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Experimental engine

Posted by mike t 
Re: Experimental engine
June 26, 2015 02:49AM
I agree that air will behave differently, and have tried to make it clear that I understand this, however I would be very surprised if the difference will swing the results in favour of the uniflow only exhaust, in this engine at least, it is currently producing only 20% of the power of the poppet exhaust configuration for a given amount of air consumption. I suspect it may improve at higher rpm, if an earlier engine I fitted to a motorcycle is anything to go by. This week end I hope to fit a belt step up drive between the engine and the alternator, so I can control the out put at higher rpm. I think the bash valve timing needs to be reduced any suggestions as to what would be a good starting point?
Thanks for your comments, Mike T.
Re: Experimental engine
June 29, 2015 04:16PM
Hello again, the air behaves more than differently, it is not even comparable, at best with the air you are releasing from some previously stored pressure tank through a device that changes the rush of air into rotary motion by interrupting its path, how you would expect using this released pressure to recompress the same air that is being released was going to give any result, I am at a loss. A uniflow engine recompresses the "wet" expanded steam, introducing heat through the compression............. it is a heat engine, and it does work, and it is efficient, a rush of escaping air is just that, I am trying to be helpful here not argumentative and sorry if it does not seem this way, I just don't want somebody obviously very capable to waste his time. also, forget bash valves, they have no mechanical sympathy and anyone who has any kind of engineering background knows collisions between components always results in failure of a part very soon.
To Kimmel says in his blog something along the lines of , if anyone is going to make a good engine he needs to start at an early age, as there are a lot of bad ideas to get out of your head first, I concur with him
Re: Experimental engine
June 30, 2015 04:22AM
Hi Chris, untill I can accurately measure the steam consumption, I will have to accept what you are saying. However my gut impressions on running previous engines on compressed air then on steam were that the performance did not vary significantly. two double acting slide valve engines at 120 and 150 psi and a single acting poppet inlet uniflow exhaust at 600 psi. The engines were all run in on compressed air then run on steam, they seemed to behave very similarly with either air or steam, however I am unable to quantify this, which is the purpose of this test engine and when I can come up with a steady supply of steam and some means of accurately measuring the flow rate without having to hold the engine at a steady setting for an extended period of time, I hope to have some results that will show the difference between steam and air but more importantly show the most efficient configuration for this size of engine at least. Have modified the dyno so it will not allow the engine to run away at higher rpm. So I can now take readings at the rpm the engine is intended to run at. The bash valve head seemed easy enough to construct and was intended to fill out the information, not as a serious contender for reliability, one other head I hope to make is a piston valve to be configured piston inlet , and piston or uniflow exhaust or both, this should help set a benchmark. By the way if you compress any gas it heats up.
Re: Experimental engine
June 30, 2015 04:50AM
Ok I give up
Re: Experimental engine
June 30, 2015 08:06PM
Don't give up Chris. I have run engines with a huge welding tank at 2400psi without a regulator using my direct injector valve (variable lift).

I could actually see ice form on the stem of the exhaust valve of the engine that was running.
Re: Experimental engine
June 30, 2015 11:41PM
Here's a video of the injector cycling about 0.07sec. And yes I do directly hook this valve on to a pressure tank with no regulator. The tolerances are over built to 12,000psi working pressure.

L912 injector test1
[www.youtube.com]



Edited 1 time(s). Last edit at 06/30/2015 11:42PM by Jeremy Holmes.
Re: Experimental engine
July 01, 2015 05:43PM
Hi Jeremy

I am only giving up trying to explain the differences between air and steam, and that putting heat into compressed air makes no difference to the air, but a large difference to steam, because it is a heat engine........and if you don't see any increase in power between steam and air......my traction engine will just about get itself out of the shed on 100psi of air, 100psi of steam and I have pulled 60 tonnes!!!!!!
anyway all I was attempting was to save Mike some time, but ....
Re: Experimental engine
July 01, 2015 09:03PM
Chris,

The big issue is thermal loss. Just the fact that Mikes cylinder sleeve has no fins is a bonus...

