Effect of other after-Treatment Systems on Particulate Emission and Composition

Summary

A passenger car engine was equipped with a NOx-trap and the engine management was changed in order to achieve an optimal trapping/regeneration cycle of the trap at constant load and speed. These modifications had a big influence on the soot formation process. At two engine mapping points the particle composition and size distribution was measured. A first idea, if there is any indication to future problems with the measurement technique or to future health problems due to unexpected particulate behaviour was evaluated from these measurements. The measured particulate composition and size distribution downstream the NOx trap are within the range of the particulate emission of engines without NOx trap even during the fuel rich (l < 1) phases. There was no indication to problems with future particulate emissions found.

Introduction

Beside the reduction of the particulate emission the NOx reduction will be important in order to reach future exhaust regulations. In principle three configurations are possible depending on the effectiveness of each component:

Reduction of the engine out emission / Reduction with after-treatment systems
Particulate reduction by Particulate trap / NOx reduction by catalyst (NOX trap, SCR…)
NOx reduction by engine Management (EGR, Injection timing…) / Particulate reduction by Particulate trap
NOx reduction by catalyst and Particulate reduction by Particulate trap

The investigations at VKA (RWTH-Aachen) concerned two questions:

Is there any additional health effect by changing physical or chemical behaviour of the particulate emission to be expected

Are there any problems due to the measurability of the particulate emission to be expected.

Due to these questions the NOx trap was selected for the investigations. The NOx trap requires every few minutes a regeneration by the CO exhaust of a fuel rich combustion (l1). This causes a varying exhaust mass flow even at steady state conditions. (Fig.1) The internal combustion kinetics and the soot formation process is changing dramatically by changing from lean to rich mode. In the moment there is very few literature, how this might change the physical and chemical parameters of the particulate emission.

Engine, engine management and exhaust after-treatment


A prototype 4 cylinder, 1.9 l (92 kW), Common Rail Diesel engine with variable turbo charger was used for the investigations. Full access to the engine management system allows the optimisation of the fuel rich combustion process at constant torque and speed. By switching from lean to rich combustion most parameters (injection timing, rail pressure, exhaust back pressure, EGR-ratio, intake air flow and boost pressure) were changed. A close coupled oxidation catalyst was used in order to increase the CO-concentration for the regeneration of the NOx trap during the fuel rich phases. The NOx trap reduces the CO-Emission efficiently by reaction with the trapped NOx.

Two engine mapping points were selected for the investigations:

1500 rpm;
2 bar bmep
(4,7 kW) / As example for
ECE - test conditions / High changes of the fuel/air ratio necessary for regeneration - fuel and air flow have to be adjusted / Temperature of NOx-trap (210°C) at lower limit of its working temperature
3000 rpm;
8 bar bmep
(38 kW) / Outside of the NEDC - load and speed map / Only small adjustments in the air flow are necessary / Temperature of NOx-trap (450°C) at upper limit of its working temperature

Sampling System

The gaseous exhaust was measured from the undiluted exhaust between the oxidation catalyst and the NOx trap and downstream the NOx trap. The EGR-ratio was determined by CO2 measurement.

A full flow dilution tunnel, controlled by a Venturi orifice was used for regulating the primary dilution ratio. Because of the dynamic exhaust gas flow, only full flow dilution is sufficient to measure the exhaust gas compounds. (Only the lambda- and the NOx sensor are quick enough to measure real time signals from the undiluted exhaust.) A low primary dilution ratio (approx.3.5) was selected in order to be comparable with future test procedures, where a low dilution ratio seems to be necessary in order to achieve sufficient accuracy for the measurement of the gaseous compounds. A secondary dilution (approx. 6.5) was used to reach the maximum Temperature of 52°C at the sample position. The secondary dilution was measured by gas meters. The total dilution ratio (approx. 23) was continually controlled by CO2 measurement of the diluted exhaust.

It was not possible to separate the exhaust of the lean and rich combustion with the impactor. The aerodynamic diameter was determined from the continuous sample during lean and rich phases.

With the DMA the mobility diameter was determined. For a period of 8 minutes (4 lean/rich cycles) the DMA voltage remained constant. One size class was sampled during this period and counted by the CPC. During the fuel rich phases the particle number reaches maximum or minimum values within short peaks. The particle number during the rich phases were evaluated from these peak values and compared to the values of the lean phases. Than the next bigger size class was measured by increasing the DMA voltage. 14 size classes were measured from 10 to 400 nm. Than the measurement was repeated by lowering the Voltage from step to step form 9920 V (400 nm) to 15 V (10 nm). The total test lasts about 4 hours.

Operating the NOx Trap

The engine was switched to the fuel rich regeneration mode for 3 seconds after 120 seconds trapping. Some engine parameters for the engine mapping point 1500 rpm; 2 bar bmep are shown for example in fig. 5.

High CO and Hydrocarbon concentrations (up to 1.5%) were reached during the rich mode upstream the NOx trap as shown in fig. 6. The time resolution of the exhaust gas analysers is not sufficient in order to reach the peak values within 3 seconds.

