[ANSWERED]Automatic Transmission Paddle Shift reaction time
12-18-2015, 04:59 PM
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[ANSWERED]Automatic Transmission Paddle Shift reaction time
Original question is here.
From jvp:
Merry Ho Ho and a Happy New Year to all. Thanks for making the first year of "Ask Tadge" a success! We're going to take the last couple of weeks of the year off, and get back on schedule Sunday, 3 January. At that point, I'll have a new poll up and we can vote for the first question of the year.
dmhines asked:
Why does the time between depressing the paddle in manual mode and the Automatic Transmission shifting seem so long. Why can't it be instantaneous?
Why does the time between depressing the paddle in manual mode and the Automatic Transmission shifting seem so long. Why can't it be instantaneous?
Tadge answered:
First of all, we on Team Corvette want to wish everyone in the Corvette community and on the Corvette Forum a wonderful holiday and a safe and happy new year!
Now on to the answer:
Although shift speeds have been a priority in racing for a long time, it is a relatively new focus in the mass production auto industry. For decades we had basically two transmission choices; a traditional manual transmission where the time to shift was 100% dependent on the driver’s skill level, and a torque converter automatic where the vehicle hardware and calibration determined the shift timing and speed. The main focus on the automatics was a smooth shift with little driveline or customer disturbance. Yes, there were niche products that featured different set ups, so pardon me for the generalization.
Some performance cars started to offer automated manual transmissions that used a traditional clutch but did the work of shifting robotically, but my experience with those early units was that they were slow and lacked the finesse of a well-shifted manual driven by a skilled driver. Over time the shift quality and speed was improved and manufacturers started talking about shift times as an important performance vehicle characteristic. For the most part auto makers have moved away from automated manuals and use sophisticated planetary automatics or DCT’s to get the best shift speed and quality. This question focuses on shift speed, so I will leave the pros and cons of the two automatic transmission types for another day.
So why can’t down shifts be instantaneous? Well, there is the transmission hardware side that faces the physical challenge of transferring the torque from one set of gears to another set of gears. That is the time most manufacturers quote when discussing shift times. Those times have been improved over the years to very small fractions of a second and the Corvette is competitive with the best on that score. To be honest, we have put a higher priority on upshifts because they occur while accelerating and will contribute faster lap times. Downshifts typically occur when off-throttle and on the brakes, so a slower shift is far less likely to affect lap times. Regardless, up or down, our 8-speed automatic transmission is capable of extremely fast shift speeds.
The paddle shifters on Corvette are controlled by on-board computers just like many other functions on the car. They are “intelligent” devices that seek to improve the safety and controllability of the car. When the driver requests a downshift, the car does not blindly obey (the same is true for throttle inputs). It checks a multitude of parameters to verify that a shift can be done safely, without risk of damage to the car and without upsetting the vehicle dynamics. The control modules work to match the speed and torque of the engine to the future gear state. Note that there are a number of ECUs (Electronic Control Units) involved communicating in a coordinated way across an on-vehicle network, not just a computation done in a single processor. The larger the engine, the more inertia and the more challenging perfect matching is. I’m sure you know that an abrupt shift and resulting torque spike while cornering quickly can unsettle the car, so the algorithms for shifting will take that into account. In addition to how the car is being driven, ambient conditions, driver mode selected and other factors are taken into account. Even the break-in state of the transmission matters. Over time the transmission actually learns about itself and seeks to improve shift performance. All this intelligence takes computational time by the on-board processors. The throughput of those processors is dependent on the workload at the time of the shift, so the latency from shift command to shift execution can vary a bit.
All manufacturers are working to improve the shift speed and quality of their transmissions. I foresee ongoing improvement in this area.
One last side note: Some companies do something that we do not do. They move the tachometer needle to the new rpm on a shift faster than the engine actually achieves it. This bit of electronic trickery improves the perception of shift speed without actually changing it. Although it seems to be effective, we don't do it because it is inconsistent with our philosophy on Corvette to convey the most accurate information possible in our cluster displays.
First of all, we on Team Corvette want to wish everyone in the Corvette community and on the Corvette Forum a wonderful holiday and a safe and happy new year!
