I did know that ! honest ! Actually I used to know a lot more about engines than I should admit - I've been out of that trade some years and thought I still had a passing knowledge at this stage.
Since Ed posted the question - I've been reading up on variable geometry turbochargers, Variable engine compression ratios, filter and emission controls
Ouch - I've been away tooooo long !
Gotta get the books out and update them.....