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Two-stroke engines offer twice as many power strokes for a given number of rotations and pistons by doing the exhaust and intake simultaneously. This normally happens spanning the end of the power stroke and the beginning of the compression stroke, though ideally it would only steal time from the beginning of the compression stroke. By tradition, nearly all two-stroke engines use a carburetor and are lubricated by oil being mixed into the fuel. The problem is pollution: unburned fuel passes right through the cylinder because the intake and exhaust valves open together, and this is made much worse by having the fuel be about 2% to 3% lubricating oil.
KTM fixes this for their two-stroke motorcycles by adding fuel injection. KTM injects into the transfer port, which isn't as good as injecting directly into the compression chamber but should still be a huge improvement. Honda has also filed a patent for a general purpose two-stroke with fuel injection. If one of them simply adds a normal oil pump, many of the problems of two-stroke pollution and annoyance should be gone. Next up, we need this small enough for chainsaws and weed cutters.
(Score: 1, Informative) by Anonymous Coward on Tuesday March 21 2017, @08:27AM (6 children)
The summary is missing one word: Gasoline or petrol. Because all the things it talks about have existed in two stroke engines for decades - in two stroke diesel engines. And because diesel is always injected near the end of the compression, the overlapping intake/exhaust ports doesn't matter.
AFAIK two stroke diesel engines have excellent fuel economy. So good that once you get above a certain size, you rarely see anything else (diesel locomotives, ships).
(Score: 3, Interesting) by Unixnut on Tuesday March 21 2017, @12:01PM (3 children)
I presume they mean Petrol. Petrol is a solvent, so it can remove/dissolve oils, which is a problem if the oil is there to lubricate engine parts. Hence you have to mix in oil and then expel the oil with the exhaust.
Diesel is a lubricant itself, so the Diesel fuel lubricates and is burnt as the fuel as well, then exhausted. Hence Two stroke and Diesel just went hand in hand. I always wondered why Diesel cars and trucks were not two stroke, due to the benefits provided in doing so. Maybe a scalability issue?
(Score: 1) by dwilson on Tuesday March 21 2017, @02:37PM (2 children)
Look up the "screamin' jimmy", otherwise known as a Detroit Diesel, series 71 [wikipedia.org]. Semi-articulated trucks and buses have used two-stroke diesels for years.
If it was good enough for the USSR to copy, it must have done something right?
- D
(Score: 2) by Unixnut on Tuesday March 21 2017, @03:57PM
Very interesting! I had no idea the USA had two stroke truck engines in mass use.
In Europe all truck/bus/car Diesel engines I have ever seen have been 4 stroke. Only 2 strokes were on boats/ships, trains, etc... big machinery.
You learn something new every day, thanks :)
(Score: 2) by dry on Wednesday March 22 2017, @01:04AM
I notice the Wiki article you reference states that Detroit Diesel switched to 4 strokes in 1995.
(Score: 2) by Kromagv0 on Tuesday March 21 2017, @03:31PM (1 child)
Came here to say just this. When it comes to efficiency only combine cycle gas turbines beat those large 2 stroke diesels but then only if you are able to use the waste heat stream from the turbine for heating. Even then combine cycle gas turbines aren't that much better with overall efficiency. They however are much more cost effective for power generation because of fuel cost which is why they are used substantially more.
You also forget to mention that 2 stroke diesels are all forced induction now so the turbos blow the exhaust gases out, then the exhaust port closes and the turbos begin to pressurize the cylinder.
T-Shirts and bumper stickers [zazzle.com] to offend someone
(Score: 2) by dry on Wednesday March 22 2017, @12:41AM
Exhaust valves (2-4 per cylinder on the Detroit diesel), not ports. Maybe more accurate timing when open. It's always a design issue with 2 stroke gas engines getting the intake/exhaust ports set right for maximum scavenging. Probably why most small 2 strokes are always supposed to be operated at full throttle.
I notice that Detroit Diesel switched to 4 stroke in 1995.