For my English speaking followers. Should we reinvent the wheel and get back into the discussion of whether the appropriate reference value for assessing thermal systems is the higher or lower calorific value?
After lengthy negotiations, DIN 4708 finally agreed on the lower calorific value, even though efficiencies of over 100% can be achieved with condensing boilers. This is of secondary importance, since we are mainly aiming for a comparison of different investments, and therefore the absolute number is secondary.
In addition, indirect efficiency determination by means of exhaust gas loss measurements is usually used in practice. In this case, the calorific value, which is often not available or is incorrectly stated in the literature, no longer plays a role.
It should also be noted that when using the higher calorific value, system components are designed too small, even in the case of a condensing boiler, as some residual water vapour still escapes with the flue gas. The introduction of two reference points would open the door to cherry-picking.
Here, however, it is a matter of evaluating the efficiency of automotive drives. Again, we should not reinvent the wheel. In the motor industry, the so-called "shell diagram" has become established.
I add an example diagram in the comments.
This shows the specific fuel consumption as a function of engine speed and torque: The curves show kg of petrol/diesel used per kilowatt hour of mechanical work performed (break power). FYI, calorific value petrol = 12 kWh/kg or 83 g / kWh!!
I am convinced that these shell curves exist for all engines that are at least mass-produced, but it is extremely difficult to get them from the engine manufacturers, because the efficiencies shown are significantly lower than the communicated values depending on the operating mode, and are not exactly exhilarating overall. In the diagram, we have just 18.2% efficiency in 2nd gear at 5,700 RPM and 110 kW engine power.
Such "shell diagrams" would also be suitable for the evaluation of battery or hydrogen EV drives. Input variable = kW electricity in, output variable = kW break power.
So for battery:
Power Source – Battery – Electric Motor – Test Bench Brake
And for hydrogen:
Power Source – Electrolysis – H2 Compressor – Fuel Cell – Electric Motor – Test Bench Brake
The graph would also show the working range of the system (RPM/Torque/Specific Consumption). Since input and output are easy and accurate kW to measure, you would get a good assessment of the efficiencies and the operating envelope. Again, calorific values do not play a role here. However, it would have to be ensured that all auxiliary units are within the system boundaries under consideration, and that energies stored in and out of the system are taken into account.
Unfortunately, even with a longer search, I didn't find any such diagrams.
Why not?
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![Effizienz und Wirkungsgrad, zwei völlig eindeutige Begriffe – ODER ETWA NICHT?]()
Effizienz und Wirkungsgrad, zwei völlig eindeutige Begriffe – ODER ETWA NICHT?
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