h a l f b a k e r y"My only concern is that it wouldn't work, which I see as a problem."
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This engine would run at constant speed in a fuel-electric hybrid automobile, and would convert an apparently impossible fraction of the chemical energy in the fuel to work. The loophole which permits this is the fact that the engine is not re-compressing the compressed air, and hence does not operate
a closed cycle. When the complete cycle is accounted for, including the heat transferred from hot to cold reservoirs during compression of the air, the thermal efficiency is no more than expected. However, since it is always necessary to replenish fuel, why not replenish compressed air at the same time? This would give a considerable boost to the total milage possible using a given amount of fuel, due to avoidance of the work required to compress air.
As the piston rises, exhaust gasses are expelled through the exhaust valve. At top dead center, the valve closes, and fuel and compressed air are injected and ignited. The temperature and pressure rise and the power stroke occurs with the exhaust valve closed. The exhaust valve opens at bottom dead center and stays open to top dead center, the end of the cycle.
I should like to know whether anyone has encountered this idea before.
Orbital's technology applied
http://www.acarplac.../gm/xv8-engine.html
[half, Apr 24 2006]
Orbital's Direct Injection: Overview
http://www.orbeng.c...tion/dioverview.htm
[half, Apr 24 2006]
Other comments on this idea
http://renewableene...sure_compressed_air
On Renewable Energy Design. Sorry for the ads. [Archimerged, Apr 24 2006]
French MDI air car
http://www.theaircar.com/howitworks.html
Car operating on compressed air alone [Archimerged, Apr 24 2006]
Wikipedia
http://en.wikipedia.../Carnot_heat_engine
article on Carnot heat engine including Carnot's theorem [Archimerged, Apr 25 2006]
The "Massive Yet Tiny" (MYT) Engine
http://www.angellab....com/mytengine.html
Has a dwell at TDC, making possible the concept addressed by [Archimerged] [reensure, Apr 25 2006]
[link]
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I'm not following how compressing the air outside the cylinder improves efficiency. Where does the energy come from to compress the air? Would the air have to be compressed to an even higher degree (vs. conventional ICE) in order to get the volume of it stuffed in to the cylinder in the appropriate, i.e. short amount of time? |
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I know that Orbital has done significant work with air assisted injection, meaning that air is injected along with the fuel. But, I think that's a bit different from this. I first heard of Orbital in the context of reducing emissions from 2-cycle ICE's. |
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The compressed air comes from any sort of compressor at all, possibly one I have suggested elsewhere in this category, or from electricity, etc. For pratical use, service stations would refill fuel and air at the same time. |
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The extra efficiency comes from leaving the exhaust valve open whenever the piston is rising, so that there is no compression being done by the piston, and the only work is due to friction and turbulence as the exhaust gasses leave. The result of this is that in theory (neglecting that friction and turbulence) all of the chemical energy could be converted to work, as far as I can see. |
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The work of compression must be done somewhere. Putting aside the gains due to better atomization with air/fuel injection (see Orbital's stuff), the efficiency gain must come from being able to compress, transfer, store, distribute and deliver to the cylinder the compressed air more efficiently than can be done directly in the cylinder. |
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Have I now grasped the concept correctly? |
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That is a lot of compressed air. Lets take a 1 litre engine as an example, running at 3000 rpms. Since each piston will fill and empty with each revolution, you are using 3000 litres of air per minute. |
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A typical air compressor can go to 150 PSI, or about ten times atmospheric pressure. So to run your engine for one hour it is going to take a 18,000 litre tank. |
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Ok, lets assume that you can compress air to ten times the pressure of a typical compressor, or 100 atmospheres. You still need an 1,800 litre tank. The gas tank of your typical car is only about 40 to 50 litres. This is going to take a BIG car. |
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What the heck is the compressed air for? Is the sole objective of the air to increase burning effeciency of the engine or to bolster volumetric effeciency at low RMPs? |
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Couldn't you just run say 2 cylinders of a 4 cylinder car as air compressors for the remaining 2? Knock-out the spark plugs and build an intake tract minus the fuel delivery and plumb the compressed volume of the remaining cylinders to your 2 firing cylinders. And why not just run a 2-stroke engine if you're worried about effeciency? |
