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A technical look at parallel axis hybrid transaxles

Monday, January 1, 2018 - 08:00
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If you are like most people I know, you have not paid much attention to what has been going on with hybrid vehicle powertrains over the years; why would you? Hybrids have been a very small percentage of overall vehicle sales, especially with gasoline prices being “so low.” The trendy vehicle to have today is a truck or an SUV.

While you may have been ignoring hybrids, there has been a quiet battle going on for the most efficient hybrid/plug-in hybrid powertrain design for the last 20 years. The results of this battle (so far) have surprised me, and I want to share with you what has happened. First, we need to define five boring — but necessary to understand — technical terms:

Parallel Hybrids: Parallel hybrids use an internal combustion engine (ICE) and electric motor/generator(s) to simultaneously provide power to the wheels through the vehicle's transmission. In addition to supplementing the power of the engine, the motor/generator can also charge the hybrid battery while the vehicle is in motion. Driving the vehicle with only the motor/generator is also possible. An example is the Honda Integrated Motor Assist (IMA) hybrid system.

Series Hybrids: A series hybrid system uses an electric motor to rotate the wheels through the transmission. The motor can be powered by a hybrid battery and/or an internal combustion engine (ICE) driven generator. The generator is used to provide an electric power source for the motor and/or charge the HV battery. An example is the Chevrolet Volt Plug-in hybrid.

Series-Parallel Hybrids: A Series-Parallel hybrid system has two electric motor/generators. Electrical power can be generated by the first motor/generator using power from the internal combustion engine (ICE). The generated electrical power is used to charge the hybrid battery and also to provide power to the second motor/generator. The second motor/generator directly drives the wheels through the vehicle's transmission (Series operation). The second motor/generator can also receive supplemental motive power from the ICE through the first motor/generator by way of a power-split gear set (parallel operation). Examples are the Toyota, Ford and Chrysler hybrid systems.

Inline Axis: A series parallel hybrid system with two electric motors that share a common axis of rotation. One motor is mounted directly above the other motor. They share a common centerline.

Parallel Axis: A series parallel hybrid system with two electric motors that do not share a common axis of rotation. One motor is mounted to the side (offset) of the other motor. They do not share a common centerline; they each have their own centerline.

Now that we understand the terminology, let’s look at the major contenders in the hybrid powertrain battle.

Contender #1: Honda

This may surprise you, but Honda was first to market a hybrid vehicle, the Insight, in the US for the 2000 model year. The Insight powertrain used an IMA, single electric motor, parallel hybrid system with a 5-speed manual transmission or a belt-driven Continuously Variable Transaxle (CVT). The IMA system used its single electric motor to “assist” the engine’s crankshaft in rotating, for regenerative braking, and it also started the engine after an idle stop event. Honda used the slow-selling IMA system for 17 model years in various vehicles ending with the 2016 CR-Z. I drove a Civic hybrid when I was considering buying a new car, but it was too gutless for my taste. In my opinion, the Honda IMA system could never quite compete with the Toyota hybrid system; it was always rated about 10 mpg lower in fuel economy than the Prius.

The IMA hybrids were plagued with high voltage Ni-MH battery problems as well. They had battery self-discharge problems and too short of a battery life. To make things worse, the 12-volt system is charged from the high voltage system. When the high voltage (HV) battery dies, there is nothing with which to charge the 12-volt battery, so it dies too, and then the engine can die while driving from lack of voltage. This was a huge safety problem for Honda so they extended the warranty on the batteries. The used car market is full of Honda IMA vehicles with dead HV batteries. The cost of the HV battery is higher than the value of the car, so avoid them if possible. As if that was not bad enough, Honda also had problems with failed push-belts in their CVT transaxles that could leave the driver stranded.

The good news for Honda is that they started replacing the IMA system in the 2014 model year with a more efficient two-motor, series-parallel transaxle system called “E-Drive.” The new Honda two-motor hybrid transaxle system is totally unique. No other manufacturer uses this design. It uses one electric motor as a generator and one electric motor as a traction motor, the two motors are not connected together. Unfortunately, the E-Drive system is only available in the hybrid and plug-in hybrid versions of the Honda Accord.

2000-2016 Honda IMA system stator and rotor

Contender #2:  Toyota

Toyota was second to market a hybrid vehicle, the Prius, in the US for the 2001 model year. The Prius used a twin-electric motor, inline axis, series-parallel, hybrid transaxle. This transaxle design has been part of the Toyota Hybrid System (THS), Toyota Hybrid System II (THS-II) and the Toyota Hybrid Synergy Drive (HSD) for the last 17 model years. This inline axis design uses two motors that share a common centerline or axis, Motor-Generator One MG1 and Motor-Generator Two MG2.

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