Help! > Specific Engine Help
Bump valve uniflow engine
astroud:
Hello
I am planning to build a simple bump valve uniflow engine to run on air. I have a 3/4" bore and a 1" stroke.
From reading up the inlet and exhaust valve opening and closing seems to be pretty flexible, I am planning the inlet to open c. 1/16" before tdc and the exhaust port to start opening half way down the stroke, does this sound OK ?
Is there any wisdom on how much dead space should be above the piston at tdc, I am thinking more than usual to allow for air compression.
thanks, Andrew
PaulR:
Following along... :popcorn:
MJM460:
Hi Andrew, I find the best way to understand the effect of valve events and clearance volume on these (or any other) engines is to construct an indicator diagram like the attached sketch. I have sketched the cylinder below the diagram so you can see how the corners of the diagram line up with the physical events as the engine moves.
An indicator diagram is a graphical representation of cylinder pressure and cylinder volume. The area enclosed by the loop the operating pressure traces out is the work produced by the engine each stroke. The operating point moves clockwise around the loop in the direction of the arrows. No need for exact pressures, just good enough to demonstrate some critical points. Anything that increases the area enclosed by the loop also increases the work produced during each stroke, while anything that decreases the area enclosed decreases the work output.
I don’t pretend that this diagram is to scale, though the volume axis should be, as all the interesting points come directly from physical measurements of your engine. However, the pressure scale is, well shall we say somewhat elastic? But adequate to show the principle.
There is still a wealth of information contained in this simple looking diagram.
First draw the cylinder cover, piston top and bottom dead centres, and exhaust outlet to scale on the volume axis. When the piston starts pushing the valve open and then allows it to fully close, changes the curve at points 1 and 5, while the exhaust port starting to uncover through to finally covered again changes the form of the curve at points 2 and 4.
Starting with the piston just finally closing the exhaust port on the upstroke, I have assumed for this purpose that the cylinder pressure exhausts to atmospheric pressure by the time the exhaust ports are closed (at point 4). Realistic for the very large ports you have proposed, perhaps not so realistic for the smaller ports I have drawn. (I have no particular reason for the size I drew, I had overlooked your proposed port diameter when I drew the sketch.)
The piston compresses the gas remaining in the cylinder through the remainder of the upstroke towards point 5. The shape of the curve is approximately determined by adiabatic compression, and is generally the shape I have drawn.
If the clearance volume is large enough, the cylinder pressure at point 5 will be lower than your supply pressure as I have shown.
On the other hand, if the clearance volume is very small, the compression stroke might reach your supply pressure before the piston starts bumping the valve open, and results in an unusual indicator diagram for the remaining part of the compression stroke, but I am sure you can see the enclosed part of the loop and hence the work output will be smaller.
On the other hand, if the clearance volume is larger than I have drawn, the pressure reached at point 5 will be lower, and the area enclosed by the loop is larger, hence more work output.
The clearance volume also affects the power stroke. When the piston moves towards the crankshaft, the gas enclosed by the cylinder expands, producing the work output.
I have assumed in the diagram that by the time the valve closes at point 1, the cylinder pressure is about equal to your supply pressure. The expansion of the gas in the cylinder as the piston moves to point 2 is determined by the volume ratio from point 1 to point 2. If the clearance volume (plus the volume change during valve closure) is very small, point 2 will be lower than I have assumed, reducing the area enclosed by the curve, while if the clearance volume is larger than I have assumed, point 2 will be higher, so more area enclosed by the curve, and more work output.
You can see that more clearance volume increases the work output on both the expansion stroke and on the compression stroke, so increases the work output of the engine.
This extra work is obtained at a cost however. It will require more steam or air to get the pressure at point 2 up to the supply pressure, hence more running cost to get that amount of work output. The cycle efficiency is lower.
Of course, I doubt if anyone reading this knows or cares how much fuel cost is involved in running a model engine, but on a large machine in industry, running cost becomes a significant issue.
I hope this answers your question, but it probably raises more questions than it answers. Please ask if anything is unclear. There is a lot more information that this diagram can tell us about these engines, but I have rambled on long enough.
MJM460
Charles Lamont:
MJM40 has given an excellent explanation. There is one point, relevant to Andrew's question, that I don't think quite right. When the valve closes at point 1, there a certain quantity of air in the cylinder. At the bottom end of the stroke some of that air is let out. On the compression stroke at point 5 we have the same cylinder volume as at point 1, but less air, so the pressure cannot exceed the supply pressure.
The point remains that a smaller clearance volume will give greater efficiency, and a large one will give more power but use disproportionately more air.
There is, however, another consideration. The exhaust is going to atmosphere. Assuming the exhaust port closes with the pressure at 1 bar. A 7:1 compression ratio would mean that the engine will not run with a supply pressure below 7 bar.
Jasonb:
--- Quote from: Charles Lamont on August 05, 2026, 09:40:10 am ---There is, however, another consideration. The exhaust is going to atmosphere. Assuming the exhaust port closes with the pressure at 1 bar. A 7:1 compression ratio would mean that the engine will not run with a supply pressure below 7 bar.
--- End quote ---
The couple of uniflow engines I recently got running had very little head clearance so the compression ratio would have been quite high and probably more than 10:1 but they run on 2 to 3 bar with no problem.
You can see how little clearance and get an idea of piston movement from the crankshaft
https://www.modelenginemaker.com/index.php?action=dlattach;topic=12854.0;attach=184323;image
Navigation
[0] Message Index
[#] Next page
Go to full version