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Inside the fuel-efficient 9-speed 9G-TRONIC from Mercedes-Benz

6 March 2014

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Design of the Mercedes-Benz automatic transmission 9G-TRONIC. Click to enlarge.

Mercedes-Benz’ new 9-speed 9G-TRONIC, the first rear-wheel drive nine-speed automatic transmission with torque converter, is making its debut in the E 350 BlueTEC as the standard transmission paired with the 185 kW (252 hp) V6 diesel. Announced in 2013 (earlier post), the 9G-TRONIC represents the seventh automatic transmission generation from Mercedes-Benz.

The E 350 BlueTEC offers an average NEDC fuel consumption (sedan model) of 5.3 liters of diesel per 100 km (44.4 mpg US). While an overall reduction in engine speed improves NVH comfort and also cuts down external noise by up to 4 dB(A), the reduced fuel consumption of the E 350 BlueTEC with 9G-TRONIC has primarily been achieved as a result of the high efficiency level of the transmission, Mercedes says.

As part of this, the broad ratio spread of 9.15 for gears one to nine allows a clearly perceptible reduction in engine speed and is a decisive factor behind the high level of energy efficiency and ride comfort. Shortened shift and reaction times ensure optimum spontaneity combined with ease of shifting. In manual mode and S mode in particular, the 9G-TRONIC responds significantly more spontaneously and enhances driving pleasure.

The ease of shifting of the new 9G-TRONIC—a focal point during development—comes from a comprehensive package of measures. These include the novel direct control system (more below) which enables short, barely perceptible gear changes. The combination of twin-turbine torsional damper and centrifugal pendulum technology in the torque converter ensures outstanding drive comfort. Together with the extended gear ratio spread, higher speeds can now be driven at lower engine speeds for even greater comfort. In practice this translates into being able to drive at 120 km/h (75 mph) in 9th gear with an engine speed of around only 1350 rpm, for example.

The 9G-TRONIC can support a range of drive configurations including: rear-wheel drive; 4MATIC all-wheel drive; hybrid drive; and plug-in hybrid drive. ECO start/stop is standard. Suitable engines for the 9-speed are 4, 6, 8, 12/in-line and V engines.

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Lightweight design and fuel economy measures. Click to enlarge.

Lightweight. The development engineers also focused on the area of “compact lightweight construction”. Despite two additional gears and a maximum transferable torque of up to 1000 N·m (738 lb-ft), the new automatic transmission requires as little installation space as its 7-speed predecessor and is also lighter in weight. The two-piece housing design has been retained: the torque converter housing is made of lightweight aluminium, while the transmission housing with weight-optimized plastic oil pan is made of an even lighter magnesium alloy.

New gearing concept. Another goal was to implement the nine gears with a minimal number of planetary gear sets and shift elements. Computer-based system analysis and mock-up made it possible to realize this goal with only four simple planetary gear sets and six shift elements. Mercedes-Benz has secured a worldwide patent for this specific configuration, which the engineers consider to be the best possible.

A planetary gear set consists of the outer ring gear, the inner center gear and between them the planetary carrier with the four planetary gears and their bearings. Four planetary gears are required in the 9G-TRONIC so that the expected torque of up to 1000N·m can be reliably transferred in future engine/transmission combinations.

Planet
Structure of the planetary gear set. Click to enlarge.

The ring gear, planetary carrier and center gear in a planetary gear set are connected by carriers and multi-disc clutches, or braked by the multi-disc brakes which are supported by the transmission housing. This enables the planetary gears to transfer drive torque to the inner teeth of the outer ring gear or to the outer teeth of the inner center gear. The result is several gear ratios, and at the same time it is possible to reverse the direction of rotation for e.g. reverse gear.

The gear ratio is the ratio between the number of gear teeth on the driving and transferring gears. Depending which planetary gear sets are connected in series, blocked or separated, multiplying the part ratios produces the overall ratio for the relevant transmission gear.

In the 9G-TRONIC, the individual gears are engaged by three multi-disc clutches and three multi-disc brakes. The purpose of the multi-disc clutches is to transfer the drive torque between two components as a friction connection. The ratio configuration of gears one to nine allows the wide ratio spread of 9.15. For the same performance compared to preceding transmissions, the rpm level is considerably lowered as a decisive factor for the high energy efficiency and NVH comfort of models equipped with the 9G-TRONIC.

