Hybrid cars are becoming a fad, because they can get better gas mileage than ordinary cars. They can do this mostly because of "regenerative braking", a way of converting the energy of the car's motion into electricity, and storing the electricity for later acceleration.
Another thing about hybrids is that they can have smaller engines than regular cars. This is because, to travel at say 100kph (about 60mph), a car only needs maybe 15 horsepower to push through the air resistance. ALL THE REST of an ordinary car's horsepower is there solely to allow the car to accelerate quickly. BUT, if an electric system is being used for acceleration, and stored energy is used to power that, then that system can be charged up slowly with a 15HP engine, and dumped as fast as necessary into the electric motor, for rapid acceleration. The net effect is that the small engine can run at its most efficient speed, all the time.
The thing is, "most efficient" does not automatically always mean "very efficient". Internal combustion engines are just about always associated with incomplete combustion (which catalytic converters were created to deal with) and the formation of pollutants such as nitrogen oxides. So the Question is, "If all we need is about 15HP, why do we have to use internal combustion to get it?"
So let's consider a steam engine, featuring external combustion, instead of an internal combustion engine. No unburned fuel! No production of NoX! We mostly all know that steam was put to use in the classic "Stanley Steamer" car. It had a 20HP engine, bigger than what we need here (Note: cars weighed a lot less in those days, so less HP was needed to accelerate them than is true today).
One advantage of external combustion is that any fuel will do. Have you compared the price of coal to gasoline lately? And a major disadvantage of steam cars (the thing that led to the demise of the Steamer), is that they tend to need to stop for more water rather often. It has always been difficult to completely recycle the water for a steam engine in an auto-sized vehical.
To clarify that last point, we need to compare the "waste heat" of external and internal combustion engines. In the ordinary auto engine, after the hot gas has done its work pushing on a piston, it is fairly simply exhausted. It takes a fair amout of heat with it, that does not have to be removed from the engine through the radiator. But in a steam engine car, the only way to do the equivalent is to let some steam escape the vehical. Either ALL the waste heat must be removed through the radiator, or you must replace water lost as steam. Making a big enough radiator has simply not been practical, when trying to remove the waste heat of a 100HP steam engine (equivalent to the standard for a smallish car today) --and even the smaller 20HP Stanley Steamer couldn't do it well enough to keep people from buying internal-combustion cars, with the convenience of not needing to make refill-water stops. (Even large steam engines, such as on railroads, had this same cooling problem, and needed lots of water tanks built along the tracks.)
HOWEVER. Here we are talking about only a 15HP steam engine. Can modern technology provide a radiator that can handle the waste heat for only that amount, which fits unobtrusively in an ordinary-sized car? Or can we AT LEAST arrange things so that the car needs water no more often than it needs to stop for fuel? Here I'm going to assume we can do that thing.
One reason I think I can make that assumption, not just because these days we likely have more effective radiator technology than is typically used in cars, is because the definition of "waste heat" itself depends on how efficient the engine is. Your ordinary IC engine maybe converts 30% of the chemical energy of the fuel into mechanical motion of the vehical; the other 70% is wasted. Obviously if we have a more-efficient-than-30% steam engine, then there will simply be less waste heat to dump through the radiator. And we can be sure that steam-power technology has improved considerably, since the days of the Stanley Steamer. Large power plants manage 50% conversion of steam energy into the mechanical energy that turns electric generators. We can't do that well on the small scale of an auto, but we should be able to do 40% or maybe a bit more.
Here's an oddball suggestion, which starts with the Question, "Did you know that a Stirling Engine is about 45% efficient?" This is not a steam engine, but it is an external-combustion engine, and perhaps I should stop and just say, "How about a Stirling-electric hybrid?"
But I have a mad idea to suggest, and so will continue. Inside a Stirling engine there is a "working gas" that is shuttled between a heated zone and a cooled zone. The gas used is most often either hydrogen or helium, because they expand and contract by very significant amounts when heated/cooled, compared to other gases.
What I'm wondering is, "Hasn't anyone tried steam in a Stirling engine?" If ANY of the steam condenses in the cool zone (cooled by the radiator, of course), then there is a 1600:1 change in volume, for the quantity of steam that does condense. This is WAY more than even hydrogen or helium can manage (when they remain fully gaseous)! And of course when the steam/condensed-steam mixture is shuttled to the hot zone and it all becomes steam again, that 1600:1 change in volume again helps the engine work. A key fact about Stirling engines is that they can work even when the temperature differential between hot and cool zones is less than 10 degrees. We would have rather more than that amount of difference here, with a steamy Stirling, but we need not over-do it. All we need is about 15HP, PRODUCED AS EFFICIENTLY AS POSSIBLE.
