mattia.bobbo wrote:So cooling the charge is the simplest way you have to improve power and efficiency, both now and in the old turbo era. You just cannot say: the higher the inlet temp, the higher is the efficiency, because this is a pure theoretical assumption. In real world you have to stay within the limit of knocking and other thousand limits.
No.
The amount of fuel you can add is limited to 100 kg/hr. Adding more air (whether by increasing revs, VE, MAP or charge density) simply makes the mixture leaner, it doesn't increase the power.
In a pure thermodynamic sense, reducing CAT (charge air temperature) will decrease efficiency and therefore decrease power. The reason is mainly due to reduction in energy of the working fluid available for expansion, both in the cylinder and in the turbine. For example, turbine power increases 18% when you increase the exhaust temperature from 800*C to 1000*C.
Another reason is slower flame speed and reduced rate of heat release. Heat released nearer to TDC can be used more effectively during the expansion stroke. This is less important for engines using TJI which seems to permit combustion rates and pressure rise rate close to the limit the engine can tolerate.
Of course increasing CAT has at least two major downsides - detonation and combustion chamber/exhaust temperatures. So at least some intercooling is required - to maintain the optimum CAT to balance the above conflicting requirements.