Spis treści:
- History of the DIWA series gearbox and how it came into existence
- Main idea behind the operation of DIWA gearbox
- The exact operation and powerflow of early DIWA series without a retarder
- The exact operation and powerflow of late DIWA series with a retarder
- The infamous Voith gear whine
- The future of DIWA gearboxes
This article is about the Voith DIWA series of hydrokinetic transmissions and small history of the DIWA gearboxes will be written and then the exact operation of this type of gearbox will be explained in detail. The DIWA series of gearboxes work on the principles of differentials and torque converters and they are used in buses that work in city and suburban routes.
Autor: TAUNUS
History of the DIWA series gearbox and how it came into existence
City buses in the 1950s generally had a manual transmission with a very long shifter running from the front all the way to the back along with a clutch pedal linkage that also ran along with the shifter linkage all the way to the back. Not only this manual transmission was very tiring to operate, it also required frequent maintenance such as the clutches would frequently wear out and would need replacement along with the shift and clutch linkages that required very frequent adjustments and greasings to keep them operating properly. They would lose adjustment and need to be greased very frequently due to their very long nature caused from having to span meters in length from the front of the bus all the way to the back.
At the time early automatic gearboxes were also being used with torque converters and planetary gearsets such as the Leyland city buses that used Lysholm-Smith torque converters, these types of gearboxes provided a very easy operation for the driver along with low maintenance thanks to the torque converter and strong planetary gears and however they were very inefficient because the torque converter always has some losses due to not being able to couple 100% and back then generally high torque multiplication converters were used along with low amounts of gears which really hurt their fuel economy and fuel in 1950s europe was extremely valuable so the manual transmission buses were very popular due to being much more efficient than the regular automatics.
During the early 1950s Voith wanted to find a medium and produce a new type of gearbox that would be both efficient and at the same time offer the ease of use and low maintenance aspects of the automatics. From this idea came the first serie of DIWA gearbox in the year 1952 and it was introduced to the public in the 1953 Frankfurt motor show. The name DIWA stands for „Differential Wandler” in German which translates as „Differential torque converter” to English. This naming implies the combined use of a differential and a torque converter.
Buses that work city routes and suburban routes barely ever see speeds over 80 km/h because of their routes not being able to support driving at higher speeds. Due to this reason city buses dont require transmissions with tall gearing to provide low rpm cruising at high speeds. The most important thing when it comes to the gearboxes of city buses is being able to get the bus moving quickly and efficiently from a complete stop which is accomplished extremely well with the DIWA gearbox thanks to its CVT like stepless operation in first gear.
The very first models had only 2 forward and 1 reverse speeds which was enough since these buses did not exceed 80 km/h speed. However as time went on it received more planetary gearsets which allowed 3 and 4 speed versions to be introduced along with a retarder function. The 2 speed versions were good in the 1950s however they were still lacking a bit in terms of efficiency and performance so the DIWA series was constantly being developed further and overtime 3 and 4 speed versions came into existence that offered better efficiency and performance.
Main idea behind the operation of DIWA gearbox
A planetary gearset thanks to its nature can have 2 different inputs simultaneously coming to it which makes it possible to vary the speed of one of the inputs to change the overall gearing provided by the planetary gearset. The details of why exactly this happens is not important in this article however for people that are interested, this phenomenon of planetary gearsets is explained in detail in this article.
This means that a planetary gearset can be turned into a CVT like stepless gearbox if there is a mechanism involved that can constantly change the input or output speeds of the planetary gearset and this way constantly adjust the gearing provided by the planetary gearset. This is where the differential part of this gearbox comes into play, the differential section is located at the front end of the whole gearbox and is followed by the torque converter as it can be seen in the schematic below shown as „B”.