Three speed sensors monitor operation and provide the transmission control system with corresponding data for effective shifting. Here it is possible for several gears to be jumped when accelerating or decelerating, should the driving conditions call for it.

Twin turbine torsion damper and centrifugal pendulum. One of the most comfort-enhancing and at the same time fuel-saving features of the 9-speed is the torsion damper, which compensates even more effectively for eccentricities and vibrations within the transmission.

A basic physical law operates in this case: the lower the rpm and road speed, and the lower the number of cylinders, the more pronounced these can be. This results in a conflict of aims between comfort and fuel-efficient operation. It is resolved by the use of a twin turbine torsion damper additionally fitted with a centrifugal pendulum. Depending on the rpm, this shifts the center of gravity and also allows comfortable vehicle operation in the most fuel-efficient operating range.

Moreover, the optimized damping enables slip in the torque converter lockup clutch to be reduced considerably, which likewise contributes to fuel economy. For the first time, a return spring has been integrated into the torque converter lockup clutch of the 9G-TRONIC. The multi-disc lockup clutch was previously only hydraulically controlled. Use of the return pressure spring allows reliable and comfortable activation even at very low rpm.

Torque converter. The drive element of a classic automatic transmission is the hydrodynamic torque converter. In the new 9G-TRONIC, the engineers have improved the hydraulic circuit in the torque converter and increased its efficiency to up to 92%. This extraordinarily good figure is important for fuel economy, as the losses imposed by physics when transferring the engine torque to the transmission’s input shaft are kept to a minimum. In the first-generation 7G-TRONIC dating from 2003, the efficiency of this component was only 85%.

The heat generated during operation is reliably dissipated via the transmission oil cooler. Conversely, the 9G-TRONIC requires no additional radiator to warm the cold transmission oil when cold-starting under Arctic conditions. The second-generation, synthetic Fuel Economy low-friction oil also performs reliably at extremely low temperatures.

Oil supply concept accounts for 54% of fuel savings potential. To ensure the reliable and at the same time highly efficient supply of the durable and shear-resistant 2nd-generation synthetic fuel economy engine oil, the 9G-TRONIC is fitted with two pumps. The considerably size-reduced, mechanical main pump installed “off-axis” lies next to the main shaft and is driven via a chain.

In an automatic transmission such as 7G-TRONIC, the main oil pump previously ringed the transmission shaft and was directly driven. This meant that the diameter of the transmission shaft prevented the pump from being reduced in size as desired. For this reason the highly efficient vane cell pump is now placed alongside the main shaft (“off-axis”), and is reduced in size to suit requirements.

The mechanical main pump, which ensures the oil supply to the electrohydraulically controlled automatic transmission when the internal combustion engine is running, is backed up by a separate electric auxiliary pump.

On the one hand this design enables the flow of lubrication and coolant to be controlled actively on demand, and at the same time also means that the 9G-TRONIC can benefit from a start/stop system. In subsequent hybrid applications, this additional oil delivery also allows so-called “sailing”, i.e. maintaining a constant speed without using the internal combustion engine.

When the engine is off—for example at a red traffic light in start/stop mode—the electric auxiliary pump is actuated, ensuring a defined basic pressure and guaranteeing that all necessary functions are maintained. When the driver wishes to move off on a green light, oil delivery by the electric pump after engine-starting guarantees immediate and agile acceleration. In certain operating states with the engine running, the auxiliary pump also assists the main pump, for example at very low engine speeds or in very high temperatures. In this case the flow of oil is added as needed to ensure smooth gear shifts or when there is a higher cooling requirement.

This innovative oil supply concept using a mechanical main pump and electric auxiliary pump, as well as demand-related control, accounts for around 54% of the total fuel-saving potential of the 9G-TRONIC. The less oil that has to be moved within the transmission by more efficient pumps, the higher the overall efficiency. The fully synthetic Fuel Economy low-friction oil also contributes to this.

Main transmission shaft with three deep-drilled holes. The main transmission shaft is another technical highlight of the 9G-TRONIC. First, at 550.9 millimeters, it is one of the longest shafts in the entire automotive industry. Second, it performs other functions in addition to its main purpose of power transmission: using a sophisticated internal ducting system, the shaft also performs various lubricating, cooling and control functions.

