# Hybrid 911



## strathconaman (Nov 3, 2017)

I am in the early stages of designing a hybrid system for the 996/997 Porsche 911 AWD. The end goal is to push my eyes further into my head apex to apex. 

These are rear-engined sports cars, with the transmission in front of the engine. What makes these cars ripe for a hybrid conversion is the transmission on them has an "output" shaft that goes to a front viscous coupled differential. 

The front differential can be removed, making the cars rear-drive only. An electric motor could then be fitted where the differential once was, and power fed into the drive train through the front of the transmission. There is possibly room for the battery where the rest of the final drive and half-shafts were. The trunk is located directly above this, which gives more space for components.

Their engines red line at 7200 RPM, and generally make the most torque between 3500 RPM and 5500 RPM. The electric motor would be turning at the speed of the engine, through the rear differential.

These are already relatively fast cars. The hybrid system would be used like a supercharger for wide open throttle (or near WOT, 75-100%) conditions. As such, even a 70-100 ft/lb increase in torque would be sufficient.

They come from the factory with 2 radiators, but the ability to fit a third radiator. The location of the electric motor in line with the front wheels makes liquid cooling the controller, motor and battery pack nearly trivial.

They have electronic throttle bodies, so control of the electric "boost" would be based off the throttle position sensors already integrated into the car. Tapping into that might not be trivial.

DIY hybridization (especially for sports car application) isn't as popular as a straight EV conversion. I am having a hard time finding resources to get started on things like battery pack sizing, and most importantly, charging. I don't see this as a plug in hybrid, but I need to get energy into the batteries somehow.

Most of the AC controllers allow for re-gen braking, but I need to charge as well when the car is just cruising.

I would love to hear input from this group, especially if you have links to resources. 

Thanks


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## brian_ (Feb 7, 2017)

strathconaman said:


> ... What makes these cars ripe for a hybrid conversion is the transmission on them has an "output" shaft that goes to a front viscous coupled differential.
> 
> The front differential can be removed, making the cars rear-drive only. An electric motor could then be fitted where the differential once was, and power fed into the drive train through the front of the transmission.
> ...
> The electric motor would be turning at the speed of the engine, through the rear differential.


The speed statement doesn't make sense. The output to the front final drive is at transmission output speed, not engine speed (which is the speed of the transmission input); as a result, the motor would be turning at the speed of the transmission output (or the input to the differentials), not at the engine speed.


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## Tremelune (Dec 8, 2009)

I suspect you don't see many hybrids because the juice often isn't worth the squeeze. It would be much cheaper and simpler to slap a turbo or supercharger onto the existing motor. I recognize that isn't the same characteristic as having an electric motor driving the front wheels from a standstill, but it's something to think about if your goal really is just more power.

Elephant Racing developed their own hybrid system that fits between the engine and transmission. The installation cost is $75k (you read that right), so I'm interested to see where you wind up with your FWD approach.

https://www.roadandtrack.com/new-ca...erformance-aftermarket-porsche-hybrid-system/


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## brian_ (Feb 7, 2017)

Tremelune said:


> ... I'm interested to see where you wind up with your FWD approach.


The proposed approach here is not FWD - it doesn't replace mechanical drive of the front axle with electric drive; instead, it eliminates any drive of the front wheels (making the car just RWD), and converts it to a parallel hybrid. That gives it more peak power, but less drive traction, more total mass, and more weight on the non-driven front tires.


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## brian_ (Feb 7, 2017)

Tremelune said:


> Elephant Racing developed their own hybrid system that fits between the engine and transmission. The installation cost is $75k (you read that right), so I'm interested to see where you wind up with your FWD approach.
> 
> https://www.roadandtrack.com/new-ca...erformance-aftermarket-porsche-hybrid-system/


This is apparently the system which inspired this project idea, as noted in strathconaman's earlier post in another thread:
Re: Project AWD KERS Viper

The primary difference is mounting the motor/generator between the engine and transmission input (Vonnen) or using the front transmission output (here).


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## Bowser330 (Jun 15, 2008)

I love the parallel hybrid idea, you could use the front motor of a Tesla model s, 300hp added to the 911.

Alternatively couldn’t you get a rwd 911 and just switch over the front suspension hardware to the C4 model?

I have seen a a few white papers online about parallel hybrids but I haven’t seen any diy hybrid performance cars.

The charging could be through regen, you could actually have regen on slightly turned on all the time to charge up what you used on your bursts of acceleration.

My question is on the power delivery control, when you press the throttle the gas motor will rev up and the electric motor will rev upas well, how does the electric motors rpm get determined by the road speed? No electronic speed synchronization is needed?