This differential planetary gearset receives input from the engine via the input shaft shown as „a”. This input shaft is connected to the input sun gear „s” and the input sun gear is connected to the planet gears of the planetary differential shown as „p” which is connected to the planet carrier „q”. The carrier is connected to the impeller of the single phase and single stage torque converter shown as „P” in the schematic meanwhile planet gears are also connected to the output sun gear shown as „r”. This output sun gear is directly connected to the intermediate drive shaft „b”.
This intermediate drive shaft is only connected to the turbine of the torque converter shown as „T” via an overrunning clutch shown as „f”. The overrunning clutch makes sure that only the turbine can drive the intermediate driveshaft and that the intermediate drive shaft can not drive the turbine when the speed of the intermediate drives shaft exceeds the speed of the turbine, the overrunning clutch allows the turbine to freewheel and not be driven by the intermediate drive shaft.
This means that when the output sun gear is driven by the planet gears the power flows a purely mechanical path which is efficient, however the other path for powerflow that goes from planet carrier to the torque converter is purely a hydraulic path because that path only goes through the torque converter as it can be seen in the drawing below.
This makes the DIWA series also a „split powerflow” type of gearbox like the original GM Hydramatic gearbox where the power coming from the engine is split into 2 different paths one of which is hydraulic through a fluid coupling and the other path being purely mechanical. The main idea behind the DIWA lies in this differential section, if the 2 different paths of powerflow can be driven in constantly differing speeds, then the differential section can act as a stepless gearbox and constantly provide different gear ratios.
The exact operation and powerflow of early DIWA series without a retarder
The early and later series differed a bit in terms of operation so the earlier version will be explained first which the D506 version shown above was also an early version. In the earlier versions as shown above the differential gearset is made up of 2 different sun gears, 2 different planet gears which are connected to the same planet carrier and there is no ring gear like in a conventional planetary differential.
The planet carrier is connected to the impeller of the torque converter and is also connected to a brake clutch pack shown as „d” in the schematic from previous section. The overall layout of the differential can also be seen in a bit more detail in the schematic below.
In first gear (also known as „DIWA gear” in German publications) there is split power flow along with a CVT like stepless operation. When the bus is stationary, the output shaft „c” and therefore the intermediate driveshaft „b” and the output sun gear „r” from schematic below are also all stationary.
With an idling engine, the power will flow from the input sun gear to the planet gears and from the planet gears the power will flow to the impeller of the torque converter however because the bus is stationary, the output sun gear and the shafts connected to it will be held stationary by the weight and inertia of the stationary bus so no power will be flowing from the planet gears to the output sun gear. The planet gears will be revolving around the stationary output sun gear because the output sun gear will be held stationary by the weight and inertia of the stationary bus.
This means that at idle, the powerflow is only through the hydraulic path. Because the engine is idling at a low rpm, it will also drive the impeller of the torque converter at a low speed and due to this the torque converter will be stalled because the impeller is not spinning fast enough to drive the turbine of the converter. For more information about torque converters check out this article:
As a result of this, the engine is decoupled from the output shaft and the engine can safely idle while the bus is stationary.
Due to the exact gearing in the differential, the planet carrier will be driven at approximately double the engine speed and this means the impeller will also be driven at twice the engine speed however this is still not enough to move the bus when the driver is pressing the brake pedal. The bus will creep forward if the driver is not pressing the brake pedal but if the brake pedal is pressed it creates a lot more resistance which stalls out the torque converter so that the bus can stand still and the engine can safely idle without stalling.

When the driver floors the accelerator pedal, the engine will jump upto around 60-65% of its maximum rpm in a very fast manner and it will stabilize at that rpm and slowly keep rising in rpm until shifting into second gear which can be heard in this video at timestamp. This 60-65% of the maximum rpm is where the peak torque of most of the diesel engines is being made which means the rpm where they are most volumetrically efficient and this means that it will be using the fuel most efficiently in that rpm range while at the same time getting the bus going at a rapid phase.
This happens thanks to the CVT like stepless nature of this gearbox in first gear, the gear ratio inside the transmission constantly changes so that the engine can be kept at its maximum efficiency range while at the same time providing a lot of power for acceleration until shifting into second gear which can be seen in the graph below.
In cases where the driver depresses the pedal a bit and not fully, then the engine will jump to a lower rpm and then slowly keep rising provided that driver doesnt smash the accelerator pedal later.
When the driver floors the accelerator pedal, the output shafts and the output sun gears will at first still be stationary because the bus will also still be stationary however now that the engine picks up speed, so does the impeller and it can now start turning the turbine of the torque converter.
As stated before, the turbine is connected to the intermediate drive shaft so as the turbine rotates, it will also rotate the intermediate drive shaft and the output shaft and the output sun gear all at the same time and this way the bus also starts moving. Notice that turbine’s rotation also causes the output sun gear to rotate along with it. This means that the output sun gear is no longer a braked element in the differential gearset which means now the engine power will also start flowing from the planet gears to the output sun gear along with the carrier which connects to the torque converter.
Now the differential gearset has 2 different output speeds which and the speed difference between them will be constantly changing which will constantly change the overall gearing. At first the speed of the impeller will be a lot higher than the speed of the turbine and therefore the output sun gear which gives a high gear ratio however as the torque converter starts coupling and turbine speed starts to near the impeller speed, the output sun gear’s speed will also be nearing the speed of the impeller which causes the gear ratio to decrease.
The more the output sun gear speeds up, the more the gear ratio decreases in the differential. Additionally, the torque converter used in all DIWA gearboxes is of the single phase type meaning the stator is completely stationary no matter what which can be seen in the picture below where the stator shown at the right is fixed to a stationary housing. The middle part shows the impeller and the left part shows the turbine.

