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posted by Fnord666 on Wednesday January 31 2018, @12:50AM   Printer-friendly
from the under-pressure dept.

Submitted via IRC for AndyTheAbsurd

It's called Spark Controlled Compression Ignition, and Mazda made it work.

Despite rumors to the contrary, the internal combustion engine is far from dead. Recently we've seen several technological advances that will significantly boost the efficiency of gasoline-powered engines. One of these, first reported back in August 2017, is Mazda's breakthrough with compression ignition. On Tuesday, Mazda invited us to its R&D facility in California to learn more about this clever new Skyactiv-X engine, but more importantly we actually got to drive it on the road.

The idea behind Skyactiv-X is to be able to run the engine with as lean a fuel-air mixture (known as λ) as possible. Because very lean combustion is cooler than a stoichiometric reaction (where λ=1 and there is exactly enough air to completely burn each molecule of fuel but no more), less energy is wasted as heat. What's more, the exhaust gases contain fewer nasty nitrogen oxides, and the unused air gets put to work. It absorbs the combustion heat and then expands and pushes down on the piston. The result is a cleaner, more efficient, and more powerful engine. And Skyactiv-X uses a very lean mix: a λ up to 2.5.

Source: https://arstechnica.com/cars/2018/01/mazdas-skyactiv-x-shows-the-internal-combustion-engine-has-a-future/


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  • (Score: 0) by Anonymous Coward on Wednesday January 31 2018, @02:35AM (6 children)

    by Anonymous Coward on Wednesday January 31 2018, @02:35AM (#630795)

    WE had lean-burn gas engine before, but it went away because of NOX emission problem, same as diesel engines, a basic chemistry problem. How does this solve the problem? I

  • (Score: 3, Informative) by requerdanos on Wednesday January 31 2018, @02:37AM (3 children)

    by requerdanos (5997) Subscriber Badge on Wednesday January 31 2018, @02:37AM (#630797) Journal

    How does this solve the problem?

    Short version, dynamic timing adjustments leading to more complete burning of the fuel.

    • (Score: 1) by Frosty Piss on Wednesday January 31 2018, @03:27AM

      by Frosty Piss (4971) on Wednesday January 31 2018, @03:27AM (#630809)

      Short version, dynamic timing adjustments leading to more complete burning of the fuel.

      Why yes, that's exactly what the article says! The parent commenter must have wondered in from Slashdot.

    • (Score: 2) by Whoever on Wednesday January 31 2018, @06:26AM (1 child)

      by Whoever (4524) on Wednesday January 31 2018, @06:26AM (#630839) Journal

      But this engine runs with excess O2 and naturally there is nitrogen in the air intake, so how does more complete burning of the fuel reduce NOx emissions?

      There is also mention of lower combustion temperatures, perhaps this is the reason?

      • (Score: 3, Interesting) by ElizabethGreene on Wednesday January 31 2018, @03:42PM

        by ElizabethGreene (6748) Subscriber Badge on Wednesday January 31 2018, @03:42PM (#630983) Journal

        NOx forms when you have heat, oxygen, and nitrogen. You can demonstrate this by firing an arc furnace in an enclosed space. The air in the vessel will turn brown from NOx compounds.

        The minimum temperature for this reaction is ~2800 F. Below that it doesn't happen in quantity.

  • (Score: 2) by bradley13 on Wednesday January 31 2018, @09:01AM

    by bradley13 (3053) on Wednesday January 31 2018, @09:01AM (#630873) Homepage Journal

    "WE had lean-burn gas engine before, but it went away because of NOX emission problem"

    NOX is the result of accidentally burning the nitrogen in the air, during combustion. This happens because of two things:

    - Availability of oxygen, which would really rather combine with something more easily combustible. Hence, a lean fuel mixture is a necessary prerequisite.

    - High temperature. Nitrogen doesn't particularly want to be oxidized - in fact, the reaction is endothermic. Diesels, for example, burn hotter than gasoline engines, which is why NOX is a problem for them.

    According to TFS: They are running lean and low temperature. Hence, NOX should not be a problem.

    --
    Everyone is somebody else's weirdo.
  • (Score: 2, Informative) by Anonymous Coward on Wednesday January 31 2018, @09:22AM

    by Anonymous Coward on Wednesday January 31 2018, @09:22AM (#630882)

    Rich of peak vs. lean of peak. It's a common topic for piston-powered aircraft.

    Peak means stoichiometric reaction ratio - exactly enough fuel for the amount of air the engine takes in. This is great for fuel economy, but it's lousy for NOx emissions, because it unsurprisingly burns really hot, and it's harder on the engine because it produces higher pressure. Cars use this in cruise mode - fuel system designers call it "closed loop" mode because the fuel mixture is adjusted based on the O2 sensor in the exhaust so it forms a feedback system. Airplanes can't use it at all because they run at full throttle most of the time and they would overheat or blow up.

    Rich of peak usually means "best power" in practice. More fuel than air. Cars run this way when they're cold, before the O2 sensor warms up, and whenever you accelerate. Safe for the engine, higher HC emissions, lower NOx than peak, bad fuel economy. Engines with carburetors always have to use this mode. Carbureted engines have an accelerator pump that changes them from "rich" to "more rich" when you step on the gas. Now you know why cars switched to fuel injection.

    Lean of peak means more air than fuel. Cars don't currently use this mode at all. Even "lean burn" engines like Honda and Mazda ca. 1980 were still rich of peak, just less rich. Lower emissions along with better fuel economy but less power. Some airplanes can use this mode, all diesels use this mode (well they're supposed to - if you see a diesel blowing a cloud of smoke, it got too rich). Problem is this mode creates a high risk of knock. Diesels don't knock so it's fine for them. There's a pretty narrow window in between "too lean to burn" and "too rich to be safe for the engine." Airplanes have a bunch of fancy engine gauges and run pretty much at the same speed and power output for hours at a time so they have time to find a lean mixture setting that works for them. And they use 100 octane gas with low-compression engines so knock isn't as big a risk either though it certainly can happen.

    NOx is produced by high combustion temperatures. Lean of peak actually lowers combustion temperatures, although it increases exhaust gas temperature because the mixture burns more slowly (more of it is still burning when the power stroke ends and the exhaust stroke begins). This slower combustion is partly why it produces less power - though if you had an engine that was designed from the ground up for this mode, you might recover some of that in the engineering. This is not unlike another unusual Mazda engine, the Wankel rotary, which also has lower combustion temperatures but higher exhaust temperatures because the mixture doesn't have time to finish burning before the exhaust stroke, though in that case, it's because the banana-shaped combustion chamber takes longer for the flame front to travel through. Mazda had a third such engine, the Miller cycle (basically an engine with a longer power stroke than compression stroke). So this is pretty much par for the course for them.

    This lets cars use lean of peak mode but in a quite new way. Possibly full-time, although lean of peak produces less power than rich of peak so they might try to include a traditional rich mode for acceleration and possibly also warmup. Maybe they're counting on the high compression to avoid the need for that. This seems very lean and very high compression - quite a bit leaner than lean-of-peak airplanes, and with a very different design that allows them to run well into what would be the "fire went out" zone for a traditional engine. They also seem to claim that the combustion is faster, eliminating one of the big problems with usual lean operation.

    I hope it works out. My sentiment is always with the rotary engine, but I think Mazda at this point is just trying to stave off hybrids until most cars can go fully electric. Mazda just likes their funny engines and trying a different way. Nothing wrong with that.