 Two-stroke engine 2
If you think this is the end of the race to improve scavenging, you're seriously mistaken. The rotary valve has been around for more than a hundred years. In the simplest case, access to the
crankcase is controlled not by the piston, but by one of the two particularly large crank webs in two-stroke engines, or even a channel through the crankshaft. Depending on the angle of
rotation, they expose an opening through which suction can take place.
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Incidentally, a diaphragm, which could also be called an air nonreturn valve, can also be used as the opening to the crankcase. It opens whenever a vacuum develops in the crankcase,
allowing fresh gases to get in. A typical example is the horizontal lawnmower engine, with the cylinder on one side and the intake manifold with carburetor and air filter on the other.
Rotary valves for the intake are obviously not as crucial for scavenging. They are more effective for the exhaust port (Yamaha, for example). However, they have to be installed separately and
driven by the crankshaft. Here, too, the slide valve control did not become established due to problems with heat, lubrication and lack of sealing.
| Such double-piston engines are considered single-cylinder |
In this case, uniflow scavenging seems more promising. Look at the movement possibilities of the two pistons, regardless of whether their connecting rods run on a single crankpin or are
directly connected. The only important thing to note is that from BDC to TDC, the left piston is higher, and from TDC to BDC, the right one is higher. Therefore, the top left exhaust port opens
and closes more often than the transfer port.
It's said that the uniflow scavenging system has an asymmetrical control diagram. So, from this side, advantages would be expected. Unfortunately, this isn't the case when you look at this oddly
shaped combustion chamber, which has disadvantages for rapid pressure buildup and exhaust gas development. Furthermore, a cylinder head extending over two cylinders is difficult to mount
and subject to unfavorable thermal stress. Leaks can result.
Finally, there's the supercharged engine. In this case, the pressure is no longer generated by the underside of the pistons, but by two specially added cylinders, in which the necessary
pressure is generated above and below the pistons. The system functions as if it only has an even number of working cylinders, in this case, only four at the bottom.
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Now you might ask why the concept of turbocharging is combined with the old-fashioned uniflow scavenging design. This reveals a disadvantage of reverse scavenging: the absolute reliance
on absolutely uniform flow through the two transfer ports. If one of the two produces more, the central inlet on the rear wall and the even emission of exhaust gases are impaired.
Just a final word on lubrication. Since gas-tight crankcases for the indiaidual cylinders aren't necessary, this engine could also run with an oil sump and pressure circulation lubrication.
Normally, the crankshaft and connecting rods are mounted on needle bearings to benefit from this, together with the air-fuel mixture with the aspirated engine oil.
It was all useless,, DKW suffered shipwreck with this engine. It was neither particularly fuel-efficient nor did it last long enough. Whether this was due to the principle or the manufacturer is no
longer clear. It remains to be noted that the supercharged engine survived magnificently, but only with turbochargers and as large diesel engines.
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It's truly the star of the present day. Container ships account for about half of the world's transport. Diesel engines struggle to match the efficiency of this 3,000-ton engine. Recently, even
ocean-going giants have had to get used to fuel-efficient driving, as it's more economical than reaching their destination quickly.
A one-meter bore and two to three meters stroke are the basic dimensions of these four-story-high structures (pictured above). Their enormous engine blocks are usually cast in one go after
weeks of preparation, using every possible means. They develop 74,000 kW (100,000 hp) at speeds of around 100 rpm. A transmission would probably not be able to handle the torque and
would also take up cargo space.
The crankshaft is directly connected to the propelling screw. To brake, reverse gear must be engaged. To do this, the engine is stopped and then restarted in the opposite direction using the
collected compressed air. You can take this opportunity to think about which engines would theoretically start if you were to engage reverse gear instead of second gear when towing.
What happens in the engine? Whenever the respective piston reaches a certain angle before BDC, inlet slots in the cylinder walls open. Powerful turbochargers blow air through these slots, forcing waste gases
out through valves that simultaneously open in the cylinder head. Fuel is then injected into the compressed air after the slots and valves close.
Large engines have an oil sump, although it is frequently drained. The mass of engine oil is in the tens of tons. Complete oil changes are almost never performed. Only partial quantities are replaced after
appropriate testing of oil samples.
The fuel is a heavy oil distillate, produced and sold inexpensively as a waste product by refineries. It must be heated significantly before it can even be used in a ship's engine. Its use is prohibited in an
increasing number of ports and entire shipping lanes, such as the North Sea. These require expensive diesel fuel.
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