That horsepower would then drive a generator which produces electricity which is stored, as previously mentioned. The electric side of the hybrid does the rest. =========== Added Sept 20 At the present time, the best rechargeable energy-storage system I know of, in terms of size and weight and energy-efficiency, is a flywheel. We can't make a flywheel good enough to simply spin up and use it to go 300 miles, like we can fill a tank of gas and go that far, but we CAN make a flywheel easily hold several times the energy needed for fast acceleration. Thus the steam engine should be almost directly connected to a flywheel, which in turn is connected to a motor/generator (so regenerative braking can charge the flywheel), and in-between that and the main drive motor/generator(s) is the electrical control circuitry. Later on the flywheel could be replaced with even-more-efficient supercapacitors, but they are too bulky for use at this time.
=========== Added Sept 16 Note that because this hybrid is partly electric, when "cold" it can start moving immediately, while the steam engine, no matter what kind, is heating its water to a boil.
=========== Added Sept 19 I should mention a couple of additional points of energy savings and efficiency that a car with an electric power train can have. This applies to vehicles that do not have a motor/generator for each wheel; this is for those who might first think that you simply replace a gasoline engine in the engine compartment of the car, with an electric motor.
First, there are various designs of electric motors, and for some, their speed can be electrically controlled. This means that no gear-changing transmission (or any other sort of mechanical transmission) is needed in the car. Electric motors can deliver maximum torque at 0 RPM, and a tire-screeching start is quite possible (has been done, in fact). There is significant weight savings when the car has no mechanical transmission, of course. There is also some immediate energy savings, because the more mechanical parts that are involved in transporting energy from Point A to Point B (like inside a transmission), the more friction points (gear contact points, shaft bearings, etc) there are, and each of those is associated with some wasted energy. Eliminating a mechanism means eliminating its associated losses, of course.
Second, if the car is either front-wheel drive or rear-wheel drive, then the differential can also be eliminated. (If the car is intended to be 4-wheel drive, it may be better to put a motor/generator at each wheel hub, but we could of course instead have one motor/generator for each axle.)
Eliminating the differential is a trick that apparently most people don't know about, although I first encountered it published in Popular Science two or three decades ago. I suppose I should first describe just what a differential does in a car, for those who might be wondering (skip this if you already know). If you drive a car in a circle, the wheels inside the circle follow a smaller-diameter circle than the wheels at the outside edge of the circle. Since the two circles have different sizes, they have different circumferences--equivalent to "lengths". If the wheels are rigidly connected to a simple axle, they rotate in unison, and easily go straight together. But when taking a curving path, the outer wheel needs to rotate faster than the inner wheel, because of the greater length. If the axle is cut, obviously the wheels can do that. A differential, located at the cut, is a messy collection of gears that allows the wheels to rotate at different speeds as necessary, while simultaneously allowing power to be delivered from the main drive shaft to both wheels.
Now, an electric motor generally has a rotating part (the "rotor") and a stationary part (the "stator"). The way that an electric motor can replace a differential starts by noting that if the stator part of the motor is NOT held stationary, it will rotate in the direction opposite to the rotor--both parts rotate at half the original speed, which, RELATIVE to each other, is still the original speed. So, what we do is connect the rotor to one wheel, and connect the no-longer-a-stator to the other wheel through a simple reversing gear. Differentiation of rotation speeds happens automatically while the wheels go around a corner--the "slow" inside wheel holds back one part of the motor, making it approximate a stator during the curve, so to maintain relative motor speed, the other part of the motor must rotate faster. Eliminating the complex mechanism of the differential saves both weight and cost (and some mechanical energy losses), of course.