Okay, I think I finally figured out the whole "low CR -> better efficiency" thing.
I recently came across this article on the Nissan/Cummins 2.8L diesel pickup:
Green Car Congress: Cummins progressing toward ATLAS Tier 2 Bin 2 fuel-efficient diesel for light-duty pickups
In the article, it talks about how they looked at two compression ratios: 16.5 & 15.3. They ended up getting better efficiency with the 15.3 CR. Here's a quote:
Quote:
Other studies have shown that lowering compression ratio can help to reduce engine-out smoke levels along with enabling premixed combustion modes favoring low NOxformation. To confirm this experimentally, the Cummins team made two combustion bowls with 16.5 CR and 15.3 CR. These were installed on two different engines with 8-hole injectors.
With higher EGR, they found a reduction in oxygen concentration for the 15.3 CR down to 15.1%, resulting in a significant reduction in NOxemissions. The additional piston bowl volume also helps to achieve better in-cylinder charge-fuel mixing resulting in lower smoke emissions when compared to the 16.5 CR bowl.
The improved turbine match with the 15.3 CR engine further helped in reducing fuel consumption by 3% when compared to 16.5 CR and this can be attributed towards reduced pumping losses resulting in an improvement in open cycle efficiency.
—Suresh et al.
The Cummins team found a significant reduction in smoke emissions via the combination of lower CR, 8-hole nozzles and high in-cylinder swirl.
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I happen to know Arvind Suresh, the Cummins engineer quoted in the article (who authored the SAE paper. I got a hold of Arvind and discussed things with him. He confirmed that it's not the lower compression ratio itself that gives a higher efficiency. In fact, the lower CR itself lowers efficiency. However, the emissions (both NOx & smoke) are quite a bit lower with the lower CR. For that reason one can meet the emissions with lower EGR, more advanced timing, etc. So the lower CR does give better efficiency
at the same ultra-low emissions levels, however, if just the CR was changed and all the other settings were left the same efficiency would have gone down.