That is right Bob, a two stroke has no cylinder suction since the only time the piston moves downward is during the power stroke. So, to achieve induction of air for the next compression and combustion stages, you have to use some other way to pump it in. There are three standard ways: crankcase induction (for small cheap 2 strokes), separate pumping cylinders (in other words a reciprocating air compressor on the same crankshaft as the power-producing cylinders) or a positive-displacement supercharger, usually a Roots blower, such as the Detroit Diesel and the TS3 used. The advantage the valve-controlled engine like the Detroit Diesel has is that the valve timing does not have to be symmetrical in the two piston strokes, which lets you achieve more power and efficiency. I once had a piston-controlled ported two stroke motorcycle that achieved the same thing by tricky design:
[Linked Image]
Original, and more information, is on this site:
http://berniesbikeshed.wordpress.com/splitsingles-and-what-they-are/
They even have a picture of my motorcycle, labelled 250SGS. Mine was red, though.

The idea is that the master-slave connecting rod arrangement places the big end bearing for the rear cylinder some distance rearward from the front cylinder, so the piston movements are somewhat different. By putting the transfer port on the rear cylinder and the intake and exhaust ports on the front one, much better scavenging can be achieved. Note there is only one combustion chamber across both cylinders, which is why it is called a "split single". For a while before WW2 the manufacturer, Steyr-Daimler-Puch, supercharged this engine for racing. The separate timing of the transfer and exhaust ports makes it (barely) feasible to close the exhaust port before you close the transfer port.

The conspicuous advantage the split single's superior scavenging had for road use, is it idled rather smoothly and two-stroked from just off idle, even at little or no load.

The 2 stroke Detroit Diesels had many supporters, Bob. One of the things the drivers loved was the exhaust sound, and the three foot flame from the truck's exhaust stack at full song. (This was not considered a positive feature when hauling cattle however, the beasts found it unsettling.) The company I used to work for had a couple of Detroit Diesels in the emergency power supply for the mainframe computer: a 16V92T to run the full system, and a 12V71 to just keep the mainframe itself ticking over while the 16V92T was being serviced. Running 24/7 in an emergency "no mains power" situation, the big one had to be serviced every Sunday. Ah, the old days.