edit: Solved. Thanks guys
In: Engineering
In addition to the air brakes, buses and trucks use air suspension, so you may be hearing them releasing air from their bags. Most times when a bus stops, it releases air to lower the bus and make it easier for passengers to hop off.
A lot of heavy vehicles rely on air pressure to actuate their braking systems. The “tsss” is excess air being released by a pressure regulating valve.
It’s the brakes.
Cars like yours or mine use hydraulic braking fluid to compress the brakes. This isn’t loud enough to provide a distinctive sound (unless something is wrong with said brakes).
Trucks and busses used compressed air. This air needs to be kept at the right pressure for the brakes to work properly, and the use of the brakes causes the pressure to increase, so a valve needs to release pressure to keep that in check. The hissing sound you hear is that mechanism at work, releasing the air.
As most have already said, trucks use air pressure to both remove the parking/secondary brake and to apply the service brake.
A compressor is constantly running which charges several large tanks which in turn provide air to release the parking brake and for service brake applications. When the system reaches a set pressure a Pressure Relief Valve will vent to atmosphere to keep the system pressure as close to constant as possible. This is one occasion where you might hear a release.
The louder releases that you can hear are more than likely as soon as or very shortly after the truck has come to a complete stop. Some of the axles have a Spring Brake Actuator fitted. These use pressure to apply the service brake and to compress a spring. When applying the secondary or parking brake, that pressure is released sharply allowing the spring to reassert itself, applying the brake mechanically. The spring also acts as mechanical fail safe in the event of an air leak. It’s required by law in the UK that all vehicles are fitted with a form of mechanical brake.
In short – apply parking brake, big release of air, gigantic spring applies brakes.
An interesting (I thought) aside – the air pressure holds the brakes off, and each brake has its own spring that is always trying to apply the brake. Then the air pressure from the control system is run to all brakes via lines. That way if there is a leak or some other failure where you loose air pressure, the brakes go on. I always thought that was a cool example of a ‘fail-safe’ design – where the design tries to predict a failure that is likely to happen, and defaults to the safest option.
*edit*: turns out this is misleading (unintentionally). I thought nearly all air brake systems worked this way, but it sounds like this is only in the case of the parking brake. See other comments below. Thank you to those who have taught me something here.
So why do these big boys use air pressure and not what ever the rest of us smaller rides use?
It’s our air brakes. They use air to apply instead of brake fluid – which is what cars use. so what you are hearing is them releasing their brake pedal which is venting the air.
I responded, but for some reason it was removed. My answer was simplistic,a nd answered the question.
The noise is from the air dryer expelling moisture from the air system. Trucks use compressed air for brakes and other functions. That requires that the system have as little moisture as possible. They have an air dryer plumbed in, and as it removes moisture, it has to empty itself sometimes.
I see a lot of right and wrong answers here. I think the TSSSSS your thinking of is the air dryer. When ever you use air in the air brake system, it needs to be replaced, with dry air. Once the air tanks are refilled the air dryer purges itself with a loud TSSSSSS.
Source: Am heavy truck mechanic
Trucker here:
That noise you hear could be a few things. All relating to the brakes. The trucks brake system relies on pneumatic (air) pressure rather than hydrolic (fluid) pressure like in your normal car.
When the driver applies the brakes air pressure is applied to the system, the air expands inside the brake chamber which turns an S-cam which moves a rod that pushes the brake pads to make contact with the disk or drumb (usually drumb) and the truck stops. When the driver releases the brakes you can hear a hiss as the air pressure is released.
The hiss you hear that most people associate with trucks is when the driver sets their parking brakes. They pull out a valve in the cab that in turn sends a rush of air pressure from the air tanks to the brakes. And that holds in place as long as the trucks compressor is running. After a long while, the air may bleed off and once it reaches a predetermined pressure (20psi) the emergency spring brakes are automatically engaged. That truck ain’t going anywhere.
The last hiss ill talk about is a safety check valve on the air tanks. You’ll hear this hiss after pressure in the tank builds to 125psi. At that point the valve opens and quickly releases some pressure and a satisfying hiss and closes again once the tank is back below 125psi. This whole thing happens in less than a second.
Its a complicated system, but when dealing with heavier loads, air brakes are the bees knees.
Latest Answers