Hybridization for handling - the G-M Differential

[dwightlooi]

To date, gasoline-electric hybrid vehicles have been designed primarily with improving fuel economy as the primary goal. Although some implementations -- such as those in the Lexus LS600h and RX400h -- have sought to create vehicles with no compromises to performance or even a little gain in this department, no hybrid vehicle to date has been designed with improving handling dynamics as the primary goal and with economy gains as a secondary (but very tangible) benefit. Here, I propose such a concept.

Concept Premises:-

(1) Hybrid drive trains can be used to enhance handling as well as improve fuel economy.

(2) By adding two motors directly to an open differential, we can create an advanced differential with the ability to not only bias torque through computer controlled resistance application on the less tractive side (traditional active differentials) but also add torque on the opposite side. The biasing of regenerative braking can also be used to provide artificial stability (modulate over or understeer) when braking in a turn.

(3) By using two DC PMMs of modest output, we can keep the entire motor and battery assembly small and light. That the motor's torque output falls linearly with wheel speed -- hence diminishing their usefulness at high vehicular speeds -- does not matter much because electric assist has the greatest utility at low to mid vehicular speeds (0-60mph), and most handling issues related to power application during on-limit cornering and braking also happens at low to mid vehicular speeds (<60 mph).

(4) On a 3000 lbs vehicle, a mild 148 volt Li-Ion battery pack and two 15 kW (20hp) can easily be fitted with a weight penalty of ~250 lbs. An estimated 20~25% improvement in fuel economy and a dramatic enhancement of the vehicle's handling dynamics can be expected.

(5) When coupled with a derivative of the 2.0 liter DI VVT turbo engine (LNF) or a derivative thereof, this may be an ideal "sporty green propulsion" package for GM's Alpha (compact RWD) platform.

The idea is very simple. This is a direct drive parallel hybrid with a twist. Instead of putting the electric motor on the flywheel (ala Honda Insight) or coupling it with the engine via a planetary resistive-assistive type CVT (ala Prius), we put two smaller motors on each of the differential output shafts. The differential itself is a traditional open differential.

This arrangement allows electric assist during acceleration and regenerative power recovery during deceleration. But on top of that, the motor totally transforms the differential into what is probably the most advanced active differential ever. When powering out of a corner, one motor will add power to the outside wheel while the other puts resistance on the inside wheel. The effects of this is amplified by the mechanical open differential which naturally wants to bias engine power to the side with lesser resistance. In doing so we can prevent or at least delay the onset of inside wheel spin while we add to the power application on the outside wheel which naturally has more traction (due to its higher rotational speed and weight transfer). When decelerating we can enhance vehicle stability by applying differing levels of power generation on the two wheel. The following diagram illustrates the setup.

[Camino LS6]

Interesting idea.

I could see it presenting some packaging challenges and I wonder about its impact on IRS systems.

[Drew Dowdell]

there is an incredibly simpler design that would/could achieve the same goals you are going for here; Motor in wheel.

that's not to say this isn't a very well thought out concept....

I don't quite follow some of the arrows though... shouldn't the arrow direction for the driveshaft always point the same direction if the car is going forward?

[pow]

Is this like SH-AWD + the AWD on a HiHy/RX400h (separate electric motor-only power for the rear wheels)?

[Z-06]

there is an incredibly simpler design that would/could achieve the same goals you are going for here; Motor in wheel.

that's not to say this isn't a very well thought out concept....

I don't quite follow some of the arrows though... shouldn't the arrow direction for the driveshaft always point the same direction if the car is going forward?

Aren't you quoting about the Mini on Steroids that they did in UK?

[Drew Dowdell]

Aren't you quoting about the Mini on Steroids that they did in UK?

yes. While 540hp is a bit excessive, that concept has proven that such a system can exist in one of the smallest cars around this side of a SMART.

[Z-06]

yes. While 540hp is a bit excessive, that concept has proven that such a system can exist in one of the smallest cars around this side of a SMART.

Weren't the mechanicals on that expensive? I guees once the technology comes up, it will be easier to reduce the price.

That system is quite innovative.

[dwightlooi]

there is an incredibly simpler design that would/could achieve the same goals you are going for here; Motor in wheel.

that's not to say this isn't a very well thought out concept....

I don't quite follow some of the arrows though... shouldn't the arrow direction for the driveshaft always point the same direction if the car is going forward?

A few points:-

(1) There are two problems with wheel hub motors... the first is that electric motors can be heavy. And, for the most parts the size and weight of the motor is a function of how much torque it can produce. If you eliminate the reduction gearing the motor's size increases. Because electric motors tend to be able to rev up to very high speeds, it is a good trade off to reduce the speed range it can achieve in exchange for torque multiplication. Let's put some numbers into play here. A 225/45 R17 tire is about 78.5 inches in circumference -- thats roughly 0.00124 miles. At 60 mph the wheel is turning at 806 rpm and at 120 mph it is going 1612 rpm. A typical DC motor will easily turn at 10000 rpm or more and their torque output decreases linearly with rpms, so it is beneficial to use reduction gearing to reduce its speed range from 0~10000 to say 0-3300 rpm while tripling the torque output. Hub motors don't do that. If you try to get 3x the torque without gearing, you'll need a bigger motor. The following diagram illustrates the torque curve of a DC motor -- I think it is self-explanatory.

The second problem is that motors can be heavy and fragile. The last thing you want, if you are after great handling and dependability, is to put all that mass in the wheel so it bounces up and down as unsprung mass and subject the motor to all the shocks and rigors of not having a suspension to ride on. Putting the motors at the differential isolates them from the shocks of wheel movements, and reduces unsprung mass dramatically -- instead of motors going up and down, only the half-shafts go up and down with the suspension.

(2) The ARROWS in the previous post DO NOT indicate the rotation of the wheels or the motors. They indicate the direction of the application of force -- which may or may not be the same as that which the components are turning. Think of it this way.... When you are braking from 60mph to 0 mph, the wheels are always rotating forward. The Brakes apply force in the reverse direction even though they don't rotate at all. In a similar vein, you can have a motor act to apply force in the opposite direction while turning in the same direction as the wheels.

[Drew Dowdell]

(2) The ARROWS in the previous post DO NOT indicate the rotation of the wheels or the motors. They indicate the direction of the application of force -- which may or may not be the same as that which the components are turning. Think of it this way.... When you are braking from 60mph to 0 mph, the wheels are always rotating forward. The Brakes apply force in the reverse direction even though they don't rotate at all. In a similar vein, you can have a motor act to apply force in the opposite direction while turning in the same direction as the wheels.

thank you, that's why I asked..... perhaps your graphic could use some sort of key to clarify. I still find it hard to follow your picture.

I don't know how detailed you intend your graphic to be in terms of packaging, but could I make another suggestion? I follow the reasoning against hub wheel motors. For packaging reasons, could the motors be mounted further away from the differential and have power transfered via additional drive shafts? Motors don't care which direction they are oriented. They could be mounted semi-vertically behind the rear seats with their output shafts facing down towards the differential.