Most efficient steam engine design?

[cbwho]

The simple Mamod traction engine has variable cut off, using the eccentric 'throttle' changes the position of the cylinder oscillating axis, which in turn changes the period of the inlet.
Maximum efficiency of a boiler is achieved using maximum superheating of the steam. The energy required to boil water (latent heat of evaporation) is required whatever the boiler pressure. Additional heat used to increase steam temperature and pressure is realised during the expansion phase and is delivered as output power.
The most efficient steam engines are supercritical, where the steam is superheated beyond the point at which the pressurised stem is as dense as water, allowing for efficient heat transfer. This happens at pressures on excess of 3000psi and temperatures in excess of 400 degrees Celsius. These figures are only realistic in flash steam boilers, where the pressure vessel consists of small diameter tubes which have high strength relative to their inside surface area. An interesting topic, which I intend to pursue at some point.

That is definitely true of the later Mamod TEs but not true on the early 60s TE. My experiment involved a non-adjustable TE frame mounted with a 1980s Mamod "locomotive" double acting oscillator cylinder. The double cylinder clearly had double the power but it also clearly consumed twice the steam. Admittedly my sample size is small.

 

[dazz]

Hi
Condensing steam does more than just recycle water. If done right, it creates a reduced pressure on the engine exhaust to below atmospheric. This improves engine power and efficiency.
Condensed steam would need filtering before it could be put back in the boiler.

There is a steamboat called the TSS Earnslaw on a lake in the South Island of New Zealand. This is fitted with two locomotive boilers and uses lake water to condense the exhaust steam.

Dazz

 

[popnrattle]

I made a "steam" engine with cylinders. 1/2" stroke and 3 inch bore is short on torque but can be geared down to meet your needs.

 

[Rocket Man]

I remember reading once how exhaust steam from #1 cylinder can be reused to power another #2 cylinder. I don't remember how well that works but I understand the physics, it appears to me back pressure on #1 cylinder will reduce its power and power to #2 cylinder should not be very high. This engine design might have a special name?

 

[Shopgeezer]

The triple expansion engine is a common marine engine. This is three cylinders. Exhaust from the first cylinder enters the second, and exhaust from the second enters the third. Each cylinder is larger than the previous one. It requires lots of pressure to the first cylinder. Very efficient. Triple expansion engines ran a lot of the commercial shipping in the world right up until the 1950’s. I know an older engineer who worked on a steamer in the Caribbean bouncing around the islands. That’s the way I would like to spend my winters! Here is a description of this engine.

https://en.m.wikipedia.org/wiki/Compound_steam_engine

Here is a model kit for one of these engines:

https://www.ministeam.com/acatalog/Martin-Triple-Marine-Steam-Engine-Martin_Triple.html#SID=289

 

[Charles Lamont]

This engine design might have a special name?

An engine with two-stage expansion is called a 'compound'. The physics is about minimising the temperature variation in each cylinder. In engineering practice it also reduces the effect of valve and piston leakage.

 

[Jules]

The Titanic ran the exhaust from its triple expansion engines into a steam turbine.
Now that’s really efficient.
The output power from the turbine was close to that of the two triple piston engines.
The turbine drove the central propeller.

 

[k2steve]

The Titanic ran the exhaust from its triple expansion engines into a steam turbine.
Now that’s really efficient.
The output power from the turbine was close to that of the two triple piston engines.
The turbine drove the central propeller.

I did not know that - Thanks

 

[Shopgeezer]

Now you have me thinking about running my own steam engine exhaust into a turbine. What a great idea. Interesting historical note. I understand that the Titanic was the fastest liner of its day.

 

[TimTaylor]

Keep in mind that the Titanic boilers (29 of them) generated steam at 215 psig working pressure - much higher than is safe for any hobby application - so they had lots of pressure drop to work with. It's triple expansion marine engines were specifically designed to provide the low pressure inlet steam to the Parsons low pressure turbine that drove the center propeller.

Steam turbines work off of isentropic enthalpy drop. At the typical pressures we use for hobby boilers, to get any useful work out of the steam engine exhaust, you would have to use a rateau type turbine designed specifically for the pressure and flow - there simply isn't enough pressure drop to use a simple curtis stage design. You'd also likely need a condenser to get enough pressure drop.

