Steam-Electric Hybrid

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.
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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.

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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.

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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.