Quote:
Originally Posted by Bobux
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).
|
As far as the turbo goes, the detailed thermo is complicated, but here's a simple explaination of where some of the advantage comes from:
Without the turbo, your intake pressure is 1 atm, and let's say with a turbo you get to an intake pressure of 3 atm. You'll have roughly 3 times the air at the same engine speed, 3x the air means ~3x the fuel means ~3x the power at the same speed. Without the turbo, you'd have to increase the engine speed ~3x in order to get the same amount of air (and power).
So with the turbo, the lower speed means lower friction losses & lower pumping losses. The heat loss to the cylinder walls should also be less because the ratio of fuel burned to surface area has increased.