I had a customer yesterday, express concern over the way their CVT (Continuously Variable Transmission or Constant Velocity Transmission) felt when compared to a relative's conventional automatic transmission.
The main concern was that the car felt sluggish, conventional wisdom would suggest that the smaller engine on the Civic Hybrid is the source of the concerns. One must figure in the way a CVT works and the instant torque provided by the IMA (Integrated Motor Assist) Motor. The HCHII can keep up with (or outpace most) traffic if watching based on one’s position relative to the other cars on the road, or based on the speedometer. Judging by feel will make the car seem slower than it actually is due to lack of shift shock.
On a conventional automatic transmission, there are four, five, six, or now seven separate gears with separate gear ratios. The Civic Hybrid when equipped with an automatic transmission uses a CVT transmission; specifically it uses a CVT with two cones and a belt. The belt is tensioned by the two pulleys, which can vary their diameters together to provide different gear ratios. They do not need to disengage to switch ratios, but on a regular automatic the engine must temporarily be disconnected from the wheels while the car changes gear ratios (by changing the physical gears)
On a conventional AT (Automatic Transmission) when you want to go faster, the engine must also turn faster (excepting overdrive and torque converter lockup to some degree). When you get out of the engine speeds where the engine’s peak power output is, you must change the gear ratio to get the engine back into its power band to continue to accelerate rapidly.
On a CVT when you demand large amounts of acceleration the transmission and engine control systems will rev the engine to one of its sweet spots (where it can make the most power for the load demanded) and hold it there (Constant Velocity). The transmission will vary the gear ratios many times per second, mostly independent of vehicle speed until the power required decreases (continual variation of gear ratios). Depending on if you are looking at the gear ratio or the engine speed provides for the two different meanings come from for the acronym.
When accelerating briskly the AT will start revving the engine up and when it leaves its power band, will shift gears dropping engine speeds back to where it makes the most torque several times. The period where the engine slows, and is temporarily disconnected from the wheels will be felt as either a small or a large jolt depending on the quality of the vehicle (luxury cars tend to minimize the jolts) this is shift shock. The CVT will rev the engine to the sweet spot and hold that RPM until the desired speed is reached and the driver backs off the throttle. The lack of shift shock tricks you into feeling that you are going slower than actuality. Glance at the speedometer and you will most likely be travelling faster than desired.
IMA shakin’ it up in the Hiz-ouse
IMA further confuses the feelings because the motor can drop torque into the mix instantly and if the driver is used to a regular CVT system may notice that the engine doesn’t rev to the same sweet spot every time. This is because the electric motor is assisting the main engine with its workload.
Imagine playing tug-o-war, if there are three people on your team and six on the other team each member of your team has to work extra hard to have a chance at winning.
Now add 6 children to your team, everyone on your side has to do much less work, still a fair amount if the other team does not cut the kids any slack, but not as much as without their help.
The IMA Motor is the 6 children, and for 2006-2009 (The Eighth Generation Civic) the IMA Motor would equate to let’s say 6 teens on the weight lifting team, and can power the vehicle on its own for short periods.
Continuing the metaphor, the adults can rest for a moment while the teens hold the opponents at bay, whereas the children would be unable to maintain against the load.
Therefore, when transitioning to an IMA Hybrid with a CVT transmission or another hybrid with a CVT style transmission acceleration will feel extremely slow, but when measured acceleration will be normal, or peppy. You just need to drive it for a while to get used to it. Then when you ride along with someone (yes, the person with the expensive luxury car too) even the slightest hint of shift shock will feel like a novice driver on a manual transmission grinding away at the gears.
How to cope
Give it time and this will feel normal just as the conventional AT feels normal now.
for the Midlands Hybrid Club
"Promoting the spread of clean personal transportation technologies."
My fuel economy, tracked at every fill-up since purchase, your mileage will vary.
The Midlands Hybrid Club, the home of the hybrid club and hypermiling of the Carolinas, based in Columbia, SC.