[FlatOut]

I think the guys on the ecomodder forum know the answer - that 'pulsing and gliding' is the most efficient way of using fuel to propel an infernal combustion engine. Such is the problem with using a nasty series of explosions to move us along - running at very low loads is inefficient, so a period of acceleration followed by a 'glide' is technically the cheapest way to travel.

Some engineering professors are suggesting that the limitations and poor emissions of explosion engines - as they call the ICE - will eventually cause the return of the 'steam' engine - the Doble steam cars of the 1920s were fast, quiet, reliable, economical and beat even California's 2020 tailpipe emission rules - which are incredibly tight.

[sgiitk]

Originally Posted by SS-Traveller

In most automatics, every time one lifts the foot off the throttle when in 'D', one coasts. Only when going downhill does one select a gear to get engine braking assist - and that's something not commonly done when going down a flyover, so coasting happens anyway!

This is why a prudent driver selects a lower range or the Sports mode to get the braking. I have owned ATs since 1999 so do have some idea.

[SS-Traveller]

Originally Posted by FlatOut

...an infernal combustion engine.

Couldn't help but burst out laughing. Was that typo intentional or unintentional?

[DerAlte]

Originally Posted by sgiitk

This is why a prudent driver selects a lower range or the Sports mode to get the braking. I have owned ATs since 1999 so do have some idea.

Unless the slope is 20% or more, a mode other than D is totally unnecessary. Sports mode will not have more engine braking than D mode. Only the acceleration profile and shift points are different in Sport mode, and engine braking is independent of that. Also, though forcing a lower gear may bring more braking (same as in MT vehicles), one uses lower gears going up a slope, especially in slippery conditions.

* If there is disciplined or no traffic, and Cruise Control is On, the CC will moderate the speed. For example, if the engine RPM is going higher than required for that speed (downslope acceleration), it will force a lower RPM by cutting off fuel (AT, MT) and / or forcing a lower gear (AT)

* If the traffic is not disciplined, like in India, the driver always has a foot on the brake pedal going down a slope - as readiness for eventualities

* If there is no CC, engine braking is always present during coasting. Otherwise, one would only require to give a burst of acceleration and travel literally kilometers on a level road! Once one lifts the foot off the acc pedal, the compression stroke is the drag and there is no further energy input from combustion. Even on a level road this is active. The deceleration depends on the speed when acc. pedal went to 0, which gear is active, weight of the car, air resistance and a couple of other factors. To put it coarsely, the car may still accelerate in 5th but not in 3rd.

[thoma]

Originally Posted by DerAlte

ECU can't do anything till you do something to come out of it.

May be I've not put the situation I encounters, clearly, in the previous post. I have a habit of using the engine braking a lot. On plane roads, say I'm driving in 3rd gear, I downshift to 2nd for a planned stoppage. Foot completely off the a-pedal all the time. In 2nd, the speed comes down quickly (heavier engine braking due to the limitation of the gearing?) as well the RPM in 2nd. And well before the idle RPM, the ECU takes over to avoid lugging (as you've mentioned). This is when the decelerating car launches forward a bit (if it was bumper to bumper traffic I need to brush the brakes here) and settles to keep on moving at a constant speed (no lugging too) endlessly without any throttle input. Condition: if the roads are plane. On inclines or in higher gear I agree that the ECU's little input is insufficient to prevent the engine from stalling.

So my doubt is whether the ECU kicks in above or below the idling RPM? I've felt it is above and that is why these posts.

[manim]

Originally Posted by DerAlte

...a mode other than D is totally unnecessary. Sports mode will not have more engine braking than D mode.

True, I have gone up and down the Mettupalayam - Ooty hill-roads 83 times in six years.
I have used only plain 'D' mode either going up or down. (after experimenting with 'S' and paddles).

Having driven only manuals earlier in my life, I find that the AT does a wonderful job, I was always in control and never felt the need for manual control.

To add, driving this stretch of road is highly stressful with frequent jams and lots of dangerous drivers.

[DerAlte]

Originally Posted by thoma

... And well before the idle RPM, the ECU takes over to avoid lugging ...

LOL I feel like saying (in typical courtroom scene style) "Objection, me lord, this is speculation"!

