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Compression braking while accelerating?

leesonic

leesonic

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I live on a somewhat busy road, and I've noticed something with some trucks and tractor trailers lately. Some of them seem to leave their compression brakes (AKA Jake brakes) switched on when pulling away from a light, causing them to make that rasping nose while accelerating. Is there any reason for this, or are they just trying to be obnoxious? I don't drive a truck, but I'm guessing there must be some sort of switch or control to turn this off and on. But why would you leave a device designed to slow you vehicle down switched on while accelerating?

Lee.
 
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There is a switch that control the compression brake, but to the best of my knowledge, the compression brake doesn't have any affect when the engine is under a load (accelerating). If the throttle is open and air/fuel are entering the combustion chamber, I don't think the jake brake is doing anything at all; it only works when the vehicle is decelerating and the throttle is closed.

Perhaps you're hearing the turbo spooling and the air rushing into the engine? Those diesels consume a massive amount of air!
 
Around 99.99% of diesels don't have throttles, so the throttle will never be closed. Every compression stroke is compressing a full cylinder full of air (and fuel), hence the need for the blocks, crankshafts, etc. to be made stronger than a comparable gas engine. Compressing the air with no fuel slows down the crankshaft, but that compressed air will push the piston back down again if there is no other route out, albeit not as strong as if there is fuel there to be burnt. Venting it through the exhaust with the piston at or near the top of it's stroke reduces that effect.

The noise of a turbo spooling is a high pitched whistling noise. This noise is like a machine gun rasping noise, and it only happens between gearshifts, and on deceleration obviously.

Lee.
 
Between gearshifts perhaps it's a wastegate unloading high boost/manifold pressure.

More stuff these days uses variable geometry turbos so wastegates I thought were mostly a thing of the past, but...maybe.

Edit: Perhaps another possibility is an engine brake kicking in to manage engine rpm overshoot as the clutch is disengaged.
 
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I've never noticed a "rasping noise" from the new trucks during acceleration. They definitely have (IMHO), some kind of waste gate that pops off when they shift gears.
 
Municipalities in my area pass and post ordinances forbidding the use of un-muffled engine brakes(what the OP is hearing, Jacobs or otherwise) in or near residential areas.
 
If there is alot of traffic, the jake brake will assist in slowing down the rig in case some asshole cuts in front. So some drivers will leave it on in the inner city or congested areas.You have to realize what it is like with tonnage onboard, even with the best of technology you can never stop "on a dime" or even close. You would be lucky to stop on the federal reserve building.

To do a brake job on those big rigs costs a bunch of money, but before you go and say "So what ?", remember that everything you have came in on a truck, even your truck. So every time you want to regulate them and fine them or tax them more, it is really coming out of your pocket.
 
I drove truck for a while....and the jake brake switch can be mounted just under the top of the shift nob so that the longer finger can activate or deactivate it. Sometimes fatigue from too many hours of driving can be noticed by how they are driving.

The driver just simply for got to hit the switch...
 
I drive a 98 Kenworth T800 chassis dump truck with a 435 CAT 6cyl. My truck has a Jake brake it vents through the muffler. It also shuts off at Idle RPM. I leave the Jake brake on almost all the time. Some city driving and a few areas where Jakes are prohibited outright. That said. If the driver completely releases the fuel pedal the jake will on many trucks engage until the pedal is depressed again. This can easily happen during a shift. And be causing what you are hearing.

When I drive one of our other trucks that does not have the Idle shut off on the Jake I end up having to fiddle with it all the time which is to me a BIG PITA.

My truck currently has about 460K on the clock and gets better loaded mileage then out 2015 Kenworth T800 with some fancy poop clean Cummings in it Granted his truck is not quite broke in yet (has less then 12K on the clock) But he gets about 4.3mpg loaded and I routinely get 4.6-4.8mpg with a 1 ton heavier load (new truck has a 31 ton working load mine has a 32.25 ton working load). His blows past me on the hills. I can run 70mph on the flats he's restricted to 65mph.
 
BigElCat said:
I've never noticed a "rasping noise" from the new trucks during acceleration. They definitely have (IMHO), some kind of waste gate that pops off when they shift gears.
Click to expand...

Yep. A blow-off valve keeps the turbos from spooling down, at every gear change. It keeps the boost up, so that the engine doesn't bog.

This is actually very good for the environment- it minimizes the amount of black soot that is discharged during a gear change, or after momentary deceleration. If the turbo is spun up, it comes up to full boost almost immediately, as soon as the throttle is pushed.

IIRC, this was something that was pretty much brought to common use, by Gale Banks, a few decades ago. I'm pretty sure he didn't invent the concept- but he was one who showed how much benefit could be had, from using one, pretty much before anyone else...

Regards,
Gordon.
 
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