Lurch when engine turns on from EV mode while parked

Some motion is unavoidable, given that the thing that cranks over the engine to start it is ... the transmission.

My Gen 3 liftback does something nifty when cranking in Park, that my old Gen 1 did not do. The Gen 1 would simply crank the engine, and as the transmission was in Park, the lurch would only go as far as the parking pawl would allow, then kind of bounce back.

But my Gen 3 liftback applies the brakes, as a way to further restrain the motion. It still moves, but it's not so much of a free-roll, bonk-into-the-park-pawl proposition. Not that it eliminates the motion, but does reduce it.

Being essentially Gen-3-like, a v would do the same, I assume. If you're listening closely, you can hear the brakes being applied (a soft click-squish sound) just before the engine cranks, click-unsquish after it has started. (Once you recognize that click-squish sound, you can also impress acquaintances with your psychic ability, by knowing when the engine is going to start, a moment before it does.)

 

MG1 doesn't mess around when it cranks the ICE like those puny 12V starter motors in gassers. It accelerates a fairly large mass around its axis very quickly. So that's a little like the torque reaction you'd get by snapping open the throttle on a BMW boxer twin or a P-51 Mustang. (I've read that you shouldn't do that close tot he ground in a Mustang as you might hit the ground with a wingtip.) That by itself won't make the car lurch, but it's noticeable.

Only this torque reaction is turned 90º so it feels different. And then there's the drag inside the power train that @ChapmanF already pointed out.

 

Well, it really does transmit force to the wheels; it's not just a torque reaction, like in the Mustang. MG1 is connected to the power-split planetary gear in the transmission, and it is only able to give the engine a shove because it is shoving against the ring gear (connected to MG2 and the wheels).

The car has a few different ways it can try to minimize the lurch. Most obviously, it can try to calculate the right amount of electrical power to send to MG2 at the same time, to resist the torque created by MG1. That's the only method it has available when it starts/stops the engine while rolling down the road, and it works fairly well; you notice if you're paying attention, but it's not super lurchy.

When you're parked, you're more likely to notice any little lurch; at the same time, there are additional ways to keep the car from moving. Naturally, with the parking pawl deployed, the car doesn't have more than a few inches it can move, but that feels like bonking into a hard stop. So with Gen 3 they added (or maybe it started in Gen 2, I've never checked) the automatic application of the brakes. I'm sure they still do also send opposite power to MG2 to resist the motion. It's a tricky problem! They work pretty hard to minimize the lurch, but can't get it to be imperceptible.

They could reduce a torque reaction by having MG1 take a more leisurely time spinning the engine up to 1000 rpm. But the engine mounts may be tuned to keep the natural frequency for engine vibrations somewhere below 1000 rpm, to keep those vibrations suppressed in normal running ... so a slower spin-up through that resonant region would leave you noticing more vibration, even if the lurch was less.

 

Set the parking brake.

 

A slower spin up when starting would likely make the transmission damper rattle everytime. I know my gen2 sounds like the hubs of hell when I trigger the "compression test" mode at 250 rpm. Probably also better for emissions to spin it fast before enabling fuel for startup.

Posted via the PriusChat mobile app.

 

Hell has hubs?

 

Atlanta.

 

Yep, and when they wear out the get REALLY noisy.

Posted via the PriusChat mobile app.