While your observation of the engine being made to maintain speed as much as possible may be / is right (I get the same thing in my Safari sometimes), this is an indirect anti-stalling measure - not allowing RPM to reach the danger band near idling RPM, where torque is low. Actual stalling tendency - which is in that danger band - is detected by severe deceleration of the crankshaft (load much greater than power produced at that time) - it can be handled by the ECU only in AT vehicles (via AT ECU). In MT the ECU can't do anything, and in severe cases (clutch not depressed) it just stalls with a thud. I am sure you haven't tried that, but you definitely can: by braking and not pressing clutch in the scenario you described, it will just go dhichka dhichka dhichka thud in 2 seconds - lugging followed by stall.

This is one reason why drivers of small goods vehicles with small-block diesel engines, like Tata Ace etc., don't want to ever drive on the left side of the road. With vehicle loaded, they can be in serious trouble in the usual conditions in the left lane - which for a major part is unending start-stop situations because of roads joining and other people stopping. This results in they irritating everyone else by driving at 30Kmph in the right lane, when everyone else can go at 50-60 steadily.

[FlatOut]

Originally Posted by sgiitk

This is why a prudent driver selects a lower range or the Sports mode to get the braking. I have owned ATs since 1999 so do have some idea.

I often knock the Mercedes back into 3rd, sometimes then into 2nd when coming up to a road junction, especially if downhill or rather too rapidly. It feels right - I also use the lower gears when descending very steep hills. Some may say this is a relic from the days when brakes could expire more easily - if nothing else, it evens out the braking force on a RWD car around all four wheels and wears your pads less quickly.

Before the 124s reach their top (4th) gear lock-up at around 85kph, the revs will drop right off (if in top gear) as you release the accelerator, allowing them to coast to a large extent. This is just one delightful aspect of Mercedes' abilities to match automatic boxes and engines beautifully well - another is the way the W124s set off in 2nd gear, unless you stab your right foot on the accelerator. This makes beautiful use of a large-ish 6 cylinder engine and the torque-multiplying effects of a torque converter. It is quite possible to set off acceptably quickly without exceeding 1500rpm in my auto-box 124 - and it makes the whole car more relaxing and significantly more economical. They succumbed to consumer clinics and sales-led engineering when they deleted this delightful aspect of their gearboxes - many found the rate at which they set off from rest a little to slow for their liking, and didn't like having to move their right leg so far to engage 1st.

VW group cars were cutting off fuel to their carburettors back in the 1980s, on the over-run. I tend to agree with Jeroen's comments about a car using less fuel with your foot off in a high gear, rather than selecting neutral which requires fuel to keep the engine ticking over. But of course, what you gain in fuel you lose slightly in speed with the drag of the engine. A bigger engined, heavier car with long gearing will benefit most from not slipping into neutral. It is possible that a small, light car with a tiny engine and low gears could do better off in neutral, since it may be slowed down significantly by the drag of the engine, and will use little fuel when idling.

In times of austerity in Britain, drivers would often coast with the engine switched off. Not a good idea with modern cars on congested roads.

This highlights the inefficiencies of an internal combustion engine, which is slurping fuel (and making the air foul) while motion-less in a traffic queue. Using regen braking to charge accumulators (hydraulic motors and gas accumulators are my preference to electric motors and batteries) which then propel the car from rest makes a lot of sense.

Originally Posted by SS-Traveller

Couldn't help but burst out laughing. Was that typo intentional or unintentional?

Quite intentional, SS-Traveller. Here in England, many engineers still refer to ICEs using the 'f' instead of 't' - harking back to steam days, when the ICE was loud, prone to breakdown, inefficient and a daft design in a steam engineer's eyes. Indeed, they were often referred to as 'Infernal Explosion Engines', highlighting their single massive drawback compared with an external combustion engine.

[sgiitk]

Originally Posted by DerAlte

Sports mode will not have more engine braking than D mode. Only the acceleration profile and shift points are different in Sport mode, and engine braking is independent of that.

Agreed. I did not feel like giving a detailed explanation. So in a Civic S means 4th and so on, unless over ridden. One can (and must) use the paddle shifters to force lower gears. Also, in other cars (like my old Santro) one had 2, and 1 available. This is why I always called the Santro AT as a 3+1 and not a 4. The reverse braking will be of the highest gear allowed in that setting. This is even described in somewhat stilted language in the Brio AT manual, where D3, 2 and 1 are available.

