Both, super-chargers and turbo-chargers, adds to the power of the engine by forcing more air for burning more fuel in the cylinders. And both can be used on petrol or diesel, alone or in combination. And in principle they don't affect the efficiency.
Why diesels are turbocharged? Basically, the turbochrger is more ideal system as it is freewheeling (not mechanically connected to engine). Therefore it can ideally match the air fuel ratio over a longer rpm range. Also any change in load will affect the engine rpm - torque may increase and rpm may reduce but power may be same (example changing of gear, going on slope). As power demand is same, the air demand will remain same. As turbocharger is not mechanically connected, it will continue to supply almost same amount of air, whereas mechanically connected supercharger will reduce the air and hence the power.
The reason why it is not popular with petrol engines is that the lower exhaust pressure at low load will cause higher turbo-lag and will considerably degrade the performance. Even worse that diesels at startup. Would you like to buy such car? On the other hand, due to spark ignition, the petrol engine have the advantage of higher operating rpm range and therefore using direct driven blowers are not able affect that much the operating range, as it would do in diesel engine.
Why combination is not used? Well ask the marketing gurus. They build vehicles for mass use and not for few guys who would prefer performance over anything else. (Combination will increase complexity, manufactering and maintenance costs etc.)
PS: Bigger diesel engines, manufactered entirely on the basis of economy has turbos matched to a very narrow operating band and at starting use a electric blowers or two blowers. |