TDIs: More Powerful, Especially on Dynos

Hi everyone,

While on vacation, I drove my mother's gas-guzzling car, which felt a bit underpowered and heavy for my taste.
Without access to a computer or diagnostic tools, I estimated the approximate time for the 0 to 100 km/h acceleration test in a way that minimized wear and tear on the vehicle.
First, determine the speedometer reading at the gear shift points (6000 rpm in 1st and 2nd gear, at the beginning of the red zone).
Then, I timed the car in first gear, accelerating from a standstill in neutral up to 6000 rpm.
Then, I timed the duration in 2nd gear, accelerating from the 1st/2nd gear shift point to 6000 rpm.
Then, in second gear, I accelerated from the 2/3 point to 100 km/h, and I stopped the stopwatch.

This test, of course, does not include any clutch slipping, and the potentially time-saving "brutal" start (revving the engine without load and abruptly engaging the clutch) is also missing. Instead, the test starts at approximately 5 km/h instead of 0 km/h. Nevertheless, the sum of the three times should be roughly comparable to the catalog value.

The acceleration times for the gasoline version were 13.2 seconds. Based on the catalog value, and adjusted for weight (due to optional equipment), a time of approximately 11.2 seconds should be achievable.

To evaluate the reliability of this testing method, I also tested our ALH-Ibiza on the same track (shift points at 4500 rpm). The result was 10.1 seconds, compared to the manufacturer's specification of 12.1 seconds.

The trend is undeniable: Gasoline engines perform approximately 18% worse than advertised, while TDI engines perform about 16% better.

A subsequent dynamometer test of the gasoline engine revealed an actual power output of 80 kW. Converted to standard conditions (1013 mbar, 20°C), the gasoline engine produces 88 kW, whereas the official specifications state 95 kW.
The actual 80 kW instead of the advertised 95 kW corresponds quite closely to the ratio of 0-100 km/h times, as indicated by the weight-adjusted catalog value and my own tests. The Ibiza was not put on a dynamometer.
The dynamometer operator (from a company like RMS Tuning or something similar in Kössen, Tyrol) mentioned, by the way, that most standard naturally aspirated engines are about 5 to 10% weaker than the specifications indicate.

In the test bench report, I particularly noticed the significantly lower air pressure compared to the standard (1013 mbar): 940 mbar, where each millibar less results in approximately 1 promille of lost power.
Once again, the obvious: engines are weaker at high altitudes.
This also applies to turbocharged engines where the boost pressure control reference is the ambient pressure (e.g., wastegate turbochargers with a standard diaphragm housing without electronic correction).

Apart from high and low-pressure systems, the air pressure decreases by 1 millibar every 8 meters of altitude.
So, if a test bench is located, for example, at an altitude of 800 meters, a correct measurement will effectively compensate for the altitude-related power loss, essentially bringing the reading back to sea level. This means that, according to standardized performance figures, the engine receives back the approximately 10% of power that is lost due to the altitude.

TDIs regulate their boost pressure as an absolute value, which means they always have full cylinder filling and therefore full power up to a certain altitude. Above that altitude, the boost pressure, and thus the power, is reduced using an atmospheric pressure sensor to prevent the turbocharger from over-revving.
Therefore, if a TDI engine is placed on a dynamometer at an altitude of 800 meters (where the full boost pressure is still applied), the automated performance calculation system of the dynamometer will also grant it a 10% power boost, even though it doesn't actually need it.

I'm quite certain that at least part of the excessive power output of the TDI engines, as indicated by dyno tests, can be attributed to these factors, and of course, the calculation error increases with altitude.

On the other hand, this does not invalidate the DZR (Dynamic Stall Ratio), because the air pressure is only used to correct for air resistance, but not for the power calculation.
Contrary to what one might expect, the additional power of the TDI engine, which is usually present according to the DZR (German type approval regulation), seems to be confirmed, at least in our Ibiza, by the 5 to 100 km/h acceleration test.

hello ulf

It's not just about the boost pressure alone. The performance of a turbocharger can only be assessed in relation to its efficiency. And that efficiency is not the same at 800 meters above sea level, even with the same boost pressure. Due to the 'thinner air,' the turbocharger has to operate at a higher speed to generate the same boost pressure.

