Honda’s Approach for US Clean Diesels: PCCI and LNC
22 August 2006
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.
Resources:
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
Once again, poor quality of US fuels causes emissions problems. Right when we were promised ULSD solved all the emissions problems, they remind us that our low and varied cetane diesel fuel makes it hard to pass emissions requirements.
Posted by: Sid Hoffman | 22 August 2006 at 04:25 PM
To remove the cause of pre-ignition of the Otto cycle engine, Randolph Diesel invented Diesel cycle diesel engine to compress the air alone. The task to reduce engine-out emissions and sfc of four-stroke engines is far more difficult than preventing pre-ignition. Reducing engine-out emissions without penalizing fuel efficiency requires low-temperature combustion without EGR. Achieving high thermal efficiency without high combustion temperature requires higher expansion ratio than the compression ratio. Obtaining high mechanical efficiency requires high power density such that mechanical losses per unit power output is reduced. A four-stroke engine having expansion ratio equal to or less than the compression ratio has no chance to reduce both engine-out emissions and sfc. Therefore an overexpanded two-stroke engine has been created. It has low-temperature combustion process designed solely to meet emission standards without EGR and an expansion ratio much larger than the compression ratio (as in a Miller cycle) for higher thermal efficiency. The difference in stroke lengths is utilized for a long scavenging process such that an overexpanded two-stroke configuration (without port on cylinder wall) is obtained to greatly increase the power density. It should be obvious that engine research programs concentrated on combustion process can not possibly increase thermal and mechanical efficiencies of four-stroke engines. For more information please contact me. My email address is [email protected]
Posted by: Pao Chi Pien | 22 August 2006 at 06:39 PM
That's quite ingenious, Pao!
Toyota and Ford are circumventing the Diesel's emission and weight problem by using 4-stroke Atkinson-cycle engines in their hybrids. The disadvantage of this is the lower power density of this type of engine, due to the lengthened expansion stroke and the shortened compression stroke gather less air and fuel volume in comparison to a Diesel compression stroke. By using a two-stroke cycle instead, then more specific power can be obtained per unit of displacement, due to more frequent power stroke.
But, then, you'll need an air blower to force air into the cylinder, which robs some power from the engine, then you will need a direct fuel injection instead of port injection, both of which will be more expensive than a normally-aspirated 4-stroke engine with port fuel injection.
Do you plan to use poppet valve(s) for air intake? If so, this type of valve will introduce swirl into the intake stream, causing mixing of intake and exhaust gases inside the cylinder, thus reducing the efficiency of exhaust gas scavenging. Poor scavenging will lead to loss of power and engine overheating. Other type of valves don't seal well nor hold up too well against the heat and pressure of combustion.
My email address is [email protected] if you'd rather reply directly.
Posted by: Roger Pham | 22 August 2006 at 09:01 PM
Surely you mean RUDOLF Diesel?
Posted by: Ruaraidh | 23 August 2006 at 01:02 AM
Roger,
The overexpanded two-stroke engine has a crankcase air compressor. Because the engine expansion stroke is the compression stroke of the crankcase compressor, a large scavenging ratio is obtained. The difference in stroke lengths is utilized for scavenging process. When the intake valve closes, the piston already has traveled more than one half of the stroke for a scavenging pressure of 22.5 psia. I am going to send you the schematic views of the overexpanded two-stroke engine with a crankcase air compressor. I Want to emphasize that engine research programs concentrated on combustion process alone can not possibly increase thermal and mechanical efficiencies of four-stroke engines.
Posted by: Pao Chi Pien | 23 August 2006 at 02:58 AM
Echoing Sid...how on earth are we to meet "silk purse" emission regs with "sows ear" fuel? This crap has got to end. Maybe sour OPECs could do the planet a favor by scrubbing their filthy crude before export.
Posted by: [email protected] | 23 August 2006 at 05:46 AM