Thermal changes in geometry, and the effect on starting, etc

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Mike Everman
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Thermal changes in geometry, and the effect on starting, etc

Post by Mike Everman » Wed Feb 11, 2004 7:00 am

I seem to want to harp on things thermal, but think about it again with me...
A common comment of those with engines (that run) is "it starts right away, but not when hot" or "I've got to pre-heat it to get it to start". Let's embrace how much an engine changes due to temperature, shall we?

Let's assume your engine goes from cold to 1300f (red hot or so). Let's also assume that the thermal coefficient of steel (close enough for jazz) is 8e-6in/in/degf, that is: it grows in every dimension 8 millionths of an inch for every inch in that dimension for every degree f.
For a delta T (change in temperature) of 1300 degrees, that translates into about a 1% change in every dimension, the worst one obviously being the length of the engine.
If your length is just over two meters (100"), then the length is going to change an inch!

OK, it doesn't sound like a lot, but stay with me.

On a side topic that is pertinent, Graham and I have been discussing boundary layer effects that are of particular interest with Locky Kazoo, that is, a crushed a pipe that imitates the geometry of a Lockwood, but didn't consider the effective reduction in area of any passage due to the thickness of the boundary layer hugging the walls. If you do the math, a normal conical exhaust throat becomes a very narrow slit in a pipe that is crushed to the same area of the formerly circular passage; a slit that may just be less than the thickness of the boundary layer, choking the engine in an unexpected way.

Further, someone needs to enlighten me, but (my sometimes unreliable) intuition tells me that this boundary layer will become thinner as the walls of the engine get hotter; a further increase in the dimensions you started with.

Upshot is now, your engine gets hot, it gets longer, every area inside grows 1%, and the boundary layer shrinks, effectively opening up every passage further, by an amount we may be able to determine, but with the inalterable geometry of engines around here, nothing you can do about it. No wonder they're finicky and no one wants to trust their life to one!

So I call for geometry that can change for cold or hot starting, an engine that will morph automatically in the presence of temperature changes, self cancel its noise, have an SFC of 1.5, makes you a sumptuous breakfast and gives you a neck rub that makes you cry. But I digress...
Mike
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Post by Bruno Ogorelec » Wed Feb 11, 2004 9:16 am

A good summary, Mike. To that, one should add the fact that the impedance (resistance to flow) of a pipe will change with temperature, also skewing the working cycle. The hotter the gas and the pipe, the greater the resistance to flow. In other words, one should optimize the engine for hot running and learn to live with difficult starting -- or develop a trick starting technique. The latter is something every self-respecting owner of an old car used to have -- at least for winter conditions.

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Post by tasdevil » Wed Feb 11, 2004 1:40 pm

This might seem obvious, but with the amount of fuel we're pumping in, could we not just use this to preheat the engine?

It's pretty obvious even to my lack of knowledge in this area, that we should be building these engines for hot running, and tuning them approproiately.
Even if it requires blowtorching the entire surface to get the things to start.

Maybe a way of preheating the engine before start, with the fuel supply would be an idea?

Cliff.

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Heat

Post by Nick » Wed Feb 11, 2004 5:29 pm

brunoogorelec wrote:A good summary, Mike. To that, one should add the fact that the impedance (resistance to flow) of a pipe will change with temperature, also skewing the working cycle. The hotter the gas and the pipe, the greater the resistance to flow. In other words, one should optimize the engine for hot running and learn to live with difficult starting -- or develop a trick starting technique. The latter is something every self-respecting owner of an old car used to have -- at least for winter conditions.
yes but acoustically speaking things get quicker(sound waves travel faster) as the temp rises, just to throw another problem in. :-)

Nick

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Post by Mike Everman » Wed Feb 11, 2004 5:36 pm

The hotter the gas and the pipe, the greater the resistance to flow.
Is this mass flow reduction, because the gas is hot?
That brings to mind the fact that the exhaust throat will have similarly hot medium in both directions, but the intake will be hot out, cool in, making a straight pipe seem conical. Faaaaascinating.

I was also thinking that Locky Kazoo is well disposed for changing the exhaust and intake throats while or before running, if all other things can be resolved.

And, yes certainly you tune for running, not starting. My point is that optimum tuning for both is painful or impossible, and gets more impossible as scale goes down.
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Post by Bruno Ogorelec » Wed Feb 11, 2004 6:53 pm

Mike Everman wrote:That brings to mind the fact that the exhaust throat will have similarly hot medium in both directions, but the intake will be hot out, cool in, making a straight pipe seem conical.
Have you just pinpointed another reason why Lockwoods have inward-flaring intake ports? To un-cone them?

