Diesel engines have been getting a lot of bad news recently, but there is a another form of compression ignition that has been lurking in the laboratory for years.
A diesel compresses the air, and then squirts in fuel, which ignites in the hot air.
The other form of compression ignition compresses a fuel air mixture until it all ignites simultaneously.
Theoretically, this could result in superior fuel economy and low levels of pollution.
This is a tremendously difficult thing to do since things like this, since the line between ignition and nothing is a very fine line, and things like ambient temperature, barometric pressure, etc. can cause premature ignition, i.e. pinging, which hits the inside of an engine like a hammer.
Nissan has come up with an innovative way to fix the timing, they have added a spark assist so that they can control the timing.
As opposed to a conventional spark ignited engine, where the flame front progresses from the spark, in their “Skyactive X®” technology, and the initial local ignition kicks up the pressure and temperature enough for the compression ignition to kick in.
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.
This is known as homogeneous charge compression ignition, or HCCI, an idea that Kyle Niemeyer covered in depth for us back in 2012. HCCI has some other advantages, too. On top of burning cooler and with fewer pollutants, the combustion event happens faster, with a higher pressure peak, so you get more work out of the same energy. All of that sounds pretty wonderful, so you’re probably asking yourself why every gasoline engine on the road doesn’t just use HCCI.
Unfortunately, it has been one of those ideas that worked in the lab but couldn’t ever quite be translated into a production engine. The biggest problem has always been controlling exactly when during the engine cycle compression ignition occurred, something that you want as close to top-dead center as possible.
Obviously, this wasn’t without challenges. The fuel:air mix needs to be a little richer near the spark for it to ignite than you want it to be throughout the rest of the cylinder. These need to be distinct regions to avoid λ dropping to 2 or below (which won’t undergo compression ignition). That’s achieved by swirling the air inside the cylinder and generating a vortex effect, where the calm center has a low enough λ to ignite by spark, surrounded by a high λ region that then undergoes compression ignition.
Mazda’s next challenge was to prevent pre-ignition, or knock. Higher compression ratios increase the potential for knock, which is why higher compression ratio engines usually also require more expensive, higher octane fuel that is knock-resistant. Now, technically, compression ignition is knock, but if it occurs before you want it to—at top dead center—bad things can happen, because a combustion event will exert downward pressure on the piston as it’s moving up on a compression stroke.
The solution here was to use less time to heat the fuel:air mix. There’s a small initial injection of fuel at first, then the bulk of the fuel is introduced into the cylinder as late as possible during the compression stroke. This is done using multiple orifice injectors to increase atomization and mixing of fuel and air.
If all that wasn’t enough, there’s the added problem of keeping track of compression ignition. In the past, this has been one of the hardest problems for HCCI engines to solve. Ideally you want combustion to happen at the same point in the engine cycle each time—about four degrees after top dead center. But as ambient conditions change—a cold day in Denver versus a hot one in Houston—the time needed for the fireball to reach sufficient pressure also changes. So the engine ought to be able to change spark timing to keep the peak pressure at the right spot.
It’s basically an ingenious use of the stratified charge engine to create an HCCI engine.