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2002 gmc envoy_slt
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wait... what? ABS making you slide into something?? ABS makes sure your wheels don't lose traction with the pavement... Downshifting, while it doesn't hurt anything, doesn't have any way to know or control whether or not the wheels have traction...

Mike
Here's a thought, ABS helps the tires to maintain traction, but does (generally) extend stopping distances. So ABS could cause you to "slide" into something if it extended the distance too far.

...A question for deeper thought. If you were slowing down using your brakes they'd get hot. Where does the heat go when you're using your tranny for braking, and what simple mod could you add to increase your ability to slow down on mountain hills?
The heat goes into the transmission fluid, which is transfered to the coolant. A simple mod would be an additional external transmission oil cooler (with a fan...ideally thermostatically controlled with a manual bypass for off-road applications). By increasing the ability to cool the fluid before it gets to the radiator, you can substantially increase the ability to use the "engine brake" method to slow down. This also reduces the work the cooling system has to do, since at slow vehicle (especially rock-crawling) speeds the airflow over the radiator is due mostly to the fan.

(Deep thoughts ahead) The heat that would be dispersed by the brakes is diverted (by downshifting) to the engine and ultimately the radiator to the surrounding air. A more efficient engine fan or radiator would help disperse it. The trans does heat up some, but is not the main heat sink for the energy. It would fry in no time if it was since it is not designed to convert heat into energy (or back).:)
The engine braking is actually using the drivetrain as a drag on the engine. Therefore, the trans fluid is being heated up. That heated fluid needs to be cooled.
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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(Deep thoughts ahead) The heat that would be dispersed by the brakes is diverted (by downshifting) to the engine and ultimately the radiator to the surrounding air. A more efficient engine fan or radiator would help disperse it. The trans does heat up some, but is not the main heat sink for the energy. It would fry in no time if it was since it is not designed to convert heat into energy (or back).:)
He was referring to the pumping losses in not only the transmision, but specifically, the torque converter, making more heat via engine braking. Specifically, he was referring to adding an aux trans cooler.

If anything, engine braking makes the engine cooler... The fuel system shuts the fuel down, and it's sucking plain old air into the cylinders... Not only that, but it's really cold air, because there is a vacuum in the intake manifold, and when the air goes past the throttle plates, the loss of atmospheric pressure rapidly cools the air. Since there is very little air that makes it into the cylinders with a closed throttle, they don't compress it to a very high pressure, and therefore, don't heat it up very much. If the air leaving the engine is cooler than the piston/cylinder/head temperature, it cools the engine.

It's a bit different, but I've got a pyrometer on my CTD truck, that has a manual transmission. Running down the highway pulling a load, my EGT is usually somewhere in the 800F range... If I engine brake down a good sized hill, the EGT will go under 200F, and when I go back to idle (where EGT is less than 200F, usually), the EGT will actually climb up to 300F or so, and work its way down... It does this, because it's sucking heat out of the cylinders; and when the engine isn't drawing as much air through, and the air is in the cylinders longer, the air has time to pick up more heat from the pistons...

Mike
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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Here's a thought, ABS helps the tires to maintain traction, but does (generally) extend stopping distances. So ABS could cause you to "slide" into something if it extended the distance too far.
ABS only extends stopping distances on a smooth, dry surface where a mere mortal can brake right at the edge of traction; as well as on gravel and snow, as locking up the brakes help you stop on those, because they create a pile in front of the tire, and the material works against itself to help you stop.

In any kind of real world, non-frozen pavement braking (uneven, wet, wet/dry, and even smooth/dry for about 95% of drivers), the ABS will stop faster than without.

Regardless of this, he was talking about compression braking on the highway, to scrub some speed... He wasn't talking about panic stopping, anywhere near the realm of the ABS/non-ABS debate...

Mike
 

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heat

Yes, in an auto trans vehicle such as a trailvoy, the trans must absorb some of the heat, but the main physical heat exchanger is the engine (and its cooling system). If you are transferring the heat exchange duties that are resulting from converting velocity to heat from the brakes to a different mechanical system, that system must be able to handle the load, or else it will fail. The trans is (mainly) designed to affect torque not convert heat into energy. The engine is designed to convert heat into energy, and it works both ways, such as during engine braking. Thermodynamics says (1st law - conservation of energy) the energy of kinetic motion must remain kinetic or be converted to heat. In a gear transmission (e.g. semi-truck) little of that heat is lost in the trans. It has to go to the air through the engine and its cooling system. But certainly we need a trans cooler to help with cooling the secondary heat inevitably converted through the trans, but it can't be the main heat sink that takes the place of the brakes, or it will fry.
 

