Hybrid Drivetrains


Do electric driven hybrids have transmissions or are they direct drive from the electric motor?


In general the parallel hybrids require a transmission as the gas engine which needs one, works in concert with the electric motor. In a series hybrid a gas engine supplies the electricity though a generator and battery pack to an electric drive motor. Now the conjecture on my part…The drive motor may be direct drive w/o a transmission or may have a simple transmission with fewer forward speeds and reverse than required by a gas motor. which is an engineering decision.


It’s direct drive with no transmission according to gm-volt.com.


The Prius uses a differential gearset and two motor/generators. Overall, to the gas engine it looks like a CVT.


Diesel-electric locomotives use no transmission. The engine drives a generator and a set of motors drive the wheels…Hybrid cars are similar, but with a fairly large battery to capture braking energy (which locomotives waste) and augment heavy acceleration. Getting rid of the transmission is a major benefit. You would think all hybrids would strive to do that to reduce manufacturing costs and looking forward, maintenance costs…


I think that the engineers who drive locomotives are well trained and know when to start coasting so that they don’t need to waste energy through braking.

If you drive a car so that you never need to brake, you can come real close to the gas mileage that most people get out of hybrids.


No current hybrids are series hybrids. The Volt will be the first. All current hybrids have transmissions.

Here’s Toyota’s (good) explanation of their 'series parallel drive’


And for good reason…everything I’ve read says that for light load, automobile application, parallel hybrids still have an efficiency advantage; until of course battery storage in cars is allowed to increase to the “magic” 100 miles allowing most to never use the gas engine until weekend trips to “grandmother’s house we go”.

The Volt is an obvious “joke” on the rest of us if you think it to be advanced technology. GM is just continuing to hold it back and use it as propaganda fodder as long as possible. When it does come out with a useless battery, it will have little mileage advantage over a Corolla. I feel that, only until the battery gives 60-100 plus miles under the most demanding conditions and cost under $20k will it be cost effective to present parallel hybrid with a transmission. Until then, GM will continue to use the previous generation Prius technology; while we play for the plagiarism.


So on the Prius, the “Power Splitter Device” is actually a CVT type transmission and a computer decides how much load it carries and how much load the electric motor carries…To me, that “power splitter device” is the weak link in this system. Owners are not only faced with replacing a very expensive battery, but they also must contend with a potential transmission failure…


The world is not flat. They do indeed brake. The energy created is turned into heat and radiated off huge resistors (radiators) mounted on top of the locomotives…When that’s not enough, the air-brakes are applied to all the freight cars, heating the massive steel wheels through friction…


The power splitter on a Prius is just a simple fixed ratio planetary gear reduction set. The engine drives the planet carrier, the sun gear drives the small motor/generator, and the large main motor/generator drives the ring gear which is also the output of the transmission.

If the small motor/generator on the sun gear is not generating, it freewheels and the engine just spins the sun gear and the transmission is effectively in neutral. When the motor control makes the small motor/generator start generating electricity, it brakes the sun gear which causes the engine to turn the ring gear instead. The electic power generated is delivered to the big motor/generator and its torque is added to the ring gear output.

Reverse is electric, the small motor/generator is also the ICE starter.

The mechanical part of the transmission is as simple as transmissions get.

Here is an animation showing how the transmission works.


Thanks for the link, BLE, now I know what that ‘power splitter’ really is. Pretty clever. I assumed when the car magazines said there was a CVT in there somewhere, it was the belt and cones type of CVT. Now I know it is a CVT, created by manipulating the three parts of the sun gear. It’s quite a bit like the Model T transmission, actually. You can see a similar animation for the Model T here:


“They do indeed brake.”

A heck of a lot coming down a mountain pass. Plus, the rolling resistance is very low and the aerodynamics is pretty good when the whole train is straight, so they coast a lot better than a car on flat ground. Imagine a car that could coast a mile from 10mph.


While the mechanical layout is very different the Prius gearset performs a similar function as the differential on a conventional car.

Imagine a RWD differential with a final drive ratio of 1:1. The ICE motor would go to the input shaft. Left axle to MG1. Right axle to MG2 and the drive wheels (through conventional diff).

Idle the ICE with the car standing still (MG2rpm = 0) and MG1 spins at 2X the ICErpm.
Drive pure electric with ICErpm = 0 and MG1 spins at same rpm and opposite direction as shaft to drive wheels.
Drive with MG1rpm = 0 and the shaft to drive wheels spins at 2X the ICErpm.

I’m not suggesting that any individual or company try to build this. I’m just pointing out the similarity. Each mechanism is well suited to its intended job.


Yes indeed…It sounds like no bands or clutches would be needed, nor any hydraulic shift controls. Just a simple planetary gearbox…The tricky part is balancing the ICE output and the drive motor output so the occupants of the car don’t feel or hear any strange sensations as the planetary finds equilibrium


Yes. The thing that makes it all work is the ability to run one of the electric motors ‘backwards’ as a generator, it seems.


The thing is there are 3 input/outputs to the PSD and 2 equations that must be continuously solved: the sum of RPMs (times appropriate gear ratios) must equal zero, and the sum of torques (divided by appropriate gear ratios) must also equal zero. Have positive and negative RPMs depending on which way the shafts are turning, and will have positive and negative torques depending on whether torque is being added or removed.

Most of the time one of the MGs (Motor/Generator) will be operating as a motor while the other is operating as a generator. The result is a propulsion path similar to a diesel locomotive or the proposed GM Volt. At the same time the engine drives gears for conventional propulsion. By controlling the pair of MGs one can control the engine RPM which is effectively what most people thing of as the transmission’s function.

Toyota did their homework and keeps the solution to the two equations stable, there is no hunting for a solution, it just works. The only instance of odd or unusual behavior is the engine often quits spinning below 42 MPH and can sometimes be felt when it starts.

Because Toyota has complete speed and torque control over the electric motors, starting the engine is not the violent jolt of conventional starters. Engine can be smoothly brought up to 1200 RPM, fuel and spark turned on, instantly starts.