The Volkswagen emissions scandal has picked the scab off a festering wound for the diesel engine that never seems to completely heal: It’s a dirty motor, always has been.
One Band-Aid after another has been applied to the diesel over the years to make it just clean enough to stay ahead of tightening global emissions regulations that target carbon dioxide (CO2) in Europe and nitrogen oxide (NOx) in the United States.
Diesels are bandaged by a number of expensive traps, filters and catalysts in the exhaust system. Some diesels require frequent inoculations of urea to clean the exhaust. These and other components reduce harmful emissions, but they also make the diesel engine less efficient and lower performance.
And VW has shown how easily these emissions systems can be manipulated to skirt regulations.
But long before the VW mess, combustion researchers and engineers at Oak Ridge National Laboratory in Tennessee started working to reduce harmful diesel emissions at the source: in the cylinder.
I spoke with Jim Parks, group leader of emissions and catalysis research, and Scott Curran, an Oak Ridge researcher who works on combustion and emissions, about the potential of curing the diesel’s dirty habits without adding expensive components downstream of the engine.
They, along with about 40 researchers and combustion engineers, are working on a pair of interesting projects that hold the potential to knock most NOx out before it’s produced.
This is complicated stuff, but it underscores how complex the job of cleaning up the diesel engine will be.
One project eliminates most NOx in-cylinder by lowering the temperature of diesel combustion. This is done by injecting the diesel fuel far earlier in the combustion cycle.
“With a normal diesel combustion event, you inject fuel almost when the piston is at top dead center on the power stroke,” Curran says. “So, temperatures and pressures in the cylinder are the highest they will ever be. That’s why you don’t need a spark plug to ignite diesel fuel.
“The earlier you inject diesel, the more mixed [with air] it will become, and the fewer rich pockets you will have, and the less NOx you can form.”
Parks says Oak Ridge has a low-temperature combustion diesel engine up and running in its lab, but it won’t yet work on the road in a car, and it might be as long as 10 years before low-temperature combustion makes production.
Curran says that while low-temperature combustion has great potential to greatly reduce NOx, it creates other problems. A diesel engine running with low-temperature combustion is louder than today’s production diesels. It would emit more hydrocarbons and CO2, Curran says. It would need revised fuel injectors and pistons, and, more important, more precise sensors. And that is part of the other project underway at Oak Ridge.
Researchers there are creating a new generation of sensors that would be installed in each cylinder, in the intake system and in the exhaust system. The goal: Improve individual management of each cylinder and get them to perform uniformly.
In today’s engines, Curran explained, all cylinders are managed collectively, not individually. The sensor technology being developed at Oak Ridge aims to ensure that each cylinder’s performance is harmonized. One of the most important sensors measures the pressure in the cylinder as well as the strength of the combustion. Some early versions are already in production.
“If you are off a little bit because of a couple of misfires, you don’t know what cylinder is responsible for that and when it happened. But you can adjust global parameters on a slow time scale,” Curran says. “But if you have this really fast measurement in the cylinder, the theoretical limit is the next [combustion] cycle, or in the same cycle, you could adjust how much fuel and how you are injecting that fuel and change it almost instantly.”
The exhaust gas recirculation system -- which routes some of the exhaust gases back into the cylinder to lower combustion temperature -- provides a good example of how a cylinder pressure sensor could reduce emissions.
“When the EGR valve opens,” Curran says, “the first cylinder that sees the spent exhaust gas will behave very differently than the one that just fired. It’s kind of like four individual engines being run in a carefully orchestrated way.”
It might not look like car diesel engines have a very bright future today, but there are a lot of creative engineers working to solve their many problems.