Rectangle steam cycle

Tell me what's wrong with this idea:
Steam cycle is a rectangle on the PV diagram. Steam is worked non-expansively in a cylinder, being supplied at the rate of evaporation in the boiler. Quality goes from Q in the boiler to 1, dry and saturated, at the end of stroke. The steam is then exhausted into an isochoric heat exchanger, where it loses heat until it reaches a very low quality. It is then partially condensed back to the original volume and put into the other side of the heat exchanger, where it is heated back to quality Q by recuperated heat. The boiler then evaporates that steam to dry and saturated, at the rate of consumption in the cylinder.
The work done is the work of that evaporation. The heat rejected is the same, the heat of evaporation of 1-Q which cannot be recouped in the heat exchanger, plus the small amount in the condenser. So the efficiency is 50%, less a small amount.
Basically, the idea is that work should be done by evaporation, not by an ideal gas engine cycle.
An isochoric heat exchanger is a difficult proposition that has been investigated for Stirling engines.
50% efficiency on gasoline is 75 mpg. Of course you wouldn't get that in actual use. On the other hand, regenerative braking would be simple.

Steam is worked non-expansively in a cylinder

Not So. Most engines, the valves close with a very short after opening, the rest of the stroke is expansion.
Rolly

Hi Rolly,

I think he's proposing a non-expansive cycle, one with about 100% cutoff. The exhaust heat is mostly rejected in a heat exchanger and then goes to a condenser, but is only partly condensed. The condenser then exhausts to the feed pump back into the other side of the heat exchanger where the partial condensate is reheated by cooling off the exhaust steam. From there it goes to the boiler where the temperature is raised back to operating levels.

It'd be interesting to see how much pump work would be necessary since you wouldn't be moving a pure fluid.

Right, full-stroke admission of steam. This has the big disadvantage of requiring a multi-ratio transmission.
The exhaust steam (from the hot side of the heat exchanger) could be completely condensed, if it turns out that the heat rejected is less than the work of pumping the steam at the low volume into the cold side of the heat exchanger.

On second thought, it's obvious that we could use a variable cutoff, instead of full-stroke admission, and recuperate whatever heat is left in the steam. Efficiency would be even better. No gearbox required. So the only new thing I propose is the isochoric heat exchanger working with wet steam.

Sounds reminiscent of the Anderson-McCallum vapor recompression locomotives. The patents can be found at:

Forum Link

The Museum of Retro Tech (an interesting website well worth the visit) also has an article:

Anderson Loco

Ken

The mechanical vapor recompression is a Brayton cycle. It has some advantages, certainly over a non-condensing engine. The design you linked to seems to have insufficient cooling for the condensers.

Well, it was real hardware and had a better than 20 percent improvement in efficiency over a standard locomotive, so it's a bit hard to judge if anything was insufficient. The patents show that they did a lot of design and development work. By all accounts, they muffed the blower; it's hard to figure out how they pulled that off. As it is, the blower difficulties were mechanical and not inherent to the cycle.

I also have in front of me an analysis received in the mail this past Wednesday by a club member where he sets up three different analysis methods for another similar cycle. It's a bit hard going because a couple of the tables aren't as intuitive as I'd like.

Regards,

Ken

On third thought, the hot side of the heat exchanger would not have to be isochoric, but instead should be isobaric. Since the steam in it is wet, it stays at the temperature of the cylinder exhaust. The feedwater pump would have to vary its output pressure in order to keep the pressure (and temperature) of the wet steam in the cold side of the heat exchanger less than that of the hot side. Until we get to the last, boiler, stage, the steam in the cold side is at a lower pressure and temperature than that of the hot side, in order for the heat to move across the exchanger; 50 deg.-F is a typical difference. The only heat rejected is that necessary to make this difference.
At early cutoff, high expansion, and low exhaust pressure, there would probably be no gain from this system. It would be a big improvement at high power and long cutoff, though, not only in efficiency but also in eliminating the need for a large radiator.

Hi Ed,
An interesting thought, the Brayton cycle will provide for a higher Carnot Efficiency than that of the Rankine Cycle. This is comparing piston engine configuration to the same in a steam engine. Gas Turbines are a different animal.

