TDI Engines: Limits and Expert Insights

Hello everyone,

No, I don't want to tune (my car), but I was wondering about the following:

How far can the fundamental principle of our TDI be pushed, assuming we have sufficiently durable mechanical components?

Up to what boost pressure (with the corresponding fuel injection) would this system still function efficiently? Or would it even become more efficient at higher boost pressures?

I'm not talking about increases of, say, 1 bar to 1.5 bar, but rather something on the order of 5 bar or more.

It's logical that neither today's blocks nor typical turbos can do this, but if we assume "hardware" that *can* do it (essentially treating this as a thought experiment): What would happen?

Without having much background knowledge, I suspect that the theoretical limit must be reached when the boost pressure enters the range where it starts to cause compression within the engine itself... but even that would still leave a very large margin.

What do you think?

Could it be feasible to build a small 0.5-liter TDI engine with immense turbo boost that achieves the power and torque output of our existing 1.9-liter engines?

Best regards,

January.

Hello,

As the turbocharger's compression ratio increases relative to the engine's compression ratio, the engine-turbocharger combination increasingly resembles a gas turbine, with the engine acting as the combustion chamber. At this point, one can conceptually eliminate the engine altogether and arrive at a pure gas turbine. The maximum compression ratio of a gas turbine is determined by the chemistry of the combustion process. Above the decomposition temperature of CO2 (everything has a decomposition temperature), carbon cannot be burned, and above the decomposition temperature of water, hydrogen cannot be burned. If the temperature in the combustion chamber exceeds these values, no combustion occurs within it (but rather beyond it).

Conclusion:
If the engine is to remain a TDI (Turbocharged Direct Injection) engine, the ratio between the turbocharger's compression and the engine's compression must be maintained. Both could be increased until the temperature becomes so high that combustion no longer occurs.

Okay, I've been sitting in front of this thread for 15 minutes now, just thinking.
While I don't have any knowledge of engine building, I can imagine that increasing the turbocharger boost pressure might bring a lot more power to your setup, but it would likely significantly increase fuel consumption.
I'm familiar with that from turbocharged gasoline engines that operate at around 5 bar of boost pressure. In those cases, fuel consumption increases dramatically.
The question is whether reducing the engine displacement to 0.5 liters and the number of cylinders to one or two would also reduce the excess weight sufficiently to achieve the fuel consumption and performance levels of a 1.9 TDI.
It would significantly reduce weight and space requirements, while also creating new possibilities for integration.
However, the technology, development, and materials required for such complex engine designs become very expensive.
I think many engine manufacturers are already considering this. They tend to be more advanced than the average hobbyist mechanic.

Hi,
I wouldn't compare it to gasoline engines, because with increasing boost pressure, you have to significantly reduce the compression ratio to avoid knocking. But Ulf probably didn't intend that when he was considering the TDI.
Currently, and likely for a long time to come, a problem with real-world gas turbines is the turbine inlet temperature, or more precisely, the blade temperature at the inlet. I don't know at what temperature water and CO2 dissociate, or why they might no longer combust, but that temperature is certainly much higher than current materials and cooling methods can withstand.
Furthermore, this increases the construction costs, making such a turbine too heavy for use in aviation and too expensive for ground-based applications.
The efficiency should initially increase further (assuming effective heat dissipation).
I also fear that the characteristics of the engine will increasingly resemble those of a turbine, meaning that power is only available within a narrow speed range. At the latest when variable geometry turbines (VTGs) are no longer possible due to the high exhaust temperatures, compromises will have to be made.

The borderline case is the turbine, as used in airplanes. In my opinion, the advantages of a piston engine as a "combustion chamber and turbine" are:

"In this design, the component temperatures of the 'expansion chamber or engine' are significantly lower than the combustion temperature due to the intermittent combustion process. This allows for a higher combustion temperature and an increase in thermal efficiency. In aircraft turbines, this effect is only partially achieved; there, the turbine blades are protected from combustion temperatures by a layer of cooler gas."

2. The partial load behavior and efficiency at partial load are much higher for a compressor than for a turbine, because the compression ratio is mechanically determined. Turbines are only used in extremely niche applications in the automotive industry (e.g., American tanks, tractor pulling competitions, rare locomotives).

Best regards,
Christian.

Hello everyone,

First of all, thank you for the competent and interesting answers.

So, if we take a scale where our TDI represents one end and the aircraft turbine represents the other... how far towards the middle do you think it would be worth moving in that direction (assuming that materials science issues can be overcome)?

Best regards,

January.

What do you think?
Could it be feasible to build a small 0.5-liter TDI engine with immense turbo boost that achieves the power and torque output of our existing 1.9-liter engines?


That's definitely the case. See the 'Renn-TDI' .

'However, the limit will settle where the effort required to generate boost pressure becomes too high. If you want 5 bar of boost pressure, that pressure has to come from somewhere.'
You would then need either compressors or multiple turbochargers in series or parallel configurations. This is something that is done, for example, in shipbuilding. However, using turbochargers like that wouldn't be feasible for a vehicle engine; it would likely make the vehicle undrivable.
However, you also need to consider that higher boost pressure will result in significantly increased compression pressures, which in turn will raise the air temperatures. This is likely to cause considerable problems with the fuel injection system.

On the other hand, the efficiency will eventually decrease again because the mechanical and thermal losses simply become too large.

When working with turbochargers, the characteristics of the flow machine will increasingly be transferred to the engine as you increase the boost pressure. This causes the engine to operate more and more like a propeller, which is generally less suitable for vehicle engines.
TKN has already mentioned it. Eventually, you'll have a free-piston gas turbine .

CU Gremlin.

Hello everyone,

I once had some documents for a diesel high-altitude engine. As far as I remember, it was planned for the GROB Strato. The engines were supposed to still provide power at an altitude of 18 kilometers. The propeller diameter was 6 meters!


This diesel engine should operate with two turbochargers in series, using a boost pressure of 4 bar.
It was probably cheaper to modify existing gasoline engines than to build a completely new engine from scratch.


I haven't found any more documents, just the link to the airplane.

http://www.grob-aerospace.de/get.php4?pageid=205

Regarding the question: I would assume that material stress will cause the process to end before any combustion-related problems arise.
It seems to work up to about 4 bar.

In my opinion, a higher boost pressure relative to the compression pressure has another advantage that hasn't been mentioned yet: The power output (kW per liter of displacement) increases, which means that the actual piston engine can be designed to be smaller for the same amount of power. This in turn means: Lower internal friction within the engine itself. This translates to: [/b]Reduced idling fuel consumption.} While a larger and more powerful turbocharger, which is also part of the overall process, does contribute more to the total losses, it is practically ineffective during idle. Therefore, you have a very small engine with small pistons, which is quite economical at idle and in low-speed zones (e.g., 30 km/h).

extreme example: idle consumption comparison between a Lupo 1.7SDI and a Lupo3L (both with approximately the same rated power).}