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:hijacked Asunder - you've got my attention now. :confused: For a long time I've been using engine braking on a dozen vehicles, and have been assuming the auto tranny torque converter was the major heat exchange element. But even my manual transmission vehicles had effective engine braking so thinking back, it can't be 100% from the torque converter. :duh: :duh: :duh:

But a quick Googling doesn't reveal any studies on what the proportion is, so now I'm confused. What a holiday gift to give somebody like me who could easily become obsessed with the question. ;) (My fault, not yours....) :p

So it seems I was wrong :sadcry: to assume it's only heating up the tranny and the aftermarket tranny cooler is the cheap improvement I was thinking of. There's definite thermodynamics going on in the engine, even with zero heat of combustion. Is the energy going into the cylinder walls and coolant by conduction from compressed and heated intake air, or out the exhaust. SOMETHING has to be the recipient of the energy that used to go into the brakes. :undecided

I'll probably need to split these posts off into their own thread, but if anybody can come up with a definitive, preferably scientific (and not a Mythbuster-class) link - please help me have a calm Christmas with this question answered. :coffee
 

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2003 gmc envoy_sle
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Discussion Starter #26
See what I mean, I knew it was more complicated than that! I kept trying to figure out the situation. Yall really help out a lot! Roadie You are 100% correct on this i Feel ya!:confused: Well i Do and i dont understand Its mixed!
 

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Lord Kelvyn's revenge...

Hey Roadie,
I respect your knowledge and appreciate your presence on this board which has helped me in sussing out some of the various things that happen to "Atrailvoyunders":). I thought that you were hinting at trans coolers with our brother from the north. I just wanted to spark some discussion to further add to the understanding of the physics of automobile systems. And sorry to ruin your holidays with questions of thermodynamics (btw, the second law, i.e. entropy always increases with time, is my favorite to ruminate on. I think it basically means that we are all doomed):eek: So you see, that happens to me also.:crazy:
I think it is back to friction that we can trace the source of the reconstituted heat...exactly what you said - the compression of air (a la the Jake brake in a more extreme form) and cylinder wall friction, along with accessory turning frictional drag that creates the heat that is radiated back to the "world" outside the vehicle.
 

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:hijacked Asunder - you've got my attention now. :confused: For a long time I've been using engine braking on a dozen vehicles, and have been assuming the auto tranny torque converter was the major heat exchange element. But even my manual transmission vehicles had effective engine braking so thinking back, it can't be 100% from the torque converter. :duh: :duh: :duh:

But a quick Googling doesn't reveal any studies on what the proportion is, so now I'm confused. What a holiday gift to give somebody like me who could easily become obsessed with the question. ;) (My fault, not yours....) :p

So it seems I was wrong :sadcry: to assume it's only heating up the tranny and the aftermarket tranny cooler is the cheap improvement I was thinking of. There's definite thermodynamics going on in the engine, even with zero heat of combustion. Is the energy going into the cylinder walls and coolant by conduction from compressed and heated intake air, or out the exhaust. SOMETHING has to be the recipient of the energy that used to go into the brakes. :undecided

I'll probably need to split these posts off into their own thread, but if anybody can come up with a definitive, preferably scientific (and not a Mythbuster-class) link - please help me have a calm Christmas with this question answered. :coffee

Non-expert opinion incoming:

Frictional losses at the engine bearings, cylinder walls, gears, shaft bearings etc. are where the energy transfer is happening. During engine braking we're reversing the flow of energy through the system from engine->wheels to wheels->engine while the engine still has a HUGE mechanical advantage. This essentially turns the motor from a power source into a huge air pump with a blocked intake source.

Where does energy loss happen on your average air pump? Through frictional losses on the wear surfaces.

Coolant is still pulling heat out of the motor, so the bulk of the energy transfer is happening to/from the engine coolant system. The frictional surface of ALL the wear parts in the system is much larger than the area on the brake rotors, so heat is easily managed. Still, all parts of the system absorb/release some of the wasted energy. Housings, gear oils etc...