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## Isaac97 (Jun 3, 2019)

Well I just went looking around for half an hour reading about 996 AWD, disassembled Tesla units, and lightweight batteries. And this thread is 18 months old. Try not to necropost please 
-Isaac


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## Bowser330 (Jun 15, 2008)

The last post before mine was 11 months ago, Sept 16th 2019, not 18 months ago. Try to get your dates right.

“Necroposting is seen as a form of spam and clogs up the forums with old and unneeded topics.”

I don’t deem this thread to be about an old or unneeded topic, nor is what I posted spam or with intent to clog the forum. I was actually searching for diy parallel hybrid setups and came upon This thread. if I’m researching it others might be too, plus you did too before you realized it was soooooo 11 months ago.

this forum has a ton of very intelligent people and if i have a damn question I’m asking it no matter what trendy new word little kids are using these days.


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## Isaac97 (Jun 3, 2019)

Sorry, I didn't see the date on the post before. Jan 10 2019 is 18 months ago, and since then the OP has been gone. In my experience necroposting usually means replying to an old thread, I didn't exactly look up the definition before using the word (probably a mistake of mine, not the first one).
I'd really appreciate if you wouldn't go calling people little kids though 

BTW, to answer your question, as long as there's a direct connection (gearbox or otherwise) between electric motor and wheels there is no problem with motor RPM synchronization.
A Tesla motor is especially painless, since it's induction rather than PMAC (no back EMF problems when unpowered) and it's torque controlled rather than speed controlled.
If you want a cheap SDU controller take a look at openinverter, for 300 euros (maker is in Germany) you can get a replacement logic board which will get tons of power out of the SDU (though maybe not matching the efficiency of the Tesla logic board.) There is also a forum with lots of bleeding edge development, mostly in repurposing OEM motors and inverters.
-Isaac


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## brian_ (Feb 7, 2017)

Bowser330 said:


> I love the parallel hybrid idea, you could use the front motor of a Tesla model s, 300hp added to the 911.
> 
> Alternatively couldn’t you get a rwd 911 and just switch over the front suspension hardware to the C4 model?
> ...
> The charging could be through regen, you could actually have regen on slightly turned on all the time to charge up what you used on your bursts of acceleration.


That's called a "through the road" hybrid, and it's very inefficient.


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## brian_ (Feb 7, 2017)

Bowser330 said:


> My question is on the power delivery control, when you press the throttle the gas motor will rev up and the electric motor will rev upas well, how does the electric motors rpm get determined by the road speed? No electronic speed synchronization is needed?


Applying "throttle" doesn't speed anything up; it increases torque output (whether it's an engine or any kind of electric motor in a car). Of course the car will likely speed up, which means that the motor goes faster, but not directly because of the accelerator pedal position. In an automotive application, the motor speed determines the frequency of the power supplied to the motor (plus a slip factor in the case of an induction motor), not the other way around. The only reason for management of the speed of the front and rear axles is for traction control (detecting and limiting wheelspin); synchronization is not needed for operation.


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## Bowser330 (Jun 15, 2008)

brian_ said:


> That's called a "through the road" hybrid, and it's very inefficient.


That may be one of the explanations why we haven’t seen them more.

Didn’t Toyota/Lexus use a parallel hybrid technology for the rear electric motor in one of their hybrid suvs?


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## Bowser330 (Jun 15, 2008)

Isaac97 said:


> Sorry, I didn't see the date on the post before. Jan 10 2019 is 18 months ago, and since then the OP has been gone. In my experience necroposting usually means replying to an old thread, I didn't exactly look up the definition before using the word (probably a mistake of mine, not the first one).
> I'd really appreciate if you wouldn't go calling people little kids though
> 
> BTW, to answer your question, as long as there's a direct connection (gearbox or otherwise) between electric motor and wheels there is no problem with motor RPM synchronization.
> ...


Thank you for the reply


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## Isaac97 (Jun 3, 2019)

IIRC the rx400h had a small rear motor. But that was powered normally by the battery pack (the car also had a FWD hybrid transaxle).


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## brian_ (Feb 7, 2017)

Bowser330 said:


> Didn’t Toyota/Lexus use a parallel hybrid technology for the rear electric motor in one of their hybrid suvs?


That is not a "through the road" system.

In all Toyota and Lexus hybrids with a transverse engine and AWD, the rear wheels are powered only by an electric motor, with no mechanical connection to the engine. This includes the hybrid versions of the Highlander, RAV4, RX, NX, UX, and now even the Prius (optionally); Honda and BMW have some models which similarly have an electric-only axle. The difference between this and the system proposed above is that the Toyota system never generates power by running the engine to drive one set of wheels to drive the motor on the other set of wheels as a generator; instead, as Isaac97 mentioned these vehicles are hybrids with motor-generators connected to the engine, so they can generate from engine power without the losses of going through tires. The electric-only axle only generates for braking.