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Little 4-stroke theory: Intake opens BTDC, closes, BBDC. Exhaust opens BBDC and closes ATDC. If you tried to employ the elementary system as described in that last paragraph you would wind up with a horribly slow, ineffecient timing structure. |
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And I'm real sorry if that seemed to go in a different direction than what the idea was intending, but I don't see the reasoning behind the need for the compressed air. |
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//In theory, all of the chemical energy of the fuel can be converted to useful work by an engine which uses both fuel and compressed air//
Normal internal combustion engines can achieve this by augmenting their port timing, velocity, diameter and tract length/volume. By exceeding 100% volumetric effeciency (cylinder filling), the volume of air thats being caught in the clyinder is indeed compressed air. Granted most times this is accomplished only at exceedingly high engine RPMS, but the concept is already out there. If you're only concern is burning 100% of the cumbustible fuel/air mixture, then look in to port resonance and taking advantage of the exhaust pulse to ensure all of the fuel is being burned while still maintaining a high level of preformance and effeciency. They've been doing that with 2-stroke pipes for AGES. Using sound to keep the unburned fuel in, let the spent exhaust gasses out. |
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[half] Yes. Also the fact that you don't have to carry around the fuel used to do the compression. In fact the compression might be done directly by solar power or a Stirling engine operating off ambient heat. So that part of the work is not done using fossil fuels. |
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[Galbinus_Caeli],State of the art tanks hold around 300 atm, and there are electric compressors (used with the air car in France) which fill a tank that will operate a car on the air _alone_ for a reasonable cruising distance in 4 hours. High speed transfer takes a few minutes given the infrastructure. |
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Also remember that the point of using fuel is to heat up the compressed air very hot, and if you double the temperature to 600K, the pressure doubles. Of course it won't be 600K at exhaust, but it will still be pretty hot, a lot hotter than the air in the tank. So I think the amount of air needed is less than your estimate. |
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After all, if the air car gets a reasonable cruising distance using no fuel at all, you could just take it and do external combustion to improve its milage. But I thought maybe internal combustion would do better. |
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[Letsbuildafort], I just noticed that the theoretical limits on conversion of fuel to work are lifted if engine does not need to compress the working gas. I don't know about the practical limits. |
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The compressed air represents some work done ahead of time that you don't need to carry around fuel to do. Compressed air is very good at taking heat and converting it to work. So you burn the fuel to heat up the air, and almost all of the heat energy turns to work. |
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A two stroke engine still has to compress the air and fuel mixture. |
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This engine would have a very high compression ratio (ratio of volume BDC to TDC IIUC) because its not hard to squeeze the last of the exhaust out an open valve. |
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Re 4 stroke theory: this is a 2 stroke engine. It does exhaust and ignition, and it has only one valve. (The fuel/compressed air injector is a tiny opening). ATDC, add air and fuel and ignite, BBDC open exhaust. There is no intake or compression. |
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Re 100% burning. I was assuming regular ICEs get 100% burning. But burning fuel produces heat, not work. Conversion of that heat to work cannot exceed efficiency = 1 - Thot/Tcold by Carnot's theorem. |
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Woo - thanks for clearing that up, [Arch]! Seriously - look in to just augmenting the 2-stroke's port timing. I don't feel as though simply having a high compression ration is going to further your goal of achieving a 100% fuel effeciency ratio. |
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You're right - burning fuel does produce heat, but it does yeild "work" by creating expanding gasses which push-down on a piston. Its simply the job of the crankshaft to convert linear motion of the piston/con rod into roational motion. Thus in effect fuel causes the bits of the engine to "work." |
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How about just getting a really really heavy flywheel attached to an already (relatively) effecient 2-stroke motor? |
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Re: 2-stroke engines don't have any "valves." And they do indeed have an intake stroke. Which takes place along with secondary compression. |
