Everyone from Nostradamus to the Department of Energy predicts that internal combustion engines will provide primary propulsion for the majority of the vehicle fleet for a few more decades. But it’s looking like the auto industry may fall short of the forthcoming fuel economy/CO2 standards landing in 2025. Rather than tweaking the standards, maybe it’s time to look at CO2 reduction from a more holistic standpoint—by designing the next generation of engines to work with higher-performing, higher-octane fuels and examining CO2’s impact on a well-to-wheels basis.
How could higher-octane fuels help? “The real key is the ability to design the engine with a higher geometric compression ratio,” Chris Cowland, FCA’s director of advanced and SRT powertrains, says. “We’d like to be running between 14:1 and 15:1 from a thermodynamic perspective to give us part-load fuel efficiency and also enable us to generate higher power densities.” Higher octane prevents harmful engine knock at high compression and enables more extreme Miller- or Atkinson-cycle operation (even higher expansion ratio with delayed intake-valve closing to reduce effective compression with or without supercharging). What’s his ideal octane rating? The answer requires an octane primer.
Most of the world classifies gasoline using only a research octane number (RON), but in the U.S. we use an anti-knock index (AKI). This averages the research and motor octane numbers—(RON+MON)/2. These two figures are obtained via different knock tests using single-cylinder laboratory engines, with the difference between RON and MON referred to as fuel sensitivity. Cowland suggests the optimal target fuel would raise the RON to 98 while reducing MON slightly for a greater fuel sensitivity. “It’s been proven on high-boosted engines that high MON levels actually hurt us from a knock perspective.”
So why not go even higher than 98 RON (which is available outside of the U.S. today)? “Higher RON values allow us to stretch efficiency farther,” Cowland adds. “But if we look at the well-to-wheels efficiency, including producing the fuel, we would not actually be environmentally positive.” In other words, it’s a bit pointless to produce very efficient engines requiring a fuel that’s more CO2-intensive to produce.
Maybe it’s time to look at CO2 reduction from a more holistic standpoint.
Octane can be boosted using metals, ethers, aromatic chemical additives, or alcohols. Good old tetraethyl lead was banned as of 1996, and methyl tertiary butyl ether (MTBE) demonstrated a nasty habit of Houdini-ing its way out of storage tanks and into ground water, earning itself an effective national ban in 2005. Aromatics do much of the octane boosting today, but some are being linked to health problems. Ethanol increases octane, and when made from bio feedstocks (as is generally the case here), it improves the lifecycle CO2 picture. But its reduced energy content erodes volumetric fuel efficiency, so any CAFE regs predicated on miles per gallon of a fuel containing more than today’s 10 percent ethanol would require revision.
Dan Nicholson, GM’s VP of global propulsion systems, is quick to point out that the U.S. Council for Automotive Research (a consortium of GM, Ford, and FCA) Fuels Working Group “is focusing on the characteristics of the fuel, not how it’s made. We’re trying to enable a spec that’s as tight as necessary but as open as possible to encourage innovation among fuels and additives companies.”
Nicholson and Cowland both envision this new fuel being dispensed via unique nozzles that prevent misfueling older cars and hence “wasting” added octane that won’t benefit older engines. They also stress that the sooner the government, auto, and oil industries can agree upon a standard, a regulatory framework, and a distribution rollout plan, the sooner the three- to four-year development clock can begin ticking for manufacturers to develop 98-RON-optimized engines.
Read more by Frank Markus here:
- Drown Your Global-Warming Sorrows in Ethanol
- Privileged With Fed Resources, NHSTA Upgrades NCAP
- Smartphone Tech Will Lighten Your Greenhouse
- What I Learned at Valvoline’s Sesquicentennial
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