front/rear hybrid torque numbers

[slashbinslashbash]

From Dailytech

The production version of the car makes use of a turbocharged 1.5-liter three-cylinder engine producing 231 hp (236 lb-ft of torque), which is sent to the rear wheels using a 6-speed automatic transmission. The front wheels are powered by a 131 hp electric motor (184 lb-ft of torque) paired with a two-stage automatic transmission. Those two engines combined give the i8 a total of 362hp and 320 lb-ft of torque.

This is simply wrong, right? The torque to the rear wheels can't simply be added to the torque to the front wheels... it's just not additive like that. You can never get 320 lb-ft from this car. However, the horsepower can be additive. Horsepower is a scalar, torque is a vector. Right?

 

[Phanuel]

No clue, but their torque math isn't additive at all anyhow.

184 + 236 = 420 I think he's missing a 1 there.

 

[Viper GTS]

It's no different than putting any other AWD car on a dyno and recording AWHP/TQ numbers. It's valid.

Other sites are reporting 420 peak torque which makes more sense, though the same could be achieved through computer control of the electric motor output (to save some weak part in the powertrain).

In this case 420 does seem more logical.

Viper GTS

 

[tweakmonkey]

Pretty sure it works.

If you put 100 tq to one wheel, and 100 tq to another wheel, you get 200 tq assuming they have traction. This is how the 918 Spyder gets its power numbers, too.

 

[slashbinslashbash]

It's no different than putting any other AWD car on a dyno and recording AWHP/TQ numbers. It's valid.

Other sites are reporting 420 peak torque which makes more sense, though the same could be achieved through computer control of the electric motor output (to save some weak part in the powertrain).

In this case 420 does seem more logical.

Viper GTS

Torque at the engine is a given. But you can vary torque at the wheels: it is dependent upon gearing. You can have a 100 lb-ft engine creating 1000's of lb-ft at the wheels by gearing it super low. That is how farm tractors have small 4 cylinder engines hooked up to giant wheels that are capable of pulling huge plows through the dirt. Of course, they are limited to very low speeds, but the torque is there.

I don't think the AWD comparison is valid. The torque in an AWD car is all coming from a single source; all of the wheels are mechanically linked. The front and rear wheels in the i8 are independent. The front wheels have no part of the 236 lb-ft from the ICE, and the rear wheels have no part of the 184 lb-ft from the electric motors.

Take the case of a traditional 2WD car. The engine puts out 200 lb-ft. Each wheel gets a portion of that. Torque at each wheel is multiplied by the gear ratios in the transmission and differential (let's say a total ratio of 4:1 engine:wheel, for 800 lb-ft), and divided by 2 because we have equal amounts of torque going to 2 wheels, 400 lb-ft per wheel (assuming a locked differential for the sake of simplicity).

Now take it to AWD. 200 lb-ft at the engine, 4:1 gives 800, divided by 4 wheels gives 200 lb-ft at each wheel. All from a 200 lb-ft engine.

In the case of the i8, we have 184 lb-ft split between the 2 front wheels (separate transmission from the rear) and 236 lb-ft split between the 2 rear wheels...

.. and now I see where I went wrong in my thinking. Ok, I guess we can just add up the numbers after all. Sorry for the silly discussion

 

[Ferzerp]

Torque at the engine is a given. But you can vary torque at the wheels: it is dependent upon gearing.

Not really in regards to published stats. Published torque is generally given in direct drive (less transmission loss), and the rear end gearing is factored out to have a standard that is independent of that.