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My 2010 Prius also lurches forward when slowing and encountering a rough surface. It did cause me to bump a pedestrian in a cross walk. I reported it yesterday when I had a recall software update. At first the guy acted like he never heard it before, then when I started to show him on my iPad he admitted I was not the first customer to report this issue. He said I could make an appointment to have it checked which I declined as it appears Toyota has yet to solve the problem.
As an EE (electrical engineer, of the electronics type, with patents on high speed circuits), in the design division of a light rail government outfit, I work alongside the electric motor traction-power team. I have discussed this Prius braking & overall engine-power control with the traction power, software types & electronic types of the EE types. As a Prius owner myself, I have experienced its odd braking behavior. First, I'd recommend Toyota dealer sales persons to stop telling the public the Prius has a "CVT" (continually variable transmission). It has no such thing. My daughters Nissan has one, & the CVTs are turning up in a number of the high MPG small cars. But to my knowledge, no Toyota hybrid on the market has a CVT.
A Toyota dealer near my home, "claimed" to me, that taxi companies are getting 400K miles on the Prius with no major repairs, including its "transmission". The Prius has no transmission.
Not having a transmission is both a blessing & a negative. The longer a person has driven regular cars, sticks & automatics both, the more weird a Prius will seem, & the more weird it is to learn to "control" the Prius. Add to this problem, that the "parallel-hybrid" has its inherent complex problems that can be solved, but only with the very best engineering. The Prius is a partially solved parallel hybrid.
Not having any clutch for a Prius, both removes a maintenance cost, & build cost. But it introduces a very noteworthy problem on “unintended torque to the tires”, as it lacks a true "neutral".
There are governmental entities actually considering laws to force a mechanical clutch into this type of vehicle. Lacking a clutch, (either manual or auto), then the electronics are left as the only way to keep unintended torque from the tires. If the Prius had its anti-lock/traction-assistance breaking feature turned off, this would help some, but introduce new issues.
Toyota almost certainly chose the "parallel-hybrid" method, as it delivers the best MPG, for the least engineering cost & lowest manufacturing cost. And lacking a transmission, it can also boast a very low failure rate of its only transmission like device; the "PSD" (power sharing device). A PSD has no slipping parts & no gears or belts to change spin ratios. All regular transmissions, that most of us are so familiar with, have slipping parts, & gears or belts.
As soon as I, (a hard core engineer type, aka nerd) began to look into the design of the Prius, I find critical components given strange marketing names like PSD (power sharing device). Also Toyota does not voluntarily tell the world how the car works. The best data is published by reverse engineering enthusiasts.
The PSD is a marketing name for a mechanical “differential”. The Prius also has typical mechanical differential, like all cars to apply power switching to the left or right front wheels. Thus, between the rubber tire contacting the road, & two sources of torque (force that moves the vehicle) are;
1) the gasoline engine (ICE) & its MG1 (the ICE’s shaft connected electric motor) &
2) the other MG2 (the bigger of the two electric motors),
that enter the first differential, that Toyota calls a “PSD”.
Thus there are TWO mechanical differentials in series, the PSD & the final differential that is closer to the tires. I’m an EE, not an ME, but in my opinion is that two differentials in series, gets a little squirrely, mechanically speaking. That is, mechanical differentials are not perfect, in their function of how humans need them to behave.
Differentials do have “sticky” behaviors. Generally, most of us drivers barely notice that sticky or squirrely mechanical differential behavior in a regular car or truck. But when two mechanical differentials are in series (like in a Prius), things do get noticeably stickier. “Sticky” regards the inertial behavior of the parts in a differential. The Prius exacerbates the differential’s imperfect behavior, with the motors limitations, & no-clutch, that creats an uncommonly high rotational spinning mass, for such a light vehicle.
I’d like to see a “ratio of spinning power-train-mass/vehicle-mass” comparison, for a Toyota Corolla & a Prius. And I’d bet the Prius has almost four to eight times the spinning mass, that is directly connected (no clutch, auto or manual type) to the final-differential & tires. The final differential is the last one before the tires.
Add the two series mechanical differentials, with no clutch to provide a true “neutral” & what Mr. Barr calls “spaghetti code” & overloaded stacks, & yikes, you have a too slow reacting computer, trying to deal with the two-series-differential & no-TRUE-neutral effect, & the list of additional parallel-hybrid issues below.
The additional issues are limitations of the inductance of the electric motors, that slow down the motors ability to change output, to a torque change request, & the permanent magnets (that improve power efficiency) that spin, that also unfortunately reduce the number of mechanical field positions that can be put in a 360 degree circle, & you have a recipe for jerky mechanical behavior at the tires, as to the needs of the driver in traffic & imperfect road conditions.
I’ll stick my opinion out into the wind here, & make a guess that there is not a good computer design requirements document for the engine-drive-train-control-computer. At least not a detailed one that states all signals, what the signals must do, & how much time (micro-seconds) that the functions must happen within. Also the engine, the motors & differentials, tires, mass of the vehicle, all need to be mathematically simulated into a complex control system. That’s no small task.
If engine-drive-train-computer decisions that are being made in 100’s of milliseconds can be made in 1 millisecond or less, that improvement should be pursued.
For instance, a buddy if mine, another EE nerd, of the FPGA type (field programmable gate array) expert, works for a subcontractor of General Electric, to make FPGA’s that control the jet engines of the biggest planes.
The jet engines need to achieve the best possible thermal efficiency (lowest fuel consumption for hoarse-power ratio, aka watts), but operate on the edge of compressor-stall, to achieve that goal. The solution is the FPGA, to compute the equations in nano-seconds, that a typical small computer would complete in 100 milliseconds (a tenth of second). FPGA’s use logic parts (like AND & OR gates & flip-flops), that are hard wired, for ultra fast decisions. Whereas the computer of the Prius is a general purpose processor, with a program written for it.
Back to the Prius, 30 mph, is 44 feet per second. Just 100 ms equals 4.4 per second feet at 30mph. Consider that a computer control output to a motor or breaking device, comes 4.4 feet, after the driver would have wanted that to happen, is not a good thing. Add the unwanted computer control delay to the overly “sticky” behaving components of the drive train, & the result is a lot of jerky, sticky, squirrelly…. (I can’t come up with better lexicon here) behavior that will happen at the tires, as compared to what the driver is expecting.
None the less, I still like the Prius, as it cut my fuel bill considerably.
Great post, thanks. A recent house guest of ours was complaining about the brakes on his '06 Prius (and six tows for various issues). We test drove a V just this week and crossed it off our list. The problem reports or how it drove weren't really the concern, we just weren't wowed by the space utilization. Not boxy enough (the C-Max gets similar EPA ratings with a boxy design; of course owners have complained that they can't match the EPA ratings).
I may change my mind about the Prius V after the next generation arrives later this year. And I'd ask you about tin whiskers and SUA, but I get enough flack around here about my conspiracy theories LOL.
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