Free piston engines

[Roger]

I came across this interesting film about Free Piston Engines, which I was surprised to see were used in an experimental French locomotive at timestamp 11:07.

I don't know if any of these designs made it through to the present day, perhaps Reg knows?

I came across this idea some years ago in a series of books I bought from a Car Boot Sale. Sulzer made something very similar in that book. I suppose the cost of the turbines and reduction gears made them uneconomic compared to a relatively simple Diesel Engine.

It looks like this Shell YouTube channel has a lot of interesting archive films.

[RGR 60130]

Jan 29, 2024 9:04:56 GMT Roger said:

I came across this interesting film about Free Piston Engines, which I was surprised to see were used in an experimental French locomotive at timestamp 11:07.

I don't know if any of these designs made it through to the present day, perhaps Reg knows?

I came across this idea some years ago in a series of books I bought from a Car Boot Sale. Sulzer made something very similar in that book. I suppose the cost of the turbines and reduction gears made them uneconomic compared to a relatively simple Diesel Engine.

It looks like this Shell YouTube channel has a lot of interesting archive films.

They never caught on at sea. Ships got a lot bigger very quickly after that moment in time so that may have had a big part to play. I’ve sailed on opposed piston jobs though they didn’t stand the test of time either. Air springs are still used a lot for exhaust valves and are quite reliable. The reed type suction valves are fairly reliable - often used in scavenge spaces. The plate type valves as used in air compressors too aren’t all that reliable over extended periods, especially when getting hot. Sulzer started using power recovery turbines in the 80s which utilised ‘spare’ exhaust gas to power a turbine geared to the crank shaft of a conventional diesel engine. The general feeling was that the installation and maintenance costs would probably exceed the saving in fuel over the life of the ship. I don’t recall any mention being made of running backwards / astern , possibly requiring a second turbine for that purpose - big money. Steam turbines are wonderful things in terms of maintenance requirements as long as the steam quality is maintained within parameters. Using exhaust gas is a different matter entirely as deposits build up on blades and nozzle rings making regular cleaning a necessity.

Reg

[andyhigham]

There was a design that took this idea a stage further.
A 2 stroke engine with reed valve inlet, combustion chamber and a resonant exhaust. It powered flying bombs aimed at London during WW2
More recently the design has been improved by removing the reed valves and fitting a resonant U shaped inlet tube.
Known as the "pulse jet" engine

[Roger]

Jan 29, 2024 20:45:13 GMT RGR 60130 said:

Jan 29, 2024 9:04:56 GMT Roger said:

I came across this interesting film about Free Piston Engines, which I was surprised to see were used in an experimental French locomotive at timestamp 11:07.

I don't know if any of these designs made it through to the present day, perhaps Reg knows?

I came across this idea some years ago in a series of books I bought from a Car Boot Sale. Sulzer made something very similar in that book. I suppose the cost of the turbines and reduction gears made them uneconomic compared to a relatively simple Diesel Engine.

It looks like this Shell YouTube channel has a lot of interesting archive films.

They never caught on at sea. Ships got a lot bigger very quickly after that moment in time so that may have had a big part to play. I’ve sailed on opposed piston jobs though they didn’t stand the test of time either. Air springs are still used a lot for exhaust valves and are quite reliable. The reed type suction valves are fairly reliable - often used in scavenge spaces. The plate type valves as used in air compressors too aren’t all that reliable over extended periods, especially when getting hot. Sulzer started using power recovery turbines in the 80s which utilised ‘spare’ exhaust gas to power a turbine geared to the crank shaft of a conventional diesel engine. The general feeling was that the installation and maintenance costs would probably exceed the saving in fuel over the life of the ship. I don’t recall any mention being made of running backwards / astern , possibly requiring a second turbine for that purpose - big money. Steam turbines are wonderful things in terms of maintenance requirements as long as the steam quality is maintained within parameters. Using exhaust gas is a different matter entirely as deposits build up on blades and nozzle rings making regular cleaning a necessity.

Reg

Thanks for that, I can see why really large Diesels are probably better suited for high efficiency and very long life, compared to these more exotic engines. It was an interesting moment in the development of Diesels that really led to Gas Turbine engines. The chapter in the book has a title something like 'Supercharging and where it leads'. It shows the effects of ever increasing boost pressure that results in more energy being recaptured from the exhaust than was being developed at the crankshaft. The further you go down that road, the less important the crankshaft output becomes, until it's just a positive displacement gas producer. Once you reach that point, you have to make the decision as to whether that makes sense any more or whether a turbine compressor stage is a better option.

[chris vine]

Yes, that is very interesting, Roger.

In gas turbines, the compressor delivers something like 4 times the amount of air needed for combustion - this is to keep the turbine inlet temperature low enough to not melt the turbine.
The trouble is that the compressor and turbine are not 100% efficient. So you are compressing a lot of air/gas and expanding it through the turbine in a sort of back-to-back arrangement, losing energy as you go.

The standard gas turbine uses a constant pressure cycle. IE, the delivery pressure from compressor equals the input pressure to the turbine. (less a tiny bit in the combustion chamber).
Again, this is inefficient because the compressor (which is not 100% efficient) has to compress the gas to a high pressure to drive the turbine.

The alternate cycle would be a constant volume cycle where the gas is compressed to a lower pressure, but this gas is then put in a different chamber for combustion. Now the pressure rises on combustion and can drive the turbine at higher pressure. Or, a lower compression pressure for the same turbine pressure.
This would be great and has been looked at, but the problem is the complexity and mass of all the extra valves and chambers to make it work. For an aero engine, increased mass gives decreased efficiency because the plane has to get off the ground!!

(This constant volume cycle is what happens in a normal petrol or diesel piston engine.)

One would think that gas turbines might have been made for stationary/ground use which used the constant volume cycle. However, almost all industrial gas turbines are based on aero engines so it probably would be very expensive. Also, I suspect that the combined cycle where the gas turbine exhaust drives a steam turbine, probably makes up most of the theoretical difference anyway.

Thanks for bringing the film to us!
Chris
Chris.