[saket77]

Ok, I tried an experiment last evening and I am posting it.

It was conducted on my 4 Stroke bike. I reached the speed of about 50 kmph in 5th gear and switched off the ignition. Here is what I experienced:

1. The rpm meter went dead reading '0' as soon as I switched off the engine.

2. With no clutch deployed, the bike sounded exactly like it sounds with engine running while coasting. (Even with the engine switched off).

3. The bike behaved entirely normal, i.e..the wheels> transmission kept moving the pistons without any judder that I expected. Felt just like normal coasting.

4. Later, just switching on the ignition, the engine came back to life with one little heavier judder.

Now what I think:

1. Since it was a carbureted version bike, I am sure that while normal coasting, fuel supply is not cut off and the fuel consumption is at the rate of an idling engine . ( in carbureted systems)

2. Since most cars today have an MID (& obviously an MPFI system), it shows lower fuel consumption, hence coasting does saves fuel; to answer the thread title. Its altogether a different thing that coasting in neutral will take you more far than coasting in a gear, but its not recommended anyway as the control on the vehicle is compromised. So, even if it would have saved fuel, I would not practice it (not recommend practicing it either) anyway.

3. Switching off a car while coasting in gear or neutral can be deadly, depending on circumstances. While you have lesser control on the car while coasting in neutral, switching off the engine means losing your vacuum assisted brakes and power steering as well, if you have one. While, switching off the engine while coasting in a gear will NOT result in loss of braking power as the vacuum will be still generated, but one would lose Electronic power assist to the steering. Also, switching off the engine while coasting in gear may damage the catalytic converter of the vehicle. Hence, switching off ignition or the engine of a running car in any circumstances is NOT recommended.

4. Technically, the ECU is NOT needed to supply ANY amount of fuel to keep the engine running while coasting in gear; but practically it does or not can be confirmed only by the members who are into the area of ECU programming. Even said this, I am sure that coasting in gear would lead to lesser fuel consumption because ECU will either cut off entire fuel supply or reduce it to a very minimal level.

Thanks for giving it a read,

Regards,
Saket

[DerAlte]

Originally Posted by saket77

... 1. The rpm meter went dead reading '0' as soon as I switched off the engine. ...

Bike electrical scheme is quite different from cars. In a bike, everything cuts off other than brake light I think

Originally Posted by saket77

... So, even if it would have saved fuel, I would not practice it (not recommend practicing it either) anyway. ...

Originally Posted by saket77

... switching off the engine means ... one would lose Electronic power assist to the steering. ...

That is the most dangerous part, as it compromises safety! Yet, many taxi drivers still do it. :(

[RM2488]

Originally Posted by DerAlte

Bike electrical scheme is quite different from cars. In a bike, everything cuts off other than brake light I think

Even the brake light cuts off. However, given that a motorcycle is usually carburetted, the fuel consumption remains the same. In a car, switching off the ignition, even in gear, will definitely cut fuel consumption to zero (No injectors, no fuel pump). This is purely an academic exercise- don't do it in practice. It's way more dangerous than coasting in neutral.

[Jeroen]

Originally Posted by saket77

1. Since it was a carbureted version bike, I am sure that while normal coasting, fuel supply is not cut off and the fuel consumption is at the rate of an idling engine . ( in carbureted systems)

Actually, not true for carburator engines. When using the engine to brake you aredrawing a higher vacuum which results in more fuel to the engine than just idling.
Jeroen

[vikram_d]

I'm hoping that people have gone through the entire thread. Anyway, posting link again for reference. The logic for ECM controlled/fuel injected cars is as follows.

http://www.team-bhp.com/forum/techni...ml#post2392914

[saket77]

Originally Posted by Jeroen

Actually, not true for carburator engines. When using the engine to brake you aredrawing a higher vacuum which results in more fuel to the engine than just idling.
Jeroen

You have got a point there Jeroen! I think you just provided some more spice to continue our discussion

I once read about CV (constant velocity) carburetors and can faintly remember that how important is the engine vacuum for the operation of this design.

It will be great if you could elaborate a little more on this aspect.

Thanks,
Saket