Air engines with fins like a Briggs and Stratton conversions actually gain performance since the fins keep the engine running at room temperature which is a source of heat, since the engine running with compressed air produces a cooling effect.
Re: Experimental engine
July 02, 2015 07:34PM
Yes Chris, don't give up I am listening to what you have to say, how ever as I mentioned it does not tie in with my own observations which may or may not be accurate. I believe that the size of an engine has a great effect on matters due to the difference in surface area to volume ratio, and the ability of a smaller engine to rev higher which reduces the time available for heat loss to occur.
Re: Experimental engine
July 10, 2015 05:48AM
Regardless of the working substance a piston device is obays the conservation of energy law.

Work = Force × Distance

Force is the product of Area and Pressure.

Work in the cylinder comes from two processes. One is at constant (ideally) pressure. Here the force is constant and equal to area times pressure. And thus work is simple to calculate.

Work = pressure × area × distance

Or:

Work = pressure × volume_change

The second process involves pressure change due to expansion or compression. Here the working substance steam or air may have an effect.

Air will contain far less thermal energy then steam per unit volume. And thus will have a different rate of pressure and temperature change per volume change.

It is the expansion and compression processes that will be different.

There is a misconception of the energy conversion of steam engines. By mathematical equivalents the formula seam to indicate that all conversion takes place in the engine. But that is not the case. In the boiler we heat water and change it into steam. That is a volume change at constant pressure that is work. The constant pressure work of admission was done in the boiler. During expansion or compression there is an internal energy change equivalent to the mechanical work. We commonly calculate work using enthalpy, H. But the enthalpy H is equal the product of pressure and volume plus internal energy, U, with appropriate unit conversion factors.

H = P × V × C + U, C a conversion factor.

Normally a full expansion Rankin cycle work is calculated as:

WORK = H1 - H2

But:

H1 = U1 + P1 ×V1

and

H2 = U2 + P2 × V2

Separating the proceses:

Inlet_work = P1 ×V1

Expansive_work = U1 - U2

Exhaust_work = P2 × V2

This is more complicated with clearance and compression. But what I am trying to get across is exectly were the difference between working substance has an effect. It is only in the engine processes involving internal energy factors. Expansion and compression. The constant pressure processes are not affectted. Except for thermal transfer where temperature differences between the working substance and the engine surfaces are involved.

I am gussing here. I think that air will have a higher pressure change for a given volume change. Thus producing less work then steam for a given expansion ratio. That difference would be less at lower expansions going to no difference at no expansion. Heat transfer between the working fluid and engine surfaces is reversed if the compressed air is not heated significantly above ambient temperature. But on the other hand heat transfer to the air during expansion may result in a more steam like expansion line.

Air might provide an idea of counter flow vs uniflow performance. But I would not relay on such a test as there is just to much unknown.

Personally I think a counter flow engine is a better choice. I believe that high compression is the advantage. And a counter flow having separate inlet and exhaust passages and valves would by a better choice for automobiles.

Andy
Re: Experimental engine
July 10, 2015 04:02PM
All physical theories, their mathematical expressions apart, ought to lend themselves to so simple a description that even a child could understand them.

Albert Einstein
Re: Experimental engine
July 15, 2015 06:07PM
That is a quote taken out of context. Having to do with aether not a part of special relativity. But even so it doesn't apply when looking for numerical results for a specific application. As, here comparing a engine operating with different working fluids over different temperature ranges. The thermodynamic properties of air and steambeing different effect the result as does the operator in temperatures.

Andy
Re: Experimental engine
July 16, 2015 08:12AM
Therefore, since brevity is the soul of wit,
And tediousness the limbs and outward flourishes,
I will be brief.
~William Shakespeare, Hamlet
Re: Experimental engine
July 22, 2015 11:01AM
I look forward to hearing more about your experiments. A reconfigurable test rig is a great way to confirm (and debunk) the various theories. I'm especially looking forward to seeing the degree of difference between the various configurations.
Keep up the good work.
Oh, and some videossmiling smiley
Chuck Whitlock
Re: Experimental engine
October 18, 2015 09:08AM
Mike, this is really great stuff. I hope you will put your data here. I have a theoretical model and it is fun and possibly useful to compare it to experiments, and then guess at where the discrepancies come from.