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Post by Mike Everman » Wed Feb 11, 2004 7:00 pm

Have you just pinpointed another reason why Lockwoods have inward-flaring intake ports? To un-cone them?
Well, I think the original impetus was for flow restriction in the exhaust phase.
Once I get a kazoo running, I want to play with all of this with jack-screw adjustments. I'll be able to change the taper (a range from negative to posetive), throat areas, and exit areas.
Hmmmm.
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Post by resosys » Wed Feb 11, 2004 7:38 pm

Very interesting thread.

If we take a look at my large lockwoods, things get weird. They fire up first pop, whether they're cold, hot, glowing, warm, frozen, wet, dry, rusted, or full of fireworks (fun!).

I haven't done any thrust performance tests, yet. The throttle range is very good.

My small engines all share the symptoms other people have expressed.

I'd ponder that the lower fundamental frequency and better flow characteristics of the larger engines may play a large part in this.

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Post by Bruno Ogorelec » Wed Feb 11, 2004 8:15 pm

resosys wrote:I'd ponder that the lower fundamental frequency and better flow characteristics of the larger engines may play a large part in this.
Yes, I agree. There's little doubt that lower frequencies work better. What surprises me is that the benefits are still obvious at fairly large engines. I'd have expected the effect to be noticeable at the scaling up of small engines, up to, say, Dynajet size. However, you can see it even in sized that are gigantic for most enthusiasts.

Bruce Simpson says that his biggest engine (160 lbs thrust) is noticeably mellower than the next smallest -- which, at some 100 lbs thrust, is already very big. The monster doesdn't need forced air at all to start. You just open the propane valve and give it a spark, every time.

The problem is that such engines are really unwieldy for tinkering in a garage....

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Post by resosys » Wed Feb 11, 2004 8:30 pm

brunoogorelec wrote:Yes, I agree. There's little doubt that lower frequencies work better. What surprises me is that the benefits are still obvious at fairly large engines. I'd have expected the effect to be noticeable at the scaling up of small engines, up to, say, Dynajet size. However, you can see it even in sized that are gigantic for most enthusiasts.
Has anyone built a fat, short engine? Sounds like a fun little project.
Bruce Simpson says that his biggest engine (160 lbs thrust) is noticeably mellower than the next smallest -- which, at some 100 lbs thrust, is already very big. The monster doesdn't need forced air at all to start. You just open the propane valve and give it a spark, every time.
I had a conversation with Bruce about this same thing. It was very odd. SRL's hovercraft engines will self fire, once they are glowing.
The problem is that such engines are really unwieldy for tinkering in a garage....
This is why I have a truck and a huge desert a few hours away!

Chris

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Post by Mike Everman » Wed Feb 11, 2004 10:15 pm

Larger engines being less sensitive makes sense, the thermal change in geometry is 1%, but the areal change goes with r squared. Likewise, the effect of thinner/thicker boundary layer is going to measure in less and less as diameter goes up.
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Post by evildrome » Wed Feb 11, 2004 11:56 pm

I think I'll have to disagree with everyone & say that I think that the main reason small engines are harder to start is the granularity of the fuel supply.

I'm a maths muppet but I think there's a relationship between CC cross section, number of injection points, their size and distribution across that cross section. So, worst case scenario is a small CC cross section with single gas entry point in the CC wall. Best case scenario is a large CC volume with many injection points arranged as a circle or X perpendicular to the air intake.

If we were graphing ease of starting, I predict...

CC cross section goes up, 45 degree line up, plateaus out.
number of injection points goes up, 45 degree line up, plateaus out.
size of injection points goes up, bell curve
distribution of injection points, bit more complex but bell curved...ish

In the end its all down to fuel/air mixing.

Cheers,

Wilson.

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Post by resosys » Thu Feb 12, 2004 12:50 am

evildrome wrote:I think I'll have to disagree with everyone & say that I think that the main reason small engines are harder to start is the granularity of the fuel supply.
I'll agree that fuel injection granularity is part of it, but it's certainly all of the above, a little voodoo, a bunch of luck, and a metric assload of magic as well.

Chris

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Post by Graham C. Williams » Thu Feb 12, 2004 1:48 am

Dear All.
I think you are all getting the right idea; a great thread. Now add in the energy stored in the wave structure as you increase/decrease the size of your motor. It's not too much of a surprise that the larger motors can be self-starting. Also you should think about how much energy is required to 'set-up' the combustion conditions (for your given motor geometry). This energy for this must come from the wave structure and ultimately the combustion energy of a previous cycle. How does this effect the starting of the motor and its transition to full running.
Great stuff.
Graham.

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Post by evildrome » Thu Feb 12, 2004 3:26 am

I'd say that as long as the geometry is right a small engine should be as easy to start as a big engine (if it weren't for the fuel problem).

However, even small deviations (like weld beads) start to become a problem when you scale down the engine.

Cheers,

Wilson.

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