Here is an empathy I just had, the Brayton Cycle will use the compressive stroke of the piston to push the air-fuel mixture into the burner. Then the expanding combustion gas goes to the down piston stroke. The rotating inertia keeps the rotation going. The benefit to Carnot Efficiency is the heat exchange at the piston-block. Like the Steam Engine, the Brayton piston engine should be insulated. This is definitely a plus for the engine. The PV diagrams are very similar on these engines.

The down side is that like a gas turbine engine, the piston version will get really hot to the point where it will melt the common materials used in the day. It wasn't until the German Engineers developed high Nickel Alloys to withstand the temperatures. Here is the thought, would you use a SS piston in a steam engine? This is probably what lead to the demise of the piston Brayton Engine. Perhaps one could make this system work with Plasma Spray, Ni-Graphite type, and provide for an abraidable seal. This is the coating used to seal the turbine blades/labyrinth seals on the gas turbine engine.

Last is that the Brayton Engine works best at high RPM to achieve the HP required. In other words, Brayton is a less torquey engine.

I enjoyed looking at these engines at the Henry Ford Museum. Thanks to Tony who got me into the museum on his yearly pass. Nick Mesmer joined us right after the 2019 Spring Meet and on our way home to upstate NY.

Sorry, I'm still sold on the Steam Engine.

Kind regards,
Rick

The Brayton cycle for a steam engine would have the same problem as the Rankine at high power and long cutoff, that of low efficiency and lots of heat rejected which requires a big radiator. Low expansion means a wide loop on the PV diagram.
Regenerative cycles are not subject to the Carnot efficiency limit. A closed-cycle Lenoir, theoretically, would get 80% efficiency with an isochoric regenerator. (Using an ideal gas. With steam, the Lenoir cannot regenerate because the exhaust temperature is the low temp.)
Unfortunately, we will need a heat exchanger that is isochoric on both sides. Otherwise, the exhaust stroke of the working cylinder will have to compress the steam into the hot side of the exchanger and will use all the energy. The way we avoid that is by moving an equal volume to the condenser at the same time. So the hot side has to be isochoric.
Using the retained heat in an internal-combustion engine to generate steam by water injection was tried in marine engines in the 1920's. Apparently it wasn't worth the complication.

What's wrong with the idea is that evaporation at the low volume takes proportionately less energy than condensation at the high volume. Same specific energy, but a proportionately smaller mass is changing phase for each change in pressure. So the cold side of the heat exchanger can only recuperate a fraction of the heat rejected in the hot side.
Too bad it doesn't work! I thought using the heats of vaporization would get around the Carnot limit.

As Professor Peabody pointed out many years ago, using different words, the Brayton cycle for wet steam IS the Carnot cycle. The heat transfers are isothermal. So the theoretical efficiency is the temperature difference divided by the (absolute) high temperature.

Hi Ed and al,

This is very similar to a concept which I have been working on, in the background, for a number of years. i think that it may have great promise. The downside is that it may take a lot of trial and error. Starting from a somewhat conservative/traditional steam car system with a record of successful running on the road would be advantageous, IMO. The heat exchangers may be much more difficult to work out than it may appear at first glance.

Peter

And, since the Rankine cycle is a special case of the Brayton cycle, the Rankine cycle using wet steam will also have the Carnot efficiency. Excluding superheat, however, limits the high temperature to 600 (or at the most 700) deg.-F, which, with the low pressure being atmospheric (in order to prevent sucking air into the engine) gives 37 to 42% theoretical efficiency. Taking 150 mpg as 100% efficiency on gasoline, we start around 60 mpg (before considering losses).
Of course, we need expansion ratios of like 30:1 and 40:1. However, that doesn't seem too bad for a compound engine.
The problem of low efficiency at high power can be solved by scaling the engine to give full expansion at maximum power. Put on an automatic valve connecting with the condenser to prevent pulling a vacuum in the cylinder at lower power and earlier cutoff.
No cylinder oil will be necessary. Engines have been successfully run with only condensed water lubrication.
This is all theory. But it is a new theory, and one which explains my feeling, over the past twenty years, that wet steam is the thing to use in an engine.