The drive to the rear wheels of these hybrid vehicles with front engines and rear electric-only drive is a series hybrid configuration.


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## Bowser330 (Jun 15, 2008)

Thank you for the information Brian but I guess I’m just especially dense on this topic. Do I have the below written correctly?

If a rear engine car had an independent electric axle upfront, it would be considered a Parallel hybrid. Because the motors are disconnected from one another and paralleled through the road. 

And remain a parallel hybrid when under braking or zero throttle the electric axle regenerates power back into the e-axle driving battery pack.

but the minute the regen is activated while the car is being powered by the gasoline rwd engine it becomes an inefficient series hybrid, because you are using rear mounted gas engine to power up the battery of the front e-axle.


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## Bowser330 (Jun 15, 2008)

Isaac97 said:


> IIRC the rx400h had a small rear motor. But that was powered normally by the battery pack (the car also had a FWD hybrid transaxle).


That’s the one, thx

also I checked out openinverter and woah does Damien have some amazing products. Currently has some boards coming out for independent model 3 motor control, very cutting edge as you pointed out.


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## Isaac97 (Jun 3, 2019)

Bowser330 said:


> Thank you for the information Brian but I guess I’m just especially dense on this topic. Do I have the below written correctly?
> 
> If a rear engine car had an independent electric axle upfront, it would be considered a Parallel hybrid. Because the motors are disconnected from one another and paralleled through the road.
> 
> ...


Yes, it would be a parallel hybrid, of the 'through-the-road' type as Brian said (I hadn't seen that term before, thanks for the new word!). 
It's not a series hybrid - in a series hybrid, there is a completely separate generator and the engine is not connected to the wheels directly.

As a matter of fact, the front axle is a better spot for regen anyways (front axle braking is preferable). But when charging via regen you're putting extra load on the drivetrain, on the tires and on the original engine -- more wear everywhere, and probably losing at least 10% of the power (plus inefficiencies in the electric motor).
For pure track use, this is probably fine, and does allow you to reduce costs. 

BTW if an onboard charger is an issue, it's very possible to add CHAdeMO DC charging, which minimizes the extra onboard weight and allows much higher charge rates. OpenInverter people (myself included) are working on a very simple setup which will enable that.

-Isaac


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## brian_ (Feb 7, 2017)

Bowser330 said:


> Thank you for the information Brian but I guess I’m just especially dense on this topic. Do I have the below written correctly?
> 
> If a rear engine car had an independent electric axle upfront, it would be considered a Parallel hybrid. Because the motors are disconnected from one another and paralleled through the road.
> 
> And remain a parallel hybrid when under braking or zero throttle the electric axle regenerates power back into the e-axle driving battery pack.


Yes, it would be a parallel hybrid... although parallel does not mean that the engine and electric motor are disconnected from each other, only that either can drive the vehicle without the other.



Bowser330 said:


> ...
> but the minute the regen is activated while the car is being powered by the gasoline rwd engine it becomes an inefficient series hybrid, because you are using rear mounted gas engine to power up the battery of the front e-axle.


No, that's still parallel, although it's an interesting point... in any parallel hybrid the energy flow over a whole cycle of battery charging and discharging does go from engine to electric system to road, like a series hybrid does all of the time. This one is particularly inefficient because of the tire losses.

The other reason that through-the-road systems are bad is that it is inherently undesirable to the dynamics of the vehicle to be braking with one axle while driving the other, particularly with the front braking and the rear driving.



Isaac97 said:


> Yes, it would be a parallel hybrid, of the 'through-the-road' type as Brian said (I hadn't seen that term before, thanks for the new word!).
> It's not a series hybrid - in a series hybrid, there is a completely separate generator and the engine is not connected to the wheels directly.
> 
> As a matter of fact, the front axle is a better spot for regen anyways (front axle braking is preferable). But when charging via regen you're putting extra load on the drivetrain, on the tires and on the original engine -- more wear everywhere, and probably losing at least 10% of the power (plus inefficiencies in the electric motor).
> For pure track use, this is probably fine, and does allow you to reduce costs.


All true 
Although even on a track, braking the front while driving the rear isn't good for stability.

The reduced cost (by not having a generator at the engine) also causes reduced functionality, and even the extremely cost-conscious major auto manufacturers don't cheap out in this way.


