Suppose you start with a hot gas at atmospheric pressure; the exhaust from an internal combustion engine or turbine engine will do; even the exhaust from your household water heater might suffice.
Expand this hot gas through a pneumatic motor (preferably through multiple stages, to avoid unnecessary heat loss), until it's temperature is just above the temperature of the ambient air. Cool this warm gas with an intercooler. Compress the cooled gas, prefereably in multiple stages, preferably with additional intercooling between pair of stages, until its pressure is back up to atmospheric pressure. Then send it up your chimney or out your tailpipe.
If used in a car, this could be a "vacuum turbobooster," which would be allowed to spin freely, without mechanical load, resulting in it producing a vacuum between it's supplier of hot air and it's own intake.
If there's a regular turbobooster, this would result in more power for the turbobooster's expander (and hence more power for the compressor, and thus more boost and more main engine power), and lower pressure in the main engine's exhaust manifold, resulting in less energy needed to force out gases during the exhaust stroke, resulting in more available main engine power.
If there's no regular turbobooster, the vacuum turbobooster creates a partial vacuum in the main engine's exhaust manifold, which mean that the main engine's exhaust strokes actually produces mechanical energy, thus giving the engine more power.
In applications where producing intake vacuum isn't a useful effect, the mechanical energy of the vacuum engine could be used directly (perhaps to produce electricity), slowing the engine enough to not suck in air too quickly.-- goldbb, Jan 20 2009 A real vacuum engine running http://www.youtube....watch?v=TjEcWoxITSE [goldbb, Jan 22 2009] Imposion engine http://www.free-energy.ws/implosionImplosion is the closest thing to a vacuum engine. [travbm, Nov 01 2015] Cool (+)
...and compressed.-- 2 fries shy of a happy meal, Jan 21 2009 [-] why not just keep it hot and run it through the fuel tank, that way you can dissolve the HC's back into the fuel and pre-warm the gas.-- FlyingToaster, Jan 21 2009 You must do work against atmospheric pressure in order to expand your hot gas.
If you wish to extract thermal energy from your already atmospheric-pressure hot air, you might do better to consider a stirling-cycle engine. For domestic use, you could even have a flow-through reservoir set up in your domestic water supply line to act as the cold-sink.
Kudos for trying to extract energy from waste heat, but as-is, your idea is bad science. Neutral vote from me.-- Freefall, Jan 21 2009 if your water heater is producing hot exhaust it's time for a new, high efficiency, unit from your local home improvement center. As for your car, the device you describe would impair overall fuel economy by adding tremendous weight far exceeding any few calories that you could harvest. This is wishful thinking.-- WcW, Jan 21 2009 I don't understand the objections to my proposal; it's basically a cycle of nearly isentropic expansion followed by nearly isothermal compression. If this heat cycle is to be modeled as a closed cycle instead of an open one, a third step is isobaric heating.
A simpler description of the engine: intake (hot air at atmospheric pressure), turboexpander, intercooler (warm air at partial vacuum, cooling to tepid air at partial vacuum), turbocompressor, exhaust (air at atmospheric pressure, at a few degrees cooler than intake).
Freefall: you say "You must do work against atmospheric pressure in order to expand your hot gas." I don't understand this. If the intake to the system is at atmospheric pressure, and the interior of the intercooler is at a partial vacuum, then expanding air through a turbine ought to produce work.
Even if you assume that we expand the air with a piston engine (which is forgivable, since I did say "pneumatic engine,") then *part of* the intake stroke (expanding the air after having moved it into the cylinder) requires the engine to do work against the atmosphere, and the *full* exhaust stroke (moving the expanded, low pressure air into the intercooler) produces work (as the atmosphere presses on the shaft side of the piston).
WcW, no matter how efficient the exhaust of the water heater, it's not going to be cooler than the temperature of the water it's heating, which is about 120F, or 49C, which is well above atmospheric temperature in most climates.
As for a car, how much weight would it add, to stick on one single turbo expander, one intercooler, and one turbo compressor? And, considering how hot car exhaust typically is, what's the theoretical maximum number of watts that could be derived from the waste heat?-- goldbb, Jan 21 2009 /no matter how efficient the exhaust of the water heater, it's not going to be cooler than the temperature of the water it's heating/
Ahem. Counter-flow heat exchanger. Thank you.-- Texticle, Jan 22 2009 thank you [Texicle] you are a gentleman.-- WcW, Jan 22 2009 "I'm just proposing that you mount two extra turbines and intercoolers in your exhaust". This would be more expensive than adding a classy twin turbocharging system and would achieve SO MUCH LESS. PLEASE!!! NON CAR PEOPLE LEAVE CARS ALONE!...ok ok calm down, be understanding,.....mutter mutter, (wanders away to tinker in shop)-- WcW, Jan 22 2009 //If the intake to the system is at atmospheric pressure, and the interior of the intercooler is at a partial vacuum, then expanding air through a turbine ought to produce work.//
And how exactly does one get the intercooler to be at partial vacuum? Answer, Work.-- jhomrighaus, Jan 22 2009 The intercooler is at a partial vacuum because air is continuously removed from it, to the atmosphere, by a compressor.