This is different from a trilok converter where after the converter goes into the coupling phase, a freewheeling clutch on the stator will allow it to freewheel inside the converter housing so that it doesnt create any restrictions to the flow inside the converter at high impeller speeds.
In the case of DIWA, after the torque converter goes into the coupling phase the stator will still be fixed and still keep directing the flow inside the converter according to the angle of the stator blades and after turbine reaches near impeller speeds and converter goes into coupling mode, this is very bad and creates a lot of restriction and losses.
However this was done on purpose in the DIWA series, powerflow will choose the path of least resistance. So in the DIWA series by making sure the hydraulic path increases in resistance after a point will make sure that the power share of the mechanical section will increase while the powerflow thourgh the converter will decrease. So as the bus speeds up more of the power will start flowing through the mechanical section, so the efficiency of the gearbox will increase as it speeds up because less of the power goes to the converter and experiences fluid losses there.
This phenomenon can also be seen in the graph above where the mechanical share increases and the hydrodynamic share decreases as the engine speeds up. So in the DIWA gearbox because of it keeping the engine near its peak volumetric efficiency and flowing more of the power through the mechanical section as it speeds up meant that it had overall very good efficiency especially when compared to the conventional automatic gearboxes at that time.
It was still not as efficient as purely mechanical gearboxes however a bit less efficient automatic gearbox was worth it since it offered a lot easier operation and less maintenance costs.
The overall tractive effort of the DIWA gearbox was also great, not only did it keep the engine around its peak torque rpm, it also multiplied the torque because torque converters also multiply the torque which is the greatest when they are stalled and decreases as the impeller speeds up and the multiplication ceases to exist once the converter has coupled. However until the coupling point is reached, it is still multiplying the torque by a good amount and increasing the total tractive effort of the bus further which can be seen in the graph below.
The blue line shows the tractive effort which is highest when the bus has no speed and starts to decrease as the bus picks up speed and the reason for this is both because of the gear ratio in the differential decreasing with speed and the fact that torque multiplication in the converter also decreases with increasing speed of the bus.
The black line shows the efficiency of the single phase torque converter, as it can be seen after bus reaches 40% of its top speed, the efficiency of the converter really starts to dip due to the stator not being able to freewheel after converter goes into coupling mode.
The purple line shows the speed of the engine and the sudden dip at 50% of buses top speed indicates the point where it shifts into second gear.

