Teillastruckeln zu Werbezwecken? | Posts 16+

Hi,

We've had this discussion about driving at very low speeds before, and if I remember correctly, the general conclusion was that it's not necessarily bad, as long as you're not demanding extreme performance from the engine (e.g., driving at high RPMs or going uphill).

For my part, I often drive at 50...60 km/h in 5th gear. The ASV (automatic speed limiter) operates smoothly in this range and consumes approximately 3 liters per 100 km, according to the MFA (multifunction display), at 60 km/h in 5th gear. At 50 km/h, it's slightly more.

To maintain a speed of around 50 or 60 km/h, very little power is required, so the pressure on the pistons is not particularly high.

Best regards,

Jan.

Quote:

To maintain a speed of around 50 or 60 km/h, very little power is required, so the pressure on the pistons is not really high.

Hi Jan,

I agree with that, especially considering the values I see on my LDA.

At 50-60 RPM, there is hardly any boost pressure built up, meaning the pressure on the piston is roughly equivalent to that of an SDI (Single Direct Injection) engine.
However, the engine is designed to handle approximately 70% more torque, which translates to more pressure on the piston.

It might be different with naturally aspirated engines, which, as suggested, can actually develop the highest combustion pressures in the low RPM range when given a lot of throttle.

However, the inertial forces are still "almost zero" when you consider that they increase quadratically with the rotational speed.

In other words, at 4000 rpm in the UT (presumably referring to a specific testing or operating condition), the speed-dependent pressure on the connecting rod and the tensile force on the crankshaft are 16 times higher than at 1000 rpm, in addition to the gas pressure.
On the other hand, is the "almost exclusively gas pressure" effect at load significantly worse at 1000 rpm...?

'Mine also uses 3L/100km even when I'm driving in 4th gear at 60 km/h. And the car sounds much better doing that. It also runs smoothly, no question about it. But it's definitely not good for the engine. After all, the owner's manual also states under the 'break-in instructions' that you shouldn't drive at too low an RPM.'

You are absolutely right. When a new engine is being broken in, you shouldn't constantly drive it at extremely low RPMs. However, once the engine is broken in and operating normally, things are completely different, and I don't see any real risk then.

At 55 mph, I'm getting around 1000-1100 RPM, and the engine sounds healthy, just like it's running in 4th gear. The fuel consumption is quite similar, but the lower RPM should result in less wear and tear.

Here's something else that might be worth mentioning: In my opinion, an engine should "get used to" things like this. When I did this for the first time after the break-in period, it didn't sound very good; it could only manage 60 mph in 5th gear, but anything less than 55 mph was difficult. Nowadays, it's no longer a problem.

Best regards,

Jan.