Because of the sulfate poisoning of the NOx trap a dependency of the fuel sulfur content is not measurable. Therefore a low sulfur fuel (comparable to D4-Fuel) was used. Nevertheless suffered the NOx trap capacity during the tests by the fuel sulfur. A desulfatisation procedure was set up at 650 °C trap temperature. Because of the rapid aging of the trap at these high temperature, it was not possible to desulfate the trap before each test. Therefore the trap capacity varies from test to test.

Fig. 6: Exhaust parameters at 1500 rpm, 2 bar bmep during lean/rich cycles


It might seems to be easier, to measure the exhaust of the rich phases by running the rich phases for a longer time. Unfortunately this is not possible because of the rapid heating of the trap during the rich phases. For example the exhaust temperatures are shown in fig.7 for a 30 sec rich operation compared to the used 3 sec regeneration mode.


For normal trap operation the trap temperatures for the selected engine mapping points are shown in fig. 8.

Particulate analysis

The particulate matter was sampled during 4 hours test time on 70mm T40 A60 Teflon coated borosilicate filter and analysed by two consecutive extractions as shown in fig. 9. The solubles were divided into an un-polar (SOF) and a polar (WSF) fraction. The insoluble part (unsol) was weighted The results of the extraction was compared with a volatility analysis by thermo gravimetry: A sample of soot was scratched off the filters and heated on a thermo balance up to 650°C under Helium. Than oxygen was added and the non-volatile fraction was burnt. While the weight of the sample was recorded. Some ash remains on the thermo balance. The insoluble part as well as the non volatile (650°C) part should mainly consist of soot. Unfortunately thermo gravimetric analysis of particles sampled at 1500 rpm; 2 bar was not possible. Although a high amount of particles was sampled during 4 hours sampling time it was not possible to scratch them off the filter surface because they deposit inside the fibres.

The results of the extraction are shown in fig.10 (1500rpm; 2barbmep) and in fig.11 (3000rpm; 8barbmep). They are compared with the total mass collected by the measurement of the aerodynamic size distribution on the impactor. A Berner low pressure impactor was used with 10 aerodynamic size classes. The mass collected on the impactor usually is lower, than the mass collected on the filter. First reason is, that a much lower volatile fraction was deposit on the aluminium foils of the impactor because of the lower pressure and the smaller un-polar surface of the foils compared with the borosilicate fibres of the filters. For the non volatile particles the collection efficiency of the impactor is about 60% to 70% because a part of the material is blown off the surface. For this reason the sum of the
collected material is usually little lower than the non soluble fraction collected on the filters.

Conspicuous is the high soluble fraction of the particulates upstream the NOx trap at 1500rpm; 2barbmep. This fraction was removed largely catalytic by the NOx trap. Differences measured with different dilution ratios are within the statistical error of the particulate measurement.


The thermo gravimetric analysis of the particulate samples at 3000rpm, 8bar shows, that the non volatile fraction is nearly identically with the non soluble fraction (fig.12). The volatile fraction consists mainly of low boiling compounds like fuel and oil compounds for the samples downstream the NOx trap and for the lean combustion. The rich combustion however produces a significant portion (20%) of high boiling compounds, volatile between 450°C and 650°C. This can not be a part of the engine oil. It might consist out of tary compounds, build at low combustion temperatures from the late fuel injection. This fraction is widely removed by the NOx trap (similar to the soluble fraction, build by the rich combustion at 1500rpm, 2barbmep

Particulate size distribution

The aerodynamic size distribution is shown in fig. 13 (1500 rpm; 2 bar bmep) and fig. 14 (3000 rpm; 8 bar bmep) In both cases the NOx trap reduces the aerodynamic diameter. It was not possible to separate the size distribution of the fuel rich phases from the lean phases. The measured size distribution from all particles emitted is similar to the size distribution of state of the art vehicles. The effect, that the aerodynamic diameter was reduced by active catalysts was observed for other catalytic systems too. Probably this effect is mainly due to a reduction of the particle density. As shown in figure 14 the mobility diameter is reduced a little bit by the NOx trap. The mobility diameter is comparable with the geometric diameter. The reduction of the particle size is not only a density effect

Especially the rich combustion don’t produce a special mode of small particles.

Fig. 13: Aerodynamic size distribution (1500 rpm; 2 bar bmep)

Fig. 14: Aerodynamic size distribution (3000 rpm; 8 bar bmep)

The mobility size distribution the rich combustion differs only a little bit from the distribution of the lean combustion. This is shown in a normalized scale (fig. 15: 3000 rpm; 8 bar bmep and fig. 16: 1500 rpm; 2 bar bmep). A small shift to bigger diameters is within the accuracy of the measurement. The catalytic NOx trap reduces the mobility diameter a little bit (fig. 15)

Fig. 15: Mobility size distribution (1500 rpm; 2 bar bmep)

Fig. 15: Mobility size distribution (3000 rpm; 8 bar bmep)