-- Vernon, Sep 01 2007 Rankine Cycle http://en.wikipedia.../wiki/Rankine_cycleDry gas thermoregenerative cycle. [reensure, Sep 01 2007] wood chip powered automobile Wood_20Chip_20Powered_20Automobileas it says [afinehowdoyoudo, Sep 01 2007] Steam Super Stirling Engine http://ieeexplore.i.../18857/00870641.pdfjust what you mention in the second-to-last paragraph, Vernon [afinehowdoyoudo, Sep 01 2007] Cyclone steam engine http://www.cyclonepower.com/As mentioned in an annotation [Vernon, Sep 17 2007] A phase-change-dependent engine Ice_20EngineIn the main text here is mentioned a 1600:1 ratio as water changes phase to steam (or from steam to water). The engine at this link involves only a 10% change in volume, between phases. So why invoke the Carnot Cycle at all, when phase changes are a superior type of pheneomenon, in terms of energy efficiency? [Vernon, Sep 22 2007, last modified Oct 14 2007] Did yer like tha'? http://www.fred-dibnah.co.uk/ [bibliotaphist, Oct 16 2007] SPARLVE and RSE Copying_20SPARLVE_20and_20RSEAs promised in an annotation, here is the referenced Rotary Stirling Engine. Also, there is a thorough description of how a Stirling might incorporate gas/liquid phase changes. [Vernon, Nov 02 2007] Popular Science article http://books.google...v=onepage&q&f=falseAbout replacing differential with electric motor, as mentioned in the main text. [Vernon, Apr 28 2010] Another PopSci article http://books.google...v=onepage&q&f=falseThe original idea of using a flywheel in a car is to try to store enough energy to make a gas tank unnecessary. In this Idea, however, we only need to store enough to allow fast acceleration. And that can be done fairly easily. [Vernon, Oct 07 2011] One more PopSci article http://books.google...v=onepage&q&f=falseSome of the key points presented in this Idea can be found here. The main difference is that I proposed a steam engine instead of an internal-combustion engine. [Vernon, Oct 07 2011] TwinFlywheel TwinFlywheelA better kind of flywheel system, for storing the energy produced by that low-horsepower engine. [Vernon, Apr 09 2012] Good idea. I had posted one for a steam turbine hybrid a while back and ended up deleting it for lack of interest. This seems much more practical, however. [+]-- acurafan07, Sep 01 2007 Bun. Potentially multi-fuel (including biomass), low pollution, & can be used as a standby power supply for off-grid or emergency use. The use of biomass as motor fuel was dissed heavily in the discussion of woodchip car, however when one considers the greatly increased risk of wildfire in north america that is the result of almost a century of over-vigilant fire suppression and the resultant buildup of woody fuels, it may actually be beneficial if responsibly implemented.-- afinehowdoyoudo, Sep 01 2007 Some caveats: the efficiency of a Stirling engine or Rankine (steam) engine depends on many factors, especially the temperature difference between the hot and cold sides of the engine. Just because an engine is a Stirling does not necessarily mean it is 45% efficient.
The power density (kW/kg or kW/liter) of Stirling engines is typically low, a disadvantage in an automotive application. Rankine much better in this regard.-- afinehowdoyoudo, Sep 01 2007 [iron horse], the key thing about steam engines, "heat of condensation/vaporization", appears to be insufficiently exploited in the Rankine Cycle. A lot of energy can be transfered at the boiling point of water, without its temperature changing at all. It seems to me that the "ideal steamy Stirling" would want ALL the steam to condense in the cool zone, somewhat below the boiling point and all to boil in the hot zone, somewhat above the boiling point, thereby providing maximum pressure differential for pushing pistons, thanks to that 1600:1 change when steam condenses (and vice-versa when water boils). Carnot Cycle temperature differences/efficiencies have nothing to do with condensation effects, and the Rankine cycle is mostly just a variation of the Carnot Cycle.
Heh, even if I'm making a mistake in the above, it doesn't really affect this Idea much. A steam engine is specified, but the type of steam engine doesn't have to be a modified Stirling.-- Vernon, Sep 02 2007 I have a bunch of old Popular Science magazines from the 70's in a closet somewhere. It's fun to look at all the failed inventions and to laugh at the inferiority of all the "state of the art" technology. I think one of them had an article about Saab experimenting with a steam powered car i just kinda glanced at it and never got around to reading the article.-- Livingfishguy, Sep 02 2007 The problem with steam is that although is expands at 100C at sea level, it will remain a liquid when under pressure, so the resistance of the engine will raise the boiling point.
[BrauBeaton]I assume the car will have a battery so as long as the engine heats up before the battrery runs flat it should be ok.-- marklar, Sep 02 2007 [mariasman], thanks for the info. For the record, I need to point out that electric motors also don't need a transmission. I added some stuff to the main text, to clarify that.-- Vernon, Sep 19 2007 @[livingfishguy] - would you consider an offer on your old PopSci magazines? My dad got rid of his in the 80's and I miss them.-- elhigh, Sep 20 2007 I was ready to bone your concept but then I started reading it more carefully.
Now I'm not sure what to think.
