close to the magnetic material as possible:

Any air-gap will reduce a magnetic field and I needed a metric.

The results were disappointing:

Based upon the generator voltages when spinning in the drill press, the surface, magnetic field is 0.477 T or 4,770 Gauss. Way to weak to generate useful voltages and power. But insights came from this effort.

A magnet has two poles so use both:

Two disks can be wired in series and double the voltage. But there is another problem.

The generated voltage is a function of the Lorenz force, the cross-product of the electron mechanical velocity and magnetic field. The fastest part of the disk is the rim so a better homopolar design is a rapidly rotating cylinder:

But a rapidly rotating cylinder needs high-speed, peripheral bearings and that led to a short-cut.

Ducted Fan, Brushless Motors

The RC community has ducted-fan, electric motors used in model jets. Some of these are rate up to 80,000 rpm and 2 kW. So I found a reasonably priced one and have ordered it for testing:

At first I was looking at three, high frequency, 10:1 transformers to step-up the output that can be boosted to traction battery voltages. But then I realized stepping the voltage up to traction battery voltages should well be within a switching power supply capability.

I may return to the high-speed, homopolar generator after rapid prototyping with this off-the-shelf motor. One fundamental question is whether we can drive the model motor enough to get its maximum, rated power out. There is no need to pursue an advanced, high-speed, homopolar motor if ordinary turbines can not even meet the 1.4 kW motor rating.

STATUS

I have the turbine but need to mount it and rig up an electric oil lubrication system. This is just plumbing and an electric oil pump . . . not a problem.

The turbine compressor shaft has to be trimmed down, a pre-stress nut configured, a shaft coupler, and motor mount fabricated. I have the aluminum stock and access to tools need to complete this build.

I've ordered the Schottky diodes needed to build the first stage, capacitor charger. My junk box has magnetics and I have an MSP-430 that should be able to drive a suitable IBGT/MOSFET for the boost converter.

By building a series of small steps, I should be able to bench test the electronics and build a suitable 'brass board.' Then mounted with the turbine and coupled to the normal exhaust, get metrics to see how practical this approach will be.

If testing shows enough promise, get a properly sized turbine, install the water injection system, and build the control electronics. Then I can proceed to system level testing and learn if there is enough recoverable exhaust heat to augment Prius performance. Right now, I don't know but I hope to find out.

Bob Wilson

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Last edited by bwilson4web; 09-05-2011 at 09:10 AM..
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Old 09-04-2011, 11:55 PM   #2 (permalink)
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...somebody has been doing his homework.
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Old 09-05-2011, 12:22 AM   #3 (permalink)
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How much water do you need to carry?
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Old 09-05-2011, 12:42 AM   #4 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
How much water do you need to carry?
Initially open-loop so a 5 gallon container should do for testing. There is no assurance it will actually work and deliver a measurable performance gain. In the off chance that it actually works, there is a lot of space in the rear hidden under the bumper cover:


My thinking is take the bumper cover off and use thin-walled, folded aluminum piping, This should provide enough area to condense a significant part of the injected and combustion water. A small pump picks up the condensate and feeds it to the water tank.

Bob Wilson
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Old 09-05-2011, 01:15 AM   #5 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...somebody has been doing his homework.
Thanks but feel free to double check my work. I've been known to make a mistake from time-to-time . . . just ask my wife. <grins>

I forgot to include that the NHW11 Prius electrical load measures ~450 W. If I can't get at least this amount, I will consider the project impractical. 'Orange4Boy' and his Hill Billy, 12 V, plug-in has already given some measure of the effect of off-loading the 12 V load.

BTW, I understand GM's "eco-mode," their current BAS system, uses a 15 kW, modified motor-generator (redesigned alternator.) I had briefly considered but rejected trying to adapt an auto alternator. Most of the specs I've found show a maximum rpm in the 7,500 rpm range. It may be possible to use just the stator and build out a custom rotor and high-speed bearings. But the large diameter rotor limits how fast practical materials can spin and increases the moment of inertia which means very slow response to turbine power. But I may revisit this depending upon how the initial testing comes out.