Quote

Lohring, I'am sensing power with the automotive alternator, the housing of which is free to rotate in bearings coaxial with the shaft, a load cell is connected to an arm.

Quote

The load cell would not hold its calibration, may be drifting with temperature changes. I am currently reading the torque from a spring scale.

I read about one experiment where bearing stiction caused an error in the torque reading. They found that a flexible coupling was better. But this was in a much lower torque apparatus.

If you gear up the alternator, how do you intend to account for the gearing loss? Superficially, I think changing to a different direct drive brake would be better. Eddy current brake perhaps? As for comparing different heads it might not matter much, but for comparing to theory, I guess the gearing introduces another unknown?
Re: Experimental engine
October 18, 2015 09:26AM
Quote

I question in the case of isentropic pressure drop what work is produced.

In the unaflow for example, the residual steam has done work from the moment the port opens until the exhaust steam is all exhausted. It has done the work of pushing the exhaust steam through the port.
The exhaust steam has done work on the condenser or receiver (in the case of a compound) fluid.
I don't see a problem with the residual steam having the same entropy as the steam at cutoff, if we are excluding heat transfer and other difficult things from the calculations.
Re: Experimental engine
October 18, 2015 09:37PM
The two boats and the motorcycle that I built did not appear to vary much in the power they put out if fed with steam or air at the same pressure, however I made no attempt to measure this. I think 100 psi acting on a piston will produce similar work regardless of whether the working gas is air or steam. Perhaps on a large engine the temperature difference makes a difference in how the valves and rings seal. Currently I do not have access to a boiler of sufficient size to run the test engine at a steady enough setting for gathering usefull results on steam. I hope to remedy that in the near future. I am thinking of making an ofeldt style boiler that could be used at a later date in a boat. Regarding the mods to the dyno, the figures I got on compressed air are near to identical comparing the direct drive to the alternator and geared drive. Only now I can expand the envelope of the test runs to higher power and rev settings. The alternator housing is very free to revolve, my troubles were from the load cell amplifier, possibly temperature related or could have been due to the proximity of the alternator itself with its high currents creating interference in the unshielded wires from the loadcell to the amplifier. The mechanical mechanical spring scale for measuring the torque has proven reliable and has retained its calibration.
Regards Mike T.
Re: Experimental engine
October 19, 2015 05:52PM
I can only think that no matter what the difference vs. steam is, the data you gather on air is going to be very valuable.

If one is sacrificing efficiency for power density in the expander, then just keep in mind that the expander is only part of the overall plant, and that the boiler, condenser, pumps, fan and blower is sized to water rate and not to mechanical power.

110 psig would require a rather low compression ratio in a uniflow with atmospheric exhaust, in order to avoid overcompression. For steam I think 6 might be too much, air I have no idea.
A variable aux exhaust gives the flexibility to reach correct compression pressure. So does variable clearance, but the latter does not completely eliminate the compression work when this is desired (for slow speed/starting). If you have too high compression ratio but pump the compressed steam into the steam chest, then there is an irreversibility associated with that (flow loss), a decrease of expansion ratio (you could spend much of the downstroke re-admitting the "old" steam), an increase in compression work and more leakage (spending more time with large delta P over the rings) compared to a full compression engine.
An engine with an aux exhaust on the other hand would benefit from a clearance as small as possible, to reduce the compression work and achieve a larger expansion ratio.

I am confused as to what your definition of compression ratio is - if you change the exhaust valve timing you change the ratio. CR = volume at exhaust close / clearance volume.
Re: Experimental engine
October 20, 2015 02:42AM
(I am confused as to what your definition of compression ratio is - if you change the exhaust valve timing you change the ratio. CR = volume at exhaust close / clearance volume. )

Good comment. I had overlooked that the true compression ratio takes into account the valve timings. I have just calculated it for the full stroke, for comparitive purposes. The clearance volume on each head is unchanged of course.
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