I'll have to do some drawings and see what I can make of it.-- elhigh, Sep 20 2007 Wow looks like I commented too soon before reading all of it. A Steam-sterling engine is one of the best engine ideas I've ever heard. You could maybe even use some of the water before it's boiled to pump over the cold side of the engine to help condense the steam. This sounds worth patenting if it works how you describe.-- acurafan07, Sep 22 2007 [acurafan07], thanks for the nice comment. In the near future I hope to be posting a description of a purely rotary Stirling engine, specifically intended to take advantage of water/steam phase changes. But I have some other things to finish working on first.-- Vernon, Sep 22 2007 /So why invoke the Carnot Cycle at all/ .. any heat engine has a max efficiency, regardless of the working fluid or mechanical arrangement. This max efficiency is related only to the hot-side and cold-side temperatures and is referred to as the Carnot efficiency.-- afinehowdoyoudo, Sep 23 2007 [iron horse], obviously you haven't read or understood the "Ice Engine" (see "Phase change dependent engine" link). It does all its work at Zero degrees Celsius, and ONLY at that temperature.-- Vernon, Sep 23 2007 This is nothing but a big vague list.
Are you proposing to burn fuel in the boiler of a 20 Hp steam engine and then route that energy into a flywheel or to an electric generator for charging up a supercapacitor? Or are you suggesting to use a steam powered sterling engine for this?
Anyway, it sounds inefficient.-- quantum_flux, Oct 14 2007 [quantum flux], the Idea here encompasses several variations on the theme. A 20HP external combustion engine is the starting point; it might be a steam engine or a Stirling engine or some combination of the two. An electrical generator is required, and so also is an energy storage system. The typical storage system today in a hybrid car is a bank of batteries, but I'm quite sure a flywheel is better--and in the future a bank of supercapacitors will be better yet. Only one of those three things need be chosen for a particular vehical. Then there is the electric motor for moving the car (which must double as a generator during regenerative braking), and the electronic power-control circuitry.
That's all: Engine, generator, storage, motor/generator, and control circuits. Not the kind of list you were complaining about.-- Vernon, Oct 15 2007 When I was taking thermodynamics in college, and learned about the carnot cycle I designed a Carnot engine that works like this and sounds similar to the rotary sterling engine mentioned by acurafan07. My design looked like a long rotary vane compressor except without openings. Each of the four chambers was sealed from the others and as the four sectioned drum rotates heat was applied on one side and taken from the opposite side. Each chamber was filled with an amount of water that tuned it to work with the temperatures sources supplied. The simplest is if you had a 22°C source and a 101°C source and the chambers were set so that the smallest volume was the same as the volume of the water and the largest is the volume of that much water as steam. If the small volume is on the bottom and the heat is on the left, cool on right, the system will turn clockwise and effect a perfectly efficient Carnot cycle as the two sources approach point sources or maybe as the splits go from four to infinite. The system can be tailored to different heat ranges by changing the position of the center cylinder to change the ratio of small to large volume and by changing the minimum pressures so that the water boils completely at that temperature and pressure. Also you could change the working fluid to ammonia, freon or mercury to match the temperature ranges. I did some work to see that it would work as a solar engine if you floated it in long trough of water. The problem I remember was power to size and friction/sealing of the moving vanes. I can give a better description if you think this is different enough to make a new topic.-- MisterQED, Oct 15 2007 [MisterQED], if your idea is different enough from this one (which is more about a hybrid car than it is about an engine), then feel free to post it.-- Vernon, Oct 16 2007 Bun for anything powered by steeeam, like, y'know. [link].-- bibliotaphist, Oct 16 2007 For what it's worth, I noticed that you put the Steamer's hp at 20 hp, which refers to boilerplate horsepower; if I remember correctly, max horsepower at the axle was closer to 125 hp -- it just wasn't sustainable (which is why steam-powered automobiles' power ratings referred to boiler power).-- myislanduniverse, Oct 03 2008 I've thought about this sort of idea for a while. One of the most obvious things would be to combine a standard internal combustion engine with a Sterling engine acting as a cooling system for it. One engine would be powered by the expansion of the combustion, the other powered by the heat generated.
After a little research, I found someone's invented a "six stroke cycle", where the two further strokes are water being injected and evaporating to drive the piston down. It's around 60% efficient, which is good, but needs a supply of water, of course.-- mitxela, Oct 03 2008 [mitxela], why not use a steam engine combined with a Stirling engine? It may be that the Stirling can help cool the steam enough to reduce the need for water stops. And the steam engine still has its inherent advantages over internal combustion, fewer pollutants and a wider fuel range.-- Vernon, Oct 04 2008 Thanks for the introduction to the electric-motor "differential": it seems to save the bother of running two wheel motors at different speeds, though it might weigh a bit more (because of the two half-axles).
The problem (that I see) with a water phase-change Stirling engine is that you have to dump or infuse so much energy to get it to change that it couldn't be done in a reasonable amount of time in a cylindrical volume with a piston moving through it.-- FlyingToaster, Jun 06 2011 random, halfbakery