By entering more air to achieve a better combination of particles with the fuel achieving better combustion. Diesel works with excess air.

For diesel engine, turbocharger increases the air pressure and consequently (with maintaining temperature) the air density at the intake stroke leading to fill more air and burning more fuel to maintain the same A/F ratio. This situation improves the mean effective pressure. Also the global efficiency of the engine is improved because  the ratio between the real cycle and the ideal one  increases more rapidly than the decrease of the mechanical efficiency. Thermodynamic and combustion efficiencies remain quasi-constants.

Variable geometry turbochargers permit us to adapt the air mass flowrate at each operating conditions of the engine (to avoid or deviate from the off-design area of the air compressor) by varying the turbocharger geometry leading to the change of its rotational speed. Hence it will be possible to achieve and control the best compromise between the following antagonist performances:

Engine power

Engine fuel consumption

Emissions.

Nota: The combustion quality is also strongly affected by the turbulence level.

Turbocharging internal combustion engines (both Diesel and Gasoline) is the most effective way to keep high power levels of downsized engines, reducing fuel consumption. In order to obtain a favorable torque curve (elastic engine) the turbocharger has to be regulated (mostly regarding turbo speed), driving basically to 2 solutions: Waste Gated Turbines or Variable Geometry Turbines.

Waste Gate Turbines allows some of the exhaust gas to by-pass the turbine -> turbine speed is regulated (and compression ratio as well) "wasting" part of the enthalpy available @ cylinder exit

Variable Geometry Turbines allows (mostly) noozle angle and effective section to be modified in order to regulate turbine speed @ each operating condition of the engine, optimizing A/R consequently. The most common solution is represented by a pneumatic or electrical actuator which rotates inlet vanes of the turbine, upstream the rotor.

The greatest advantages of VGT are:

- Low turbo-lag (due to high speed @ low engine load)

- Low consumption / High power output @ full load (lower back pressure due to high effective Area)

- Low consumption overall (due to optimized turbo efficiency and full use of energy content of exhaust gases)