A higher RPM at the same boost pressure, however, means a lower efficiency and, consequently, a higher proportion of unusable hot air, which should be reflected in a lower engine power output.

It must also be considered in the back-calculation whether the engine itself has already reduced the injection quantity per piston stroke and, consequently, the overall power output due to the increased intake air temperature. Without the corresponding amount of fuel, an excess of air is no longer beneficial!

Furthermore, your assessment may not apply to newer TDI engines, such as all PD engines. These engines have an additional map that limits the maximum injection volume based on altitude.

I believe that the TDIs actually have more horsepower than what is stated on paper. Measurements with k=0 regularly confirm this.

Hi everyone,
'This doesn't seem to align with the 20% air excess and the engine's robustness. What's the manufacturer's or developer's benefit in reducing the engine's performance at relatively low air pressures (in mountainous regions)? Such a measure could only be explained by widespread failures of older, unregulated turbo-diesels or turbos in mountainous areas. And that would then prove that these components are operated very close to their performance limits in the lowlands, a limit that is exceeded with just a few meters of altitude. In my opinion, this doesn't make any sense. If a turbocharger has to build up a boost pressure of 1.2 - 1.3 bar every time the accelerator is pressed, and then is regulated back to 0.9 bar, it should also have to rev extremely beyond its 'normal' speed each time. And it does. I can clearly hear it in my ALH 90PS engine. How harmful can a slightly increased speed in the thinner air of the mountains actually be?'
On the other hand, there's also a certain German thoroughness. If something can be regulated, it will be, regardless of the purpose.
Regards,

Eike.

audifan wrote:

The performance of a turbocharger can only be assessed in relation to its efficiency. And the efficiency, even at an altitude of 800 meters, is not the same as at sea level, despite having the same boost pressure. Due to the "thinner air," the turbocharger has to operate at a higher speed to generate the same boost pressure. A higher RPM at the same boost pressure, however, means a lower efficiency and, consequently, a higher proportion of unusable hot air, and this should be reflected in a lower engine power output.

Hi.

Generally speaking, that's probably correct, but the performance of a TDI engine seems to degrade less quickly than that of a (petrol) naturally aspirated engine when the ambient air pressure decreases.
Probably because the slight increase in exhaust back pressure for a more powerful turbocharger charge is less significant than the effect of lower external pressure reaching the cylinders --> resulting in less air mass being injected, limited by the soot control map.

Quote:

Furthermore, your consideration may not apply to newer TDIs, such as all PD models. In those models, there is an additional map built in that limits the maximum injection quantity depending on altitude.

Are you sure about that? At what altitude (or, more precisely, below what external pressure) do PDs shut off the valve?

Quote:

I believe that the TDIs actually have more horsepower than what is stated on paper. Measurements with k=0 regularly confirm this.

By doing so, you are agreeing again to the last paragraph in the original post .

I'm currently working on an extension of the DZR (presumably a software or tool) to allow for the determination of rolling or frictional resistance (roughly corresponding to the traction power on a dynamometer) through experimental measurements using an additional spreadsheet.
IMO, this could shed some more light on the "mysterious" extra power of TDIs.
Preliminary calculations suggest lower performance gains than those calculated using the latest versions of the DZR.

I still need to conduct a few more measurements in the field to obtain reasonably reliable reference values (instead of the usual essential self-measurements).
Approximately, I will be able to email Rainer the thing as update number 238 sometime in the middle or end of next week.

....Ulf
Are you sure about that? At what altitude (or, more precisely, below what external pressure) do pressure differential controllers shut off the valve?


Yes, I am.

For a PD100, the pressure range is 800, 900, and 1000 hPa. The reduction from 800 hPa to 1000 hPa starts at 3250 RPM with a value of 1.2 mg/h. At 4200 RPM, the reduction is 3 mg/hub, which is almost 9% and should be noticeable. Up to 3250 RPM, the values for all three ranges are the same.

@Audifan

The lower efficiency of the turbocharger at higher altitudes is compensated by the improved efficiency on the exhaust side (due to a higher pressure difference).

Best regards,
Uwe