Hi, took a long 500+ mile trip in upstate NY with lots of climb ups and downs. On several occasions while rolling DOWN the slope with adaptive cruise control, the engine will get stuck over 3500rpm. Only after disengaging the cruise control the rpm would go back to usual 2k range.Has anyone else faced this issue?
CX-9 stuck at 3500-4000 RPM while going downhill with adaptive cruise control
- Thread starter new2maz
- Start date
Correct. Cruise control keep your requested speed when going uphill, but also prevents you from going over your requested speed when going downhill. Instead of constantly applying the brake when going downhill it tries to slow down by downshifting and using compression braking from the engine.
This saves your brake a bit and can prevent them from overheating. I do this manually often by selecting my own gear when going down a steep hill, even though this small engine does not provide that much in terms of compression braking capability.
Also this technique does not use more fuel (or only does so in a negligible way) because your throttle input is zero. (i.e. it revs faster, but there isn’t much fuel being sent to the engine if any at all).
This saves your brake a bit and can prevent them from overheating. I do this manually often by selecting my own gear when going down a steep hill, even though this small engine does not provide that much in terms of compression braking capability.
Also this technique does not use more fuel (or only does so in a negligible way) because your throttle input is zero. (i.e. it revs faster, but there isn’t much fuel being sent to the engine if any at all).
We are not used to it anymore with all the automatic engines keeping the revs so low for fuel economy purposes, but revving at 3500-4000 rpm with low throttle isn’t really damaging to an engine. The higher rev are not really hurting the engine.
It is actually harder on an engine to go full throttle at 2000rpm than mid throttle at 3500 rpm. It is counter intuitive, but going up a hill in 5th gear with the engine at low rpm is a lot harder on the engine than going up the same hill in fourth gear at higher rpm. Even if it makes more noise and sound like the engine is working harder, it is actually having a easier time.
It is actually harder on an engine to go full throttle at 2000rpm than mid throttle at 3500 rpm. It is counter intuitive, but going up a hill in 5th gear with the engine at low rpm is a lot harder on the engine than going up the same hill in fourth gear at higher rpm. Even if it makes more noise and sound like the engine is working harder, it is actually having a easier time.
GTEyes
2022 CX5 Signature - Machine Grey
- :
- NS, Canada
I've modified my cruising speed on the highway to avoid the annoying buzz the engine makes when downshifting on declines. Most of my commute is on 2-lane, 2-way highway with posted speeds at 100 km/hr. I used to go 105 km/hr for better fuel economy but found the engine did this downshifting behaviour on the declines (I think it actually drops down two gears when travelling at this speed), so I've bumped my speed up to 110 km/hr and now it seems to only go down one gear which isn't really noticable. Ironically, because of this "feature", I drive faster on the highway, which is actually harder on fuel. Go figure.
Recall the formula to calculate power > (torque [in lbs-ft] x rpm) ÷ 5252 = horsepower. Power gets work done. Torque is just a force. Increasing the rpm reduces the torque which means less twisting force on the rotating parts doing the same work but at higher rpms. (The constant, 5252, is valid only for the English measure of torque. The constant is different for the metric measure. No, torque and hp are not equal at 5252 rpm, that's just an artifact of the formula. The numbers match. Torque and power are different things and not subject to being equal.)
Just to add a clarification to how you explained it (I know you know but to make it easier to understand for others). Increasing the rpm doesn’t reduces the torque. Increasing the RPM reduce the torque required to do the same work.
It is like if you need to move a block a certain distance in a minute, you can give the block a single slow hard push (high torque, low rpm) or 10 fast small push (lower torque, higher rpm). The small pushes are easier on the engine than a single hard push.
Going full throttle is a different case, where you push as hard and as fast as you can. (High torque, high RPM, max power)
It is like if you need to move a block a certain distance in a minute, you can give the block a single slow hard push (high torque, low rpm) or 10 fast small push (lower torque, higher rpm). The small pushes are easier on the engine than a single hard push.
Going full throttle is a different case, where you push as hard and as fast as you can. (High torque, high RPM, max power)
