Yes turbo is engaged all the time but below 1700 or 2000 (dont know exact rpm) the effect of turbo is negligible because there r not enough exhaust gases to make the turbo spin fast so when driving economically within city like u said at 50km/h, it would have the consumption of a 1.5L engine but when u floor it or put it in S and the rpm rises, the turbo starts to spin faster and u get that high torque (fuel economy will go down significantly to provide that power)