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Old 11-15-2019, 08:20 PM   #1 (permalink)
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Intake vacuum energy recovery

Hey guys I'm wondering if anyone has experimented with trying to recover energy from the difference in pressure between atm and the intake manifold. Based on some back of the napkin math there seems to be about 800 Whrs available from the 15 ish inches mercury pressure drop across the throttle plate for every gallon of gas burned.
There is a whole host of engineering problems trying to recover this, given that the power available is dependant on engine vacuum and rpm. As well as the engine control problems because the ecu is getting input from the throttle position and the manifold pressure or air flow.
Anyway I was thinking a roots blower coupled with an electric generator that you could use to both pull energy out of the air and also control the air flow by torque controlling the generator might be an interesting experiment.
I'd love to know if anyone has tried this thanks.

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Old 11-16-2019, 12:22 AM   #2 (permalink)
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If you found a way to take all of that energy, there would be no air movement past the throttle. The more restrictive the intake, the more energy is needed to pull the air into the cylinders so more fuel is needed.

It is not a source of ‘free’ energy.
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Old 11-16-2019, 09:02 AM   #3 (permalink)
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Quote:
Originally Posted by cliffordlj View Post
Hey guys I'm wondering if anyone has experimented with trying to recover energy from the difference in pressure between atm and the intake manifold. ...
Hi Clifford,
I don't remember anyone here actually running any experiments. There is an old thread here that discusses the idea, before getting side tracked. There are a few other threads, maybe.
Your estimate of 800 Wh per gallon works out to about 2.4% of the energy in the gas.
My contribution was to point out that for an Otto cycle engine, the maximum amount of power lost to throttling is always less than 5% of the engine output. At idle that's about 5% of almost nothing, and at WOT there is no throttle loss.
This lost power has been fretted over since the first gasoline engines, but no viable method has been found to use it. Yet.
-mort
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Old 11-16-2019, 02:13 PM   #4 (permalink)
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I think with modern control techniques it can be achieved, you are looking at atleast 300 usd in hardware. So the expense for auto manufacturers is just not worth it.
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Old 11-16-2019, 06:29 PM   #5 (permalink)
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Quote:
Originally Posted by matt36415 View Post
If you found a way to take all of that energy, there would be no air movement past the throttle. The more restrictive the intake, the more energy is needed to pull the air into the cylinders so more fuel is needed.

It is not a source of ‘free’ energy.

Matt, the first part of your quote is not correct.

If an engine is pulling say 7 psi of manifold vacuum then if the throttle plate was replaced by a turbine driving an electrical generator then energy could be extracted.

The correct gas flow will still occur.

The only time energy cannot be extracted is at WOT.

For the system to work the turbine would have to spin at precisely the right speed that will result in the requested manifold vacuum.
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Old 11-16-2019, 06:51 PM   #6 (permalink)
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A turbine that is restrictive enough could be a challenge.
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Old 11-17-2019, 09:05 AM   #7 (permalink)
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Screw a turbo on the engine, turn a minus into a plus, and enjoy the benefits.
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Old 11-17-2019, 05:54 PM   #8 (permalink)
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The answer you seek is out there. DDG/?q=potential+vs+kinetic+energy
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Old 11-17-2019, 07:21 PM   #9 (permalink)
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The only uses for vacuum as a power source I can remember right now, besides the brake boosters, are door locks in some German-designed cars and those automated clutch kits frequently used by disabled drivers in countries where manual transmissions still outnumber automatics.
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Old 11-18-2019, 07:21 AM   #10 (permalink)
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the concept is valid.

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electric generation, intake vacuum, pumping losses

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