On the engine side, a large axial hollow-drilled hole measuring a few centimeters supplies the front planetary gear set with oil, which reaches the right places via smaller transverse holes. The drilled holes on the output side of the main shaft are far more interesting, however. Three parallel holes each measuring 6.1 millimeters are deep-drilled into the transmission shaft with a core diameter of 16 millimeters to a depth of up to 361.5 millimeters. These three deep-drilled holes have various functions in the 9G-TRONIC: via transverse holes, they ensure a defined oil flow rate to lubricate and cool the planetary gear sets and shift elements. They also perform an important control function, and transfer the set gearshift pressure to the multi-disc clutches and brakes.

The machining of such a shaft is a masterpiece of production engineering. The requirements are particularly exacting when drilling the three deep holes. To date no other automotive manufacturer or machine tool producer has undertaken such a task, with such a ratio between shaft diameter and hole depth.

Over their entire length of up to 361.5 millimeters, the deep-drilled holes must precisely meet the requirements to just a few thousandths of a millimeter. Machining must follow precisely defined geometrical specifications:

  • Distance and parallelity of the holes versus each other;

  • Distance and parallelity of the holes versus the outer surface of the shaft avoidance of twisting during the drilling process;

  • Radial positioning of the three drilled holes to ensure a free flow to the transverse holes;

  • Correct depth of the individual holes; and

  • Residue-free drilling with no microfine swarf remaining in the holes.

The machining operation is carried out with the minimum quantity of cooling lubricants. In addition, complex guidance of the 370-millimeter long drill bits was to be avoided and the processing time was to be considerably reduced.

To achieve this, Mercedes-Benz dispensed with conventional cooling lubricants are completely dispensed with when producing the 9G-TRONIC drive shaft. During the drilling process, a fine oil/air mist is sufficient for lubrication; this reaches the drilling face through a duct in the single-fluted drills. The generated heat and swarf are conducted away with no residue via a bead in the side of these single-fluted drills.

The savings made possible by this so-called mist-cooling technology are enormous. The requirement is reduced by 99.9% compared with conventional production processes. Whereas around 18,000 liters of cooling lubricant per hour were previously needed, the mist-cooling technology requires only 0.3 liters. In other terms: rather than the capacity of an entire road tanker, the content of a household drinking glass is sufficient to ensure the high quality of the machining process.

The processing time per shaft has also been shortened, while improving production quality. To minimize cycle times in the production process, the production engineers eliminated the usual guide sleeves when spot-drilling. The machines drill the deep holes “free-hand”, so to speak, while maintaining absolute dimensional accuracy. In addition, the single-fluted drills of cemented carbide allow high working speeds.

Whereas the figure was around 125 millimeters per minute in the case of 7G-TRONIC PLUS, the new drills allow a speed of over 250 millimeters per minute. The net result is that it takes just under three minutes to process the main transmission shaft—around 63% less than the cycle time during the production of the 7G-TRONIC PLUS.

Fully integrated transmission control. All the components for gearshifting, lubricating and control processes are fully integrated into the transmission housing, improving the control quality and reliability of the 9G-TRONIC. The advantage of this new direct control is more efficient use of the hydraulic power.

In this direct control system, the hydraulic gearshifting element is directly linked to the electromagnetic valve. The hydraulic control slides now only have one third of their original diameter. This means that control of the six shift elements (three multi-disc clutches and three multi-disc brakes) can be much faster and more efficient.

The actuating unit for the electric transmission oil pump, the control unit, all the electro-magnets, as well as the complete sensor system comprising rpm, temperature, pressure and position sensors, are combined together on a single mounting bracket. The control unit becomes the command center for the 9G-TRONIC, and is incorporated into the electronics architecture of the vehicle.

Apart from data obtained from the transmission itself, the integrated 9G-TRONIC control system uses information from the drive control (e.g. engine speed, accelerator position), the dynamic control systems (steering angle, linear and lateral dynamics) and the safety systems (interventions by ABS, BAS Plus, Collision Prevention Assist, DISTRONIC), and is able to control all shift processes optimally using these data.

There are also advantages in terms of electromagnetic compatibility (EMC), as mutual influencing by various electronic components is avoided. Extensive tests in the EMC laboratory showed this to be the case.

The software necessary for all control processes was developed in-house, at the Mercedes-Benz Technology Center (MTC). Only the control units, i.e. the hardware, come from a supplier.

In addition to the temperature, pressure and position sensors, three rpm sensors continuously monitor the operating state of the 9G-TRONIC and provide the transmission control system with the following data for effective gear shifts: internal transmission rpm (rpm of the main transmission shaft); rpm of the turbine (output rpm of the torque converter); and output speed.

