EPA designed its Clean Diesel Combustion technology (earlier post) to prove the potential for a diesel engine design, using innovative air, fuel, and combustion management and conventional particulate matter aftertreatment, to achieve lower NOx levels without the need for NOx aftertreatment.
Clean Diesel Combustion technology began to take form and be appreciated as we were looking for alternative paths to supports EPA’s HD [Heavy Duty] 2007 rules. More specifically, we were looking for combustion approaches that enabled the engine to exhaust the combustion products with engine-out NOx emissions at or below a 0.2 g/bhp/hr level at every point where the engine was required to operate.
This NOxemissions target is the ultimate level of the HD 2007 standard, and will be required for HD engines sold after 2010. From a light duty perspective, this is approximately equivalent to a diesel SUV or light pick-up truck emitting at the Tier2-bin5 level.—David Haugen, EPA Advanced Technology Division, at DEER 2004
“Engine-out” refers to the exhaust gases emerging from the combustion cylinders. The EPA, in other words, wanted to handle NOx in-cylinder to the greatest extent possible, rather than relying on aftertreatments.
The conceptual basis of EPA’s approach is to prevent NOx formation in the first place. Since NOx is formed at high temperatures as a byproduct of hydrocarbon combustion, EPA sought to keep the local temperature below critical NOx formation threshold—around 2,100°K (1,827°C or 3,320°F—yeah, it gets hot in engines).
The EPA team discovered that it could achieve this by reducing oxygen concentration to manage the oxidation (combustion) of the fuel in the diffusion flame region of the cylinder.
There are four key features to the CDC technology:
A hydraulically-intensified fuel system to lower PM and smoke emissions, and improve engine efficiency.
A boost system to increase engine power and the efficiency of the combustion process, thus reducing emissions and increasing fuel economy.
Low Pressure Exhaust Gas Recirculation (EGR) to lower the peak combustion temperature to reduce the formation of NOx.
PM Aftertreatment to reduce the remaining smoke, unburned hydrocarbons (HC) and carbon monoxide in the exhaust to levels required for future emissions standards.
Our work has found that by operating in a region of 11% to 14% intake oxygen, we are able to achieve engine-out NOx levels low enough to meet the level of the diesel emissions standards. Of course, in order to reduce the intake oxygen from 20.9% down to this target range of intake oxygen, substantial levels of EGR [Exhaust Gas Recirculation] are required. And as we all know, moving more charge mass—the fresh air plus higher EGR rates—puts burdens on the boosting system and can make smoke reduction more challenging.—Haugen, DEER 2004
No free lunch. By managing the combustion to minimize NOx, PM, hydrocarbon and CO reduction become more of an issue. On the PM front, after almost two years of work the engineering team managed to cut this type of emission by approximately 70%. At this point, the CDC technology will still require the use of some PM aftertreatment, and the use of oxidation catalysts to handle the CO and HC.
From the NOx point of view, the initial test results as presented at the DEER conference were good. The test vehicle was a minivan, using the same displacement engine as the stock unit.
The Tier 2 Bin 5 specs are highlighted in the middle for comparison, and the different tests represent different types of standardized driving cycles. You’ll not that the CDC technology reduced NOx emissions by an order of magnitude, with a minor penalty in fuel consumption.
|Initial CDC Vehicle Test Results (emissions in g/mi)|
|Tier 2 Bin 5||–||0.018||4.20||0.07||0.010|
The partnership with Ford now will help determine whether or not this technology foundation can be further refined, balancing performance, cost and effectiveness, to the point where it makes sense to deploy in commercial passenger car applications.