swirl and tumble in ic engine


Turbulence is very important for close to complete combustion. Swirl, Squish, and Tumble are used to create turbulence in internal combustion engines.

Kirk Harris discovered that a static fan shape with overlapping 70 degree blades creates lots of fast moving swirl at the approximately one to two meter per second velocities found in TLUDs.

1. Swirl:

The rotational motion of air within the cylinder is called Swirl. Swirl enhances mixing and makes the flue air mixtures homogeneous. Swirl is the main mechanism to spread the flame within the combustion zone.

2. Squish:

The radial inward movement of air is called Squish. Squish can be defined as an inward flow of air towards the combustion recess. 

3. Tumble:

Squish generates secondary motion about the circumferential axis near the outer edges. This motion is called ‘tumble’. To achieve this either the fuel is directed towards air or air is directed towards the fuel.

In the last Newsletter, we announced the publication of Aprovecho’s recent research on the Jet-Flame, “Retrofitting stoves with forced jets of primary air improves speed, emissions, and efficiency: Evidence from six types of biomass cookstoves” Here is a simplified summary of the findings:

When the goals for biomass cook stove interventions were raised to include protecting health, it was obvious that adding a chimney or cooking outdoors continued to be the historically proven solutions. USA heating stoves create more smoke than cook stoves but the smoke is transported outdoors in the chimney and diluted by clean air to meet EPA outdoor air standards for PM2.5. Cooking outdoors, especially in a bit of wind, directly dilutes the PM2.5.

When the outdoor air is cleaner, the emissions from the stove can be higher. When the outdoor air is dirtier, the emissions need to be cleaner. Simple! Aprovecho published a model that estimates emissions based on the quality of the outdoor air. See: http://aprovecho.org/portfolio-item/project-planning/

ISO Tier Mapping for CO and PM2.5 per MJdelivered for the natural (blue triangle) and forced draft (orange dot) cases. Note the log scale on both axes.

As seen on the upper right side of the graph above, stick burning stoves (even in the lab) emit very high levels of PM2.5. That can be OK when used with a functional chimney or outdoors in rural locations with limited numbers of cooks per hectare. But in many more crowded situations the emission rates need to be much lower to protect health.

Adding forced draft mixing to many types of stoves, including the open fire, can be very effective in reducing the emission rates of PM2.5. The Jet-Flame shoots primary-air-only jets into the bottom of the fire and this simple technique reduces emissions of PM2.5 and CO, while reducing fuel use and time to boil. We hope that technologies like the Jet-Flame can assist stove projects to protect health especially when combined with chimneys and/or outdoor cooking.

Kirk Harris has been investigating TLUDs for decades and, as far as I know, his natural draft TLUD burning pellets achieved the lowest natural draft recorded score for PM2.5:  0.7mg/minute at high power (Lawrence Berkeley National Laboratory). This video shows Kirk in China at Shengzhou Stove Manufacturer where Mr. Shen built a copy of his stove to start the process of possibly manufacturing it.

The fascinating aspect in the video is how fast the flame is swirling, keeping the flame below the level of the pot and increasing dwell time.

Adding a fan shaped static mixer between the hole in the concentrator ring and the bottom of the pot has become commonplace in various TLUDS since Kirk invented the technique. We recently added a fan shaped static mixer in a natural draft TLUD to get rid of creosote. The tars were burned up in the hot, swirling flame.

Keeping the flame below the cold surface of the pot is always helpful and can be achieved with the Jet-Flame and in both natural draft and forced draft TLUDs.

To last long enough for commercial/carbon success, the combustion chamber has to be made with cast refractory ceramic. Making the static mixer and combustion chamber from cast refractory ceramic dramatically increases longevity. The Oorja stove in our lab has lasted for about 20 years!

I imagine Dr. Tom Reed smiling in heaven as the stove community moves closer to optimization with:

  • clean burning pellets 
  • a well-engineered TLUD 
  • a refractory ceramic combustion chamber 
Illustration of how an electrostatic precipitator works
Illustration of how an electrostatic precipitator works
Electrostatic Precipitation: Smoke particles are negatively charged and attracted to positively charged metal plates that are automatically cleaned.

