Pneumatic depressurizing sound on trucks? - Page 1
Forum section: electronics / general

author: vlad777
author_rank: Frequent Contributor
timestamp: February 26, 2017, 03:46:54 pm
content: Hi,What makes that "psssss" sound on trucks and buses?It happens regularly at low speed/rpm (?) during operation.Edit: And lasts about a second.I am very interested what kind of a system needs to vent regularly on a truck ??Many thanks.

author: kony
author_rank: Regular Contributor
timestamp: February 26, 2017, 04:04:29 pm
content: Overpressure valve blow-off. Compressor keeps pumping the reservoir for numatic actuators, and when there is no consumption of the air, once in a while it releases the excessive accumulated air.

author: Ampera
author_rank: Super Contributor
timestamp: February 26, 2017, 04:05:01 pm
content: Air breaks? I mean large vehicles often have MANY things that using pneumatics, especially buses. It's possible the air compressor is venting pressure if it gets too high.I mean over here we have GILLIG buses that have air driven doors, breaks, and kneeling (Yes our buses kneel, the front lowers to make it easier to get on)

author: Delta
author_rank: Super Contributor
timestamp: February 26, 2017, 04:11:00 pm
content: Releasing the pneumatic brakes.

author: vlad777
author_rank: Frequent Contributor
timestamp: February 26, 2017, 04:24:51 pm
content: I am not sure but I think venting happens when releasing accelerator.Is there any pneumatic system related to engine operation?

author: Ampera
author_rank: Super Contributor
timestamp: February 26, 2017, 04:42:59 pm
content: Quote from: vlad777 on February 26, 2017, 04:24:51 pmAir breaks.

author: StillTrying
author_rank: Super Contributor
timestamp: February 26, 2017, 04:49:45 pm
content: Air breaks?Is that when the driver stops for a few minutes rest.I am not sure but I think venting happens when releasing accelerator.Probably the pressing of the brake pedal.https://en.wikipedia.org/wiki/Air_brake_(road_vehicle)

author: SeanB
author_rank: Super Contributor
timestamp: February 26, 2017, 05:59:27 pm
content: Pneumatic systems on a truck/bus are brakes, both service and parking ones, power shifting of the gearbox ( very common on ZF, MB and MAN gearboxes amongst others), power clutch for manual gearbox ( you just press to disengage, and release the pedal to give a controlled smooth application as the system bleeds air out at a controlled rate), wipers, suspension ( both air bag suspension for trucks and ride height control for buses), door openers for buses and then assorted ancillaries, like a wheelchair lift on some buses, retracting steps on others and on some the windscreen washer as well.This is all supplied by an engine driven compressor, filling a few pneumatic reservoirs, and also include a manual or automatic drain valve to release condensed water from the air, inlet and outlet filters for the compressor, oil coalescing filters to get the dirt and oil vapour from the compressor out ( it is fed in many cases from engine oil for lubricating the pistons, bearings or other parts like variable displacement devices, and can have oil traces in the outlet air), some pressure burst disk, a emergency pressure valve ( burst disk is if the emergency valve fails to operate) and a pressure regulating valve that dumps excess air pressure when the compressor has filled the tanks to operating pressure. Thus the regular dumping when idling.Brake systems generally require pressure to release ( aside from some older systems grandfathered in decades ago and generally now required to be modified) so that if you do not have enough pressure ( big red brake warning light in the console) and attempt to release the parking brakes ( pressurise the system over 2bar with main system still under 6bar) there is a loud warning buzzer in the dashboard to tell you brakes are still applied. All other sytems require pressure to operate, just the brakes must be fail safe with burst piping or disconnected trailer lines.

author: Vgkid
author_rank: Super Contributor
timestamp: February 26, 2017, 06:25:42 pm
content: Quote from: vlad777 on February 26, 2017, 04:24:51 pmIf it is related to the engine, then it could be the wastegate.If it is the louder noise (sounds like a bunch of popping noises)often heard when a large diesel engine is deccellerating then it is the 'Jake Brake'. Which is used to slow an engine by opening the exhaust valves.

author: MarkS
author_rank: Unknown
timestamp: February 26, 2017, 06:56:45 pm
content: Former CDL holder and bus driver here. Those that have mentioned pressure release valves are correct. In the industry they are known colloquially as "pop off valves". They are a safety feature that keeps the air system below a set point; under 150 PSI in the U.S. This release of air happens regularly while the engine (the compressor is powered directly by the engine) is running, but is only audible when the vehicle is idling.

author: W2NAP
author_rank: Unknown
timestamp: February 26, 2017, 06:58:40 pm
content: air brakes.the compressor that fills the air tanks usually kicks on when the ap drops to about 100psi and kicks off at about 120psi

author: MarkS
author_rank: Unknown
timestamp: February 26, 2017, 07:06:27 pm
content: Quote from: W2IBC on February 26, 2017, 06:58:40 pmQuite right! 150 PSI is the maximum pressure allowed by the USDOT. If it ever reaches that high, something is wrong with the system. It's been five years since I drove a commercial vehicle and I forgot the details.

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2017, 07:31:43 pm
content: SeanB's middle paragraph describes perfectly what's happening, especially the bit about the auto drain valves on the bottom of the accumulator tanks that are there to release the condensed water and oil vapour from the system. Older trucks had a keyring on the valve or even a small tap for for manual use to drain the condensates.

author: W2NAP
author_rank: Unknown
timestamp: February 26, 2017, 07:57:28 pm
content: Quote from: MarkS on February 26, 2017, 07:06:27 pmI still drive a CMV. drove big truck, tanker, and now driving a rollback tow truck

author: MarkS
author_rank: Unknown
timestamp: February 26, 2017, 08:05:56 pm
content: Quote from: W2IBC on February 26, 2017, 07:57:28 pmI would be as well, but the rotating schedules required by charter bus operators was causing me to fall asleep at the wheel. It got to the point where my body was desensitized to caffeine. I wasn't going to be one of those drivers you see on the news that fell asleep and crashed his bus.

author: W2NAP
author_rank: Unknown
timestamp: February 26, 2017, 08:09:52 pm
content: Quote from: MarkS on February 26, 2017, 08:05:56 pmcan always become a tow truck driver.