Stop and Smell the Tailpipe


#1

In the last segment of this week’s Car Talk, Tom and Ray discounted the possibility of exhaust gases being pushed forward into the cabin from the trunk area of a vehicle while the vehicle was accelerating, presumably because when you accelerate a car things are “pushed backwards” inside the car. I would like to state that this is not always the case.
If you have ever been to a birthday party and taken home a helium balloon in the car, you will find that the balloon does not move towards the back of the car when the car accelerates, but psuedo-paradoxically moves towards the FRONT of the car.
The reason this happens is because when you accelerate your car, not only do you, and your family, and the left over cake you have move to the back of the car, but so does the air in the car. Because the air is now at the back of the car, there is more mass of air at the back of the car and the balloon, which is lighter than air, is forced to move towards the front of the car.
I submit that this is the same principle that would cause Helen (the caller in the last segment of Show #1350) to smell exhaust gasses when she accelerated.
I am aware of the fact that the component exhaust gases are heavier than air, however I believe that the exhaust gas temperature lowers the density of the exhaust plume causing it to rise into the trunkspace, and because the trunk is an enclosed space the gas does not cool and condense or disperse into atmosphere. When the car accelerates forward, cabin air rushes into the trunkspace via the back seat pushing forward the less dense exhaust gas, which then causes Helen to smell exhaust gas.


#2

Not for that reason. The reason the balloon move forward when the car accelerates is because the balloon is less dense than air, moving or not. So when the car accelerates forward the balloon ‘floats’ forward as the effective gravity force (gravity + car acceleration) becomes slightly off of vertical.

As for exhaust gas getting into the passenger compartment, it’ll diffuse there over time, or it could be pulled in if the air circulation creates lower pressure in the passenger compartment than in the trunk. But it’ll have nothing to do with acceleration.


#3

Texases, Gravitational force alone is not enough for the balloon to move forward (if the car acceleration experiment occurred in a vaccum, or atmosphere of hydrogen then the result would be different). The g-force is what causes the air pressure differential (the proper name of what I described) to move the balloon forward. A good way to imagine this is to think of what water in a bottle does, and then expand that to a tank of water that is full enough for the balloon to touch the roof of the tank while the car is not moving.
In the show’s case, Helen was experiencing a strong smell of exhaust gas immediately when she accelerates from a stop, while only smelling it a little bit at other times. Tom and Ray didn’t go into the timing of how quickly she smelled it or for how long she smelled it


#4

I did not hear the episode and agree with texases, but my feeling is that exhaust gas enters the cabin (assuming it’s from the tailpipe) due to vehicle design and/or a trunk/hatch seal that is not sealing completely.

Any moving car creates a low pressure area at the outer rear which has a tendency to bring exhaust gas up. This is more prominent in vehicles such as wagons, SUVs, minivans, hatchbacks, etc and is why many of them may have a window lip, small spoiler, etc. It’s to disrupt that low pressure area in the rear a bit


#5

jonjoy: Air does not “move to the rear” of a vehicle in any appreciable manner. Put an altimeter inside the car; it won’t indicate a climb/descent with acceleration/deceleration.

What WILL happen is a modest reduction of cabin pressure due to velocity in accordance with Bernoulli’s principles. This will “suck in” stuff like exhaust gases in areas of locally higher pressure.


#6

Spoilers on vans, wagons, and SUVs? Can’t say I’ve seen them. And unless they are scooping air to pass it UNDER the spoiler, they aren’t going to impact the low pressure area behind the vehicle.
Ford Country Squires (station wagons) of the late 60s actually had scoops built into the D pillars which directly fed air into the low pressure area.


#7
Spoilers on vans, wagons, and SUVs? Can't say I've seen them

All the SUV’s I’ve owned had/has spoilers. But they have NOTHING to do with performance. Their sole purpose is to back-wash the rear window. This helps prevent the rear window from getting real dirty - especially during the winter.


#8

@mikeinnh @tony carlos

I second what mike said. My Prius wagon has a large “spoiler” above the rear window, minivans also tend to have a lip at the rear for the same reasons mike mentioned.


#9

My Matrix has a small overhanging lip above the rear window.


#10

Rick, you’re going to have to help me out here.
Here’s a shot of the Prius V showing the rear spoiler:


Now it appears to me that that spoiler is attached across its leading edge to the glass and/or the hatch, without any openings. If that is the case, how the heck is it going to direct any air onto the read window?
My guess is that it extends and directs the airflow over the roof, increasing the length of laminar flow, and decreasing the amount of turbulence that follows the car. That would reduce the aero drag coefficient and help with fuel economy.
Unless I’m not seeing some openings, the window-wash aspect is missing me.


#11
My guess is that it extends and directs the airflow over the roof, increasing the length of laminar flow, and decreasing the amount of turbulence that follows the car.

It increases the length of atttached flow. Flow ceased being laminar at the first body seam, if not a long time before that.

As an aero guy, the mis-use of the term “laminar flow” really bugs me…