The extensive, networked sensor system with its continuous comparison of all rpm values makes it possible for several gears to be skipped when accelerating or decelerating, should the driving situation call for it.

Diagram
The following is an example using the power flow in 1st gear. Planetary gear sets 1 / 2 / 3, the multi-disc brake A / B and the multi-disc clutch E are involved:
  • The inner center gear of the 1st gear set is part of the drive shaft, and therefore permanently connected to it.

  • The planetary carrier of the 1st gear set is connected to the outer ring gear of the 2nd gear set via the 2nd multi-disc clutch.

  • The multi-disc brake A brakes the inner center gear of the 2nd gear set. This increases the torque while reducing the rpm.

  • The outer ring gear of the 2nd gear set has a mechanical connection to the inner center gear of the 3rd gear set.

  • The planetary gears of the 3rd gear set rotate in the outer ring gear braked by multi-disc brake B, and transfer the increased torque at reduced rpm to the drive shaft.

  • The output shaft rotates with reduced input rpm and a significant torque increase in the engine’s direction of rotation.

Power transfer follows the following principles:

  • The output rpm is reduced in the lower gears. This leads to lower speeds at the drive wheels while increasing tractive power and drive torque.

  • Conversely, the output rpm is much higher in the higher gears. This leads to higher speeds at the drive wheels, accompanied by lower drive torque.

Three modes. The three transmission modes of the 9G-TRONIC allow an individual control strategy depending on the traffic situation or the driver’s personal preferences.

In ECO mode the control is equivalent to a very economical driving style: upshifts are performed sooner, and the handling is gentler overall to support an economical driving style at lower engine speeds. In SPORT or MANUAL modes the response and shift times are shortened, and there is higher revving in the gears to support a dynamic and sporty driving style.

Like the 7G-TRONIC PLUS before it, the 9G-TRONIC also has the automatic mode “short-term M”, which makes operation even easier and more comfortable. The driver can now also engage the required gear using the shift paddles in ECO and SPORT mode, without first activating MANUAL mode. “Short-term M” remains active if there are repeated manual gear shifts or a sporty driving style is maintained with higher linear and lateral acceleration levels. In contrast to permanently activated MANUAL mode, however, “Short-term M” is deactivated after a certain period without higher power requirements, and the transmission reverts to the original mode.

Operation of the 9G-TRONIC is unchanged compared with the preceding 7G-TRONIC: R, N and D are selected using the DIRECT SELECT lever on the right of the steering column, while the three transmission modes ECO, SPORT and MANUAL are activated with a switch on the center console. The well-proven positioning and operation of the steering wheel shift paddles are also unchanged. The central display in the center dial instrument reliably informs the driver which gear is engaged, and which transmission mode is active (e.g. “D9 E” = ninth gear, ECO mode).

March 6, 2014 in Fuel Efficiency, Transmissions | Permalink | Comments (32) | TrackBack (0)

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Comments

Wow!, Mechanical engineers running amuck, designing an expensive, complicated device that no one can fix except the factory. Come now do we really need all that gearing? How about using a simple high torque CVT until the drive train goes full electric and we can concentrate on practical engineering instead of mechanical experimentation.

It may look complicated but I am sure that I would rather have this than a constantly slipping, constantly wearing CVT which I consider to be an out-dated idea. The only question that I would have is whether the lock-up torque converter is better than an dual clutch transmission. Maybe the lock-up torque converter is smoother and the dual clutch has slightly better performance. The dual clutch transmission seems to less complicated and has only 2 friction clutches.

This kind of complexity bodes well for the future of plug-in vehicles. I just wish the rate of improvement and cost reduction would hurry up.

Son has a BMW Mini with CVT...works well at over 160,00 miles without problems. May be what you think is slipping is normal operation. Also, we aren't talking race cars here...street cars. You really don't need all that complexity for what should be a simple transportation appliance.

Alan:
I'm with you on batteries in cars...bought a Leaf in 2011; been waiting for three years for Nissan to improve the battery range...still waiting.

Complex 300+ parts very expensive 10-gear auto-transmissions are the latest way to get a few more mpg from old tech ICEVs. Will we see 12-gear units with 1000cc boosted ICE soon?

One or two gear extended range e-vehicles will replace those beloved mechanical master pieces soon?