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I guess, the question really is, how much energy is used to do the actual air compression in the piston? |
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And, if you do this remotely, how much weight in compressed air would you need to carry around, and how would that excess weight (minus perhaps weight in fuel savings) hurt your efficiency? |
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Some of the compression in a standard ICE is likely done through momentum of the rotating cam. If you do this through use of pre-compressed gas, you may not be saving much at all, as the cam is still going to put the piston back in place. |
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And, can you inject this compressed gas fast enough? 3000 RPM is pretty quick, so you'd need to have a big hole to pipe the gas in that fast. |
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//I guess, the question really is, how much energy is used to do the actual air compression in the piston// |
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Bore² x Stroke x .7854 = Clyinder Volume
(CV + CCV) / CCV = Compression Ratio. |
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Compression ratio is an expression used to compare the volume of a clyinder when the piston is at TDC and BDC. While its much easier to determine actual combustion chamber pressure using a compression/leak-down gauge, pressure is expressed as: |
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//Some of the compression in a standard ICE is likely done through momentum of the rotating cam// - Sorry, the camtrain does little to directly affect compression if at all. The cam facilitates the timing of the valves. Thats it. Valve timing can have a bearing on compression, but this is where the correlation ends between "cams" and "compression." |
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//And, can you inject this compressed gas fast enough// Certainly - imagine pulling a syringe as to draw in air as fast as you can. The piston insinde the clyinder has the same effect. The faster the piston moves, the velocity of air increases proportinally (barring turbulence, valve diameter, etc.). |
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What we're dealing with here is using compressed air to help compliment volumetric effeciency. An engine operating at 100% VE is ingesting its' total displacement every cycle or rotation of the crankshaft (depending if its a 2 or 4 stroke engine). Exceeding this can theoretically be done with the aid of compressed air, for low RPM operation as in this application. Overcoming the 100% VE is relatively easily done at higher RPMs where the piston speed and port timing affect intake-port velocity and subsequently directly affect cylinder filling. |
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Anytime, [sopocles]. Woo - I'm winded. |
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I propose running a constant speed opposed V-twin 2-cycle engine with 2 additional compression cylinders in conjunction with a heavy crankshaft or flywheel, effectively making a V-4 configuration. Leaving two of the cylinders to compress and inject air into the 2 remaining firing cylinders, would reduce or eliminate the need for carrying a separate large-volume air tank. Proportional compressable volume is achieved due to the fact that the same displacement cylinder is being used uniformly. |
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The opposed 90-degree V-twin configuration is capable of generating torque more effectively than a twin-arrangement who's angle is either more acute or more obuse. I understand that compressing these "dead" clyinders is going to comsume a fair bit of rotational energy from the crankshaft. Thus, a constant, low RPM of a torque-indictive motor spinning a heavy rotating mass would best. |
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<stepping off soap box>Please tell me I was on track with that idea.<sosb> |
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These are replies to earlier comments, not those directly above. |
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[Letsbuildafort]: 2 cycle engines expend energy compressing the air/fuel mix in the crankcase so it will flow into the cylinder. This one doesn't; that energy was expended while compressing the air at the stationary site. So this engine doesn't need an open crankcase and oil in the fuel. This engine just injects fuel as liquid, and highly compressed air, and ignites it, all around TDC. It might have to do some trickery like MDI does (see link) to extend the time at TDC. Looking around a little, I see there is a two stroke diesel engine but it still does compression which is how the fuel gets ignited. |
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Burning fuel produces heat. The amount is measured in joules, same as work. The joules of heat produced can never all be converted to work by an engine that goes through a closed cycle. Some of the energy has to stay heat. If you have a closed cycle engine which doesn't work this way, you can build a perpetual motion machine with it.
