I was riding in a car recently and noticed the engine speed changing fairly erratically - first from the sound, then from the tachometer. The owner of the car, who was driving, told me that this is because it was equipped with a CVT, and it had done this forever - since the test drive and in the years he’s owned it.
Now, it seems to me the point of the CVT is to allow the engine to operate close to its most efficient speed all the time - a discrete transmission has the same job, but coarser resolution in the engine speed to wheel speed mapping. But surely wild, continual fluctuations in engine speed aren’t especially efficient?
Why do cars with CVTs experience these swings in RPM? Is there some ambiguity in the control system?
For a gasoline engine, the best-efficiency RPM depends on how much power you’re trying to deliver. For a given power output requirement, a low-to-middling RPM is generally better because heavy throttle application is required (remember, power = RPM * load).
Note that “best efficiency” is not always what the driver wants. When you stomp the accelerator to the floor, the ECU has been programmed to understand that efficiency is no longer the top priority. Instead, it will adjust the CVT’s drive ratio to get/keep the engine up into the RPM range associated with maximum power output, efficiency be damned.
I would expect a smooth blend between those two extremes, so that the closer the accelerator is to the floor, the more the engine moves away from max-efficiency RPM and toward max-power RPM.
Bottom line, if the driver is very unsteady on the accelerator pedal, a CVT-equipped vehicle is likely to have its RPM vary quite a bit. I can attest to this; a relative owns a CVT-equipped vehicle, and his throttle management results in exactly this engine behavior.
What kind of car? I own two cars with CVTs and I don’t experience the wild fluctuations you describe. The closest I’ve experienced to what you describe is that in one vehicle when going down hills it will kick up the RPMs a bit to keep the car from speeding up.
I do know that Nissan has had some issues with their CVTs. I’d recommend to the owner that he/she join a message board specific to that model vehicle and poke around a bit. Chances are good there is lively discussion about theCVT including what’s normal and what’s not for that particular vehicle.
My wife has a 2012 Nissan Altima with CVT. I’d say that the RPM changes much less than a conventional automatic. The RPM will change some when going from a flat road up a hill whereas a conventional transmission may stay at the same RPM for the speed until it struggles to a downshift.
My Insight had a CVT but acted like a regular transmission. I could still sort of feel it ‘shift’. It was almost as if they programmed it to run through imaginary gears.
Also, it’s possible it’s low on CVT fluid, he should check that out. They do act wonky when that happens (low or dirty fluid), IIRC RPM fluctuations are one of the symptoms of needing the tranny looked at as it can start slipping. If the engine suddenly (for any number of reasons) doesn’t have any resistance, it’s going to race. If the car is under warranty, I’d get it looked at or ask them to take it for a test drive the next time it’s in for an oil change.
What kind of car, maybe there’s people here with a similar one that can tell you if it’s common or not.
I read that for the first CVTs, consumers interpreted the lack of “jump” when you lay on the gas as the engine having no power. So they did exactly what you say – introduce some lag so it has the same “feel” as a regular transmission.
It always bugged me though, and maybe it’s because I understand how a CVT works.
Remember that commercial a few years ago for a car with a CVT. The guy pulls out of his parking spot, the girl grabs her lipstick but waits for the car to get up to speed to put it on, clearly assuming the car is going to shift through a few gears, but it never does (until the guy lets off the gas a few times as a joke). My car never ‘glided’ like that, so to speak. It bugged me because I paid money for a CVT, but I might as well have just had a traditional 4 speed AT that I wouldn’t have had to replace the ATF (actually, very expensive Honda CVT fluid) at about 36,000 miles.
For those asking about the car model, it’s a Nissan mid-size SUV (the Murano, I think). It’s about five years old. And these fluctuations happen even when cruising down a country road - often, but not always, after adjustments in power by the driver.
Seems like it might be a mismatch between controller settings and operator inputs?
I have a Subaru with a CVT and there are no erratic fluctuations. The RPM is dependent on speed, incline and throttle position, but quite predictable. Depress the throttle and the engine goes faster to deliver more power. Let off the throttle and the RPMs decrease. Cruise at a steady speed on a flat road and the RPMs are steady. I have heard that Nissan’s CVT is not as well liked as Subaru’s, but I don’t have any firsthand experience with a Nissan CVT. FWIW, I like the CVT in my car, as it tends to keep RPMs down while cruising on the highway.
