Advanced Low Temperature Diesel Combustion: Low Emissions and Better Fuel Economy

29 April 2008

Researchers at Wayne State University and the US Army’s TARDEC (Tank-Automotive Research, Development and Engineering Center) are proposing a strategy to reduce the penalties in diesel engine performance, fuel consumption and HC and CO emissions associated with a low temperature combustion (LTC) operating regimes.

LTC spans a number of approaches designed to reduce engine-out emissions of NO_x and PM. The researchers devised an Advanced Low Temperature Combustion (ALTC) strategy to counter the downsides of one of those, the LTC Modulated Kinetics (LTC MK) approach. LTC MK reduces NO_x and PM by burning a pre-mixed fuel-air charge, low in oxygen concentration, at low temperatures.

LTC MK accomplishes this by retarding injection timing to have combustion start late in the expansion stroke; applying high EGR rates; using medium injection pressures; and increasing the swirl ratio to increase the rate of burning. However, while NO_x and PM are lowered, LTC MK causes fuel penalties, higher HC and CO emissions, and unstable engine operation. To address that, the researchers are proposing a four-step approach.

• Combustion phasing. The first step advances the start of combustion (SOC) to improve fuel economy and to stabilize the operation of the engine. This also has the effect on increasing NO_x and PM.

• Increased EGR. To address the increased NO_x, the second step is to increase Exhaust Gas Recirculation (EGR) from 60 to 64%—made possible by the early combustion phasing in the first step. This results in an increase in PM emissions.

• Increased injection pressure and/or swirl ratio. To deal with the increased PM, the third step they took was to increase injection pressure. In work presented at the recent 2008 SAE World Congress, they boosted pressure from 800 bar to 1,200 bar at a swirl ration of 1.44. Alternately, another approach is to maintain the 800 bar pressure and to increase the swirl ratio from 1.44 to 7.12.

• The fourth step is to optimize the combustion phasing, injection pressure and swirl ratio to give targeted engine-out emissions and fuel consumption, with stable combustion and operation at different operating points.

In their paper at the SAE World Congress, the researchers noted:

...optimization should take into consideration the effect of varying LPPC [location of peak rate of heat release due to premixed combustion], EGR, injection pressure and swirl ratio on fuel economy and engine operation. For example, there is no need to go to the LTC regime at the part load used in this investigation...In such a case a lower rate of EGR can be applied, reducing the need to use higher injection pressures or swirl ratios. Producing higher injection pressures consumes more energy in driving the injection pump. Also, higher swirl ratios might affect fuel economy, because of the increase in heat losses...In addition, higher swirl ratios reduce the volumetric efficiency of the engine, increase the load on the cooling system and limit the power rating of the engine.

It should be noted also that the optimization should also take into consideration the effective operation of the aftertreatment devices under the different operating modes of the vehicle. Thus both the engine and aftertreatment devices should be treated as one system that is required to meet certain targets in engine performance, fuel economy and tailpipe emissions.

Resources

• N. A. Henein, A. Kastury, K. Natti and W. Bryzik (2008) Advanced Low temperature COmbustion (ALTC): Diesel Engine Performance, Fuel Economy and Emissions (SAE 2008-01-0652)