Throttle Cut-Off: RPM Engagement Explained

Hi!

I only use engine braking for longer, unavoidable, and constant braking situations, such as on a long downhill stretch with a speed limit.

'My goal isn't to protect the regular brakes (an engine is more expensive than brake pads), but rather to reduce the idling fuel consumption to zero without turning off the engine (which is, of course, prohibited).'

The conclusion is that the engine should be allowed to idle at the lowest possible RPM to minimize wear and tear during thrust operation.

On a mountain that I regularly descend at a speed of 65 km/h, I now have the option of using engine braking in 4th gear (approximately 1800 RPM) or in 5th gear (approximately 1300 RPM).

Here's my question: At what engine speed, above the idle speed, does the torque cutoff become effective? Does it engage abruptly, or does it start gradually (softly) as part of the idle speed control system?

Or, to put it another way: Is the fuel cutoff a separate control system with a fixed switching point, or is it simply an extended idle speed control that continuously adjusts the fuel quantity until it reaches zero, while the engine speed is constantly increasing? Can the fuel quantity actually be continuously adjusted down to zero? (The spray pattern from the nozzle would likely be quite poor just before reaching zero fuel injection.)

Unfortunately, I don't have a power consumption display that could help answer this question (at what speed above idle does the power consumption become zero?).

Thank you in advance for your answers.

You're asking some really interesting questions, and I can only offer my perceptions and observations, so it's not verified knowledge! If anyone knows better, please feel free to correct me.

The MSG (presumably a device or system) distinguishes clearly between "clutch pedal pressed" and "clutch pedal released." This difference can be easily observed by giving a few short bursts of acceleration while the clutch pedal is pressed, or by doing the same while the transmission is in neutral with the clutch pedal released.
When the clutch is partially depressed, the driver's input signals are dampened (reducing jolts, a comfort feature). The engine responds less readily to the accelerator pedal and also revs down more slowly.
Since this is the typical behavior during normal driving, I believe there is no abrupt transition in "glide" mode (unlike when the clutch is depressed. Here, you can notice that the engine doesn't inject fuel again until around 900 RPM).
Therefore, I suspect that starting around 1200-1300 RPM, the injection amount gradually increases from 0 to the idle injection amount.
This should be verifiable using VAG-COM!

Furthermore, I am convinced that the engine itself experiences very little wear, even at higher speeds in coasting mode, because there is no ignition. It only functions as an "air pump," which does not put it under significant strain.
I always utilize the engine braking effect on downhill stretches and have never had any concerns regarding wear and tear.

Regards,
Alex.

P.S. The MSW can continuously adjust the injection quantity from 0 to the maximum injection quantity.

@ Arosa

Hey, your acoustic knowledge base is perfectly suited to answer all your questions.

- Park next to a house wall, start the engine and warm it up, then lower the window.
- If you give it some gas now, the SDI engine will start sputtering.
- If you lift your foot off the accelerator, it doesn't spray any fuel (which you can clearly hear by the absence of the ignition noise during the AKU test).
- When it reaches the low-RPM range, it starts sputtering again because it's injecting fuel.

The exact timing depends on the pump's characteristic curve, which can also be adjusted using the diagnostic system (see the Ruckel article).

The gear in which you should use engine braking depends too heavily on other factors (e.g., road conditions, engine temperature) for me to provide a recommendation in this forum.

The signaling system can actually achieve a true 0.0 mg/H setting, because the shut-off mechanism is essentially redundant; the motor could be stopped directly by the signaling system itself.

"Fuel cutoff" simply means that it is possible to inject absolutely nothing.
That's a characteristic that diesel engines have always had, and that's partly why they were more fuel-efficient back in the days of carburetor-based gasoline engines. Carburetors always draw in gasoline.
Nothing is done abruptly.

If you can, try logging the fuel flow rates using VAG-COM; that should help you understand a lot.
If the engine speed is higher than what the engine control unit (ECU) calculates based on the accelerator pedal position (and other components), the amount of electric power delivered is reduced. This reduction continues gradually until it reaches zero.
When you press the accelerator, it increases until some limit is reached: engine speed, torque, or accelerator pedal position.

If there are any questions, please don't hesitate to ask.

m;
The car, which I had to take to the workshop again on Sunday, needed another 500 km of driving before the problem could be properly diagnosed (and that after having already driven 300 km to get to the workshop, all on cheap gas *sigh*).

I can only partially understand that, as my clutch disengages below 1200 rpm.

But until then, the ALH will definitely be in a thrust-reducing mode, which is audible and visible (MFA).

When free revving, one must be careful. The engine control units evaluate the rate of deceleration. If the engine slows down gradually, the system will re-engage it later than if it decelerates quickly.
reason: the engine speed must not fall below the idle speed, therefore, when the speed drops rapidly, fuel injection is resumed earlier to prevent it from dropping further.
The same applies to gasoline engines (in fact, this has been the case since the beginning of engine electronics).

