In developing its promised Tier 2 Bin 5 diesel solution for the US (earlier post), Honda is concentrating on advanced combustion management with Premixed Charge Compression Ignition (PCCI) and a trap-type lean NOx catalyst (LNC), according to Yasuyuki Sando, Honda Senior Manager, Advanced Powertrain.
In a presentation at the Diesel Engine-Efficiency and Emissions Research (DEER) 2006 conference, Sando noted that Honda has made a great deal of progress with its advanced combustion management and aftertreatment systems, but that hurdles remain.
Premixed Charge Compression Ignition is one of a number of variant approaches to delivering lower engine-out NOx. Others in this category include HCCI (Homogeneous Charge Compression Ignition), CAI (Controlled Auto Ignition), and LTC (Low Temperature Combustion).
For its approach to PCCI, Honda designed a new piston bowl and optimized the nozzle, further cooled the Exhaust Gas Recirculation (EGR), and initiated timing at close to TDC (top dead center). At very light loads, Sando said, soot and NOx are almost zero. At higher loads, the level of engine out emissions can be lowered drastically.
With that engine-out emissions capability, Honda opted for a trap-based Lean NOx catalyst system to meet Bin 5 NOx levels.
Overall, the industry is converging on two basic approaches to NOx reduction for clean diesels: the lean NOx traps (LNT) and urea-based SCR. LNT is a simpler, less costly solution, but it also offers reduced efficiency in NOx removal.
As a result, manufacturers are tending to look to LNT systems for smaller vehicles, while the urea SCR systems—even with the additional burden of the urea systems issues—will handle the larger vehicles.
Tim Johnson from Corning, in another DEER 2006 presentation, calculated that the crossover point for LNT versus urea SCR was at about the 2.0-liter engine size (factoring in cost as well as regulatory requirements). Johnson also noted, however, that implementing advanced combustion regimes in engines to reduce engine-out NOx could push that crossover point up to a 5.0-liter displacement.
Despite some of the challenges faced by LNT—such as increased fuel consumption and durability—Sando said that Honda believes the technology is promising for passenger cars.
Sando was also very clear that the recently-patented Honda plasma-assisted catalyst system for NOx reduction (earlier post) is not factoring in to the company’s product plans for the upcoming clean diesel.
In addition to solving problems with durability for the LNC, Sando pointed out two other areas of challenge: meeting the stringent US On-Board diagnostic (OBD) requirements, and US fuel quality.
Echoing comments made by other presenters at DEER 2006, Sando noted that US diesel fuel has a lower bottom threshold and wider variation in cetane numbers compared to European and Japanese fuels.
For our emissions test procedures, we use EPA test fuel with low cetane, but in the actual market, the fuel can have much higher cetane. Essentially we need diesels that can cope with that variation of cetane number, which has influence on high EGR rate combustion.
An engine calibrated on 57 cetane misfires on 47 cetane, but if calibrated on 47 cetane, soot increases on 57 cetane.
A control system is required, and Honda is currently studying a method to detect the cetane number of the fuel using an in-cylinder pressure sensor.
SAE paper 2004-01-1316: Development of New 2.2-Liter Turbocharged Diesel Engine for the Euro-IV Standards
SAE paper 2005-01-0378: PCCI Operation With Early Injection of Conventional Diesel Fuel
SAE paper 2006-01-0920: PCCI Operation with Fuel Injection Timing Set Close to TDC
SAE paper 2006-01-0180: Study on Ignition Timing Control for Diesel Engine Using In-Cylinder Pressure Sensor