An Engineer Explains the Toyota Prius
We were enjoying our monthly “Electric Vehicle Coffee, Cake and EVs” session at the Bracken Ridge Tavern in Brisbane, Queensland. Brian, a new member of the group, got my attention by telling us a story about hitting a kangaroo on his drive from Winton to Charters Towers in north central Queensland. He then said he was driving a Toyota Prius. I was really curious now. As an engineer, he was able to answer some of my questions about why Toyota didn’t make the leap from the Prius hybrid electric vehicle to battery electric vehicles, and also to share the difference between driving the Prius and his new Hyundai Ioniq BEV. This is what he told me:
To an engineer, the original development of the Prius was a fascinating process, particularly the interaction between the engineering and the business model. Toyota handed over the design to a board member who was a brilliant engineer and gave him carte blanche — development money no object!! There were 3 main areas which had to be developed individually & then work together optimally:
1. Battery and Battery Management System (BMS). Batteries were not part of the Toyota skillset, so they tied up an agreement with Panasonic as the principal manufacturer of nickel–metal hydride (NiMH) batteries. This chemistry was the current “state of the art” in battery chemistries, and logistics in raw materials was unlikely to be an issue at that time. However, the anticipated lifetime of batteries at 200–300 cycles was insufficient, so a BMS had to be developed to substantially extend this. The technique used to extend battery life was to break the battery into a large number of smaller modules and use computer control to ensure that the normal operating state of charge remained within the range 30% to 80%. That is, no battery module was allowed to be overcharged or to discharge completely.
2. Transmission. This needed to combine the attributes of the internal combustion engine (ICE) and electric machines in a complementary way. The ICE motor gives high energy and torque over a narrow speed range. The electric motor gives maximum torque at zero speed (ideal for fast takeoff) and allows energy to be recovered and put back into the battery during braking, thus reducing overall energy consumption. The Toyota solution was to develop the electronic continuously variable transmission (eCVT). This is the only CVT with no sliding parts, so has a life expectancy comparable with that of a fixed ratio gearbox and is unique in that respect. Toyota tied this up with patents so that other manufacturers were forced to find other (less effective) solutions to the transmission problem, giving Toyota a major competitive advantage. In simple terms, the eCVT was a fixed ratio gearbox with one input from the ICE engine, one input/output from the electric motor/generator, and one output/input connected to the wheel(s). The direction and proportions of energy flow were controlled in operation continuously by computer to optimise performance.
3. ICE Motor. Pure ICE vehicles use the Otto thermodynamic cycle, which has the widest usable operating speed range. Hybrids generally use the Atkinson cycle, which is much more efficient but has a much narrower operating speed range. In the original Prius, Toyota optimised sizes and power-to-weight ratios of the 3 major components such that the Prius became the most efficient people mover on the road. The battery was just large enough to provide regenerative braking and starting torque, but gave minimal (or zero) “battery only” range. It also could not run on ICE-only mode (as I found to my chagrin when the battery eventually died and the car was donated to a training college).
I feel that this became the Achilles heel of the Prius, as it could not evolve into a pure EV when technology allowed — or even efficiently evolve into a plug-in hybrid electric vehicle (PHEV). Toyota argued that the earliest Prius model was sold at a loss and that they only broke even on the second model. The “Hybrid Synergy Drive,” which was the combination of the eCVT and the power electronics, and computer systems were easily transitioned to other car models (Corolla, Camry, RAV4, etc.), so this allowed profit taking with minimal extra development.
Toyota had put all their eggs in the hybrid basket, and when lithium battery technology eventually resulted in the BEV becoming the optimal solution, Toyota had no competitive advantage. Pure EVs have starting torque much better matched to requirements, and regenerative energy range which is much more usable. (I particularly noticed the latter when returning from Toowoomba down the range — the Prius very quickly would reach 100% battery capacity requiring friction braking for most of the run. There is always plenty of room in the Ioniq battery.)
From the viewpoint of an engineer, the things I most like about the BEV are:
- Zero emissions. I charge at home and pay extra for purely renewable power (so the trolls cannot accuse me of using any coal-fired power). I initially became aware of the problems associated with CO2 emissions in 1965.
- Simplicity and elegance of design and lack of moving parts. This means low maintenance cost.
- One-pedal driving means that I am unlikely to replace the brake pads in the lifetime of the car. (Although, I only replaced them once in 400,000 km in the Prius.)
- Energy efficiency. On a long trip I actually spend more on coffee than I do on energy!!
- Maximum torque at zero speed. Even in ECO mode, the EV slips into spaces at a busy roundabout that would be impossible in an ICE car.
- Stop/start driving on the Bruce Highway during congested peak hours has at least the consolation of hardly any energy consumption — unlike ICE.
Featured image: 1993 Toyota Prius, courtesy of Toyota.
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