CU Gremlin.

Now, let's move on to control engineering.

PID controllers and their subgroups are the key terms here.

Meiner spirtzt auch 0 ein, wenn ich ihn beim Ranrollen an die Ampel leicht ueber LL bringe (z.b. im 2. Gang)
Otherwise, it starts working quite early (with free fall at 1300, and with 10 cents, the speed decreases very slowly ).

m;

Now we are going to discuss control engineering.

PID controllers and their subgroups are the key terms here.


Yep, but I didn't want to overwhelm anyone with that information here .

...which cannot be achieved with traditional PID controllers.

CU Gremlin.

Okay, so my automatic start-stop system engages and the car rolls, regardless of the gear, with a fuel consumption of 0.0 liters, at around 1000 RPM. Therefore, the "boost shut-off" definitely kicks in almost immediately above the idle speed, because the engine control unit (ECU) is only trying to keep the engine running, and at 1000 RPM, it doesn't need to inject any extra fuel for that.
Otherwise, the nail test can be performed well (if no MFA is present). Open all the windows (ideally, position the vehicle near a guardrail/wall/tunnel/building) and listen carefully for the sound of the nails.

@ Matthias

You can also hear the AKU even with the window closed!



m;

matthiasTDI96 wrote:

So, my ASZ engine idles, regardless of the gear, at around 1000 RPM with virtually no fuel consumption. Therefore, the torque cutoff definitely engages almost immediately above the idle speed, because the engine control unit (ECU) is only trying to keep the engine running, and at 1000 RPM, it doesn't need to inject any extra fuel. Otherwise, the nail test is easy to perform (if no MFA is present). Open all windows (ideally, position yourself near a guardrail/wall/tunnel/building) and listen carefully for the sound of the nail.

"Similarly, as soon as you release the accelerator and the engine speed hasn't yet reached the idle level, the MFA (Multi Function Display) shows 0.0, and the sound of the fuel injectors disappears. This allows you to, for example, coast downhill almost at 0.0 if you're parked near a hiking lodge on a constantly descending road and heading home."

Hi,

The same thing happens with the ASV - when idling, the value remains at 0.0 until it reaches just above the idle speed, at which point fuel injection resumes.

Best regards,

Jan.

Okay, first of all, thank you for the numerous answers. The conclusion is probably that I can also use the thrust cutoff at 1300 RPM.

You can also hear the AKU even with the window closed!!!

So, that's quite a serious accusation! It's not quite as bad with the AKU (vehicle inspection). I don't hear the ignition noise at idle and partial load when the windows are closed. not

However, a noise is constantly present inside the vehicle, even when the turbo is not engaged: a deep hum caused by the engine vibrations. I used to think that this hum could only be so strong when there was ignition. But recently, I turned off the ignition at 1300 RPM while driving downhill with the turbo engaged, and the hum remained completely unchanged.

Furthermore, I am convinced that the engine itself wears very little, even at higher speeds in coasting mode, because the ignition is not active. It only functions as an 'air pump,' which does not put it under significant strain.


Hmmm, but the compression pressure is also quite high, isn't it? Besides, there are parts whose wear is relatively independent of the operating pressures: piston rings and piston liners. I think that the slower these parts slide in the cylinder bore, the lower the wear will be, right?

Regarding compression pressure, I had another thought: My two-stroke engine has an intake manifold valve before the airbox, which I have disabled. However, if it were controlled to close only when the power is cut off, it could reduce the compression pressure and make the braking torque curve 'smoother' per kilowatt-revolution, thus reducing vibrations. The question is, how much vacuum can the airbox withstand? When I held the intake manifold closed with my hand while idling, it visibly collapsed.

Sure, here is the translation of the text from German to English:

'cu' is a Spanish abbreviation for 'con usted,' which means 'with you.' It is often used in online communication to indicate that the sender is available to chat or talk.

Here is an example of how 'cu' might be used in a conversation:

Person A: 'Hey, I'm bored. Want to chat?'
Person B: 'Cu!'

This means that Person B is available to chat with Person A.

'Cu' can also be used to indicate that the sender is thinking about the recipient. For example, if Person A is thinking about Person B, they might send a message that says, 'Cu.'

In general, 'cu' is a friendly and informal way to communicate with someone online.

Here's another crazy idea to make the engine cut-off even more fuel-efficient:

While the alternator is generating power during acceleration, it essentially provides electricity for free*. Therefore, during acceleration, you can push the alternator to its limits to extract even more of this 'free' energy.

What to do with it? Here's an idea: An electronic control could be used to lower the charging cut-off voltage or the nominal voltage of the alternator to 13V when the engine is running. This would be just enough to prevent the battery from becoming a power source.