Tractive Effort and Efficiency Graph for 2 Speed D506 | Voith D506 Brochure
Second gear is engaged when the output sun gear and therefore the intermediate drive shaft start to spin faster than the carrier of the differential gearset at which point the overrunning clutch connected to the turbine will let go and the very fast spinning intermediate drive shaft wont be accelerating the turbine further which would cause a lot of fluid losses and make it inefficient.
Additionally the brake clutches connected to the carrier of the differential gearset are also engaged so that the impeller of the torque converter is brought to a halt and the turbine no longer receives any fluid motion coming from the impeller. This means that the carrier will be halted and the planet gears will only spin around their own axes and not walk around the teeth of the sun gears.
Furthermore, since the freewheeling clutch also prevents the intermediate drive shaft from driving the turbine, the turbine has nothing driving it, so it will lose all of its speed and also be brought to a complete halt which means the entire torque converter is completely out of the equation in second gear and this way in second gear it is ensured that the powerflow is purely from the mechanical range with no losses from a fluid coupling which means in second gear the DIWA gearbox is as efficient as a regular manual gearbox.
This can also be seen in the graph above with the black line showing the efficiency of the converter which becomes a flat line at 100% efficiency after shifting into second gear because the converter is not driven in second gear.
The D506 version shown above came in 2 speed or 3 speed versions and the 2 speed version was shown here but the 3 speed version is basically same, it has one extra planetary gearset at the back of the gearbox which gives a lower gear ratio 3rd gear. The 3rd speed is exactly same as second gear where the torque converter is completely out of the equation.
While pure city route buses wont really need the third gear, suburban route buses will still benefit from a lower ratio third gear so voith offered both 2 and 3 speed versions. The 3 speed version can be seen below with an extra planetary gearset at the back and below it the tractive effort and efficiency graph can also be seen. The reverse gear works exactly same as the late versions so the reverse gear will be explained in that section.
One final note in this section is that the DIWA series is also kind of unique because the torque converter has cooling jackets around it so the torque converter is directly cooled by water at least in buses that came with liquid cooled engines, in very hot climates the cooling of the converter is also supplemented with a separate oil radiator that flows the ATF from the converter into the radiator to cool it in a separate radiator along with cooling provided by the coolant jackets.
In air cooled buses they came with this extra radiator as standard because there was no coolant since engines were air cooled.


The exact operation and powerflow of late DIWA series with a retarder
In 1973 at IAA the next generation of the DIWA known as D851 was presented and it changed the design quite a bit and the versions starting from D851 and later fall into the late style of DIWA gearboxes. In these versions the differential and the torque converter were changed a lot and a retarder function was added.
The differential was now a regular planetary differential incorporating a single sun gear, single set of planet gears and a ring gear. No more twin sun gears, twin planet gears and no ring gear arrangement like in previous DIWA series. In the torque converter the direction of the turbine blades were reversed to make the turbine spin in opposite direction to the impeller and the reason for this will be explained later.
The overrunning clutch that connected the turbine to the intermediate drive shaft was also now removed which now allowed the torque converter to be used as a retarder to slow down the bus. The cutaway schematic below provides a good view into each component of the D851.
Taking a look at the front planetary differential it can be seen that the power from the engine comes into the gearbox through the input shaft „1 ” and then goes to the „clutch pack carrier” shown in „4”. This clutch pack carrier connects to the ring gear of the differential shown as „6” via the input multi plate clutch pack shown as „5” and is also connected to the planet carrier of the differential shown as „8” via a multi plate clutch pack which is shown as „9”.
In the late models, the sun gear „7” of the differential is connected to the impeller „P” of the torque converter and the planet carrier „8” of the torque converter is connected directly to the output shaft „16”. Although the differential is there to still accomplish the same goal, in this generation the layout of the differential is completely different to the earlier versions.

