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.

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

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

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.

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.