Transmission jolt

Thats normal...

 

I think that's normal, my 2010 does this too (and does it in other gears too, sometimes even Park, when the ICE comes on). This is because there's no clutch, and the engine is permanently connected through the planetary gear set and electric motors. MG1 has to suddenly adjust its speed when the ICE starts and stops to compensate for the sudden start or stop of the ICE, and a bit of this shock can be felt through the drivetrain at times. I often feel a similar slight jolt while driving, which feels like gears shifting in a conventional transmission, but in the Prius, you're actually feeling the ICE start and stop. You may notice this when you step harder on the gas pedal, and also when you coast downhill and your speed reaches 46 or 47 MPH (at this speed in Gen 3, the ICE always starts, even if no power is needed, to avoid over-revving MG1). This thread may help you understand how it works: This is for Gen 2, but Gen 3 is similar except it has no chain: http://priuschat.com/forums/gen-ii-...sion/29352-introduction-prius-power-flow.html

 

When you switch to 'R' with the ICE off, it will be smooth, but at the initial time of start-up its normal to feel a slight jolt or vibration from the trans-axle, whether you are in 'R', 'D' or PARK...

 

Sir Isaac Newton's laws on conservation of momentum and energy.

The engine has mass, and sudden changing of RPM (being spun from 0 to 1000 RPM by the electric motor, then being fed gas and spark and suddenly producing its own power during startup), results in a corresponding force that must be absorbed by MG1 and the planetary gear set, and if the match isn't perfect, you'll feel some of this torque as a slight jolt, both because there's no clutch, and the sudden surge of power can even make the engine shake within its mounts - just as any other car engine does when its started.

In other cars, you feel slight jolts as the gears shift and the engine speed changes, but in the Prius with its "continuously variable" transmission you don't feel these, but instead you feel the ICE start and stop because ICE start is a rather sudden event (the ICE can't idle at extremely low RPM). Because these start/stop events happen at different times than a conventional transmission shifts gears, you have to get used to feeling this at unusual times, like in reverse.

It's a complicated computer-controlled balancing act, but I imagine it's hard even for the computer to react to the precise moment the ICE fires, and how much of a kick it gives as it fires up. While you're backing up and driving the car with MG2, MG1 is starting up the ICE, then must quickly react and stop driving the ICE when it fires.

Here's another thread that discusses ICE start: http://priuschat.com/forums/gen-ii-prius-technical-discussion/96528-question-how-engine-starts.html

 

"Transmission jolt" is a bad name for this phenomenon. Since it is caused by the ICE starting, it might be better to call it "engine starting jolt", or something to that effect. It has nothing to do with the transmission, except that the transmission transmits the torque pulse to the drive wheels.

Tom

 

How about "Power Surge" ?

 

If I understand the system correctly, if the engine is already running and your foot is on the brake, nothing mechanical happens when you shift. No gears are engaged, no clutch is thrown, nothing moves. You've merely set a control mode that will be used once the car is told to move, a mode that will control the speed and direction of the engine and the two electric motors so as to move the planetary carrier forward or back. This begins only when you lift off the brake, which has the effect of stepping on the accelerator a bit (a simulation of creep for a normal car). Even then, gears simply start spinning - there is no engagement of teeth, bands or anything like you might expect in a normal car. Driveline snatch, perhaps.

In other words, there should be no jolt at all (if your foot is on the brake and the engine is already running). The worst case jolt is when the engine starts up from cold while you are backing; in my unit, I don't feel a real jolt, just a general minor shudder (although it is huge compared to all other disturbances in a Prius).

 

It's becomes obvious that some Prius owners are very delicate if you listen to them talk about conventional automatic transmission's neck snapping jolts.

 

If this is the result of taking up slack in the driveline (I guess you're calling this driveline snatch), it should not happen just in reverse, but might depend on if you're starting uphill or downhill (which may determine if the driveline has any initial slack or not). I wouldn't expect this much play in the driveline on a new car (I don't notice any on mine), so I'm not sure if what you're describing is anything out of the ordinary or not. The Prius should be very smooth going between Drive and Reverse, without noticeable jolts.

 

OK, glad you got an answer.

 

In that case you can minimize the jolt by pressing the brake before shifting into D or R. Parking pawls are designed to keep cars from rolling away. By necessity, they all have some play. Firmly pressing the brake before shifting into gear will remove the load from the pawl and keep your Prius from moving.

Tom

 

If Prius engine/transaxle mounts are designed like they are in most other cars, the resonant frequency of the engine/transaxle mass on the mounts occurs below idle rpm. So, no matter how the engine is started (conventional starter, flywheel motor or MG1) the engine has to go through this resonance to get to an operational rpm and the transition will be noticeable.

 

Yes, that's a good point. I'd never thought about this for a car engine. I was once in the jet engine business, where resonances are intolerable in the operating range. If you listen to jet engines start up, you might hear pronounced resonances on the way up to idle speed (CFM56's in 737s seem to be especially loud). It's a huge part of jet engine design to make sure nothing resonates, although in this case its all the little bits inside, not the whole engine (the aircraft manufacturer is responsible for that sort of thing).

 

I never felt the jolt until I was turning on a side street going uphill and the car surged as if I was rear ended. At first I thought I was rear ended until I looked into the rearview mirror and saw nothing. It was the first and the last time I felt "the jolt".

 

Or set the parking brake/ensuring the car is stationary and won't roll before engaging Park, that will ensure there's no load on the park motor.. After that then having your foot on the brake before disengaging the parking brake and engaging a gear position..