While going downhill at approximately 80-90kph, when I release the throttle pedal I noticed my car would automatically downshift from 4th to 3rd. When I step on the throttle again it goes back into 4th. It doesnt happen at lower speeds.

At first I thought it might be an anomaly but a quick search online suggests that this is a feature of modern auto'boxes.

I find this quite interesting and thought I'd share


*My car is an Axia

5 year ago I do have waja campro auto, downhill auto downshift from 4th to 3rd end up the fuel consumption really sucks, because from 4th downshift to 3rd RPM from 2k to 3.5k really really bad fuel consumption lucky that vehicle no longer drive

Until today I not even know is that normal or part of the design

This is a good practice. For manual car also, going downhill need to use lower gear to slow down car. Continuous on brake will heat up brake fluid causing brake failure. THis is what happened to lorries and buses accident claiming brake failure. I guess the driver has no knowledge. Probably not mentioned in driving school

this feature is what they call HDC (Hill descent control).. similar emulated effect of downshifting for engine brake

Looks like you also have no knowledge. Auto car is by design and manual drivers probably just using brake.

Heavy vehicle very fast to get brake hot unlike car unless the hill is steep. If you watch F1, they are very cautious if brake too hot. Cannot stop fast. If too cold also not optimum.

HDC is not really engine braking, but use ABS braking system. This is to control at constant very slow descent normally on rough terrain

Its a safety feature done by ECU ,no worries it is normal in auto gearbox.

Okay so is part of the design but the waja effects too obviously really bad fuel consumption, then my driving style always normal hard to reach 100km/h and at normal 80km/h road not doing any close distance not much brake apply, sometimes downhill no vehicle ahead of me I try throttle maintain the speed or throttle off auto downshift begin dang , except if I throttle more during downhill but is not safe at all

End up sell it 8 year old only 60k km been drive XD

The ECU algorithm done by Mitsubishi, are you saying mitsubishi did a bad job? When you say bad, can you give the number of how bad was the FC. Anyway you have sold your car.

The waja last time I drive not Mitsubishi engine, under campro then taxi driver told me Mitsubishi engine version have no related issue that what i heard so in reality I'm not sure since I don't own the waja under Mitsubishi engine, fuel consumption ah erm every month I have to pay extra 150 for waja compare to my old altis , and the old altis have much more smoother experience on road compare to waja

The only complaint for the waja just the fuel consumption and the downshift issue the rest I'm happy with it ,

Some autoboxes have it, some don't. Those that don't, you need to manually shift to 3/S/L gear to engage it.

Waja engages it automatically on downhills, Persona 4AT doesn't, Vios doesn't. All Proton CVT does it.

For most part it is a safety feature, like when you go down from genting the road signs tell you to engage low gear - it's the same thing.

Rolling down hill with high RPM doesn't consume more fuel than idling. High RPM caused by engine braking is not the same as high RPM caused by throttle.

You Waja fuel consumption is caused by simply because it is Campro.

I used to own a Mitsu Waja and it does the same thing, downshifting when going downhill.

This post has been edited by dares: Oct 23 2021, 10:32 AM

This is correct but I wonder how people say higher consumption.

The problem is staying at D causing the car to go downhill too fast but shifting to 3 makes the car go slower than necessary.

So using lower gear then need to press gas pedal causing high rpm maybe the cause but not much power needed by engine still. I usually stay on D and apply brake once in a while though you still need to judge whther 3 or D gear is the best depending on slope.

But all the autos I have driven dun auto downshift in idle downhill unless told to (i.e. D2,D3) - unkers own ride is manual sendiri can apply on demand la... The ECU know its going downhill meh? Or measuring speed vs no throttle? Mebbe is new feature kot. Unke rprob diving 20yrs old auto haha

This post has been edited by Dezs: Oct 24 2021, 06:45 PM

becos increase rolling resistance. during slope bmw eco mode straight changr into neutral kasi free gear je. it is proven low rpm save fuel.

not a new tech. just not being implement widely

Are you sure change to neutral? I don't think so. Even pro drivers said DO NOT cruise down hill in neutral. This is dangerous practice. There is reason to it.

They said Axia but like you all my auto car do not downshift. My Axia or MyVi also .... maybe I just did not go down Genting or Cameron Highland to test

It is easy for the car to detect. It can sense the engine is dragging the car or being push by the car.

First of all, understand what is engine braking.
Once you understand it, see this youtube link from Engineering Explained.

https://www.youtube.com/watch?v=_bZlb62VVlw

There is 1 exception unless your car is older type which use carburetor, then it will consume higher fuel as it is not control by ECU.
But modern rides, usually will cut off the fuel when doing engine braking.

This post has been edited by littlefire: Oct 25 2021, 09:16 AM

speed above 120kph. rpm below 1000. which gear from zf8 is design for that u tell me.
bmw get stars for top safety (dangerous??) and fuel saving (ppl say??). there is reasong to it

inject "less" fuel i believe. cut off i dont think so lah.
u meaning down hill the exzos no smell (no fuel) at all?

Per video from Engineering Explained, the engine cuts the fuel injection when the car moves down a hill; at various times, the engine will use no fuel at all to power the engine, since the wheels themselves—connected to a transmission—make the engine spin.

How do you know if the car is not put in N? Below 1k rpm is engine on idle which normally when car in P or N position.

Safety rating is just rating. It is still not safe if you drive dangerously. Do you know that coasting (or going downhill in free gear N) is even illegal is some states in the US?

yup just theory.
and turning key off will stop faster rite. why not equal when both are same condition cut off

becos i drove one and it do improve fc.

official channel mention usa version got coasting function ya

Not just theory. If you have OBD scanner, you get get fuel flow rate from the ECU. When engine braking downhill with no throttle, the fuel flow rate is 0.