In slide mode, the control system automatically switches to a 'brutal' 14.5V, allowing the battery to receive a significant charging current (significant, at least as long as it's not 100% full).

Of course, I understand that the fuel savings are only in the percentage range and also depend on the driving cycle. This special system won't make much difference when driving at a constant speed on a flat highway, but it will be beneficial when driving on a long downhill slope or when there are many frequent stops at traffic lights.

Furthermore, this energy recovery system requires that the battery's charge level is always kept below 100% so that it can still accept energy. However, this also means that this system is only useful in the summer, when the engine can be started safely even with very low battery charge levels.

The optimal result is achieved when you are a regular commuter, as you can estimate the charging cycle in advance based on the predetermined and recurring route. Idealized example:
Early morning commute, 20km uphill -> Battery discharges to 70% during the ride, and the alternator is practically not being loaded (controlled partial discharge).
End of workday: 20km downhill -> The battery is charged as much as possible using regenerative braking -> When I get home, the battery is 100% full.

cu
and a happy Labor Day to you all .

*free, insofar as this energy is inevitably available, and you have the choice to dissipate it only through friction or by converting it into electrical energy. In reality, however, you have already paid for this energy when driving up the hill due to the increased consumption on the incline. So it depends on the perspective .

Furthermore, I am convinced that the engine itself wears very little, even at higher speeds in idling mode, because the ignition pressure does not occur. It only functions as an 'air pump,' which does not put it under significant strain.


Hmmm, but the compression pressure is also not insignificant, right? Besides, there are parts whose wear is relatively independent of the operating pressures: piston rings and piston liners. I think that the slower these parts slide in the cylinder bore, the lower the wear, right?

With an air pump, the piston rings aren't subjected to the ignition pressure that you inevitably have when idling. I think it probably doesn't make a difference whether you're running the throttle cutoff at around 2,000 RPM or at 900 RPM in idle. Based on my personal interpretation, the former might even be better.

Best regards, WarLord.

In an air pump, the piston rings are not additionally stressed by combustion pressure.

Hi Warlord!

How can working pressure possibly affect the wear of the piston rings? Increased pressure simply presses the rings against the lower wall of the ring groove – that's all there is to it. Where is there any wear occurring?

Are the sliding speed and the radial pressure exerted by the rings against the cylinder liner the primary factors determining piston ring wear? (due to the spring tension of the rings)

In my opinion, the radial wear of the piston rings is solely dependent on the engine speed.

Perhaps there's an effect where high ignition pressures cause wear on the piston ring groove wall, as the ring acts like a hammer against the groove wall with each ignition, effectively 'loosening' the grooves. Do you think that's possible? However, I find it hard to believe that this would be significant, since the piston ring only has a very small amount of axial play in the groove. Maybe this effect exists. But, returning to the original question, it also suggests lower sliding speeds (e.g., 1300 in 5th gear instead of 1800 in 4th gear) within the context of 'thrust reduction,' which might be relevant.

Best regards,

Hi,

The idea about using a Lima is interesting, but it's likely to only make a very small difference, probably affecting a decimal place far out.

A fully charged 70Ah battery stores 12 * 70 Wh, which is 840 Wh, or 0.84 kWh.

Let's assume the ASV's efficiency is such that it consumes about 12 liters per 100 km at full throttle and around 200 km/h, which is 24 liters for 200 km in one hour. This translates to 24 liters for 81 kWh (which is approximately the power output, maybe slightly less, since it's above 4000 RPM). Therefore, one liter of diesel provides 3.5 kWh.

Now, let's compare this and see that a FULL battery charge is equivalent to just a quarter of a liter of diesel. Since your method might only utilize 1/2...1/4 or less of the battery, a complete cycle would save between 50 and 100 ml of diesel. To achieve this with a 90A alternator, you would need to run the engine for about a quarter to half an hour at maximum power.

Conclusion: It's not worth it; a much more efficient energy storage system would be needed.

Best regards,

Jan.

Yes, there's probably some truth to that. Moreover, the partially completed charging cycle is likely not beneficial for battery life. Lead-acid batteries actually last the longest when they are kept fully charged.

To achieve a noticeable fuel saving, one would likely need a crankshaft generator from Sachs in combination with large ultracapacitors. When starting, the energy from the ultracapacitors is used as a booster, with the crankshaft generator acting as a motor for a few seconds to increase torque. To allow the generator to capture as much energy as possible during braking, the internal combustion engine should be decoupled with an additional clutch.

According to research, such a generator would cost an additional 500€ from Sachs, if it were ever offered. The engine and transmission block would definitely have to be specially designed for it; it cannot be retrofitted. For engines with a displacement of up to 3 liters, a 12V on-board electrical system should be sufficient.

Ultracapacitors for vehicles are also available for purchase, but they start at a price of €500.

Best regards,