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NOₓ Emissions

In many countries around the world, the emissions of NOx from diesel and gasoline vehicles are restricted by legislation. NOₓ is formed in the combustion chamber of engines, when high temperatures cause oxygen and nitrogen (both found in the air supplied for combustion) to combine.

Exhaust Gas Recirculation

Cut-away view of engine cylinder and external EGR loop with intake & exhaust sampling points

A widely adopted route to reduce NOₓ emissions is Exhaust Gas Recirculation (EGR). This involves recirculating a controllable proportion of the engine's exhaust back into the intake air. A valve is usually used to control the flow of gas, and the valve may be closed completely if required.

The substitution of burnt gas (which takes no further part in combustion) for oxygen rich air reduces the proportion of the cylinder contents available for combustion. This causes a correspondingly lower heat release and peak cylinder temperature, and reduces the formation of NOₓ. The presence of an inert gas in the cylinder further limits the peak temperature (more than throttling alone in a spark ignition engine).

The gas to be recirculated may also be passed through an EGR cooler, which is usually of the air/water type. This reduces the temperature of the gas, which reduces the cylinder charge temperature when EGR is employed. This has two benefits- the reduction of charge temperature results in lower peak temperature, and the greater density of cooled EGR gas allows a higher proportion of EGR to be used. On a diesel engine the recirculated fraction may be as high as 50% under some operating conditions.

Graph of EGR delay during gear change of a diesel passenger car

Advantages of EGR

  • Reduced NOx
  • Potential reduction of throttling losses on spark ignition engines at part load
  • Improved engine life through reduced cylinder temperatures (particularly exhaust valve life)

Disadvantages and Difficulties of EGR

Since EGR reduces the available oxygen in the cylinder, the production of particulates (fuel which has only partially combusted) is increased when EGR is applied. This has traditionally been a problem with diesel engines, where the trade-off between NOₓ and particulates is a familiar one to calibrators.

The deliberate reduction of the oxygen available in the cylinder will reduce the peak power available from the engine. For this reason the EGR is usually shut off when full power is demanded, so the EGR approach to controlling NOₓ fails in this situation.

The EGR valve can not respond instantly to changes in demand, and the exhaust gas takes time to flow around the EGR circuit. This makes the calibration of transient EGR behavior particularly complex- traditionally the EGR valve has been closed during transients and then re-opened once steady state is achieved. However, the spike in NOₓ / particulate associated with poor EGR control makes transient EGR behavior of interest.

The recirculated gas is normally introduced into the intake system before the intakes divide in a multi-cylinder engine. Despite this, perfect mixing of the gas is impossible to achieve at all engine speeds / loads and particularly during transient operation. For example poor EGR distribution cylinder-to-cylinder may result in one cylinder receiving too much EGR, causing high particulate emissions, while another cylinder receives too little, resulting in high NOₓ emissions from that cylinder.

Although the term EGR usually refers to deliberate, external EGR, there is also a level of internal EGR. This occurs because the residual combustion gas remaining in the cylinder at the end of the exhaust stroke is mixed with the incoming charge. There is therefore a proportion of internal EGR which must be taken into account when planning EGR strategies. The scavenging efficiency will vary with engine load, and in an engine fitted with variable valve timing a further parameter must be considered.

Application of Cambustion Analyzers to EGR Development

Cambustion's CLD500 NOₓ analyzer offers two channels of simultaneous NOₓ measurement, with a T10-90% of 10ms or less. This allows NOₓ concentrations in the exhaust to be measured for each firing cycle, allowing cyclic variability to be observed.

Cambustion's NDIR500 CO & CO₂ analyzer offers two channels of simultaneous CO & CO₂ measurement, with a T10-90% of 8ms. This allows a variety of applications:

Sampling with the NDIR500 in the intake allows measurement of CO₂ concentration in the intake charge. Measurement of exhaust CO₂ with the other channel of the NDIR allows calculation of the external EGR rate, on a cycle by cycle basis.

Depending on the location of the intake probe, either the overall EGR rate or the EGR rate specific to one cylinder may be measured. This allows verification and improvement of EGR modeling and EGR distribution, including transients.

Sampling with the NDIR probes at different points through the EGR loop allows characterization of EGR system delays and behavior.

Comparison of the CO₂ concentration in the pre-combustion gas with the exhaust gas from the previous cycle allows total EGR (internal + external) to be calculated. This technique can therefore reveal cyclical variation, as well as cylinder to cylinder variation. Such a capability may also be useful when verifying the effects of variable valve timing.

Cambustion's DMS Series particulate analyzers are capable of making exhaust particulate concentration measurements (both particle number and particle mass) and have a T10-90% response time as low as 200ms. While this is not fast enough for cycle by cycle resolutions, the DMS series allows fine tuning of EGR for particulate emissions, and the ability to measure directly in the exhaust allows comparison of different cylinders.

DSA on chassis dynamometer

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