The reason for 9-speed transmission is due to the ambitious design objective of MB's vehicle: maximum acceleration with maximum efficiency. Formerly, cars had 3-4 speed transmission for about 3-4:1 ratio, in which economy cars lugged their engines to squeeze the most out of a gallon of gas, while sport cars reved their engines to get the most acceleration and top speeds but consumes a lot more fuel per mile. The Chevrolet Corvette addressed this issue partially by having a six-speed transmission, enabling 26-mpg hwy cruise by running the engine barely 1,400 rpm at 60mph, an almost unheard of numbers in sport cars of the days.

MB has to do one-upmanship in order to stay in the competitive business of performance luxury car, by offering a phenomenon 9.17:1 ratio transmission, and barely rev the engine at 1,300 rpm at hwy cruise. At that engine speed, the engine is lugged nearly to the max, meaning that it has almost no room for acceleration without downshifting while cruising.
Thus, a downshift is needed with nearly every pressing down of the gas pedal, which may result in accelerated wear of the wet clutch UNLESS very close gear ratio is available that can minize the speed differential between one gear vs the next to avoid rapid wear of the wet clutches and to have very smooth, almost imperceptible shift, something that would require of a high-price luxury performance vehicle. So, in a high gear-ratio transmission, a large number of gear change is a necessity.

@Harvey,
Once 5-5-5 batteries will be available, I would predicted that 3-4-gear BEV will be offered, simply because in a relentless pursuit of perfection, if 2 gears is better than 1 gear, then 3-4 gears will be even better in a luxury high-end vehicle, than 1 or 2 geared vehicle. One-upmanship is the name of the game, and one do not do things always because of necessity, but because one can and one wants to beat the other guy!

Tesla's Model S uses a 1-speed fixed gear (9.73:1) transmission. It probably cost less than 1000 USD whereas this 9 speed transmission probably cost between 4000 and 8000 USD. Model S does better in acceleration and efficiency that any of the cars that will get this 9 speed transmission.

Tesla is also able to fit a 225kW engine, transmission and cooling system in the floor between the rear wheels. That is impossible with a combustion car that will need nearly the entire space under the front hood to fit its engine, cooling system and transmission. I think Tesla will never use anything but a 1 speed transmission because it is cheaper, more reliable and more compact. If Tesla needs more acceleration they can just add another engine system and do a 4 wheel drive. I suspect the efficiency and thereby range gain that can be made from using a multi-speed transmission in a Tesla is also partially lost to the weight gain of such a system. It would be cheaper to add more kwhs to the battery to get the range rather than add a multi-speed transmission.

Lad: I also have a LEAF and really looking forward to the next generation with a bigger battery. I think it will be released next year or possibly 2016.

Good idea, Henrik.
I looked at the weight breakdown for the Model S and found that the differential unit itself weighs 175 lbs, while the motor AND the inverter only weigh 350 lbs for 416 hp output! So, the differential can be replaced by having two smaller motors, 210 hp each, for each rear wheel in order to do away with the differential. With 2 motors, you can sorta simulate a 2-speed transmission, in that you can turn off one motor at cruise in order to increase the efficiency of the remaining motor, while using both motors at fast acceleration or at real high speeds. Just a simple lockup clutch is needed to connect the two rear wheels together to make one unit instead of relying on a differential unit that is heavy and contributes to more friction loss. In a tight enough turn (at low speeds) the lock-up clutch can be declutched in order to function as a differential unit by allowing the rear wheels to turn independently.

Or, alternatively, instead of a lock-up clutch, the two rear wheels can be connected by a limited-slip link or a wet clutch that allows a little slip (in case the two rear wheels are slightly different in diameter) but prohibit major slippage, so that one motor can power both rear wheels. Persistent slippage of the rear wheels when not turning is indication of something wrong with the rear wheels, and the driver is notified, while the wet clutch will be declutched to allow independent movement of the two rear wheels by two motors.

Remember that NASCAR's do not have differential units and they are running just fine, at very high speeds.

Lad

I was very surprised to see that BMW had offered a CVT with the Mini but they did for a few years for the first generation Mini. The current models have either a 6-speed manual or a 6-speed automatic. And, yes, slipping is the normal operation for a CVT. All traction type devices slip and in the realm in which they are designed to operate the slip is proportional to the torque transmitted in the same way that your tires are always slipping if they are transmitting any force to the road and the amount of slip depends on the amount of force.