How to MEASURE all of this? Above my pay grade.


Edit: looks like Asunder said basically the same thing... and threw in a couple laws of physics to boot...
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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Yes, in an auto trans vehicle such as a trailvoy, the trans must absorb some of the heat, but the main physical heat exchanger is the engine (and its cooling system). If you are transferring the heat exchange duties that are resulting from converting velocity to heat from the brakes to a different mechanical system, that system must be able to handle the load, or else it will fail. The trans is (mainly) designed to affect torque not convert heat into energy. The engine is designed to convert heat into energy, and it works both ways, such as during engine braking. Thermodynamics says (1st law - conservation of energy) the energy of kinetic motion must remain kinetic or be converted to heat. In a gear transmission (e.g. semi-truck) little of that heat is lost in the trans. It has to go to the air through the engine and its cooling system. But certainly we need a trans cooler to help with cooling the secondary heat inevitably converted through the trans, but it can't be the main heat sink that takes the place of the brakes, or it will fry.
Well aware of the laws of "fermo", as we called it... My point is that the heat that is being created in friction and through compressing air is offset by the fact that the engine is not fueling... So yes, it's making heat just by turning, and sucking against a closed throttle body, but my educated guess is that the engine is putting fewer joules into the cooling system than if it were at high idle (which is basically what it does when you are coasting in OD, and the transmission input sprag is acting as a 1-way clutch), using fuel to stay at that rpm, instead of using the back-flow of torque from the driveline.

Mike
 

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Well aware of the laws of "fermo", as we called it... My point is that the heat that is being created in friction and through compressing air is offset by the fact that the engine is not fueling... So yes, it's making heat just by turning, and sucking against a closed throttle body, but my educated guess is that the engine is putting fewer joules into the cooling system than if it were at high idle (which is basically what it does when you are coasting in OD, and the transmission input sprag is acting as a 1-way clutch), using fuel to stay at that rpm, instead of using the back-flow of torque from the driveline.

Mike
Hi Mike,
I think you are correct that the engine is probably putting fewer joules into the cooling system in "engine braking" mode than high idle. I suspect the efficiency of the energy transfer is better in that direction (wheels->engine) than the other way, else we would not need a radiator, which essentially is a nod to the INefficiency of internal combustion. Were ALL chemical energy converted to kinetic energy, no need for a radiator to bleed "waste" heat. Diesels have no throttle plate and still have substantial engine braking, and that gets substantialer when you switch on the Jake brake solenoid actuating the valves. I think we basically agree:).
Steve
 

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Hmmmm, interesting thread.........I'll bet the OP didn't expect this :rotfl:

Just as an aside, my wife has an '08 Saturn Astra (re-badged Opal Astra), with the 4 speed auto tranny.
When the PCM sees the foot brake applied while descending a grade, it forces a downshift to 3rd.
The tranny stays there until the PCM sees throttle applied (or the vehicle stopped)

A very useful parm in my opinion :yes:
 

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He was referring to the pumping losses in not only the transmision, but specifically, the torque converter, making more heat via engine braking. Specifically, he was referring to adding an aux trans cooler.

If anything, engine braking makes the engine cooler... The fuel system shuts the fuel down, and it's sucking plain old air into the cylinders... Not only that, but it's really cold air, because there is a vacuum in the intake manifold, and when the air goes past the throttle plates, the loss of atmospheric pressure rapidly cools the air. Since there is very little air that makes it into the cylinders with a closed throttle, they don't compress it to a very high pressure, and therefore, don't heat it up very much. If the air leaving the engine is cooler than the piston/cylinder/head temperature, it cools the engine.

It's a bit different, but I've got a pyrometer on my CTD truck, that has a manual transmission. Running down the highway pulling a load, my EGT is usually somewhere in the 800F range... If I engine brake down a good sized hill, the EGT will go under 200F, and when I go back to idle (where EGT is less than 200F, usually), the EGT will actually climb up to 300F or so, and work its way down... It does this, because it's sucking heat out of the cylinders; and when the engine isn't drawing as much air through, and the air is in the cylinders longer, the air has time to pick up more heat from the pistons...

Mike
Mike, do you know if there is a way to confirm that the fuel is being cut off during deceleration on my '03 TB 4.2L? I have literally zero engine braking under about 50 MPH, it takes quite a bit of braking pedal pressure to slow the thing down in town.
 
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