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## Isaac97 (Jun 3, 2019)

For a more efficient drivetrain, how about the Tesla SDU up front and then a smaller generator connected to the transmission's front wheel output shaft? Well that's more weight and more complexity... Hybrids are hard. But it would still offer better generation capabilities, although the ideal is connection directly to the flywheel (allows generation while idling/at any speed/in neutral).


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## Bowser330 (Jun 15, 2008)

I think the dynamics and stability statements have made it very clear that it can be straight dangerous to be regenerating while driving, so then how to keep the smaller battery pack charged? 

Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.

a lot of work but could make quite the performance hybrid!


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## brian_ (Feb 7, 2017)

Isaac97 said:


> For a more efficient drivetrain, how about the Tesla SDU up front and then a smaller generator connected to the transmission's front wheel output shaft? Well that's more weight and more complexity... Hybrids are hard.


That would work. Of course, with an AWD transaxle just using the stock AWD is much lighter and cheaper, and the stock AWD system plus a single motor-generator would provide a temporary power boost and regenerative braking, using all four wheels. Yes, hybrid vehicle design is inherently complex and difficult. 



Isaac97 said:


> But it would still offer better generation capabilities, although the ideal is connection directly to the flywheel (allows generation while idling/at any speed/in neutral).


At the engine output is a good place for a motor-generator because it turns at a suitable speed without additional gearing, and stays in a suitable shaft speed range even with changes in road speed. On the other hand, generation while not driving isn't typically an important feature. Most hybrid vehicles use the hybrid system to avoid running the engine at all when stationary, in neutral, or driving at very low speed; the rare exception is someone using the vehicle as portable generator set for a work site or emergency power backup.


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## brian_ (Feb 7, 2017)

Bowser330 said:


> Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.


My understanding is that in the 996 generation of 911 this is not an issue, because the output shaft to the front is the same output as to the rear differential: there is no centre differential. The required speed difference between front and rear axles is accommodated by a viscous coupling between that shaft and the front differential, mounted at the differential end of the shaft. That means that a generator (or motor-generator; see my previous post) can be mounted at either end of this shaft (in front of the transaxle or behind the front unit) and will act like it is on the transmission output. The generator can be controlled to produce whatever power is desired, regardless of the AWD system operation. Both earlier and later 911 AWD systems are different - this is the easiest generation to work with.
Hagerty: _Behavior modification: All-wheel drive tames the 911_


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## Isaac97 (Jun 3, 2019)

Bowser330 said:


> Isaac97's idea of using the now unused output shaft of the AWD transmission is a good one if a generator can be appropriately sized for the RPM range. I'm not sure on the SW controls the cars have regarding torque split between the rear diff and front diff... i assume some custom ECU work would be required to determine the right amount of torque split to the generator based on the state of charge of the battery.


The 996 has no torque split controller -- it's just a viscous coupler to the front end. Some versions have stability control (keeps the car pointing straight), others have traction control (anti-wheelspin). Traction control is rear-only, it's a virtual LSD using the brakes. But there are no electronics controlling the torque split, only the viscous clutch/coupler - someone quoted 15% torque normally, up to 45% after extended rear slippage.

EDIT Looks like we've got the same ideas here.


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## Bowser330 (Jun 15, 2008)

Thank you gents. Very interesting about using an older 911 for this project. I haven't verified which years used the VC vs. the MC.

_The newer 911 Carrera 4 uses an entirely different AWD system. Instead of a viscous coupling, which responds to slip mechanically, a computer-controlled multiplate clutch actively engages the front differential to transfer torque forward. 






Behavior modification: All-wheel drive tames the 911 | Hagerty Media


These days, you can’t walk through a Porsche dealership without bumping into an all-wheel-drive-equipped vehicle. Customers for Porsche’s SUVs, the brand’s best-selling vehicles, expect the all-weather traction of four driven wheels. On the other hand, 911 loyalists remain split over rear-wheel...




www.hagerty.com




_


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## Isaac97 (Jun 3, 2019)

A few minutes of Wikipedia checks (might be wrong) tells me that 996 and 997 have viscous clutches, no data on 991 (generation after).

EDIT 993 (1995-1999) had viscous too, generation before that had a real differential.


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## Bowser330 (Jun 15, 2008)

Isaac97 said:


> A few minutes of Wikipedia checks (might be wrong) tells me that 996 and 997 have viscous clutches, no data on 991 (generation after).
> 
> EDIT 993 (1995-1999) had viscous too, generation before that had a real differential.


Thanks for the research. I think that might apply to the carrera only, the 997 turbo i think used the multiplate clutch, but the 996 turbo did use the viscous. Very cool.


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