Unlike a 4-stroke, which introduces air via intake poppet valves during the intake stroke, a 2-stroke diesel introduces air by a rootes-type blower throughports in the cylinder:
As you can see, the ports are on a slight angle, so incoming air has some swirl to aid complete combustion.
Wouldn’t ports with a more pronounced angle cut provide more intense swirl, thus cleaner combustion?
Are you looking at this from the perspective of comparing a 2 stroke diesel with a 4 stroke diesel or a 2 stroke diesel with a 2 stroke gasoline? A diesel does not need the same turbulence to atomize the fuel because it is not introduced until just before the piston reaches TDC.
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I always thought diesels used most of their funky airflow tricks to promote efficient scavenging, i.e. removal of the burned exhaust gases while retaining maximum heat and wasting as little fresh air as possible. It may be the angled ports prevent the incoming air heading across the cylinder and out of the exhaust ports without flushing the sides of the chamber.
There’s only so much swirl you can induce with a ~6mm thick liner. I don’t have a picture, but I’ve seen in a Diesel engine repair manual that the early (1940s) Detroit diesel sleeves had the ports canted at an angle; that is, instead of an upright window they “leaned over” in a parallelogram shape. They went to the simpler design you linked later.
In the end, given modern standards it doesn’t matter. A two stroke has only about 100 degrees of crankshaft revolution to both clear the cylinder of combustion gases and simultaneously charge it with fresh air. A four stroke has about 180 degrees to push out burned gases and another 180 degrees to pull in fresh air. And runs a lot quieter.
For atomization, no, but for mixing, yes. Atomization (and some mixing) is achieved by injecting fuel at ridiculous pressures, 25,000-30,000 psi, into a very dense parcel of air (as much as 50X atmospheric density). But immediately after injection the fuel is still very unevenly distributed in the combustion chamber, and so air motion (swirl) is used to continue the mixing process. 2-strokes shape the charging port to achieve this, and 4-stroke diesels likewise angle the intake port so as to induce a high degree of swirl in the combustion chamber. Without swirl, you’d end up with high levels of soot AND NOx in the exhaust - the soot arising from the over-rich areas, and the NOx from the boundaries that combine high temperature with excess oxygen.
Here’s a slow-motion video of a diesel injection/combustion event in a 4-stroke diesel simulator (according to the comments, this is not a real engine, just a laboratory device that moves a piston to produce air motion/compression, so speeds may not fully match those of a running diesel engine) . In this video, the camera is looking up through a transparent piston crown; the edges of the field of view extend to the edges of the hollow “bowl” in the center of the piston crown. Because of the high injection pressure, the injection velocity is extremely fast; no swirl is apparent in the atomized plumes of fuel. It’s not until eight or nine seconds in that the swirling motion becomes apparent. The first signs of combustion appear at around 14 seconds; the video ends before combustion really gets rolling.
Here’s a video of high-speed diesel combustion. It’s over in just four seconds, but in the second half of the video you can see the swirling motion helping to twist/distort the gas clouds, assuring more complete mixing to help keep soot and NOx down.
mixing turbulence also comes from the “squish” area (the flat top of the piston crown surrounding the bowl.) that portion of the crown comes within relatively few thousandths of an inch from the cylinder head, and the air being squeezed from there into the bowl induces a lot of turbulence which promotes better fuel-air mixing.
True, this is the key design impact of the angle of the inlet ports, scavenging efficiency, not just combustion efficiency as some other posts suggest. Assume as is the case for all newly built 2-stroke diesels now that it’s uniflow scavenging, ie charge air comes in through ports distributed 360 degrees around the lower part of the cylinder liner and exhaust gas goes out through a poppet valve or valves in the cylinder head. Then some degree of swirl around the cylinder axis results in more of what you want: the new charge air acting like a quasi-solid pushing out the similar solid of the exhaust gas with minimal mixing of the two. It also helps cool the cylinder liner to have faster flow across its surface. But too high a degree of swirl will create a zone in the middle where the exhaust gas is displaced more by mixing and dilution with the charge air, not just being pushed out. That requires more charge air, hence more wasted energy propelling it. So there’s some optimum, very roughly speaking maybe 20 deg angle.
In the old days where many large low speed 2-strokes used loop* or even cross** scavenging, this was a factor. All designs still produced use uniflow scavenging. In US context the assumption might be old General Motors designs, either the small Detroit Diesel types or larger EMD type, latter of which is still produced, or other older designs which still exist like the Fairbanks-Morse opposed piston types. But counting by horsepower 2-stroke diesel nowadays mainly means big low speed crosshead types primarily for ship propulsion: MAN B&W and WinGD (inheritor of the Sulzer name) designs.
*exhaust ports on same one side of liner as intake ports, but above, scavenging flow loops around from the intake ports to the top of the (valveless) cylinder and out. This was Sulzer’s standard approach till the mid 1980’s and some such engines still exist.
**exhaust and intake ports on opposite sides, flow loops up back down and out the other side. Only the Italian Grandi Motori Trieste made it to recent decades with a design of this type, likewise produced till the '80’s but a minor factor in the market and few if any are still in operation.