A few years ago the automotive and engine industries presented a team of researchers at Sandia National Laboratories with a mystery. Notably, the companies did so separately, each approaching their national-lab research partners at the Combustion Research Facility (CRF) in Livermore, CA, with in-house lab engine test results that none of their highly experienced experts had yet encountered.
Every time the company engineers and combustion specialists “changed the operations of their engines to try to achieve some environmental gain like lower emissions, they ran into the same issue—increased carbon monoxide (CO) and unburned fuel levels in the exhaust,” said combustion scientist Mark Musculus. “Everybody had the same problem, and nobody knew why it was happening.”
“We tried to replicate the same conditions that they were experiencing in our optical engine, which has a quartz window that lets us observe the fuel burn,” he said. Via laser and other high-tech diagnostics, “we showed that the fuel that ended up near the fuel injector was ‘over-mixed’”—i.e., there was too much air and not enough fuel in that region.
Over-mixing slows the combustion process, producing a less-vigorous reaction that tends to remain incomplete, which results in excess CO and unburned hydrocarbons in the exhaust. Both are not only toxic, but they also result in less fuel efficiency.
It turned out that “no one had bothered to monitor the end of the combustion process because they’d never found it necessary to ask that question before,” Musculus said.
Path to better diesels
Not only can such research solve unforeseen mysteries, it also can point the way to successful design. The CRF group has continued its studies, focusing largely on low-temperature combustion (LTC) diesel, which many researchers believe could lead to a new generation of cleaner heavy-duty and light-duty diesel engines that may not require costly, complex exhaust aftertreatment systems.
“It would be great if we could figure out how to clean up the diesel engine, and LTC could offer a solution,” Musculus said.
Landmark work by CRF researcher John Dec in the late 1990s and elsewhere had showed that burning the diesel fuel-air charge at a lower temperature cuts emissions of smoky particular matter (soot) and nitrogen oxides (NOx), the two main pollutants in the exhaust of diesels.
Sandia’s laser-based diagnostics indicated that soot formed in regions in which the fuel concentrations were too high. NOx arises from high-temperature flames inside engines as oxide production rates increase exponentially with rising heat.
In-situ heat sink
LTC addresses the formation of NOx by recirculating some of the exhaust gases back inside the engine, where it acts something like an in-situ heat sink, soaking up the heat from combustion, Musculus said. This EGR-dilution effect causes combustion temperatures to fall, so significantly less NOx forms.
The other aspect of this LTC strategy is to spray fuel into the combustion chamber earlier (or much later) in the engine cycle to give the fuel more time to mix with air before it burns. LTC has been shown to largely prevent formation of the fuel-rich regions that lead to soot as well as the high temperatures that lead to NOx.
In a nominal diesel with no EGR, he explained, the flame front temperature—the important parameter in this case—ranges from 2225 to 2425°C (4037 to 4397°F). If you then add EGR, depending on your emissions levels goals, the required temperature is around 1625 to 1725°C (2957 to 3137°F). “At least that’s where the NOx levels start to become interesting,” he said.
LTC has been demonstrated in many research labs using many different strategies, Musculus said, but there are other trade-offs that limit the design options. While NOx and soot output typically drop, other emissions of pollutants go up. Which ones? Why the same CO and unburned hydrocarbons discussed earlier. Combustion noise also can be an issue, and fuel consumption can be higher than for conventional diesels.
Engine designer’s guide
The Sandia researchers recently published a summary of their research on diesel LTC in the journal Progress in Energy and Combustion Science. The paper provides what they claim is the basic science for automotive and engine manufacturers to build the next generation of clean and efficient LTC engines.
Through their paper, Musculus and his colleagues hope to communicate the details of how LTC works to the engine research community.
“This is the kind of scientific research and data that engine designers, who help to guide our research, tell us they need so that they can build the kind of fuel-efficient diesel engines that consumers will want,” he said. The team views the work as an extension to Dec’s earlier results.
He stressed that their conceptual models target a specific subset of LTC regimes, namely, low-load, single direct-injection, partially premixed compression ignition (PPCI LTC) conditions that are diluted with EGR to oxygen concentrations in the range of 10 to 15%.
The models describe the spray formation, vaporization, mixing, ignition, and pollutant-formation and pollutant-destruction mechanisms during burning. Factored in are parameters such as the mixing time, turbulence, how flows move around the cylinder, and piston head shape, while adjusting for various aspects of the spray beyond timing, pressure, shape, and droplet size.
One model is applicable to heavy-duty engines, in which the flow structures of the fuel jets are less perturbed, whereas the second pair is for light-duty engines with either early or late injection, in which in-cylinder surfaces and flows significantly deflect the fuel jets.
“In the engine community,” Musculus said, “LTC means different things to different people as the lines between different kinds of combustion have become blurry. That’s because it’s a new concept that hasn’t had time to settle down.”
LTC generally refers to a broad range of in-cylinder combustion strategies that aim to suppress the maximum heat that is generated.
“There are multiple ways to operate these engines, and we’re not promoting any particular technology,” he noted. “We’ve only mapped out the space of the most common aspects of the approaches, the common denominators—what’s shared by many approaches.”
The authors categorized the numerous LTC strategies that have emerged recently into two broad groups, according to the degree of pre-mixing and whether EGR is added or not, although many implementations use both.
“From the Sandia and maybe the general-consensus perspective, homogeneous charge compression ignition (HCCI) falls under the umbrella of LTC,” he said.
HCCI strategies have typically employed long in-cylinder mixing times prior to combustion or mixing strategies such as intake port injection to produce relatively uniform, lean mixtures. As the technique has matured, HCCI has evolved toward more inhomogeneous charge gas mixtures and temperatures for better control of the rate of heat release.
PPCI, the other major LTC subdivision, uses direct injection with shorter mixing times. The charge distributions for PPCI are less homogeneous at ignition than for HCCI and include both lean and rich regions. Low combustion temperatures are generally achieved using EGR. Both fuel-injection and combustion timing schemes are being tested, as well as multifuel reactivity control compression ignition (RCCI) strategies.