Bob Wilson
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Old 09-05-2011, 05:17 PM   #6 (permalink)
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...look for one of the "new" GM alternator-generator e·Assist units at a junkyard.

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Old 09-06-2011, 11:54 AM   #7 (permalink)
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It's been done. See here: Green Car Congress: TIGERS: Exhaust Gas to Electricity for Reductions in Fuel Consumption

I'd really question the need for water injection. After all, the turbo is designed to extract power from the hot exhaust gas stream.
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Old 09-06-2011, 12:36 PM   #8 (permalink)
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...has anyone (successfully) tested a thermo-pile for converting exhaust HEAT directly into ELECTRICITY?

...maybe integrate the thermo-pile 'into' the exhaust manifold/header?
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Old 09-07-2011, 03:34 AM   #9 (permalink)
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Quote:
Originally Posted by jamesqf View Post
I'm not having much luck with that link.

Quote:
Originally Posted by jamesqf View Post
I'd really question the need for water injection. After all, the turbo is designed to extract power from the hot exhaust gas stream.
The installation location was inspired by the Squires Turbo System but we have different goals. They want to stuff more fuel and air into the engine and I want to use the residual exhaust stream energy to generate electricity. However, I agree with many of their claims about a rear-mounted turbine:
  • lower turbine inlet - more so with water injection to further convert waste heat into more exhaust stream pressure.
  • turbine outlet closer to lowest possible outlet pressure - to improve the pressure ratio and energy extraction.
  • simpler plumbing - since I'm converting the turbine power to electric power near the traction battery, no compressed gas piping to the engine. It will be much easier to work on as the muffler volume and location is easily reached.

By injecting water, I'm making the exhaust pipe into a steam boiler. Then the turbine can extract the additonal mechanical energy from the enhanced exhaust stream. But this is why we test, to find out if it is necessary.

In a normal turbo installation, significant turbine power occurs when the engine is operated in high-power regions. But our Prius typically operates in low power regions with more modest gas flows and heat. Water injection allows this lower flow rate, lower energy stream to get a kinetic boost to generate hopefully a significant power boost that we add to the traction battery.

We really won't know the true metrics until the system is integrated. Obviously when in hybrid mode and the engine is off, there won't be any turbine power extracted. My expectation is in normal urban traffic with more modest exhaust-gas and heat flow, water injection will extract usable energy. Then when accelerating or climbing a hill, water injection will not be needed.

SOME TESTING EQUIPMENTS
  • lower left - 3" magnet with 1" center hole
  • upper left - early, low rpm test spindle that became unstable at higher rpms
  • right - brass plumbing valve stem used as solid shaft for faster rpm and early, aluminum disk test

Bob Wilson
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Last edited by bwilson4web; 09-07-2011 at 11:02 AM.. Reason: Added photo of some test articles.
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Old 09-08-2011, 12:23 AM   #10 (permalink)
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Quote:
Originally Posted by bwilson4web View Post
I'm not having much luck with that link.
Try this one: Switched Reluctance Drives

Quote:
By injecting water, I'm making the exhaust pipe into a steam boiler.
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?

Quote:
Then the turbine can extract the additonal mechanical energy from the enhanced exhaust stream.
But there is no additional energy. All it has is the heat/pressure coming from the combustion chamber. Inject water, and you just use some of that energy for the water/steam phase change, no?

Quote:
In a normal turbo installation, significant turbine power occurs when the engine is operated in high-power regions. But our Prius typically operates in low power regions with more modest gas flows and heat.
But you can only get out the energy that's in the exhaust stream. In low power operation, there's not much energy to get out.

I really do think you'd do much better to start (at least) with just the turbo/generator, without the complexity of water injection. I'd be willing to bet that you'd get as much or more energy recovery that way.

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