First gear is engaged a bit differently in this generation, because the planet carrier is connected directly to the output shaft means that it initially acts as a braked member of the planetary differential when the bus is stationary which causes the output shaft and the planet carrier to also be stationary and act as a braked member of the differential so when the driver pushes the accelerator pedal the powerflow in the differential will be from the ring gear to the sun gear and therefore to the impeller.
The carrier will initially still be stationary because of the weight and inertia of the stationary bus. But since this is a true planetary gearset unlike the earlier versions which lacked the ring gear, now braking the planet carrier also means reversing the rotation of the output. In planetary gearsets holding the carrier as a stationary element causes the output from the planetary gearset to be in the opposite direction.
This means that the sun gear and the impeller actually rotate in opposite direction to the engine and the input shaft of the gearbox, this is why in this generation the direction of the turbine blades were also reversed so that impeller spinning in reverse direction will rotate the turbine in forward direction because the blades of the turbine are also reversed which will reverse the motion of the reverse rotating impeller making sure the turbine rotates in the same direction as the engine which can be seen in this video at the timestamp, the whole video is also a great visual representation of the first gear in the later generation of DIWA gearboxes .
The turbine in this generation is not connected directly to the output shaft, it connects to its own planetary gearset shown as „13”. The turbine connects to the sun gear of this planetary gearset whose output is through the carrier which is connected to the output shaft „16”. The ring gear of this gearset is connected to a brake clutch pack „12”, whenever this brake is engaged it forces the ring gear of this planetary gearset to be held stationary.
This is important because if a planetary gearset has only 1 input and has no braked member then it will simply be in neutral and freewheel, there will be no power transmission to the output so this means that when the brake „12” is disengaged, this planetary gearset will be in neutral and it will freewheel and therefore the turbine will be disconnected from the output shaft.
This is why in this generation there is no need for an overrunning clutch for the turbine because this planetary gearset can also accomplish that role while also making the gearbox be able to incorporate a retarder function into the converter. So in first gear that brake clutch is engaged and holds the ring gear as stationary which means the turbine will be connected to the output shaft with some gear reduction in that planetary gearset.
In this generation as well if the driver smahes the accelerator pedal the engine will fastly rise to its peak torque rpm and stabilize there a bit and then keep slowly rising in rpms. When this happens, the turbine will drive the output shaft and because the output shaft is connected directly to the carrier of the planetary differential up front so now the planetary differential will have 2 different outputs.
As the bus speeds up, the carrier will also pick up speed and like in previous generation, this way constantly decrease the gear ratio inside the differential. The stator inside the converter is still completely fixed so its still a single phase converter like in previous generation so after converter goes into coupling mode it will create resistance and cause most of the power to flow through the mechanical path.
The exact powerflow for the D851 in first gear can be seen in the drawing below. Additionally the gear ratio in 1st gear will peak at around 7.3:1 ratio which includes the torque multiplication from the torque converter and it will start keep decreasing from 7.3:1 as the speed of the bus increases.

Reverse gear in DIWA gearboxes is exactly same as first gear except for the small difference that the last planetary gearset in this gearbox is the reverser planetary gearstage shown as „14”. This gearset also has a brake clutch pack shown as „15” but this brake clutch pack connects to the carrier of this gearset and recall that when holding the carrier as a stationary member in a planetary gearset the output will be reversed. When engaging the brake clutch pack to hold carrier as stationary the output will be reversed and the gearbox will be in reverse gear with a similar CVT like stepless operation with split power flow like in 1st gear.

Second gear is engaged at a similar time to the previous generation of gearboxes, shortly after the converter couples the brake clutch pack „12” will be disengaged which will shift the planetary gearset of the turbine into neutral so that turbine will receive no motion from the output shaft.
At the same time the brake clutch pack shown as „11” will be engaged and this brake clutch pack is connected to the impeller of the converter so when it is engaged it will brake the impeller and make it a stationary member and together with the turbine not receiving any motion the torque converter will once again will completely stop and the power will flow purely through the mechanical range.
When the impeller is braked this means that the sun gear of the planetary differential „7” connected to it will also be braked so the planetary differential will just become a regular planetary gearset with its sun gear braked so that it will provide some gear reduction while driving the carrier and overall in second gear the total gear reduction will be around 1.36:1 or 1.43:1 depending on the exact D851 model. Below the powerflow in second gear can be seen.

Third gear is engaged when the bus reaches roughly around 2/3 of its top speed and to engage third gear the input clutch „5” which is connected to the ring gear of the differential is released and the other multi plate clutch pack shown as „9” which is connected to the planet carrier of the differential is engaged. Doing this basically directly couples the input shaft to the planet carrier meaning there is no gear reduction and the gear ratio in third gear will be a direct 1:1.
The sun gear will be still blocked by the brake clutch pack 11 but because now the carrier which is always connected to the output is now also directly connected to the input shaft, the power will just flow straight without any reduction and the planet gears and will just walk around the teeth of the sun gear while the ring gear will freewheel. Below the powerflow in third gear can be seen.

In 1976 a 4th gear in the form of an overdrive gear ratio was added to make it a bit more efficient especially for the suburban route buses where they see higher speeds than pure city route buses. Below is a cutaway drawing for the 4 speed D854 and it can be seen that there is an extra planetary gearset added right behind the planetary gearset for the turbine. This planetary gearset has its input side from the carrier and the ring gear is again connected to a brake clutch pack which will make the ring gear stationary and with the input from carrier and output from the sun gear it will cause an overdrive gearing.