Both automobiles and the biomass industry rely on improving combustion efficiency and post combustion reduction of PM2.5 to achieve “clean burning.” It’s really hard to rely on the combustion chamber to burn up enough of the harmful smoke to protect health, especially in large scale applications. Of course, all efforts should be made to be as efficient as possible. The goal is to burn up everything! In cookstoves, with limited space and a pressing need to be affordable, the problem becomes more acute.

Biomass heating stoves are larger and can cost a lot more than cookstoves. Industrial technologies are even less constrained. For decades, home heaters have tried catalysts to reduce emissions. Factories have used a wider array of technologies including filtration, catalysts, and electrostatic precipitation. Chapter 8 in “Clean Burning Biomass Cookstoves, 2nd edition, 2021” includes explanations of these technologies. 

Generally, filtration can work very well to capture dust and smoke with reported efficiencies of up to 99% (Frisky, et al., 2001). Catalytic converters are placed into the hot exhaust path where temperatures are hot enough (above 426°C). They work well with CO (30% to 95%) but not so well to remove PM2.5 (30% to 40%) (Hukkanen, et al., 2012). The Swiss electrostatic precipitator (ESP) called the OekoTube has been measured to reduce PM2.5 by 80.2% to 97.7% (Brunner, et al., 2018). However, as in industrial uses, routine cleaning is necessary to remove creosote and other coatings that interfere with proper function. Unlike filters and catalytic converters, the low wattage ESP does not reduce the draft in the stove, which could be potentially advantageous. 

ARC has been experimenting with post combustion of PM2.5 since 2017 as a result of the EPA SBIR funded work to create a clean burning biomass heating stove. We believe that if ESP is to be useful, automatic self-cleaning must be included, as in some industrial products. The hope is to invent super clean combustion but it’s great that post combustion approaches already exist. On the other hand, forced draft mixing, which is relied upon for combustion efficiency in industry, is largely missing in both cookstoves and residential biomass heaters. Perhaps its addition will be sufficient to reach the goals of protecting health and carbon neutral fuel use with renewably harvested biomass?

The Gates funded Global Health Labs and ARC/SSM invented the Jet-Flame

Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame
Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame

Shengzhou Stove Manufacturer manufactures the Jet-Flame (jet- flame.com) that is being field tested in over 30 locations. Our lab helped to create this accessory that is designed to reduce emissions while increasing thermal efficiency and reducing time to boil. 30 pre-heated primary air jets shoot up into the fire resulting in increased molecular mixing and elevated temperatures. Smoke is reduced by about 90% compared to an open fire.

A forced draft stove can be very clean burning, but start up may create a lot of PM2.5. This is because the cold combustion chamber can allow a higher percentage of the smoke and Carbon Monoxide to escape unburnt. David Evitt, COO of ASAT (the for-profit arm of ARC), invented a method for lighting the Jet-Flame that can be a lot cleaner.

  • Wet 30 grams of left over charcoal with 10 grams of alcohol.
  • Place the small pile of charcoal on top of the holes in the Jet-Flame.
  • Light the charcoal.
  • Turn on the Jet-Flame.
  • Push the tips of the sticks of wood against the pile of burning charcoal.
  • Keep on pushing the sticks into the fire as the tips are consumed.

Here’s a video showing how we light fires in the lab:

The high mass CQC stove with Jet-Flame inserted from the side.
The Jet-Flame in the CQC high mass brick Rocket stove

I ask for help when moving the CQC stove. We built it on a piece of plywood and two folks can, with care, move it around the lab but it is heavy. The sand/clay/cement bricks are dense at 1.4 grams per cubic centimeter after being baked many times in the stove. Dr. Winiarski advised that, when possible, Rocket stoves should float in water at less than 1 gram per cubic centimeter.

