Motor Etiquette: VP and PD Explained

Hello,

Compared to the ALH and ASZ engines, I repeatedly notice that the PD engine, with its characteristics, is much closer to gasoline engines than the VP diesel engine (the AFN in the Golf 3 and the ALH in the Ibiza 6K, on the other hand, are very similar).

The ALH engine in the Ibi is still clearly recognizable as a diesel at speeds below ~1500 rpm, due to the noise and vibrations. The ASZ engine in the Polo 9N, on the other hand, is at least one class quieter – not only when it needs to provide power (which, compared to the ALH, mostly remains at the "should" level ), but also at idle.
And that's despite the fact that the Polo has already lost about 7 kg of insulation material from its interior, unlike the Ibi.

Interestingly, the intermediate throttle (ASZ) responds more readily when downshifting, for example, from 5th to 3rd gear at around 50 km/h. With the ALH engine, you have to keep your foot on the accelerator for a bit longer to bring it from 1000 to around 3000 rpm without any load.

I find it quite remarkable how much you can alter (or distort?) the character of a diesel engine to make it more like a gasoline engine.
I'd be interested to know what differences between VP and PD, and what the }causes} for those differences are. Is it simply the more favorable course of the PD pre-injection, or also the higher PD infusion rates, or is there something else that plays a role?

And of course: How do other TDI enthusiasts (in which vehicles) experience the differences in characteristics between the 1.9L VP and PD engines?
I know we've discussed this partially before, but I think it's still relevant here.

Hi Ulf,

your favorite topic

The issue with the intermediate gas might be related to the reaction of the clutch switches, which could lead to different effects. There was once a "tuning idea" to trick the engine into thinking it's always engaged by disabling the clutch switch, thereby deactivating the corresponding control system. Perhaps this is happening here when you give gas while disengaged (or are you doing it in a more traditional way, by releasing the clutch?).

Best regards,

Jan.

Hello Ulf!

So, I think that the combustion process in a PD engine occurs within tighter tolerances compared to a VP engine.
Due to the improved atomization (fuel-air mixing), more precise fuel metering, and more accurate injection timing, the combustion process occurs within much narrower limits. Furthermore, the atomization is already better at lower loads compared to conventional port injection (VP).
The higher injection rate you mentioned concentrates the combustion process within a smaller angular range, resulting in a higher degree of uniformity.

All these factors contribute to a very precise combustion process, which cannot exhibit as much variation as in the case of VP (Valve Pneumatics). The noise generation is therefore short and abrupt in the case of VP, whereas it occurs over a longer period with a wider range of frequencies.
I have deliberately written excitation spectrum here; the vibration spectrum of the engine with its auxiliary components and drivetrain is a completely different topic.

The sudden excitation of the PD (partial discharge) excites all the inherent frequencies, but the excitation is very short, so the oscillations (depending on the damping level) quickly decay.
In a VP (ventricular premature beat) with a rather undefined stimulation spectrum, which may persist for a longer period with modulated intensity, this can lead to various hum frequencies.

I hope that my explanations were somewhat understandable. If not, just ask again .

Regards,
Alex.

Jan6K wrote:

or do you do it the classic way, by releasing the clutch?

Exactly like that .

My first car was a Fiat 500 with a non-synchronized gearbox. Shifting normally resulted in corresponding noises, and that classic shifting pattern became ingrained in my movements...
"But I rarely drive the Ibi, so I'll pay more attention to the differences between when it's in gear and when it's not, on another occasion."

@Alex:
Good explanation, and it also seems plausible compared to gasoline engines: in those, the combustion is even shorter, and they run (a little ) more smoothly.

Hello Ulf!

The differences between being coupled and uncoupled are gigantic when it comes to VP.
I believe the PD engine is more negatively affected by fuel quality than the VP engine, as it is more sensitive to boost pressure and has been more severely restricted by stricter emission regulations.
My experiences with company cars have mostly shown that they tend to be less responsive to the accelerator (when the clutch is depressed).

The target value (throttle position) is so strongly dampened by the EDC when the clutch is disengaged that target value jumps are transformed into very harmless ramps .

But woe betide if the clutch is depressed, then things get very serious .

Regards,
Alex.