Roger Pham

Nascar gets away without running a differential because the cars are always turning in one direction and never turning sharply during normal racing. Also, they tune the turning by running what they call stagger where the outside tires are a little larger than the inside tires. As the outside tires have more cornering load than the inside tires, the outside tires are also slipping more.

Roger,

You would replace the differential with a lock-up clutch differential. What makes you think it would weigh any less?

Nascar's premier series uses differentials. You are thinking of dirt track cars.

Roger I did not suggest replacing the rear differential with two smaller electric motors. I said adding another motor for the front wheels in order to get a 4 wheel drive with ultimate handling and power. As I understand the Model S transmission it has a fixed gear build right into the differential. I believe the 175 lbs you mention is the weight of the entire transmission both differential and fixed gear. The link below is showing a picture of model S engine system. The differential and fixed gear is in the middle with the motor and motor controller on each side. Extremely compact.

Replacing one large electric motor and a differential with two smaller motors will most likely be more expensive and produce a less reliable system. That is just what I think and it could explain why Tesla has not done it. After all Tesla's engineers are the kings of technical EV design. The 416 hp electric motor or 305kW motor you mention is of cause the performance edition of the 225kw motor I mention. It is identical but get more current from the larger 85kwh battery and its coils are hand-welded more precisely than the machine welded 225kw engine. I am pretty sure that Tesla will do away with the handmade 416hp edition as it is too expensive to produce. Instead it will be replaced by two 225kw engines one for the rear and one at front giving the 4 wheel Tesla a combined 450kw or 611hp! That will be the future performance edition and I expect it can easily do 0 to 60 mph in less than 4 sec.

I do not think you are right about increasing efficiently by turning off one electric motor in a dual motor system without a differential. They are in a fixed gear and efficiency is a function of rounds per minute which does not change whether one or two motors pull. It would also produce terribly handling only being pulled by one wheel and asymmetric and/or accelerated wear down. Moreover, electric engines are more efficient the more hp so two smaller electric motors with the same combined power is less efficient.


https://www.google.com/search?q=tesla+differential+with+gear&rls=com.microsoft:en-US:IE-Address&source=lnms&tbm=isch&sa=X&ei=ySYbU9j6K8mAywPlzYIo&ved=0CAoQ_AUoAg&biw=1358&bih=856#facrc=_&imgdii=_&imgrc=2DhIrcMh4K6yGM%253A%3By3jhKqU0YjzsWM%3Bhttp%253A%252F%252Fmedia.caranddriver.com%252Fimages%252Fmedia%252F51%252Funder-the-teslas-skin-2013-tesla-model-s-january-2013-issue-photo-493013-s-original.jpg%3Bhttp%253A%252F%252Fwww.caranddriver.com%252Ftesla%252Fmodel-s%3B429%3B475

A link showing the Tesla motor, gear and controller a bit better. The video makes me doubt wether I am right above saying the 416hp Tesla motor has hand-welded coils. I read it somewhere but it could be it was for the Tesla roadster normal versus performance edition. In the video the motor seems to be machine made although the coils of the rotor is not shown and they could still be hand-welded. If it is not also precision hand-welded coils that take the engine from 225kw to 306kw it could be just better cooling and more current.

http://www.youtube.com/watch?v=NaV7V07tEMQ

Good point, Henrik, regarding one motor in front and one in the back to produce 4WD effect that is very valuable in slippery roads and will increase the sale value of the car.

In induction motor, the efficiency deteriorates when motor load decreases below 20%. The Model S has a motor over 300 kW, but in cruise at 60-65 mph, only ~15 kW is required. This is 1/20th of the maximum power, but since the motor is only turning ~1/2 the rpm of maximum speed of 130 mph, then the load is about 10% of maximum load. Clearly, the load is too low for maximum motor efficiency. However, if two small motors are installed and if only one motor is energized, then the load on that motor is ~20% of maximum and can barely reach the threshold of maximum motor efficiency.

If two motors are put in the rear axle, then a simple lockup clutch can be used to lock the rear wheel drive shaft together during straight forward cruise. During a turn to the Right, then the lockup clutch is released and the left motor is energized more than the Right motor, thus assisting the turn, and vice-versa for a Left turn.

@Bernard,
Lockup clutch, as is in torque converter lockup clutch, can be small and light, especially when the rpm's are synchronized, and pegs in holes are used to transfer torque instead of multi-disc wet clutch or dry clutch that are heavier. Conventional differential with housing and pinion gears are heavier.