See Carnot Heat Engine article on Wikipedia (see link). |
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[sophocles], It's not just the compression, it's also the heat. (Hmm, but I've not read up on the thermodynamics of ICE's. Since they take fresh air in and exhaust hot gas, it's not exactly a closed cycle). |
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Any energy removed from a flywheel has to be replaced. |
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MDI extends TDC in order to have more time to inject the air (see link). But if the gas is going into a really tiny space (extremely high ratio of BDC volume to TDC volume) it would move in fast through a small hole, e.g., if it were injected around full pressure of 300 ATM and the volume ratio is 300/1. |
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Re: camshaft vs crankshaft. I think sophocles meant crankshaft. |
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Re: V4. Sounds interesting, although not what I was talking about. |
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I'm pretty sure this is bad science-- the same energy is used to compress the air--- and/or is reinventing the turbocharger/fuel injector. |
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//injects fuel as liquid, and highly compressed air, and ignites it, all around TDC//
I sincerely hope you're not intending on using gasoline for this. As the pressure of whole (non-atomised) gasoline increases, it becomes harder to ignite.
So you're talking about something similar to a horizontally opposed engine with air being injected along side fuel? What about it's exhaust gasses? Do you just build-in an exhaust port in the bottom of the cylinder (somewhere around the BDC piston position)? Its gotta be a wet-sump lubricated beast. Unless its working on magic or Klingon blood or something. And aren't perpetual motion devices still well within the realm of theory and Star Trek episodes? |
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You're always going to be losing energy somewhere.
// doesn't need ... oil in the fuel//
Only if you want to run. Thats how 2-stroke motors work these days. |
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//compressing the air at the stationary site//
What?! I still don't get that ... |
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I would agree with [sninctown] in so far that this is a little unrealistic today. Not BAD science - just theoretical science. And within the bounds of this idea we are no more inventing, nor reinventing a system for induction of fuel or air as to be used in an internal combustion engine. Simply theorizing a way of complimenting the volumetric, mechanical and thermal efficiency of such devices as to result in a more effective mode of generating power. |
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[sninctown], The compressed air contains relatively little energy. After all, PV/T is constant. PV is energy, nRT is energy. A given number of moles of gas at a given temperature contains the same energy regardless of pressure. |
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It may well be reinventing the turbocharger / fuel injector, except for the fact that you don't have to (1) use fossil fuel to operate the turbocharger or (2) carry the fuel with you. And (3) maybe it would be injecting the air at a much higher pressure. But I'm not terribly interested in engineering small mobile engines, I was just asking if anyone knows of a previous example of this, or if anyone can give the thermodynamic analysis of an internal combustion engine. What is its PV diagram like? |
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[Letsbuildafort] Yes, it is theoretical science. In fact, I'm not much interested in auto engines or trying to propose something practical. But sometimes what is theoretically possible turns out to be simply possible -- it can be done. |
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The exhaust port is supposed to be at the top of the cylinder, so what would be the compression stroke empties the cylinder because the valve is open. |
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// //compressing the air at the stationary site// What?! I still don't get that ... //
Air is compressed into tanks somewhere, or by a compressor in the garage operated by electricity, or whatever. The energy to do the work of compressing air does not come from gasoline or fuel that has to be carted around. Less fuel is needed to go a given distance because none is used to compress the air. |
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The main advantage is that compressed air is good at absorbing heat and converting it to work. If you have more molecules in the cylinder at high temperature, the force on the piston is higher. As the gas expands, it continues absorbing heat from combustion. I suspect this would work better with lower RPM and a long stroke, so that more time is spent doing expansion. |
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And it might be that an external combustion engine using compressed air in an open cycle, with very hot cylinder and piston and a lot of heat flow into the cylinder as the gas expands woud get even better efficiency than an ICE. |
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[rcarty] I do tend to get obsessed about things... :-) |
