MPG with C/Control on

[Dieselman]

I'm smiling as I am typing because if you use engine braking then you will surely be changing down ......... which means the engine will rev higher for the given speed which will mean that the engine is using more fuel??

Remember using engine braking on a modern 5, or even 7 speed box will require downshift or shifts to have any real effect and then the engine will be revving faster which to my understanding will use more fuel than a slower revving engine.

John

Nein....

When decelerating and the engine revs are higher than 800rpm it will use no fuel as the fuel is cut off on the over-run..

 

[Dryce]

Remember using engine braking on a modern 5, or even 7 speed box will require downshift or shifts to have any real effect and then the engine will be revving faster which to my understanding will use more fuel than a slower revving engine.

Modern cars cut the fuel when you lift off the accelerator and there is enough momentum to keep the car moving and the engine turning.

If you coast down hill in neutral with the engine idling and brake you will in principle use more fuel than just letting the car coast down in gear with your foot off the accelerator.

This certainly applies to petrol engines and I assume it applies to diesels too.

 

[Number_Cruncher]

>>This certainly applies to petrol engines

Going one step further, modern petrol engines on the overrun not only cut the fuel, but also open up the idle control valve / rotary idle actuator / drive by wire throttle to reduce pumping losses - yes, the engine braking effect is reduced, but, the vehicle's kinetic energy is not wasted in pumping losses.

 

[glojo]

Modern cars cut the fuel when you lift off the accelerator and there is enough momentum to keep the car moving and the engine turning.

If you coast down hill in neutral with the engine idling and brake you will in principle use more fuel than just letting the car coast down in gear with your foot off the accelerator.

This certainly applies to petrol engines and I assume it applies to diesels too.

I accept what you are saying but if the gearbox changes down to third gear to take advantage of engine braking then are we saying the engine is not taking ANY fuel? If it is taking sufficient to keep the engine running then I accept the car is not under load but surely an engine freewheeling at 2000rpm will still burn more fuel than an engine over running at 3500 rpm?

I have NO idea of gearing ratios but at 60mph going downhill in fifth gear there will be very little retardation and the engine will be revolving at X rpm. If we put the car into possibly third gear to get retardation then are we saying NO fuel goes to the engine, or the exact same fuel as when it is in its highest gear? If the engine is turning at 3500rpm and is receiving a very restricted amount of fuel is this healthy? These are questions that I am asking to better educate myself and not being asked because I know the answer.

I am from the old skool and as an old fashioned HGV driver we had to engage the correct gear at the top of the hill that was correct for the slowest speed we wanted during the downhill journey and that engine needed fuel!! I accept times have changed hence my need to edumakate myself.

The nitty gritty though is that the owner's handbook for all our modern Mercedes all quite clearly state brakes are used by the cruise control to reduce speed. It then goes on to suggest if not enough braking is applied the driver should consider changing down and use engine retardation. Surely we all must agree with that?

I take on-board what Number_Cruncher is saying and that would perhaps support the handbook that states the brakes are applied?

Incidentally if we find out the handbook is wrong then great, lets show it and I will write to Mercedes and tell them there book is wrong.

Regards
John

 

[stevesey]

Fuel cut off on overrun has been common place since fuel injection became standard with the introduction of cats. Although several years ago I did read about it not being done because of emissions issues (possibly to meet some specifc regulation in very specific conditions - as generally not using fuel is pretty good for emissions).

 

[Dieselman]

Fuel cut off on overrun has been common place since fuel injection became standard with the introduction of cats. Although several years ago I did read about it not being done because of emissions issues (possibly to meet some specifc regulation in very specific conditions - as generally not using fuel is pretty good for emissions).

Because the thermal shock on the Cat was shortening life. Hot, cold, hot, cold, etc.

Our old Fiat had over-run fuel shutoff on the carburettor. Webber TLDE carb T'was the first to have it.

 

[stevesey]

Because the thermal shock on the Cat was shortening life. Hot, cold, hot, cold, etc.

Our old Fiat had over-run fuel shutoff on the carburettor. Webber TLDE carb T'was the first to have it.

Was thinking I could remember some carbs having it, just none I've fiddled with (much easier with ECU/injection).

 

[Dieselman]

It used the idle jet fuel shutoff and had a control ecu so when it detected the engine at higher revs with the throttle closed, it shut off the fuel. Came back on at 1200 rpm. Worked a treat.

It was a very advanced car for it's age, it had a semi automatic choke which you pulled out then pushed in on start and it took over, also a manifold heater...not forgetting Child Killer Electric Windows.....

 

[Number_Cruncher]

>>are we saying the engine is not taking ANY fuel?

Yes! - absolutely none. The injectors on a petrol engine are simply not activated during overrun fuel cut off. All the engine is doing during this period is pumping fresh air.

The engine won't suddenly stop dead because there's no fuel being injected - it will simply take the required power to keep turning the engine from the vehicle and driveline.

The (petrol) engine expends energy in drawing the air past the throttle.

The engine expends energy in compressing the injested air.

The compressed air gives some of that energy back in what would have been the power stroke.

These losses - the pumping of air in and out, and the energy lost as heat from the air compressed in the cylinder is the effective mechanism by which the engine acts as a brake during over-run.

As all of the energy used drawing air past a partially closed throttle is lost, and only some of the compression energy is lost, it's better to open the throttle on overrun, and allow more air to be compressed to reduce the engine braking to a minimum.

 

[glojo]

So there is no chance of doing the old explosions in the exhaust system anymore??

 

[Number_Cruncher]

>>and that engine needed fuel!!

