Has GM struck gold, 30 years later?
HCCI, which stands for Homogeneous Charge Compression Ignition, is currently under development at General Motors. Under the watch of Paul Najt, who worked on the technology during his days as a PhD researcher, HCCI is coming to the forefront of GM’s work on eco-friendly gasoline technology. The technology was put on the shelf years ago, but has made a comeback with several important improvements and a promise from GM to make it viable for use in passenger vehicles in the near future.
I recently had a chance to try out this new technology in a working Saturn Aura. You can check out my post-ride review of the technology. But right now we're going to dive in and talk about exactly what HICCI is, and how it works.
What is HCCI?
At its most basic level, HCCI combines the best properties of diesel and gasoline engines into one. Gasoline engines use spark ignition, whereas diesels are compression ignition, meaning that instead of using spark plugs to ignite the air-fuel mixture within the cylinder they use the pressure of the piston to compress the mixture, creating heat and ignition. Because compression ignition is inherently more efficient, diesel engines are predisposed to good fuel economy, but at the cost of high NOx emissions. Gasoline engines, on the other hand, are less fuel efficient, but are also much cleaner in terms of NOx emissions.
HCCI partially bridges this gap by creating a gasoline engine where compression ignition is possible. This gives the engine both the operation and feel of a diesel, but without many of the associated pollution problems. GM was careful to say, however, that they were not planning on a convergence between diesel and gasoline engines, but rather that they were working to make each as efficient as possible, since both are already well-suited for particular modes of operation (i.e. diesels make better large trucks, but gasoline engines are a better fit for sedans).
There are several new tech advances that have come together to produce the modern, 4-cylinder, 2.2-liter, 180 HP engine that I test drove. This includes direction injection, a dual electrical cam phaser (as opposed to hydraulic), 2-step valve lift, and combustion pressure sensors. The combustion pressure sensors are extremely important because they allow the engine to make minute adjustments on the fly, which is necessary because HCCI is extremely sensitive to operating conditions. These combustion pressure sensors have fallen in price rapidly, part of what makes an HCCI engine more commercially viable now compared to even a dozen years ago.
Another important advance is the ability to run in HCCI mode in low-load and idle conditions. The biggest thing making diesel and HCCI-equipped gasoline technologies different is that HCCI engines are not always using compression ignition. Because of noise, vibration, and technical issues, engines can only operate in HCCI mode in low-load situations. This range has recently been improved, allowing HCCI operation in idle as well, but the Aura I test drove could only maintain HCCI up to about 55 MPH, at which point it would switch back to standard ignition (SI) mode. Similarly, the Aura would enter SI mode during brisk, and usually stayed in HCCI mode when cruising.
Part of the excitement over HCCI technology is that it offers very little negatives for all the positives that are promised. Of course, not all promises and expectations are likely to be met, but for now the technology seems like one of the best things GM has to offer.
As I already mentioned, the HCCI-equipped engine that I test drove was able to cram 180 HP into a (relatively) small, 4-cylinder, 2.2-liter form factor. Because HCCI is inherently more efficient, it not only results in improved fuel economy, but also increased power output. As far as fuel economy is concerned, there likely have not been any real-world or EPA-like tests done, but GM is quoting a modest 15% increase in gas mileage. I say modest not because I’m unimpressed with the number, but because new technologies often come with wild claims about how quickly they will solve the world’s problems, and this does not seem to be the case here.
On top of the improved efficiency, HCCI mode operates at a lower temperature than diesel, below the range where harmful NOx pollutants are created, making the engine extremely low-emission. Emissions are reduced across the board when in HCCI mode compared to bother diesel and gasoline engines. HCCI mode is also possible with ethanol blends up to E85, making the engine fit in well with GM’s near-term plan for increased E85 use.
Perhaps the most salient upside of HCCI technology is the cost / benefit ratio it offers. Though GM isn’t talking specifics yet, I was told that it would be cheaper than current hybrid and diesel technology, putting it within range of the average consumer and positioning it to make the largest impact on fuel consumption and air pollution.
Definitely exciting technology to watch. Hop on over to my post-test drive review to learn more on how it performs in the real world.
written by Troy, May 15, 2008
written by Luciano, May 15, 2008
written by Ray ODonnell, January 19, 2013
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