Because the air being removed from the intercooler is cooler and denser than the air coming into the intercooler, it requires less energy to remove (compress) the air from the intercooler than was produced adding (expanding) the air into it in the first place.
I, of course, am assuming that the engine is in a steady state... if you're asking how it *starts*... well, that's a tiny bit different. (It's sort of like asking, where does a car engine get the momentum it needs to compress air for the compression stroke, when the engine isn't moving? Or, for a better analogy, where does a turbine engine get that "high internal pressure" that it needs to produce mechanical power, when it *starts* at atmospheric pressure?)
If used in a car, it's started by the force of the car's exhaust being pushed through it by the internal combustion engine, in a way that's very similar to how a normal turbocompressor starts moving.
If it's used in a stationary application, a small starter motor could do the trick.
By the way, there *do* exist some types of vacuum engine; see the link. However, they don't use a seperate cooler, therefor aren't as efficient as they could be.-- goldbb, Jan 22 2009 what color are the clouds where you live? They must be cool cause in your world the laws of physics and reality must not apply. For any system such as this to work you need an energy gradient for things to work. Without an engine pumping out exhaust at high velocity you have a large paperweight.
All you have here is a Turbo charger with its exhaust connected to its inlet and the outlet connected to the tail pipe.
The power released by the expanding gas will turn the compressor, which will create a vacuum, which will require more energy to maintain, which will draw more power out of the driver etc etc etc, what will happen, it will grind to a halt.
Most vacuum pumping systems are highly energy intensive as the need for tight seals means friction which will immediately shift your energy budget into the negative and make your system work even less than it wasn't going to work at the outset.
As to your link it is a great illistration as to why your idea is doomed, in order for the engine to run it requires a 400 degree temperature differential just to maintain its free spinning motion, in order to do any real work it would require considerably greater amounts of energy than it generates.
At best this is a energy negative boondogle at worst this is a perpetual motion machine. Next you will be telling us that once the engine is running you can connect the exhaust from the compressor to the inlet of the expander and the heated gas will cool as it expands, then heat up as its compressed etc etc etc.
Please give up while you are ahead.-- jhomrighaus, Jan 23 2009 Standard water heater exaust has to be hotter than water, true when it sits there heating the same bit of water in a take but heating water in general is not a safe assumption because you can run this heat by the incoming cold water and it can be a lower temp than the hot water. I am not sure if high effeciency on demand systems do this but you can. To do this to a car I would replace the body panels with seperate vacuum tanks that you fill up individually with exhaust. Over a long period of time heat disappation is the primary ingredient in generating power you already have drag and wind on the outside and having a hot car might reduce drag? You might want to consider a titanium engine that you don't need a radiator for? Lubricate with buckyballs and an airless environment? have it only be cooled by intake air and gasoline.-- MercuryNotMars, Jan 23 2009 jhomrighaus,
/All you have here is a Turbo charger with its exhaust connected to its inlet and the outlet connected to the tail pipe./
It sounds like you're confusing the turbocharger's exhaust with the car engine's exhaust, and that you're thinking that the turbocharger is taking it's own exhaust into it's own intake.
My design takes the car engine's exhaust into the input of the expander, sends the output of the expander into a cooler, the output of the cooler into a compressor, and the output of the compressor to the tailpipe.
/The power released by the expanding gas will turn the compressor, which will create a vacuum,/
Correct.
/which will require more energy to maintain,/
No. Because the air coming out of the cooler is, well, cooler (and denser), than the air coming into the cooler, it takes *less* energy to maintain this vacuum than is produced when expanding hot air into it.
/which will draw more power out of the driver/
How do you see this as drawing power out of the person driving the vehicle?
More seriously, your suggestion that it takes more energy to remove cooled air from a partial vacuum than can be extracted from hot air expanding into that same partial vacuum is contrary to the laws of thermodynamics.
All my engine is, is an open cycle, brayton cycle, heat engine, with one single difference from the usual type; specifically, in most open cycle engines, the "cooling" step is open, and the "heat addition" step is enclosed, whereas in mine it's the opposite.-- goldbb, Jan 25 2009 If you're so sure it will work, build one. Your description uses only off-the-shelf parts the way I read it. Silence the doubters!-- Alterother, Jun 05 2011 This might be better with a gas-turbine engine. The stages would be: compress, heat, expand, cool, compress. It's conceptually a little simpler, and results in a single mechanical output, rather than the two imperfectly correlated outputs of the piston + turbine version. That's surely been done, though.-- spidermother, Jun 05 2011 random, halfbakery