Affordable light weight AWD e-drive trains will become common place in another 10 years or so.

Most will have ECO Drive as standard option.

@Roger you might be right about better electric motor efficiency at higher loads than 15 kw but I think the gain in efficiency is minimal say from 88% to 91% going from 5 kw to 20kw and then on to 93% at 40kw after which it could slowly falls. I tried finding this graph on the web but it is not there. Think it is proprietary information that Tesla will not publish. Still there might be some meaningful range to gain by switching off one motor say the rear motor and then propel the car with just the front motor whenever less than 40kw is required and the car also does not need the four wheel drive for handling. To be sure, if any spinning of the pulling front wheels are detected the 4 wheel drive should activate immediately regardless of the level of the power consumption.

It will be extremely interesting to see how Tesla will implement that 4 wheel drive that Musk has told will be coming for the Model S but probably only after the 4 wheel drive Model X is launched sometime next year. Now my best guess is that the four wheel drive Model S/X will probably get a smaller motor for the front wheels that is optimized for high efficiency at loads below 20kw and that Tesla will leave the rear engine and its power options from 225kw to 310kw as it is. The rear motor will activate only when the power or the extra road grip is needed.

Roger,
If you had two motors, as per your speculative layout, you wouldn't need a lock-up clutch. You could just run the two motors at the same speed.

A centre "peg-in-hole" lockup clutch would only be useful when the car is going dead straight on perfectly flat pavement. Any slight deviation would result either in huge driveline stress, or in huge wear on your centre (non-)diff. It would be like driving an old-style part-time 4wd on dry pavement: you would get a huge repair bill after just a few miles.

@Henrik,
The following reference on page 1 will show that the peak efficiency at 90% is at above 30% load for 100-hp motor, while at 10% load, the efficiency is only ~75%, while at 20% load, the efficiency is 87%. So, between 75% and 87%, the difference is quite significant. A bigger motor will shift the curve to the Left slightly, such that the difference will be perhaps 78% and 90%, but still is a significant difference when an AC-induction motor is operated at 10% of load vs at 20% of load.

http://www.powerefficiency.com/pdf/NT_Motor%20Efficiency%20Controllers.pdf

@Bernard,

A lockup clutch is needed so that one motor can operate both wheels, thereby increasing (doubling) of the load and increase the motor efficiency. The stress on the lockup clutch would not be much because the single motor is operating only at low load. The lockup clutch will not lock if there is significant difference in rotational speeds between the wheels.
At higher acceleration, the lockup clutch will be released and both motors will provide acceleration, or likewise, during a turn, both motors will operate while the lockup clutch will be released.

Roger nice graph. It is the right type of graph I was searching for. But of cause this is a schematic of a typical AC induction motor. We need the real graph for the specific motor used by Model S. Preferable one for each of the three power levels the Model S motor comes with and one made for its peak performance (310kw) and one for its continuous performance (probably half of 310kw). There could be a huge difference to this schematics as Tesla may have developed an advanced power controller that can operate their AC induction engine always at its peak efficiency load by say 40kw at 93% efficiency. If only 4kw is needed for the propulsion of the Model S it simply pulse power the induction motor switching it on and off in rapid succession leaving it off 90% of the time so that you on average get exactly 4kW of propulsion while the motor is powered only by 40kw when it is powered.

If Tesla's motor controller can pulse operate like described then my idea in the previous post to power a 4 wheel drive Tesla Model S/X by a smaller front engine most of the time will not be preferred. It will be better to simple keep using the larger and therefore more efficient rear motor to do all of the propulsion except when better road grip is needed or maximum power is needed then the smaller front motor should also kick in.

Roger,

I trust that you understand that your lockup-clutch idea is heavier and more complex than a normal two-motor system, which itself is heavier and more complex than Tesla's efficient single motor system.

Any theoretical energy savings gained during the <1% of the time that a car is driving dead straight on a perfectly flat uncambered road would be minuscule compared to the cost of hauling added dead weight.

Side note: no public road should be perfectly flat and uncambered, because such a road would never drain. Perhaps a driveway made from an unusually smooth concrete pad could meet this criteria.

@Henrik,
Good idea to cycle the motor on and off at part loads below peak efficiency, this is also known as "pulse and glide" method of improving efficiency in ICEV, when the ICE has a specific high efficiency range not reached at cruise load. The challenge is to implement this in ways that will not result in noticeable power pulses that may disturb the passengers in the car.