Now on to the answer:
Although shift speeds have been a priority in racing for a long time, it is a relatively new focus in the mass production auto industry. For decades we had basically two transmission choices; a traditional manual transmission where the time to shift was 100% dependent on the driver’s skill level, and a torque converter automatic where the vehicle hardware and calibration determined the shift timing and speed. The main focus on the automatics was a smooth shift with little driveline or customer disturbance. Yes, there were niche products that featured different set ups, so pardon me for the generalization.
Some performance cars started to offer automated manual transmissions that used a traditional clutch but did the work of shifting robotically, but my experience with those early units was that they were slow and lacked the finesse of a well-shifted manual driven by a skilled driver. Over time the shift quality and speed was improved and manufacturers started talking about shift times as an important performance vehicle characteristic. For the most part auto makers have moved away from automated manuals and use sophisticated planetary automatics or DCT’s to get the best shift speed and quality. This question focuses on shift speed, so I will leave the pros and cons of the two automatic transmission types for another day.
So why can’t down shifts be instantaneous? Well, there is the transmission hardware side that faces the physical challenge of transferring the torque from one set of gears to another set of gears. That is the time most manufacturers quote when discussing shift times. Those times have been improved over the years to very small fractions of a second and the Corvette is competitive with the best on that score. To be honest, we have put a higher priority on upshifts because they occur while accelerating and will contribute faster lap times. Downshifts typically occur when off-throttle and on the brakes, so a slower shift is far less likely to affect lap times. Regardless, up or down, our 8-speed automatic transmission is capable of extremely fast shift speeds.
The paddle shifters on Corvette are controlled by on-board computers just like many other functions on the car. They are “intelligent” devices that seek to improve the safety and controllability of the car. When the driver requests a downshift, the car does not blindly obey (the same is true for throttle inputs). It checks a multitude of parameters to verify that a shift can be done safely, without risk of damage to the car and without upsetting the vehicle dynamics. The control modules work to match the speed and torque of the engine to the future gear state. Note that there are a number of ECUs (Electronic Control Units) involved communicating in a coordinated way across an on-vehicle network, not just a computation done in a single processor. The larger the engine, the more inertia and the more challenging perfect matching is. I’m sure you know that an abrupt shift and resulting torque spike while cornering quickly can unsettle the car, so the algorithms for shifting will take that into account. In addition to how the car is being driven, ambient conditions, driver mode selected and other factors are taken into account. Even the break-in state of the transmission matters. Over time the transmission actually learns about itself and seeks to improve shift performance. All this intelligence takes computational time by the on-board processors. The throughput of those processors is dependent on the workload at the time of the shift, so the latency from shift command to shift execution can vary a bit.
All manufacturers are working to improve the shift speed and quality of their transmissions. I foresee ongoing improvement in this area.
One last side note: Some companies do something that we do not do. They move the tachometer needle to the new rpm on a shift faster than the engine actually achieves it. This bit of electronic trickery improves the perception of shift speed without actually changing it. Although it seems to be effective, we don't do it because it is inconsistent with our philosophy on Corvette to convey the most accurate information possible in our cluster displays.
Merry Ho Ho and a Happy New Year to all. Thanks for making the first year of "Ask Tadge" a success! We're going to take the last couple of weeks of the year off, and get back on schedule Sunday, 3 January. At that point, I'll have a new poll up and we can vote for the first question of the year.
12-19-2015, 09:37 PM
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With any luck, this will quiet some of the whiners a bit.
12-20-2015, 08:09 AM
#4
Thank you Tadge, much appreciate the information and the thoughtful write up. Merry Christmas to you too.
12-20-2015, 01:21 PM
#6
Melting Slicks
Thank you Tadge. And Merry Christmas to you and the rest of the Corvette Team. You have engineered an ASWESOME car! I've always been tempted, but you got me with the C7. And I really like my A8.
12-21-2015, 01:08 AM
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Thanks, Tadge. Merry Christmas and a happy and prosperous Corvette New Year to you and all the Corvette gang! See you in BG in April!
