Turbochargers, in essence, increase the amount of air that can be pushed into the combustion chamber (when they are working).
Petrol engine: Petrol engines run in a very narrow band of stoichiometric air:fuel ratio (exact amount of air needed to completely burn the fuel) as they are homogeneous charge (or well mixed air & fuel) engines. When the spark ignites the charge, it burns and produces exhaust gases at a much higher temperature resulting in higher pressure for the same volume of combustion chamber. This results in work done on the piston and you get power.
In addition, most petrol engines use throttle valves to control air flow. At a given engine speed, this controls the amount of charge sucked into the cylinder based on the accelerator pedal position, thereby controlling the power output.
When you boost a petrol engine – in other words, put in extra air – you also have to inject extra fuel in order to maintain the air:fuel ratio in the narrow operating band. So for the same volume of combustion chamber, you get extra power but also burn more fuel. On the other hand, to gain fuel economy, you can downsize the combustion chamber volume (or engine displacement) for the same desired power output. You can even choose a middle path. So I would say the statement ‘… turbos are added to petrol cars as performance extra’ is a partial myth. Most modern engines do that in order to gain fuel economy through downsizing.
Diesel engine: Diesel engines mostly run lean (more air & less fuel) and have a much wider band of operation in terms of stoichiometric air:fuel ratio as compared to petrol engines. This is because they work with heterogeneous charge (fuel not well mixed with air) achieved through air compression before fuel injection. This causes fuel to ignite at multiple places simultaneously where a local near-stoichiometric air:fuel ratio is achieved. The combustion not only produces hot exhaust gases but also heats up the excess air thereby doing work on the piston.
In addition, most diesel engines do not use throttle to control air flow. At a given engine speed, nearly same amount of air sucked into the cylinder irrespective of your accelerator pedal position. Only the fuel injection quantity is varied to control power output.
When you boost a diesel engine – in other words, put in extra air – you need not necessarily add extra fuel as you might still be able to run* at the resulting leaner air:fuel ratio. This tends to improve the volumetric efficiency of the engine and that solely accounts for most of the additional power (which is not as high as an equivalent petrol). So there is a gain in fuel economy even without downsizing the combustion chamber volume (or engine displacement) and there is some gain in performance as well. So again, turbos actually do improve the effective performance of the diesel engine.
Contributed by Pro PavanRaj Urs, Bangalore, ARPRO 3