In condensing applications, the condensate may or may not require filtering for re-use, depending on the nature of the contaminates, i.e. oil from displacement lubricators.

 

[Andy Munns]

Many people think they started with the High Pressure and added the IP and LP, but historically they started with LP atmospheric engines where the vacuum did most of the work and added higher pressure cylinders fed by higher pressure boilers. The reluctance to embrace high pressure steam was a bit like the reluctance to embrace DC power (pro kerosene lobby) AC power (pro DC lobby) and today's renewables and EVs. Compound, triple and quadruple expansion reduce the temperature (actually heat) loss in each cylinder and this made transatlantic steamships possible. Titanic was a bit old fashioned and slower cf Cunard ships that had already gone 100% turbine. They still expand the steam over multiple stages. Recip engines cannot sensibly build LP cylinders with big enough exhaust ports, hence the push to LP turbines with their huge exhaust ports that can get the last bit of energy from huge volumes of low pressure steam.

 

[deverett]

They were still building vessels with triple expansion engines and low pressure turbines in the 1950s. I know because I sailed on one in my later Merchant Navy career.

Dave
The Emerald Isle

 

[Shopgeezer]

Twenty nine boilers! Did Titanic burn bunker oil? I can’t imagine shovelling coal for that many boilers but she was old enough that maybe so. You would need a lot of stokers.

 

[Peter Twissell]

She did indeed burn coal.
One of the coal bunkers already had a fire burning in it before she set sail.

 

[redryder]

One of the most efficient designs ever was the steam-diesel for marine use. While very efficient compared to other existing power plants, it was a commercial failure given that it's operation required tending and maintaining by both a diesel engineer and a steam engineer. The limited number of engineers who were certified for both steam and diesel stopped going to sea and became marine surveyors. Not installing it vs hiring 2 full time engineers for every shift, I believe, kept the installed fleet at just two ships.

 

[Richard1]

There is partly described in "Audel's" engineering encyclopedia. Exact name and publication date forgotten a single acting quintuple expansion engine as I recall the HP piston was 1" diam and the LP was 8.5" with an inlet pressure of something like 2,500 PSI. I have no idea if it was ever built or what the efficiency would have been.

Very rough back of envelope calculations suggest the cylinder bores would have been
1", 1.75", 3", 5", 8.5" and would have expanded the steam 3 times in each stage.

 

[Vietti]

Don't forget that the D valve can be balanced and reduce sliding friction.

 

[Richard1]

Don't forget that the D valve can be balanced and reduce sliding friction.

My guestimated inlet pressures are:-

HP 2500 PSI
1st IM 833 PSI
2nd IM 269
3rd IM 92 PSI
LP 31 PSI

Condenser inlet 10 PSI absolute.

With these pressures I don't think they would be using any sort of a slide valve balanced or otherwise at least on the first 3 cylinders. I would be very curios to know what sort of valve would stand up under those sort of pressures and temperatures. Without any superheat at all the inlet steam is 668°F I assume if you are going to the trouble and expense of such a complex engine you would want a lot of superheat.

Lubrication would be interesting to.

 

[Andy Munns]

Not sure about 'simpling valve' for a marine compound or triple... I teach that these were usually called an Impulse Valve and only gave a shot of steam into IP or LP receiver for nudging a stuck HP piston off dead centre - Rarely needed if you had some vacuum as working the links was faster. The impulse valve does introduce back pressure on the exhaust of the previous cylinder so the indicator diagram would be somewhat irrelevant.

Then, in the last days of recip steam, designers went back to double compounds with poppet valves + serious superheat + HP to LP reheat + an exhaust turbine on marine jobs. Sometimes also a exhaust turbine from the LP exhaust to the LP receiver. Search for Lentz engines.

 

[Richard1]

Not sure about 'simpling valve' for a marine compound or triple...

I did briefly work on a paddle steamer where the simpling valve simply turned full boiler pressure onto the LP valve chest and diverted the HP exhaust up the stack. I don't think this is usual though.