12-21-2015, 09:29 PM
#10
Tadge, thank you for all of your input here. As a fellow engineer I understand the limitations, compromises, and deadline constraints that are part of every engineering endeavor. Yet, you and your team have engineered and constructed an awesome machine. I never considered a Vette before but this car is simply superb. Merry Christmas to you and all of your staff.
12-22-2015, 10:12 AM
#12
Melting Slicks
This is a good explanation by Tadge, but it raises a couple questions:
1. Does the Porsche DCT do the same types of calculations to avoid upsetting the dynamics of the car before shifting, or does it "blindly" downshift (As Tadge said that the Corvette does not do)? If the Porsche DOES do these kinds of calculations, how does it do so faster?
2. Since it is performing a series of calculations and this contributes to shift time, would it not make sense to use a more powerful CPU that would perform the calculations quicker, and remove the computing time from the equation? I'm sure owners would be willing to increase the price of the A8 by $20 in order to accommodate a faster CPU.
1. Does the Porsche DCT do the same types of calculations to avoid upsetting the dynamics of the car before shifting, or does it "blindly" downshift (As Tadge said that the Corvette does not do)? If the Porsche DOES do these kinds of calculations, how does it do so faster?
2. Since it is performing a series of calculations and this contributes to shift time, would it not make sense to use a more powerful CPU that would perform the calculations quicker, and remove the computing time from the equation? I'm sure owners would be willing to increase the price of the A8 by $20 in order to accommodate a faster CPU.
12-22-2015, 10:25 AM
#13
Burning Brakes
Better yet ... when you have the Selector in Manual Shift Mode ... the car should be always calculating and ready to shift -- in other words the shift algorithm should always be running .. not just run when the Paddle is pressed. And why does the automatic have to be so smart ?? Putting the transmission on Manual Mode should actually DUMB things down to closer replicate the M7 experience.
Last edited by dmhines; 12-22-2015 at 10:28 AM.
12-22-2015, 12:11 PM
#14
Instructor
Good topic and good response from Tadge. Most of this transmission engineering stuff is over my head, but he didn't mention the ever-increasing CAFE standards that the auto manufacturers have to meet. I'll bet that they play a big factor in how GM designs these transmissions (automatic as well as manual), especially given the fact that the vast majority of driving (even for Corvettes) is on streets/highways versus the track. I don't even know how to drive a stick, but there's the infamous 1-4 manual shift lockout that they put in for better MPG, etc.
12-22-2015, 05:36 PM
#15
Instructor
This is a good explanation by Tadge, but it raises a couple questions:
1. Does the Porsche DCT do the same types of calculations to avoid upsetting the dynamics of the car before shifting, or does it "blindly" downshift (As Tadge said that the Corvette does not do)? If the Porsche DOES do these kinds of calculations, how does it do so faster?
2. Since it is performing a series of calculations and this contributes to shift time, would it not make sense to use a more powerful CPU that would perform the calculations quicker, and remove the computing time from the equation? I'm sure owners would be willing to increase the price of the A8 by $20 in order to accommodate a faster CPU.
1. Does the Porsche DCT do the same types of calculations to avoid upsetting the dynamics of the car before shifting, or does it "blindly" downshift (As Tadge said that the Corvette does not do)? If the Porsche DOES do these kinds of calculations, how does it do so faster?
2. Since it is performing a series of calculations and this contributes to shift time, would it not make sense to use a more powerful CPU that would perform the calculations quicker, and remove the computing time from the equation? I'm sure owners would be willing to increase the price of the A8 by $20 in order to accommodate a faster CPU.
12-23-2015, 02:01 AM
#17
12-23-2015, 01:43 PM
#18
Pro
Thank you
Maybe other manufacturers have their Chief Engineer answering customer questions... I can not be sure; but in my following of the automotive industry, Tadge is the only one I know of who regularly fields customer questions and provided well written, engineering focused responses - - not just some 'marketing babble'. In my opinion, we a very fortunate. Thank you Tadge, and in today's litigation adverse corporate culture, thank you to GM's Legal Folks for allowing Tadge to speak his mind.
Al Engel "Save the Wave"!
Al Engel "Save the Wave"!
12-24-2015, 01:04 AM
#20
