The Mk 2 engine was built and tested. It had the same piston with 50 mm diameter and 120 mm stroke. However, there were several modifications (in fact its a completely new machine):
- A crankshaft was added
- The flywheel had a diameter of 320 mm, with a 3:1 transmission to increase the speed.
- Three different condensers were tested, the old one ( a tin can), a tube with cooling fins and finally a heat exchanger
- The control system was re-designed from scratch.
- An electric brake with torque transducer was added to improve power take-off.
- The monitoring system included four pressure transducers (boiler, cylinder top and bottom and condenser) as well as temperature measurements and torque measurement.
And here’s the engine running. The first part of the video shows the engine with the old condenser, the second with the heat exchanger:
As you can see, the heat exchanger worked much better.
We have now finished testing the Mk 2 engine, with some reasonable results and a lot of information of where things can be improved. First the good news:
- Steam expansion is working, and it does increase the engine efficiency.
- The overall efficiency at the piston ranged from 2.0% for no expansion to 6% for an expansion ratio of 1:4. This corresponds roughly to 1/3rd of the theoretically achievable maximum.
- By accident (the steam production of the boiler appears to reduce when water levels go low), we ran the engine with a boiler pressure of 1 bar (atmospheric pressure) in the beginning of the test run, and finally with -0.25 bar or 0.75 bar (abs) at the end. So, the boiler pressure was a good bit below atmospheric, and the boiler temperature at around 90C. This was a world first, a steam engine where the boiler has a negative pressure! And, before you think “well, so what” – this of course widens the area of application of the engine into a temperature range below 100C whilst still using water as working fluid.
The two figures below show the situation. Fig. 1a shows the difference in work conducted at the piston between down- and upstroke. In Fig. 1b, the pressures in boiler, cylinder and condenser are given, the pressure drop between boiler and cylinder, and cylinder and condenser is very clear.
a. Work during up- and downstroke b. Boiler, cylinder and condenser pressures
Fig. 1: Engine performance
The other news:
- The condenser pressure was higher than expected with 0.13 to 0.45 bar (abs.). The lower pressure occurred for expansion ratios of 1:3 and 1:4, the highest pressures for the case without expansion. This of course indicates that the condenser needs to be improved, at the moment it cannot reject the heat from the condensed steam.
- There was a significant pressure drop at the valves, between 0,35 and 0,4 bar (of a total pressure difference of 0.6 to 0.87 bar). This limits the achievable power further.
- The engine had significant internal friction, caused by unsuitable bearings, the piston rod seal and alignment difficulties. Also, it is expected that the parallelogram had significant friction.
Good news about this: all these problems can be addressed by improving the detail design of the engine, none of them is linked to conceptual issues. So, the plan is to design slide valves made from plastic. Commercial solenoids (electrically actuated valves) do not appear to be suitable for the specific purpose.
The condenser needs to be designed as either a heat exchanger or a spray condenser. In addition, we need to start to think about a complete heat rejection chain so that we can circulate the cooling water. We are currently preparing a rather exotic potential solution for this problem, more about that in eight weeks.
Internal friction needs to be reduced by better detail design.
In general I can say that we are now ready to design a larger engine with a good base to predict performance. We will not achieve a power output maximum, but we can certainly guarantee an efficiency of 3% for no expansion, and 9% for an expansion ratio of 1:4.