For a long time, people have added sawdust and other lightweight materials into earthen mixtures to try to lighten up stoves. I ended up at Shengzhou Stove Manufacturer (SSM) in China because for hundreds of years ceramicists had manufactured (and sold in Africa since 1407) durable earthen stoves that weighed around 0.7 grams per cubic centimeter. Their amazing clay floats when dug out of the ground! It is full of diatomaceous earth. The Shens own a 100 year supply of clay in two mines next to the factory.

Why go to all of this trouble to lighten stoves?

The heat from the fire is diverted into the mass of the stove body and less heat is available to cook food. It is harder to start a hot, intense fire in a high mass combustion chamber. In a natural draft stove, this can be disadvantageous. The open fire has other problems but, out of the wind, the hot gases from the flames directly contact the pot and it’s common for open fires to have higher thermal efficiencies compared to high mass stoves, including Rocket stoves. Lightening the bricks helps to address this difficulty. Heat is still diverted into the stove body, but less. Well insulated, mostly metal, Rocket stoves successfully avoid most of these losses.

Indigenous cooks, experts at using fire, often use grasses and twigs to start a hot, fast fire in a high mass stove. You need to pour the BTUs into the stove to quickly prepare food. Speed to cook is almost always the first priority when talking to cooks around the world. When the SSM Jet-Flame is added to the high mass stove, the mini blast furnace immediately starts a hot, over 1,000°C fire that delivers relatively hot gases into the channel gap around the pot created by the pot skirt. (The CQC skirt creates a 5mm channel gap that is 7cm high.) 

The Jet-Flame creates a surprising result

The thermal efficiency in the first CQC/Jet-Flame test (see below) was 33%. The 5 liters of water boiled in 12.5 minutes. After the first 12.5 minutes of heating, the over 1,000°C fire started to heat up the mass and the water boiled more quickly in 10.2 minutes at 38% thermal efficiency. Three more short, but intense, heating phases resulted in the thermal efficiency incrementally rising to 41%, 42%, and 45%. The progressively hotter gases scraping against the sides and bottom of the pot in the small channel gap were more and more successful at transferring heat through the metal walls of the pot into the water.

When thermal efficiencies are in the 40% to 45% range, the performance of the high mass stove is similar to low mass, insulated Rocket stoves. This similarity was completely unexpected at ARC.

Results of five tests of the CQC Stove with Jet-Flame.
CQC stove set up for testing under the LEMS hood

ARC is investigating how to optimize the performance of the SSM Jet-Flame in the CQC earthen brick stove. Forty six thirty-minute ISO 19867 Water Heating Tests were completed under the LEMS hood at seven fan speeds. Two 4 cm x 4 cm douglas fir sticks were burned side by side. Five liters of water in a seven liter pot were heated, and the CQC pot skirt was used in all tests.


Tier 4 ISO Voluntary Performance Targets:

  • Thermal Efficiency           40% to 49%
  • CO                                     <4.4g/MJd
  • PM2.5                               <62mg/MJd

Time to boil: The time to boil decreased with an increase in fan speed.

Thermal efficiency: The thermal efficiency stayed close to 35% in most cases and was higher at 3 and 8 volts (around 40%).

Firepower: The firepower rose to 6.8kW at 8 volts, starting at 2.6 kW at 2 volts.

Emissions of Carbon monoxide: Generally emissions decreased with increasing fan speed.

Emissions of PM2.5: 7 and 8 volts scored the best, at half of the result of 5 volts.

Combustion chamber temperatures: The mid combustion chamber temperatures rose with increases in fan speed from 382C to 730C.

Excess air:  Lambda fell as voltage increased from 4.1 to 1.9.

We recommend that the project do enough field testing to determine what settings are preferable to local cooks, remembering that higher voltages consume more power. In this way, the Jet-Flame/CQC stove can be tailored to regional cooking, keeping in mind the power output and use patterns of the CQC photovoltaic solar system.

Here’s what the flame looks like when varying the voltage:

cover of Clean Burning Biomass Cookstoves 2nd edition
Click here to download the free pdf files

If stoves pollute in the lab, they certainly will in the field. We estimate at least 3 times more. Commercially available biomass cookstoves that meet WHO standards are very rare. ARC continues to be committed to doing research and development to help to get the needed new stoves to market so that field studies will show success in sales, protecting health, saving wood, and making cooks happy. We believe that sharing what we learn is very important! So, we updated our “textbook” and it’s available for free here. The chapters have been updated and rewritten to try and share everything that we have learned in the lab in the last five years.