The way the 4th overdrive gear is achieved was changed a bit in more modern versions (2000s and later versions) of DIWA gearbox. The extra planetary gearset was moved from the section where it was after the converter to the front section of the gearbox right after the differential as it can be seen in the cutaway drawing and powerflow schematics below from D864.5.
Additionally this video is also a great visual demonstration of 2-4 gears in the 2000s DIWA gearboxes. It should also be noted that in the 2000s and onwards versions, when idling, the input clutch shown as „5” below is disconnected so that there will be no converter creep and because of the engine not having to idle with the impeller it will have less drag on it and will idle using less fuel so this function was incorporated in modern versions to save fuel and cut down on emissions.


Retarder function when braking can only happen in mechanical stages so it can be used in every gear except first and reverse because it uses the torque converter itself as the retarder mechanism, there is no seperate retarder added at least in the versions that were talked about in this article.
The retarder function is very simple, the reverser planetary gearset is engaged via its brake clutch pack which means now the turbine is also coupled to it and since reverser planetary reverses the rotation, the turbine will now be driven in the opposite direction by the reverser planetary gearset while the pump is still held stationary by its brake clutch pack and since the stator is always fixed anyway, this will cause the turbine to act as an axial pump inside the converter and this will make the turbine experience massive losses due to the impeller and stator being stationary and the turbine trying to pump the fluid in the converter against those 2 stationary elements.
This will turn the kinetic energy of the output shaft into heat via fluid pumping losses inside the torque converter and this way slow down the bus without using the main brakes. Below is a schematic that shows the turbine during retarder function.
This extra heat created in the converter will be then sent to the water/oil heat exchanger shown in „9” in the cutaway above where the cool coolant coming from engine’s radiator will cool the heated oil of the gearbox.
The coolant jacket method from earlier versions were dispensed with in the later generations because it wouldnt be able to keep up with the immense amount of heat generated with the retarder function and a water/oil heat exchanger is more efficient than a seperate oil radiator as well so this was the choice ever since the D851 series came out.
The drawings below show the powerflow in 2nd and 3rd gear while the retarder function is being used. The retarder function is also shown visually in this video. At timestamp in this video the iconic sound of the retarder can also be heard.

The infamous Voith gear whine
If you have ever ridden inside a bus equipped with a DIWA gearbox you will instantly know that very loud gear whine sound like in this video. This is caused by the low helix angle of the gears used inside the transmission.
The early versions such as the DIWABUS 200D version below even used spur gears on the differential with 0 degrees helix angle and the spur gears really whine a lot, the rest of the gears were helical but had low helix angles of around 15 degrees like the Lamborghini Miura which also has a 15 degree helix angle and due to that also has that infamous gear whine.

This is kind of understandable in the old days for example the Muncie M22 from the 1960s earned the nickname „Rock Crusher” because of its 20 degree helix angle creating a lot of gear whine but even in 2000s models such as the DIWA 864.3 series from 2000s MAN buses you can still hear a lot of gear whine. Even in those modern versions the helix angle is still around 15 degrees.
There should be around 30 to 45 degrees of helix angle for very quiet operation without any whine but the bigger the helix angle the more axial force the gears are going to generate which means the gearbox will require bearings that are more capable in bearing axial loads and needs a stronger gearbox case because all of that axial force is going to end up being absorbed by the case as well, so a lower helix angle helps in making the gearbox cheaper and lighter at the cost of gear whine.
It is not clear why this was done even in 2000s but thats just how it is and i think they sound great with that gear whine and is a great characteristic of these gearboxes. Below is a cutaway picture of one of the DIWA.6 generation gearboxes and a 15 degree helix angle can be observed in all gears.
Additionally, these gearboxes before the introduction of the idling mode where the input clutches are disengaged, whined even at idle because of the differential being ahead of the torque converter and still flowing power through those low helix angle gears meant that even sitting still at idle there was still a good amount of gear whine which can be heard in this video at the timestamp.

The future of DIWA gearboxes
Even currently 7 speed versions of the DIWA gearbox exist with 2 retarders however there is not much material about them on the internet so i did not have much to go through to add them into this article. Voith is currently also developing a hybrid capable version of the DIWA gearbox so the next generation stuff has a lot of gears and hybrid integration to save on fuel and cut down on emissions as much as possible.
Enormous advanced article. Of course, that’s good idea if reader have knowledge about automotive technology (knowledge about torque converter, epicyclic gearing, etc.), for better understanding the article.