Actually, it didn't. But, fuel systems of the time couldn't just automatically switch the fuelling off.

Mercedes full size trucks since the 70's or so until modern computer controlled fuel systems were fitted in the mid / late 90's have used an an air operated exhaust brake mechanism which also (via another pneumatic cylinder) moved the fuel rack in the injector to the no fuel position.

In the trucks of the era which I think John is talking about, you could do the low-tech thing, and pull the stop cable while on overrun, and not feel any significant difference.

 

[Number_Cruncher]

>>doing the old explosions in the exhaust system anymore??

Alas, no!

 

[glojo]

>>are we saying the engine is not taking ANY fuel?

Yes! - absolutely none. The injectors on a petrol engine are simply not activated during overrun fuel cut off. All the engine is doing during this period is pumping fresh air.

Hi,
So are we saying that on a petrol powered fuel injected manual car that is going downhill at 50mph the driver having taken their foot off the throttle, they then put their foot on the clutch..... The engine is dead and allthe warning lights come on?

I am asking this to get things straight, I am certainly not out to trick anyone or be clever.

John

 

[Dieselman]

Hi,
So are we saying that on a petrol powered fuel injected manual car that is going downhill at 50mph the driver having taken their foot off the throttle, they then put their foot on the clutch..... The engine is dead and allthe warning lights come on?

The fuel switches back on as the revs drop below 1000rpm with the throttle closed.

 

[Number_Cruncher]

>>put their foot on the clutch

As the clutch clears, the engine slows down quickly - but, once the engine speed drops below a threshold, for example, 900 rpm, the ECU gradually switches fuelling on again.

You have to be fast to be faster than an engine management computer that monitor and respond to signals from all the engine sensors, and can individually time the spark and individually fuel each cylinder at full engine speed!

 

[Ted]

Soooo, (and, like John, genuine question here)
On my way to work in the morning there is a long downhill stretch where I can take the car out of gear and coast, while keeping up a safe speed (manual car).
Will I use more petrol doing this or staying in gear on a whisper of throttle and frequent times on overrun?
Ditto the above when coming off on a slip road - do I coast in neutral to the give way sign or just back off the throttle?
And last of all, to be most fuel efficient, do I let the engine run freely a 40mph in fourth, or labour slightly in fifth?
TIA.

 

[Bobby Dazzler]

Don't coast!! It will use more fuel and you will have much reduced control of your vehicle. Nothing to be gained - much to be lost!!

 

[Diesel Benz]

Hi Dieter,
I have no idea of how the system works but this is an extract from the 211 manual

Cruise control

Cruise control maintains a constant road speed for you. It brakes automatically in order to avoid exceeding the set speed. On long and steep downhill gradients, especially if the vehicle is laden or towing a trailer, you must shift to a lower gear in good time or select shift range 1, 2 or 3 in vehicles with automatic transmission*. By doing so you will make use of the braking effect of the engine, which relieves the load on the brake system and prevents the brakes from overheating and wearing too quickly.

We use Cruise control on long journeys and really like it so I have no issues either way, but I wonder how much engine braking there is at 60mph in the highest gear when going down hill?

Regards
John

Glojo, is this an extract from your own manual or did you pick it up from the current on-line manual (is your S211 a face lift model)?

The attached page shows what my W211 manual says. Nothing mentioned about braking.

While I would not take any wikipedia info seriously, specifically if it was not written for one particular car model, I would not take the user manual text too literally either. Like my pre-face-lift W211 does brake when I drive downhill with CC, but it does not use brakes, only the engine (even if an SBC car could easily use brakes). Still my manual does not mention (engine)braking.

The face lift car might use brakes but I'd like to see that explained in WIS or a similar document, the user manual may be written in a simplified form, taking into account average readers (so far I do not take any position for the face lift model behaviour, even if it does not have SBC brakes, the new brake hold system would allow the car to use brakes on its own).

 

[Diesel Benz]

Soooo, (and, like John, genuine question here)
On my way to work in the morning there is a long downhill stretch where I can take the car out of gear and coast, while keeping up a safe speed (manual car).
Will I use more petrol doing this or staying in gear on a whisper of throttle and frequent times on overrun?
Ditto the above when coming off on a slip road - do I coast in neutral to the give way sign or just back off the throttle?
And last of all, to be most fuel efficient, do I let the engine run freely a 40mph in fourth, or labour slightly in fifth?
TIA.

My opinion is that you would use less fuel if you select neutral. You save fuel by braking with the engine instead of using brakes but any form of braking means lost energy. But if the gradient is reasonable, you will end up gaining too much speed and then it is better to use engine braking (at least to the extent that fuel injection is cut off). The point is that even if the engine would not use fuel, you gain less miles when the speed is not increasing, instead of gaining speed, the energy preventing the speed increase is spent by the engine (pumping air against the resistance at intake and exhaust).

For the steady speed case it must be better to use the highest gear. Look at the fuel efficiency plot I posted earlier. If you estimate the engine load at 40 mph and map the engine speed for the two gear options, you would most likely see that the engine load does not reach the point where efficiency starts to degrade. From fuel consumption point of view, in practise it is best to drive with the highest possible gear up to "almost full load" (pedal almost floored). For small 4 cylinder engines this may result to inconvenient vibrations but that is another issue, even that does not mean higher fuel consumption.

 

[Rovinghawk]

surely an engine freewheeling at 2000rpm will still burn more fuel than an engine over running at 3500 rpm?

If it stays at 2000rpm whilst freewheeling then I'd suggest that the whole cruise control economy issue is a bit of a moot point & you need the engine looked at. Wouldn't it drop to 800rpm or thereabouts?

RH