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[bigsleep] This can easily be accomplished with a normally aspirated 2-stroke. I don't believe there is an immediate need for forced induction at all. |
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As part of the technical side of the V-4 idea, you could incorporate the two compression cylinders into the crankcase without the ports (as is needed for conventional 2-stroke powerplants). Just smooth cylinders with a reed valve assembly to bring-in and hold air, and plumbing to the other firing cylinders. Seeing as how they're incorporated into the crankcase that also facilitates the two firing cylinders, they are lubricated by means of the oil/gas pre-mix and the added moving postons serve to increase intake (primary) compression. |
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I don't see why this wouldn't work, but then again I'm not ready and willing to make a workig model. (My fear of catastrophic mechanical failure is still too great.) |
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how do you make up for the heat from compression of the air in the cylinder. A properly tuned Gasoline engine is just short of spontaneous ignition when the spark plug fires. For Diesels this is essential to thier operation. With such a scheme you would actually reduce the efficiency of the reaction. Also you would still require off board compression of the gas which would actually be less efficient as the heated and then cooled air would increase the amount of energy required to bring it up to temperature or vise versa lower the amount of energy that could be extracted from the engine. You would also require an additional vacuum pump in order to run the accesories on the car and provide brake boost. This vehicle would be significantly more expensive do to all the electronic accuators that would need to be installed to make up for the loss of engine vacuum. Also the pressure of the air injected mixuture would need to be in excess of 600psi in order to maintain optimal compression at TDC. This would require Diesel like injectors and an external fuel pump to get the fuel up to pressure all of which are heavy and cause a power drag on the engine. Cooling would occur at the injector nozzle which would lead to uneven heat distribution within the cylinder which would impact the propogation of the flame front. |
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Will it Run? Sure it will. Will it be much better than what exisits? Not really it would probably not work as well. |
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I am still confused about where you are going to carry all this compressed air. Even at 300 atm of pressure, one hour of operation of a 1 litre (tiny) engine is going to need a 600 litre tank. |
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A 600l tank is a bit more than one metre in diameter and one metre high. Or about 20 cubic feet. Which is slightly more than the entire trunk capacity of a luxury car. (Twice the capacity of a compact.) |
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One litre of air at atm weighs 1.3 g. |
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So 600 litres of air at 300 atm weighs 600*300*1.3g or 24 kilograms, so mass is not really a problem. But volume is. |
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Oh and a 1.0 litre engine is tiny tiny by today's standards. And a one hour between recharges is below what most people want. So you probably want to do is multiply these things by four. |
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a 1 litre engine refers to the full size of the cylinders. You would be injecting only a small percentage of that volume when the piston was at TDC. You would effectively be turning the 4 cycle engine into a 2 cycle since the compression and intake strokes would be removed. The air could be compressed via stirling or wind or solar. I like the idea. |
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What [jhomrighaus] said. Compressed air cools/expands to lower pressure at is released. The heat/pressure will need to be returned, either in the cylinder before ignition, or before injection into the cylinder. |
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I doubt that conduction of engine block heat in the cylinder will occur fast enough between injection and ignition, and it has the additional problem of cooling the cylinder and fuel before ignition, perhaps below optimum temperatures for a clean burn. |
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So in an ideal world, the air has to enter the cylinder both hot and at high pressure. |
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But of course, there's a lot of -waste- heat available from an ICE. The compressed air could be heated before injection by running it around the engine block, where it would actually complement the existing cooling system. The exhaust is another potential source of waste heat. |
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If this was implemented with a camless engine (ie solenoid-driven tappets) the efficiency gain could be significant. |
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As far as air storage goes, I'm thinking liquid oxygen. |
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So: the equation is: how much energy are you actually saving -vs- how much fuel does it cost you to cart around the compressed air (or LOX) tank. |
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