“Sorry everyone,” said my friend Chris. He was driving. I was sitting shotgun. Two other pals were in the back seat of his car as we cruised down the highway.
“For what?” everyone asked.
The look of trauma
“You’ll see,” Chris replied.
There’s an audible click as Chris engaged the power window lockout switch. Then, sinister laughter.
Moments later, all hell broke loose. Choking. Gagging. All passengers praying for a quick and painless death.
“I CAN SMELL IT THROUGH MY SHIRT!!”
“OH MY GOD HAVE SOME FIBRE!!?!”
The air was green.
Chris had released a category-five hyperfart into the cabin of his car, quickly filling it with one of the most potent and nauseating odours in existence. If the average fart is a firecracker, this one was a neutron bomb. It could have been harvested for biological warfare.
And, with his complement of passengers unable to use their window switches, Chris had instantly turned a pleasant road trip into a world of malodorous, heavy stink.
Moments later, Chris choked back tears.
“OH MY GOD,” he shouted, waving at the air, squinting through teary eyes. The offending fart had recirculated back to its owner, and the windows went down, and the sunroof opened, and even our driver was turning green. Thing is, for a moment anyways, the added airflow through the car only seemed to make things worse. And things were pretty bad to start with.
After this ordeal, I figured on investigating the science of clearing a horrifying fart from a moving car. I talked to Rob Murdoch, a technical expert at Mazda, about how air and farts travel around and through a car, to help develop some tips to help effectively clear the air after the driver has an emissions-related incident.
If only they had one of these...
JP: How does a fart carry through the cabin? Does it expand in a circular pattern? Slowly ooze through the cockpit? Actively seek out nearby nostrils?
RM: The proliferation of offensive gastric odours throughout a vehicle interior will be predicated on a number of factors, including windows open or closed, the settings of the HVAC system, and the strength of the cheesy bean burrito which initiated the event. In a closed environment with the HVAC off and windows closed, the air in the interior of a car is static. In such a situation, the gaseous emissions expand as they merge with the interior air: the odour rising into the olfactory area of the passengers. There’s also a gravitational effect at play, which causes air to compress slightly into a higher-pressure area towards the bottom of the cabin, thereby creating a slight upward trajectory of the emission, into the lower pressure area.
Other factors may include the temperature of the fart relative to the ambient air, the effect of varying weights of the different gases at play, and the degree of offensiveness of the odour itself. But, in simple English, farts expand outwards from the source, with a slight upward path.
JP: Can you tell us about the aerodynamics of a car going down the highway at speed, and how they might relate to odour evacuation?
RM: Speed has little effect on the interior dynamics of the air of the vehicle when the windows are closed. If the HVAC system is on, and the system is on ‘fresh air’, then the air entering the cabin must be able to push air out of the cabin, or pressure within the vehicle will increase, which causes passenger discomfort (offensive odours notwithstanding). All vehicles have an air extractor, which facilitates this air displacement. Notably, this extractor is situated in the rear of the vehicle, thereby minimizing the entrance of road noise, and maximizing the distribution of air movement within the vehicle.
The extractor should not be relied upon for odour evacuation, and unless you open windows or the sun/moon roof, the speed of your car doesn’t play much of a role.
JP: Does an aerodynamic ‘boundary layer’ around the vehicle at speed seal the smell into the car? Or will opening a window actually suck the smell out if I experience a failure to contain gas, and wish to be courteous?
RM: All vehicles are not equal, and the boundary layer (air that adheres to the body of the vehicle) is dependent upon smooth airflow across said vehicle. Any protrusions tend to break up the flow of air, causing turbulence, which impacts this flow. Side view mirrors are the notable culprit. Remember this.
Air moving past the vehicle is moving at a greater speed than the air inside the vehicle. This means the outer air pressure is greater, which causes air to try to enter the vehicle, to fill the lower pressure area, when windows are opened. However, if the side mirror interrupts the flow sufficiently, the low pressure and turbulence caused by the mirror may minimize the pressure differential.
In the case of a very aerodynamic vehicle, like a sports car, the air behind the mirror may not be as turbulent, and therefore the pressure differential is great. In this case, air tries to rush into the vehicle, and out of the vehicle, at a similar rate. As the air extractor isn’t designed for high-volume air flow and pressure relief, the low-frequency “whump, whump, whump” sound, referred to as the Helmholtz effect, often occurs, which can be disturbing to the human ear. And that fart will have a hard time being sucked out of the car, if only the front windows are opened slightly for evacuation.
JP: Can the odour released when someone has a post-digestive occurrence be filtered by the cabin air filter? Should I use the ‘recirc’ function to let the filter do its job?
RM: Unless you are trying to impress a date, and your date is canine, I would not recommend this method.
Sometimes, even a dog will run away...
JP: So, at 90 km/h, is it more effective to try to ‘suck’ the offending vapors out by rolling down the windows, or to ‘push’ it out, with the HVAC fan, and a slight window opening?
RM: Oh, definitely use the windows – the volume of air you can move is much greater than what the HVAC can move at that speed. But the more aerodynamic the car, the more window opening you’ll need. This is also known as “clearing the chamber”. For instance, “Rob farted! Quick – clear the chamber! Clear the chamber!”
JP: What about the sunroof?
RM: The sunroof may be a good alternative to opening the rear passenger window, if you have sensitive or nauseated passengers in the rear, or passengers who have been recently exposed, perhaps by emissions events of a sequential nature. However, note that opening the sunroof will draw the stank-mass up and past the faces of front-seat occupants, with unpleasant consequences.
JP: What if I’m stopped in a drive-thru or at a red light?
RM: At slow speeds or at a stop, open the windows and crank the fan. In fact, wave your arms and anything else you have in sight. This is the most serious of situations.
Optimal airflow; fresh air represented in blue, unholy stench in green.
JP: For many, the panic reaction often involves rolling down all windows, all the way, though in practice, this often seems to irritate and intensify the fart. According to science and aerodynamics, which windows should be rolled down? How much?
RM: All else being equal, a general rule is to open the driver’s window a couple of inches, and the rear passenger window half way. This causes a turbulent diagonal cross-shear of the air-mass through the cabin, meaning the offending smell will be drawn across and to the rear of the vehicle, and out the window expediently.