Yes BMW coasting mode will disconnect the transmission from the engine.

But by doing so, it will need to continue to inject fuel to keep the engine running compared to engine braking which requires 0 fuel.

On the flip side, engine braking will slow down the car and you will need more fuel to re-accelerate back to speed, which the BMW coasting mode doesn't thus saving fuel.

It's a give and take, and either one would work depending on the condition.

But knowing Europe's strict emissions regulation, this coasting function is probably designed to meet those requirements instead of being a genuine effort to save fuel. Like those silly cylinder deactivation tech.

This post has been edited by dares: Oct 25 2021, 11:10 AM

Because those people mistakenly assume lower gear = high fuel consumption

Then give throttle to accelerate to force it to upshift
Then car too fast on downhill, lift off and brake
Then back to step 1, trying to accelerate again

They themselves is the one to blame for the high fuel consumption. Think about it, with 4AT even when drop to 3rd gear on let's say Genting downhill, it would still be 80km/h+ at 3000-4000rpm if you don't apply brake at all. That's plenty fast for normal road user.

Even when I go down from Genting with 7 speed DCT, I also manually downshift 5->4->3->2 approaching the bump. Or 5->4->3 when approaching most hairpin.

the auto downshift thing, normally you fake brake and it will kick in.

for low gear at down hill, it won't increase consumption, if it does, it is your car issue due to when downshifted, your oil valve is not open if you are not accelerating

Actually both are correct, and both are more efficient in respective use case. In the modern days of advance electronics with multiple axis accelerometer to detect acceleration, pitch angle, yaw angle, the ECU will decide coasting vs engine braking.

ECU make decision for efficiency and safety, that the decision would work for most road user. Unless one try to race downhill, or mistakenly assume lower gear = more fuel and do something funny to override it.

Today even 4.0L Audi RS6, RS7 have 48V mild hybrid. The PHEV and 48V cars would coast with engine off. Downhill all modern cars will do engine braking or/and regenerative braking, with 0 fuel used.

Here's something counter intuitive.
Fuel flow rate has directly relationship with throttle body opening, which influence airflow rate/volume. Raise of Engine rpm is the result of increase airflow, and ECU inject more fuel due to more air to maintain close to 14.7:1 ratio (give and take direct injection car would burn super lean at low load, all car burn rich for high load to reduce temperature and prevent knocking). That's why on race series like GT3, TCR, WTCR, BTCC racing etc that don't get standard engine design like F1, everyone get air restrictor (essentially a plastic cone, or metal plate with standard diameter) for balance of performance.

This post has been edited by constant_weight: Oct 25 2021, 12:37 PM

^thanks for ur support.
i pick free gear becos the gliding sensation feel goods.
while engine braking is useful when im racing mood

and i always getting max stars in eco pro mode

I personally think you maybe confused people a bit lah.

Your car would coast during relatively flat or minor down slope.

Definitely not downhill, maybe anything more that 2-3% gradient it would switch to engine braking instead.

Everyone must be thinking about Genting downhill.😅

There is another smarter (At I least think it is smarter) implementation of coasting that doesn't disconnect transmission from engine.

What it does is simply keep the throttle body wide open and don't inject fuel. This case, the engine rpm still high but car would glide pretty long distance, although not as far as transmission disconnection.

Essentially when we do engine braking we are using air to brake. With throttle body fully closed, the pistons are pulling vacuum. Wide open throttle will remove the restriction.

One thing for sure is again if the driver trying to act smart by shifting to neutral him/herself. The engine burn extra fuel for both coasting and engine braking cases literally render the technology useless.

This post has been edited by constant_weight: Oct 25 2021, 01:37 PM

Not really, race car is race car, normal cars cylinder pressures and heat are no where near those race cars you're relating to

On Coast cruise, even with high RPM, injectors typically fully turn off, regardless throttle opening.

And if close, negative pressure induces more engine braking as the RPM can no longer be maintained.

Just ignore the coasting scenario here for a bit. Let's assume the driver is stepping on throttle.

The intention is to tell people engine rpm only has indirect relationship with fuel consumption, not direct relationship.

We have no control on engine rpm. We only control amount of air when we step on throttle. The corresponded fuel is base on the air, and engine rpm is the effect.

The reference to race car is to show with air restrictor, on ball park, that is the max amount of air everyone get. Try to put bigger turbo or whatever, the max performance can't deviate far from the average. Air is the key to to fuel consumption, engine speed is the result.

Aye, I also wanna highlight the amount of air we also no control in modern DBW cars, but that's a topic for another day as you deviate to cruise control/stability controls/hill whatever assist/anti stalling assist (I appreciate this!)/auto rev matching (auto also got ya!) etc

The reality is modern cars take out a lot of driving than what most people think and worse, how "good" of a driver they may think they are.

That's true!!! Bloody bastard electronic throttle.
Most of them are trying to make our life easier, some are super annoying. One example is rev hang, especially on low gears during manual shift.

But think deeper, in the old time people need practice and training to handle 300hp.
Now 200hp+ is entry level, 600hp+ family saloon is not abnormal. Without those electronic assist most of the modern car are probably un-drivable, at least not for normal driver.

I think for flat roads or slight downhill gradient, the coasting function would be more efficient.

But for steeper downhill, engine braking would be more efficient because we are using gravity (which is free) to move the car forward while using no fuel.

Maybe it is not done because even with WOT the TB and valves are still restrictive and the pistons will still be pulling some amount of vaccum? Definitely not as much as closed throttle, but I'm guessing enough to cause deceleration.

Lol reminded the first week of so after my low power car ownership, hero mode turn off everything and spun out even at low speed from traffic light launch.

Lucky no one else was involved and only a slap on the wrist lesson learned fixing back front/rear bumper and wheels scuffed up.

noob