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Diesel engines

Transient Emissions Analyzers

Vehicle emissions legislation around the world requires the testing of vehicles against various standard drive cycles, all of which are transient in nature. (Examples include the NEDC, FTP, 10 and 15 mode cycles).

Conventional emissions analyzers have time responses ~1 second T10-90%. As an engine undergoes many firing cycles during this time period, and since engines often exhibit significant cyclical variation (even at steady state), fast response emissions analyzers are vital tools to calibrate engines for emissions compliance during transient operation.

Cut away view of an engine cylinder showing valves and spark plug

Residuals / EGR

The NDIR500 CO and CO₂ analyzer is able to measure the concentration of residual gas inside the cylinder at the end of compression. The fast time response (T10-90% 8ms) allows this calculation to be performed on a cycle by cycle basis, during engine start and normal operation.

For information on the application of Cambustion analyzers to EGR measurement, please visit our EGR page.

This technique is useful for calibrating EGR systems, and also to reveal imbalances in EGR in multi-cylinder engines, which may occur only at certain speed/load points or during transients.

Particulate Emissions, NOₓ and Diesel Particulate Filters

Diesel engines inject liquid droplets of fuel (finely atomized) into the hot air inside the cylinder after compression has occurred. In the high temperature environment, the droplets vaporize on the surface and burn. The rate of combustion is normally limited by the rate at which fresh oxygen can diffuse into the combustion zone.

Due to the limited time available for combustion before expansion cools the cylinder contents and prevents further reactions, complete combustion does not occur. In rich regions such as around the droplet, some of the carbon from the partially burnt fuel condenses to form soot particles, which also absorb other combustion products which may be liquids.

The production of particles is therefore heavily related to the air fuel ratio. Although Diesel engines normally operate with excess air, the nature of the combustion process is such that soot is always produced. Higher fuel injection pressures (resulting in smaller droplets) and better control over fuel and air have greatly reduced particle emission.

The use of EGR to reduce NOₓ results in a reduction of the available oxygen, which leads to an increase in particle emissions (both in mass and number). The trade-off between NOₓ and particulates is a familiar one to Diesel engine calibrators.

It is during speed/load transients (where accurate air, fuel and EGR control is necessarily much harder to achieve) where a significant proportion of total cycle emissions may occur.

The CLD500 is a fast response NOₓ analyzer, with a time response (T10-90%) 10ms. This is fast enough to resolve cyclical variation in NOₓ, which can be very significant due to variations in EGR distribution.

Historically particulate emissions from Diesel engines have been restricted according to filter paper (gravimetric) measurements. In Europe the adoption of a particle number limit (n/km) has further reduced permissible tailpipe particle emissions, and encouraged engine manufacturers to adopt Diesel Particulate Filters (DPFs). These filters can reduce the particle concentration by greater than 99%, but this means that measurements of tailpipe particulates are no longer indicative of the engine emissions.

The Cambustion DMS500 offers a faster method of measuring both particle mass and number, which correlates with legislative techniques. With time responses as low as 200ms T10-90% and the ability to sample raw exhaust (not requiring a CVS tunnel, and able to sample upstream or downstream of a DPF) Cambustion's particulate analyzers reveal features of engine operation which offer calibrators and developers crucial data to meet emissions legislation.

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