Torque Converters & Diesel Engine Breaking!
#1
07-04-2007, 02:15 PM
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Torque Converters & Diesel Engine Breaking!
I posted here Posts # 34, 36, 37, 39, 40, and 42 https://www.ford-trucks.com/forums/628367-using-engine-to-slow-down-when-towing-not-an-exhaust-brake-question-3.html#post4897266 trying to reconcile what I believe to be some serious misunderstandings about my above title. Below, I've added some additional thoughts on the matter, and I'd appreciate inputs from others regarding their thoughts on this topic, so please join in on the fun!
The pic shows a basic TC=Torque Converter, which provides a "fluid coupling" between the flywheel and the input shaft to the transmission. The pump is the impeller, and it's attached to the flywheel. Forward torque from the flywheel causes the impeller to couple fluid energy to the turbine which in turn applies forward torque to the transmission input shaft to accelerate the truck forward against the forces of inertia, drag, and rolling resistance.
This same "fluid coupling" also works fairly well for providing engine braking during coast, where reverse torque from the driveline is applied to the turbine, and "is fluid coupled" to the impeller and flywheel to accelerate the engine against the forces of compression and friction. Note that the direction of flywheel and driveshaft rotation is unchanged, but the sense of the torque being applied changes from a forward direction for accelerating the truck to a reverse sense for accelerating the engine. A plain old TC will couple torque in both directions without the aid of a TCC=Torque Converter Clutch and its mechanical lockup capability.
There seems to be a basic misunderstanding about diesels not providing compression braking. Actually, they provide better "compression braking" than gassers because diesels ingest more air at idle (no closed throttle butterfly) than gassers, and compress it to a higher compression ratio, and this takes more work, and causes a larger retarding torque on the crankshaft. Gassers with a closed throttle butterfly act as a vacuum pump, and these pumping losses give them some additional retarding torque. An exhaust brake on a diesel provides additional retarding torque as the pistons push the exhaust gas out against the resistance of an exhaust restriction.
I can easily judge the relative amount of engine breaking by coasting in neutral and getting none, coasting in gear with my exhaust brake off and feeling the natural compression braking of the engine, and applying my exhaust brake which adds pumping loss to the compression breaking, but reduces the amount of compression breaking some because less air is ingested with the exhaust brake on.
Slippage in a TC is desired for providing no stalling at a stop, and for torque amplification. In addition to allowing your truck to come to a complete stop without stalling the engine, slippage in the TC provides additional torque to the driveline when you accelerate away from a stop. TC's can multiply the torque of the engine by two to three times. This effect only happens when the engine flywheel is turning at a much higher rpm than the transmission input shaft.
Unwanted slippage in a TC causes inefficiency and wasted power. At higher road speeds and more moderate driveline loads, the transmission input shaft rpm catches up to that of the flywheel, and the differential rpm between them is only about a 100 rpm or so. However, this slight residual slippage wastes a little power, and is the main reason why automatic transmissions got worse mpg than manual transmissions.
Back in my day as a hot rodder kid that raced away from every stop light (didn't full stop for just signs), we liked a lot of slippage because that's what got us off the line quicker. Of course gas was a X10 cheaper then. A TCC with lockup capability was added to a TC much later in the game, to improve its efficiency during cruise when fuel economy became a concern. A TCC is not required for transmitting reverse torque for engine braking, and neither is a stator which I'll discuss below. We didn't have a TCC in my day or in some cases even a stator, and we got plenty of engine breaking from a plain old slush box TC!
The stator was added to the basic impeller-turbine design to improve efficiency long before a TCC was added, but the stator does it's job at low speeds when slippage is large whereas the TCC works at higher speed when slippage is small. The stator is in the very center of the torque converter between the impeller and turbine. It redirects the fluid returning from the turbine before it hits the impeller again. This increases the efficiency of the torque converter. There is a speed around 40 mph at which both the impeller and the turbine are spinning at almost the same rpm. Now, the fluid returning from the turbine enters the impeller already moving in the same direction as the impeller so the stator is not needed. Under these conditions the fluid actually strikes the back sides of the stator blades causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it.
I was going to drone on about the key difference between a manual and an automatic transmission. The manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, whereas in an automatic transmission the same set of gears produces all of the different gear ratios, and how the planetary gearset is the device that makes this possible, but that post will have to wait awhile!
The pic shows a basic TC=Torque Converter, which provides a "fluid coupling" between the flywheel and the input shaft to the transmission. The pump is the impeller, and it's attached to the flywheel. Forward torque from the flywheel causes the impeller to couple fluid energy to the turbine which in turn applies forward torque to the transmission input shaft to accelerate the truck forward against the forces of inertia, drag, and rolling resistance.
This same "fluid coupling" also works fairly well for providing engine braking during coast, where reverse torque from the driveline is applied to the turbine, and "is fluid coupled" to the impeller and flywheel to accelerate the engine against the forces of compression and friction. Note that the direction of flywheel and driveshaft rotation is unchanged, but the sense of the torque being applied changes from a forward direction for accelerating the truck to a reverse sense for accelerating the engine. A plain old TC will couple torque in both directions without the aid of a TCC=Torque Converter Clutch and its mechanical lockup capability.
There seems to be a basic misunderstanding about diesels not providing compression braking. Actually, they provide better "compression braking" than gassers because diesels ingest more air at idle (no closed throttle butterfly) than gassers, and compress it to a higher compression ratio, and this takes more work, and causes a larger retarding torque on the crankshaft. Gassers with a closed throttle butterfly act as a vacuum pump, and these pumping losses give them some additional retarding torque. An exhaust brake on a diesel provides additional retarding torque as the pistons push the exhaust gas out against the resistance of an exhaust restriction.
