How Gas Compression-ignition Engines Work


Pros and Cons of Compression-ignition Engines
The SKYACTIV-G 2.0-liter has the power of a 2.5-liter engine and the efficiency of a 1.5-liter diesel engine. Mazda

Compression-ignition engines have a few advantages and at least as many drawbacks. Among its benefits are:

  • It uses less fuel than a spark-ignition engine
  • It uses the fuel more efficiently (in other words, less power is lost to the actual ignition and to excess heat)
  • Since less fuel is used, the car pollutes less

"As a rough analogy, spark-ignition is akin to starting a fire by lighting just one edge of the newspaper kindling and letting the flame gradually climb across the paper," explains Mazda powertrain engineer Jay Chen, via email. "[Compression ignition] is more like spontaneous combustion where the fuel and air has reached critical pressure and temperature, and the entire charge changes phase at the same time thus releasing all the energy at once. By releasing all the energy nearly at once, [compression ignition] can extract more power (since it happens well before the expansion ratio is used up) from the same amount of air while using two to three times less fuel and at much cooler combustion temperatures, which further reduces wasted heat energy and emissions formation."

Sounds great, right? The problem is that these engines are really finicky — if they were easy to design and use, we'd be driving them by now. Even if you're not familiar with diesel engines, you may have heard that they can be inconvenient under sub-optimal conditions. Part of that is due to the diesel fuel itself, which has a tendency to "gel" in very cold temperatures. We don't have that problem with gasoline, which stays liquid even in sub-freezing conditions. But compression ignition can still be affected by the weather and other ambient conditions, as well as other factors like the quality of the fuel.

"Until now, compression-ignition internal combustion engines existed only in stable laboratory conditions or raw vehicle prototypes too rough to be applied in production," Chen says.

In other words, if the pressure and temperature in the cylinders isn't carefully maintained, the process won't work. Temperatures that are too cold can damage the engine's sensitive components. If the engine gets too hot, it can start to knock — a condition that occurs when the fuel-air mixture gets too hot and detonates at the wrong time, which wastes fuel and results in a poor-running engine. A spark-ignition engine can also get too cold or too hot, but has a much higher margin of error.

Making a compression-ignition engine work reliably depends on a precise combination of air, fuel and exhaust gases mixed in the perfect ratio, at the perfect compression, with just the right amount of heat applied at the correct time. As we know, no one's been able to build a car with a compression-ignition gas engine yet, so this process needed to be further refined.

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