There have been a few questions as to why I changed my plan from strongly for keeping the clutch to removing it from the system. There are a number of factors that contributed to the decision.
- One of the founders of Tesla Motors, JB Straubel converted a Porsche 944 to electric using a pair of 8″ motors, a 1200A controller and 20 yellow top optima batteries. In his case current was limited to 900A because the clutch would slip. Now it’s hard to directly compare the torque from a pair of 8’s vs my single 11 however I would estimate them to be in the same ballpark. So without an expensive racing clutch I would expect my stock clutch to slip. (and require replacement on a regular basis).
- Jack R. of EVTV also found clutch slipping on his Mini Cooper with an AC setup of over 100kw, (somewhere between 120 and 150kw).
- After reading about these two cases things started to look bad for the reliability and power transfer through the stock clutch.
- Clutch jobs in a 944 aren’t exactly easy, so I would have to design in a complex method to allow me to replace the clutch without having to remove the whole electric drive system.
One of the goals of the electric car is increased reliability and reduced maintenance, so having to replace the clutch at an increased rate vs the gas version was bad in all respects. The negatives didn’t end with reliability and power transfer though.
- The stock flywheel/clutch/pressure plate assembly of a Porsche 944 weighs just over 38lbs, this isn’t just dead weight that I have to haul around, it’s rotating mass that has to be accelerated by the motor and will resist a change in speed (slow acceleration) The electric motor doesn’t need the smoothing effect of the flywheel to even out the power produced, it simply adds weight that must be supported and accelerated.
- The clutch on the other hand is something that any manual transmission driver wants, needs and would be stuck on the side of the road without it. However in an electric car there is no need to idle the motor, the electric motor is happy to start from 0rpm with full torque available. There is no need to rev the motor to bring it into the power band and start the vehicle rolling. So I no longer need the clutch to start and stop.
What about shifting gears?
- My Porsche 944 has the 5 speed manual out of a 944 Turbo. With all of the torque available from 0rpm I no longer need 1st gear to get the car moving. The car will quickly and happily start from a stop in 2nd gear.
- The electric motor is quite different from the gas engine it’s replacing. To get the most torque from a gas engine you have to rev it up somewhere between 2k – 3k rpm and power will build with higher RPM’s, fuel economy is drastically reduced at higher rpm though so you want enough gear ratio’s to allow reasonably low rpm’s at cruising speed. So in short you have minimal torque at low rpm, you have high torque/power at higher rpms but you have poor fuel economy at those high rpms.
- With an electric motor you have full torque from 0rpm so you can start in a higher gear and accelerate nicely. The torque is constant for as long as you can provide the same current to the motor, so in theory you have full torque from 0-6000rpm. Power builds like a gas engine because (Power = Torque X RPM / a Constant), so RPM is an integral part of power regardless of the type of motor/engine you are using. The main difference comes from the fact that electric motors are more efficient at higher rpm. WHAT?!?!? So for efficient cruising you want to be around the mid point or slightly above in the RPM range. So not only do you not need first gear to pull away from a stop sign, you don’t need 4th or 5th gear to efficiently cuise at 100km/h. This leaves 2nd and 3rd gear, (and possibly 4th gear if you plan to cruise at 140km/h).
So I’ve eliminated the need for the flywheel, and I don’t need the clutch to start or stop, this leaves the only purpose of the clutch as the shift from 2nd to 3rd, or 3rd to 2nd.
So what does the clutch really do when we shift?
- The clutch is a physical disconnect between the engine and trasmission. Many of us have shifted a manual transmission without the clutch, you just let off the gas, move the shifter and slowly slide it in gear as the syncro’s match the speed of the engine to the transmission and let everything come together without grinding. One of the reasons this is difficult for the syncro’s is due to the rotating mass of the engine. Not only do you have the whole clutch/flywheel/pressure plate, you have the crankshaft/connecting rods/pistons/camshaft and valve train/damper/alternator/A/C/Power steering and some other things that I’ve left out. This is a lot of rotating mass that wants to gradually slow down. If you do it correctly as the engine slows down it will hit the perfect rpm and your shifter will slide into gear. There is a problem if you miss the point and the engine slows down too much and you may need to touch the gas just a bit to let the engine speed up and slide into gear.
- In the setup I’m planning I have an aluminum coupler, 7.5lbs and the armature of the motor and the A/C pulley. There is also no “backpressure” from the compression of the engine so there is no resistance to speed up the motor (even without power) The entire rotating mass is drastically reduced and because of the lack of backpressure there is minimal resistance to increase or decrease speed. This starts to enter the range of what the syncro’s can handle with less manual input from the driver. I probably won’t be able to shift as quickly as it was possible pressing the clutch but it should be smooth and comfortable even for novice drivers.
- Down shifting a gas engine clutchless is virtually impossible without delicate input to the throttle, you need to increase the rpms to the sweet spot where the transmission will slide into gear. With the electric motor, you wouldn’t need to shift out of 3rd if simply coming to a stop, you could press on the brakes and simply stop, then shift back into 2nd. The car will even start in third gear, although with higher current draw and possibly slower acceleration. However down shifting on the fly should be easier than a gas engine because there is no longer the backpressure slowing the engine, the motor will simply coast smoothly and it wouldn’t require the same energy to increase the rpms to allow the shift to take place. There will probably be the need to gently touch the throttle to make the shift happen faster, but this is one thing I’ll have to learn.
What about safety? What if I need to disconnect a runaway motor from the wheels to avoid driving through the store at the end of the street?? Well simply pull the transmission out of gear into neutral. However the safetly features implimented in the controller should eliminate the need for that without multiple system failure.
So by eliminating the flywheel/clutch/pressure plate assembly (plus a coupler to attach the flywheel to the electric motor) I simply need a coupler to attach the electric motor directly to the torque tube shaft. The coupler is machined from a solid piece of aluminum that attaches to the 1 1/8″ keyed motor shaft on one end and uses the center section of a clutch disk bolted to the other end of the coupler that meshes with the torque tube splines. The clutch springs are left functional and will absorb some of the shock that either the motor could transmit to the drive train on quick acceleration, or road bumps could transmit back to the motor. There is a weight savings of roughly 35.5lbs in rotating mass, since I would have required 38lbs of flywheel/clutch/pressure plate and an estimated 5lbs of coupler to attach to the motor. This is replaced by a 7.5lb coupler that directly attaches the motor to the torque tube.
This eliminates the possiblility of clutch slip and wear, increasing performance but possibly putting more strain on the transmission. There is also a small increase in useable space since the factory bellhousing is also removed and replaced with a much smaller diameter adapter tube that will connect the torque tube and motor directly.