I can easily judge the relative amount of engine breaking by coasting in neutral and getting none, coasting in gear with my exhaust brake off and feeling the natural compression braking of the engine, and applying my exhaust brake which adds pumping loss to the compression breaking, but reduces the amount of compression breaking some because less air is ingested with the exhaust brake on.
Slippage in a TC is desired for providing no stalling at a stop, and for torque amplification. In addition to allowing your truck to come to a complete stop without stalling the engine, slippage in the TC provides additional torque to the driveline when you accelerate away from a stop. TC's can multiply the torque of the engine by two to three times. This effect only happens when the engine flywheel is turning at a much higher rpm than the transmission input shaft.
Unwanted slippage in a TC causes inefficiency and wasted power. At higher road speeds and more moderate driveline loads, the transmission input shaft rpm catches up to that of the flywheel, and the differential rpm between them is only about a 100 rpm or so. However, this slight residual slippage wastes a little power, and is the main reason why automatic transmissions got worse mpg than manual transmissions.
Back in my day as a hot rodder kid that raced away from every stop light (didn't full stop for just signs), we liked a lot of slippage because that's what got us off the line quicker. Of course gas was a X10 cheaper then. A TCC with lockup capability was added to a TC much later in the game, to improve its efficiency during cruise when fuel economy became a concern. A TCC is not required for transmitting reverse torque for engine braking, and neither is a stator which I'll discuss below. We didn't have a TCC in my day or in some cases even a stator, and we got plenty of engine breaking from a plain old slush box TC!
The stator was added to the basic impeller-turbine design to improve efficiency long before a TCC was added, but the stator does it's job at low speeds when slippage is large whereas the TCC works at higher speed when slippage is small. The stator is in the very center of the torque converter between the impeller and turbine. It redirects the fluid returning from the turbine before it hits the impeller again. This increases the efficiency of the torque converter. There is a speed around 40 mph at which both the impeller and the turbine are spinning at almost the same rpm. Now, the fluid returning from the turbine enters the impeller already moving in the same direction as the impeller so the stator is not needed. Under these conditions the fluid actually strikes the back sides of the stator blades causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it.
I was going to drone on about the key difference between a manual and an automatic transmission. The manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, whereas in an automatic transmission the same set of gears produces all of the different gear ratios, and how the planetary gearset is the device that makes this possible, but that post will have to wait awhile!
#2
07-04-2007, 02:49 PM
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#4
07-04-2007, 04:12 PM
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#5
07-04-2007, 04:20 PM
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you can ask any cat, cummins, detroit or mack builder..they will tell you the same as i just did..i drive a peterbuilt with a 425 cat in it full of logs..(little over 100,000 lb a load) i want the jake over the exhaust brake..it holds alot better..but for cost reasons over the long haul..the exhaust brake is much better for the engine...
#6
07-04-2007, 05:01 PM
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Due to the way the Jake Brakes operate, they are much more effective at producing braking horsepower. In some cases the Jacobs systems can almost produce as much braking HP as the engine can produce on acceleration. The exhaust brake is not nearly as effective and i dont see why someone would want that when they could have a Jake Brake.
#7
07-04-2007, 05:04 PM
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Originally Posted by strokin_it7.3
Due to the way the Jake Brakes operate, they are much more effective at producing braking horsepower. In some cases the Jacobs systems can almost produce as much braking HP as the engine can produce on acceleration. The exhaust brake is not nearly as effective and i dont see why someone would want that when they could have a Jake Brake.
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#8
07-04-2007, 05:14 PM
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Originally Posted by strokin_it7.3
Due to the way the Jake Brakes operate, they are much more effective at producing braking horsepower. In some cases the Jacobs systems can almost produce as much braking HP as the engine can produce on acceleration. The exhaust brake is not nearly as effective and i dont see why someone would want that when they could have a Jake Brake.
a lot more effective yes..i am not denying that...but is more harmful on a motor then an exhaust brake....if you run the jake in too high of an RPM range..you will bend some valves..a lot of fleet owners buy trucks with no jakes or exhaust brake at all cause you get more miles with out repairs...like I said I haul 100,000 lb loads..and I wouldn't do it in these mountains with out a jake..Mack and Mercedes have what they call a dinotard..its just an exhaust brake..a lot of new big trucks are coming out with them now cause it is better then nothing..but more cost effective in the long haul to have the Jacobs..
#9
07-04-2007, 05:21 PM
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#11
07-04-2007, 05:26 PM
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#13
07-04-2007, 05:39 PM
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Originally Posted by ron's power stoke
valve jobs at around 800,000...or its strickly a highway truck...the jake dont get used that much..up here we are lucky too see a flat road a mile long..the jake gets used all the time...
#14
07-04-2007, 05:45 PM
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we do a in frame rebuild..thats it.. you dont need to take my word for it,,if you really want to know..call cat or one of the other guy's...ask them witch motor will last longer with no head or valve problems...or ask them why fleet owners buy trucks with no jakes..by the way happy 4th...
#15
07-04-2007, 05:54 PM
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I don't make these statements with no experiece with them I have two log trucks in northern alabama pulling 90k one cat one cummins both in excess of a million miles with only lower bearings replaced recently
my cousin who has seven that run with them has the same with both cummins and cat
my cousin who has seven that run with them has the same with both cummins and cat