Here are some highlights:

  • With clean outdoor air, doubling the air exchange rate halves the concentrations of PM and CO in the kitchen.
  • Using an EPA model of Oakridge, Oregon, the outdoor air concentration of PM2.5 would only be increased from 13.1 μg/m3 to 13.3 μg/m3 if homeowners used an ISO Tier 4 PM2.5 cooking stove.
  • A catalytic converter works well with gases (30-95% reduction of CO) but not with smoke (30-40% reduction of PM2.5) (Hukkanen et al., 2012).
  • We think that the Harris TLUD is perhaps the first “close to optimal” cookstove. It scored 0.7mg/minute PM2.5 with pellets at Lawrence Berkeley National Laboratory. It has a 3 to 1 turn down ratio. Large natural draft static mixers create thorough mixing. Decreasing primary air reduces the rate of reactions (production of wood gas) if the air/fuel mixture becomes too rich. A stationary fan blade spins the flame for longer dwell time. And cooks at ARC love to use it.
  • When carefully tested at ARC, the SSM Jet-Flame in the CQC earthen stove scored Tier 4 for thermal efficiency, CO, and PM2.5.
  • Renewably harvested biomass can be a carbon neutral energy source when burned very cleanly.

We are getting closer to practical solutions! The ones we know about are in the book.

Rocket Stove 2021 - Pot Skirts

In this video, Dean Still explains why a pot skirt – a sheet of metal wrapped around the cooking pot – is a simple yet important way to improve the fuel efficiency of a rocket stove. He also explains how to calculate the appropriate distance between the skirt and the pot. Stay tuned to the end of the video to find out who is causing all the ruckus in the background…

Helpful references:

simplified diagram of constant cross sectional area
Simplified drawing of the concept of constant cross sectional area.

This is a very simplified illustration of what “constant cross-sectional area” means. The top circle represents the cross-sectional area of a stove riser. The bottom ring shows the same area translated into the space around a pot. It’s important to keep the cross-sectional area that the hot gasses flow through consistent, so they don’t slow down. Hot, fast flowing gasses transfer heat most efficiently. 

graph helps calculate proper skirt gap for best heat transfer efficiency
Chart for calculating channel gaps, from Dr. Samuel Baldwin’s “Biomass Stoves: Engineering Design, Development, and Dissemination.” 1987, Volunteers in Technical Assistance.

This is the chart for determining efficient channel gaps, explained towards the end of the video. It was developed by Dr. Samuel Baldwin in 1987.

Here is the Ten Stove Design Principles poster referred to in the video. Many more helpful documents are also linked on the Publications page.

sticks and charcoal start to combust in a rocket stove

The Jet-Flame was developed from combustion concepts used in fluidized beds and TLUDs.

Fluidized Bed

fluidized bed combustion diagrams

“In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which (under air) jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.”   https://en.wikipedia.org/wiki/Fluidized_bed_combustion

Top Lit Up Draft

diagram explaining how a top loaded up draft stove works

The TLUD uses under air flowing up through the fuel to transport wood gas into the hot layer of charcoal and flame above the fuel assisting more complete combustion efficiency.

Cleanly Starting the Jet-Flame

High velocity under air jets blow up into the lit charcoal placed on top of small sticks of wood. When the charcoal and wood are on fire, long pieces of wood are pushed into the made charcoal to start a Rocket Jet-Flame without making visible smoke. The sticks of wood are burned at the same rate as the continual production of charcoal creating a cleaner combustion process related to a fluidized bed and the TLUD.

sticks and charcoal start to combust in a rocket stove

Charcoal over wood is lit.

bed of charcoal in rocket stove

The charcoal becomes superheated with jets blowing up into the pile.

sticks burning in rocket stove

After 30 seconds, long sticks of wood are pushed against the burning charcoal creating flame.