LMM Acceleration: A Detailed Explanation

Hello everyone.

"In practice, wavy acceleration often occurs in conjunction with a faulty mass airflow sensor (MAF sensor), or it disappears after replacing the MAF sensor."
So far, I haven't understood what's behind it, but now I had the following thought:

If the mass airflow sensor (MAF) reading (compared to the actual airflow) has already dropped to the point where the soot limit is being triggered, the injection quantity is precisely adjusted for each reported milligram of air mass.
Now, the airflow rate is also crucially dependent on the boost pressure, so that if the mass airflow sensor reading is too low, the boost pressure essentially directly affects the injection quantity, bypassing the soot limit.

Now, if we allow the boost pressure to fluctuate slightly when accelerating or during full-throttle operation (overshoot or normal regulation), we essentially create a "boost pressure popometer."

The resulting "feedback loop" involving boost pressure, fuel injection volume, and the amount of power required to drive the turbocharger will naturally amplify boost pressure fluctuations, and consequently, the boost pressure control system will have to work harder to compensate for these fluctuations.

The same applies, of course, to cases where the boost pressure is too low and the mass airflow sensor (MAF) is functioning correctly.

However, if all subsystems are functioning correctly, the soot limit at full throttle and high boost pressure will be higher than the torque limit.
In other words, fluctuations in boost pressure and air mass do not affect the injection quantity or acceleration – unless the air mass drops to a point where the soot reduction system is activated again (for a brief moment).

From your perspective, is this theory sound, or are there any problems or inconsistencies with it?

Hi Ulf,

That sounds plausible to me!

Best regards, Rainer.

@ulf,

I think so too.

Hello,

It's good that you see it that way too.

Okay, here's the million-dollar question:

Why don't healthy turbocharged gasoline engines also tend to exhibit surging acceleration?
In this case, the amount of fuel injected is likely even more closely tied to the boost pressure, because it's crucial to maintain a lambda value of 1 as precisely as possible.
Or do gasoline engines with turbochargers also exhibit wave-like behavior, but are drivers just more relaxed about it than we are?

@ ulf:

Quote:

In this case, the amount of fuel injected is likely even more closely tied to the boost pressure, because a Lambda value of 1 must be maintained as precisely as possible.

IMO, that's not entirely correct. The lambda value is only kept at 1 during partial load operation to prevent oxygen from entering the catalytic converter and to avoid the reduction of NOx within it.
Under full load, the engine can definitely be operated with a richer mixture, i.e., a Lambda value of 0.9 or 0.8. This is also better for cooling the engine and prevents the catalytic converter from being hindered in its reduction process!

This means that the Lambda regulation doesn't need to be as strict for gasoline engines as the soot limit is for diesel engines, because the engine can still operate reliably even with a Lambda value less than 1.

Regards,
Alex.

donalexo wrote:

At full load, the engine can definitely be operated with a richer mixture, i.e., a Lambda value of 0.9 or 0.8.

This is also better for the engine's internal cooling and prevents the catalytic converter from being hindered in its reduction process!

This means that the Lambda regulation does not have to be as strict for gasoline engines as the soot limit is for diesel engines, because the engine can still operate properly even with a Lambda value less than 1.
Hi Alex,

I somehow thought that the days of "dirty" full-load enrichment in gasoline engines were long gone... but now it's clear why TDIs can sometimes feel more sensitive.

Thank you for the tutoring }.

Hello everyone!

A few days ago, I had a very interesting conversation with an engine control unit developer from Bosch, specifically regarding the 1.8 Turbo engine used in current VW, Audi, Seat, and Skoda cars. Under full load, it is indeed true that the engine tends to run rich, as described. However, if the turbocharger becomes too hot (almost glowing!), a so-called component protection system is activated. In this case, the turbocharger is cooled internally with fuel. This can reduce the lambda value to as low as 0.6. As a result, fuel consumption increases dramatically (12-14 liters), but the excess fuel lowers the temperatures, preventing engine damage. The reason so much fuel is injected is that a slight increase in the injection volume increases the turbocharger's boost pressure, and therefore, beyond a certain point, even more fuel must be injected (specifically for cooling).
Best regards, Frank.