Low Temperature Combustion (LTC) combustion researcher Mark Musculus has compiled knowledge over several years about diesel engines to form the “LTC strategy”.  Musculus, the lead author along with Sandia researchers Paul Miles and Lyle Pickett prepared the paper “Conceptual models for partially premixed low-temperature diesel combustion” published in the journal Progress in Energy and Combustion Science.

The Sandia National Laboratories, researchers’ article provides what the authors say is a necessary science base for auto and engine manufacturers to build the next generation of cleaner, more fuel-efficient engines using LTC.

Low Temperature Combustion Research. Click image for more info.

Low Temperature Combustion Research. Click image for more info.

Musculus sets up the explanation, “Diesel engines are generally more efficient than gasoline engines.  When long-haul truck drivers are burning thousands of gallons per year for cross-country freight runs, or when consumers are faced with high fuel prices, a more efficient engine becomes very important.”

While diesel engines are more efficient, they still have serious pollutant emissions problems.  Gasoline-powered engines have become ever cleaner by inserting better and better catalytic converters between the engine and the tailpipe to clean up pollutants created by the engine.  But the same catalytic converter that works so well for gasoline engines will not work for diesel engines.

Other more complicated exhaust aftertreatment systems are deployed in modern diesel engines, but engine designers and operators would like to avoid the cost and efficiency penalties imposed by those systems.

Musculus exclaimed, “It would be great to find some other way to clean up the diesel engine if we want to enjoy its full efficiency advantages and LTC might just be the solution.”

Largely due to landmark work in the 1980s and 1990s at Sandia’s Combustion Research Facility (CRF) in California, researchers already understand how pollutants are created during conventional diesel combustion. Details of how conventional diesel combustion works – research that took advantage of special optical engines and diagnostics with lasers and scientific cameras to probe the combustion processes – were consolidated into a much-referenced conceptual model developed by Sandia’s John Dec in 1997.

The laser-based diagnostics showed that one pollutant, smoky particulate matter, or PM, was formed in regions where fuel concentrations were too high. Another serious pollutant, nitrogen oxides, or NOx, arose from a high-temperature flame inside the engine. NOx emissions are not only toxic, but also once released into the atmosphere and exposed to sunlight, they react with other pollutants to create ground-level ozone, or smog.

LTC addresses the NOx emissions by recirculating some of the exhaust gases expelled by a diesel engine back inside the engine, where they absorb the heat from combustion. With this dilution effect, the combustion temperatures are lower so NOx formation is significantly reduced. The other part of the LTC strategy, Musculus said, is to spray in fuel earlier in the engine cycle to give the fuel more time to mix with air before it burns. LTC thereby avoids much of the fuel-rich regions that lead to PM as well as the high temperatures that lead to NOx.

While LTC helps reduce PM and NOx pollution, it is not without its own problems. While NOx and PM are reduced, other pollutants go up, including carbon monoxide (CO) and unburned hydrocarbons (UHC) from the fuel. Both are not only toxic, but they also result in a loss of fuel efficiency.

The CRF research team identified the sources of these emissions from LTC engines using new optical diagnostic techniques. In a breakthrough measurement, researchers used two-photon laser-induced fluorescence to map in-cylinder CO, a difficult measurement that had never been achieved inside a diesel engine.

Detecting UHC is also problematic because many different chemical species make up the overall UHC, and their composition evolves during combustion. So, instead of detecting UHC directly, researchers used laser-induced fluorescence of other markers of combustion, such as formaldehyde and hydroxyl, to observe and understand the chemical processes that lead to UHC. The combined measurements showed that the fuel that ended up near the fuel injector was “over-mixed” – there was too much air and not enough fuel, so the fuel couldn’t burn to completion, leading to the CO and UHC in the exhaust.

With this new understanding of UHC and CO emissions, Musculus and Sandia post-doctoral researcher Jacqueline O’Connor looked for a way to increase the fuel concentration in that area. One way, they discovered, is to add post-injections, which are smaller squirts of fuel after the main spray, which add more fuel in just the right area. With the post-injections, the zone of complete combustion extends over a larger region, leading to lower UHC and CO emissions while increasing efficiency by making sure that “less fuel is wasted by not even burning it,” as the press release writer put it.

Musculus and his colleagues, through their latest research paper, hope to communicate the details of how LTC works to the broader 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.

Seems like a good start and the description is rather counter to the titling – it would seem easier to explain that the combustion ignition and burn happens at a lower instead of overheated circumstance so thermal efficiency isn’t challenged.  Nor is there a suggestion of a load of energy consuming apparatus. Perhaps fuel injectors will modify to optimum performance at little cost.

Keep the work coming, those streams of black smoke seen commonly a couple generations ago are not gone, and today they represent lots of money just pumped dirtily into the sky.


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