truck exhaust flappers

I was in my office parking lot today when I heard a TING-TING-TING. I looked around and saw a truck with a tall exhaust pipe at the corner of the cab, as trucks are wont to have. On the top of the exhaust pipe was a metal cover that kept flapping up and down, as the exhaust would blow it up and then the dispersion of pressure would allow it to drop down again.

I’ve been seeing these as long as I can remember on trucks and construction equipment. Do they serve any practical purpose at all? I can’t imagine what it might be, other than to give a quick visual confirmation that the engine is running (although how you could not know that a diesel engine is running is beyond me).

I would think they are to keep rain out of the engine when it’s not running.

Around here they are called “weathercaps”. They keep rain out.

Rain caps

Or weather caps, if you prefer.

Remember that the opening of the exhaust pipe is above the engine in a big truck, so water can flow downward if there’s no cover. In a car the engine is above the exhaust opening so there’s no worry about water flowing “uphill”.

What I’ve always wondered is why even bother putting a hinged cap on it like that to keep out the rain?

Why wouldn’t they just take the pipe and bend it 180 degrees before it ends? Seems much more simple and less restrictive.

Have you seen the thick black smoke that can come out of those things? I think you want that directed up.

What I want to know is, why does it have to be hinged? Why not just a mushroom shaped end cap with openings on the side? It would have no moving parts to replace.

A mushroom shaped cap would direct the exhaust downward, or sideways. Up is better for the immediate area’s ground level air quality.

Because visible moving parts are cool. :wink:

There are probably a number of solutions that direct exhaust up while still keeping rain out, but for each one you come up with, ask yourself:

  1. Is it more or less likely (than a weather cap) to get particulate build-up from the high volume of oily smoke coming out?

  2. Is it more or less expensive to manufacture than a section of straight pipe, a hinge, and a flap of metal?

  3. Does it restrict exhaust flow (and therefore engine efficiency) more or less than the current solution?

  4. Does it still keep water out of the engine?

Personally, I’d put a 360-degree “sink trap” at the bottom of the pipe, and build in a check valve that drained built-up water right onto the roadway. The downside to this is that by the time you’ve had enough water build up to open the check valve, exhaust has been flowing over it for a while, and it’s loaded with oily petroleum contaminants. The EPA would probably find this objectionable.

I don’t buy the pollution explanation because hinged rain caps were used long before EPA was ever thought of.

I suspect that the cap is used in place of a mushroom top to reduce the back-pressure in the exhaust. A 90[sup]o[/sup] bend probably raises the pressure in the exhaust more than does a hinged cap. The same would be true of putting a 180[sup]o[/sup] bend in the pipe at the bottom so as to collect the rainwater or snow.

Exhaust back pressure reduces efficiency by quite a bit. It seems to me that I remember learning that some very large engines with a long exhaust system even have a scavenging pump to help keep the exhaust back pressure down. Like a big diesel electric generator prime mover that has a 50’ exhaust stack going out of the building.

Ultimately the same amount of junk is being spewed into the air, so I don’t see how it makes much difference. It’s not like a vertical column of smoke shoots directly into orbit and goes exploring the solar system. It dissipates into the surrounding air after only a few yards at most.

Perhaps, but the exhaust has to push against the hinged cap to get it to open. The cap is either weighted or spring-loaded to close if there is no exhaust coming out, so there is resistance there. I suspect the hinged cap is cheaper to manufacture, or at least it was when it was introduced.

Believe me, it makes a difference and that is why the exhaust goes up. Exhausts that point down are a huge nuisance if you are a motorcyclist or just located near it. You are suddenly hit by a sudden blast of soot and other junk. You want that stuff as far away as possible as fast as possible.

      • Alternately, you could just drill a small hole at the bottom of the pipe’s run. In my younger days (when Volkswagen Beetles with baja pipes were still popular) you’d occasionally see one parked with a cup hung over the exhaust pipe–but the sly-dog method was to drill a 1/16-inch hole at the bottom of each pipe right before it joined the exhaust header, to prevent rain from running down into the engine when parked (if anyone removed the cup). I dunno if this would work for diesel trucks, but for a bug the amount of noise increase from such a small engine and such a small hole was pretty minor.
        ~

There is an initial torque that has to be applied by the exhaust stream to open the cap. Once it is standing nearly straight up and the truck is going to the road at 55 mph (carefully obeying the speed limit as all truckers do) the force that is required from the exhaust is negligible. On the other hand, a mushroom cap or 90[sup]o[/sup] bend is always there and at the velocity of the exhaust stream there is quite a pressure differential around a 90[sup]o[/sup] bend.

Ever wonder why smokestacks from factories are so high? Same concept, different scale. It DOES make a difference in the immediate area’s ground level air quality, as I said the first time.

I studied Civil and Environmental Engineering in college. Trust me.

Hey, I studied engineering too.

Ok, so it was software engineering. You win.