Quote:
Originally Posted by mort
Hi Bobux,
On a Diesel, especially something like you have, a turbocharger will improve efficiency substantially. At high power the Diesel engine is injecting fuel through most of the power stroke, and that fuel burns slowely so the equivalent expansion ratio is quite low. For a compression ratio of 16:1 at full power, (but not black smoke - does it do that?) the expansion ratio is about 4:1. Although the mechanical efficiency of the turbine is lower than the piston engine, there is a lot of power there. Using that power to increase combustion pressure and temperature is like increasing the compression ratio. So you waste less and improve thermodynamic efficiency win - win. Right up to the point that something blows out...
Roughly, the thermodynamic efficiency of a Diesel with 16:1 CR and 4:1 ER is 53% Increase the ER to 15:1 and efficiency goes to 66%
-mort
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Can I ask for an explanation here? I am not an expert in thermodinamics, but I came to a different conclusion. I thought of it like this:
Example 1:
Assume I had an engine working under athmospheric pressure, CR of 10:1. When the piston reaches it's top position I have 10atm pressure. Burning all air in the chamber the pressure will rise to
40atm because the air will try to expand 4:1. This way
extra 30atm will push the piston down and generate torque.
Then I will shave the cylinder head and raise the compression to 100:1. Initially, (about) same amount of air will enter the chamber and be compressed to 100atm. Burning all air pressure will rise to
400atm because the expansions is still 4:1.
Extra 300atm generate torque burning same amount of fuel, which is much more, so efficiency will obviously rise.
Example 2:
Same naturally aspirated engine as before, CR of 10:1. This time I add a turbocharger with ideal intercooler (intake air will be at same temperature). The charger will compress air 10:1, so 10 times more air will enter the chamber, combustion pressure will be again 100atm.
Injecting same amount of fuel I will be able to burn 1/10 of air in chamber, and it will expand 4:1. Once the pressure (and/or temperature) redistributes equally in chamber we will get
1/10 * 4:1 * 100atm + 9/10 * 1:1 * 100atm = 130atm. That is same
30atm extra pressure as in case of burning this amount of fuel without the turbocharger.
Conclusion:
Higher compression does increase torque "pulled" out of given amount of fuel. Turbocharging itself (not counting side effects like temperature change and change in fuel distribution in chamber) does not increase efficiency, but only pushes more air through the engine resulting in higher engine potential (ability to burn more fuel).