When I went to UC Berkeley, studying psychology as an undergrad, lots of people made fun of Freud, Jung, and Adler who wrote (a lot) about divergent theories of what makes people tick. UCB liked to think of itself as a scientific institution, and facts are proven by statistical validity. Why should we believe speculation from the dear departed? I eventually agreed and I was intrigued by the big question: How do we know the truth?

The paradigm that captured my thinking was the BLACK BOX. In black box theory facts are like an elephant in a box. Scientists poke sticks into small holes in the box and one stick hits a toenail. A scientist exclaims, “Why, what is in the box is hard and slippery!” Other sticks hit the softer belly or the trunk, resulting in very different observations. Hopefully, after many experiments an accurate description of the elephant can emerge. (Although, what the elephant is thinking may well remain private.)

I was really excited by statistics!

If you’re trying to know the effectiveness of something, calculating the statistical significance can help. It gives you a measured amount of confidence in the hypothetical conclusion. At UCB we did experiments and 95% confidence was the minimum that students had to achieve to get an “A.” To achieve 95%, the sample size and the size of the effect had to be big enough.

Here’s the application to stoves

When we try to use found wood sticks from the forest in our experiments, one stick, for example, has two inches of bark on it. Another stick has three inches of bark and the two sticks make very different amounts of smoke. So, the variable of using sticks that emit different amounts of smoke makes it more difficult to know the truth: Did changing the air/fuel ratio, for example, result in the stove making less smoke? Using wood with no bark, we can achieve confidence in five to seven tests. We have to do a lot more tests when the fuel has added variables.

When trying to understand heat transfer or combustion efficiency in the lab (not what happens in the field) limiting variables has a great appeal to lazy researchers like me at ARC. So, we do not design stoves in the lab!

We realized quite a long time ago that we could only investigate heat transfer and combustion efficiency in the lab, and then with great relief go to an amazing place, work with wonderful people in a new culture, eat incredible meals, etc. in order to help a local team evolve a stove using found everything (and testing/statistics).

CQC stove with swinging door
CQC stove with swinging door
A door was added to the CQC stove to block some of the air entering the combustion chamber.

Last week we shared some thoughts about the importance of gathering detailed data and making direct observations when testing stoves. Here’s a question we recently considered.

Did a partial cover over the sticks entering the combustion chamber help reduce emissions in the CQC stove with Jet-Flame?

No, not in this preliminary test series, although lots of interesting things happened! Placing a swinging door over the fuel opening into the combustion chamber has often occurred to Rocket stove designers and it appears now and then in stoves. Dr. Winiarski liked the swinging door.  The thought is to get the excess air down by reducing the amount of air coming into the combustion chamber through the door. With sufficient but less excess air, the temperatures needed for cleaner combustion should rise.

Excess air did go down when a metal cover was near to touching the tops of the sticks in our experiments. It fell from an average of 2.91 times stoichiometric to 2.54. The average temperature in the combustion chamber did rise as a result, from 557°C to 596°C. The partial cover was doing its job. Firepower went up (4359 watts up from 4012 watts) and that might have helped with heat transfer efficiency. However, in this particular case, the thermal efficiency was unchanged (36% covered and 37% uncovered).

The positive changes in excess air and temperature also did not affect the emissions. PM2.5 (56mg/MJ-d covered and 55mg/MJ-d uncovered) and CO (2.63g/MJ-d covered and 2.72 g/MJ-d uncovered) were not changed enough to show a difference. Of course, using a Jet-Flame that is introducing lots of changes into the combustion chamber, including more excess air, probably makes this an unusual set of circumstances.

The swinging door may be great in a natural draft Rocket stove and as usual, Dr. Winiarski was right. It didn’t seem to be needed in this scenario. That’s OK with me, because the cover obscured the visual clues that help to tend a fire, and make tending more fun. I felt like I was flying a plane through the fog and was glad to land.