@Bernard,
Lockup clutch is not heavy...just look at a diagram of a lockup clutch of a torque converter, which will show that the clutch is just a thin and flat disc actuated by a hydraulic piston into contact with the mating surface of the turbine housing. NASCAR's ran fine without differential, at least when I was aware of it many years ago.

I was under the impression that only the differential unit weighs 175 lbs, which is a lot of weight...However, if as Henrik stated, the 175 lbs include the 10:1 gear reduction also, then it isn't so bad, and not much weight can be saved by going without the differential unit.

But perhaps you both did not see the importance of having independently-driven wheels in traction control. The efficiency difference between a single 300 kW motor and two 150 kW motors is negligible, however the advantages are much more significant. To address your concern regarding slight differential rotation rate of the wheels during normal cruise, perhaps a wet and slippable clutch can be used to connect the two wheels during cruise using one motor. The clutch force is weak and can allow limited slip when necessary, thereby keeping the clutch small and light, but at cruise with low motor torque, the weak clutch force is adequate. When more power is necessary, both motors will be energized.

Even if no weight will be saved using the two-motor-driven rear axle with a small limited-slip wet clutch, higher efficiency will result from the operation of only one motor during cruise, plus the ability to energize the motors differentially during turns that will aid in avoiding loss of traction and facilitating the turn.

Even in 4-WD application, the use of 4 motors each powering a wheel, with limited-slip wet clutch for differential to allow one motor to power the vehicle during cruise, will be more advantageous than the use of only one motor per axle. The obvious advantage will be improved electronic traction control and enhancement of the turn, especially in slippery road conditions.

Correction to my previous posting:
The peak efficiency difference between a 300 kW motor vs. a 150 kW motor is negligible. However, the use of a smaller 150 kW motor during cruise will place the motor load nearer the peak efficiency point and will result in perhaps 15-20% gain in cruise efficiency.

This is my previous posting that somehow has not been posted:
"@Henrik,
Good idea to cycle the motor on and off at part loads below peak efficiency, this is also known as "pulse and glide" method of improving efficiency in ICEV, when the ICE has a specific high efficiency range not reached at cruise load. The challenge is to implement this in ways that will not result in noticeable power pulses that may disturb the passengers in the car.

@Bernard,
Lockup clutch is not heavy...just look at a diagram of a lockup clutch of a torque converter, which will show that the clutch is just a thin and flat disc actuated by a hydraulic piston into contact with the mating surface of the turbine housing. NASCAR's ran fine without differential, at least when I was aware of it many years ago.

I was under the impression that only the differential unit weighs 175 lbs, which is a lot of weight...However, if as Henrik stated, the 175 lbs include the 10:1 gear reduction also, then it isn't so bad, and not much weight can be saved by going without the differential unit.

But perhaps you both did not see the importance of having independently-driven wheels in traction control. The efficiency difference between a single 300 kW motor and two 150 kW motors is negligible, however the advantages are much more significant. To address your concern regarding slight differential rotation rate of the wheels during normal cruise, perhaps a wet and slippable clutch can be used to connect the two wheels during cruise using one motor. The clutch force is weak and can allow limited slip when necessary, thereby keeping the clutch small and light, but at cruise with low motor torque, the weak clutch force is adequate. When more power is necessary, both motors will be energized.

Even if no weight will be saved using the two-motor-driven rear axle with a small limited-slip wet clutch, higher efficiency will result from the operation of only one motor during cruise, plus the ability to energize the motors differentially during turns that will aid in avoiding loss of traction and facilitating the turn.

Even in 4-WD application, the use of 4 motors each powering a wheel, with limited-slip wet clutch for differential to allow one motor to power the vehicle during cruise, will be more advantageous than the use of only one motor per axle. The obvious advantage will be improved electronic traction control and enhancement of the turn, especially in slippery road conditions."

Uh Oh! My earlier posting did not post. Perhaps it will appear later! I've tried to post it twice without success. Will split it and repost in segments if won't shhow up later!

This is the first part of my previous posting that somehow has not posted:
"@Henrik,
Good idea to cycle the motor on and off at part loads below peak efficiency, this is also known as "pulse and glide" method of improving efficiency in ICEV, when the ICE has a specific high efficiency range not reached at cruise load. The challenge is to implement this in ways that will not result in noticeable power pulses that may disturb the passengers in the car.

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