okay what you are refering to is a loop flow scavenge where the charge comes in from the side gets compressed and then ignited and then goes out another set of ports on the same side, very simple and hugely ineeficent due to a large dollop of charge going out the exhaust port. the under piston pressurisation is kind of used in a big diesel but not really.

What we have is uniflow scavenging where the charge comes in from ports in the sides of the liner near the bottom and then gets compressed by the rising piston and then ignited then pushes the piston down and then when the piston is near the bottom but before the inlet ports are uncoverd the single exhaust valve near the top opens and lets the exhaust gas out the piston then travels down further and the ports ar uncovred and then the charge air pressure pushes or scavenges the exhaust gas out and then you are ready for another cycle.

To supply the chagre air we have normally two turbo chargers which give alomost 1 bar of boost pressure but if any one says their car has a big turbo they don't out turbos are about 5 foot in diameter and the compressor wheel will be about 3 foot across. i think from calculation the exhust turbines them selve if they were not powering a compressor would produce about 4000Bhp each on the big engines.

to prevent our crankcase from being pressurised of containated with combustion crap (technical term) we have a diaphram that seperates the crankcase from the liners and pistons and through this diaphram we have a piston rod that goes onto a crosshead. The crosshead moves up and down with the piston and the connecting rod attachs to that which allows us to have the very long stroke to bore ratios we operate at.

if you download the PDF i point to in the big things thread that will show you a good cutaway on page 6 on page 9 there is a picture of pistons lying on their side and on page 13 there is a piston hanging with the gland in place.

the engine shown in that pdf is actually a relativly short stroke at 2.5 metres with a 960mm bore the ultra long stroke that are used on tankers for the best efficency are 3.5 metre strokes to a 840mm bore.

Those engines are about 555 effiecent where as the mainum theorectical effieceny is only about 57%

So hopefully that should explain it