Transient Emissions Challenges
T
he drive for improved fuel economy has
spawned development of a new generation of
turbocharged GDI engines. Simultaneously,
emissions legislation requires ever lower
emissions levels and since 2014 includes a Particle Number
constraint which will tighten further in 2017.
Two common scenarios arise in which there is a significant
and disadvantageous effect on emissions.
Initial emissions calibration of engines and their
aftertreatment systems is undertaken at various
representative steady state points, but it is in investigating
transients during drive cycles where the use of fast-response
emissions analyzers becomes crucial.
Recent studies at Cambustion tested a 2.0-litre
turbocharged GDI engine on a variety of drive cycles.
Data from a WLTC Class 3 (intended for the European and
Japanese markets) are discussed here. Although the vehicle
is unlikely to have been developed for this cycle (it is Euro
6 compliant), identification of features of interest can show
changes that could be used to ensure compliance.
The longer cycle duration (WLTC is 1798s, NEDC is 1198s)
means that the cold start is less significant for the total
cycle emissions- the aftertreatment is working for a greater
proportion of the cycle. Attention to the transient operation
which dominates the rest of the cycle is therefore of
increased importance, reinforced by the higher loads and
more aggressive accelerations.
Cambustion fast-response analyzers for HC, NOx, CO&CO2
and particle number / size were used. Several features of
interest were identified by the fast analyzers, the most
significant one being the effects of blow-through on NOx and
Particle Number.
Effects of Blow-through on Tailpipe NO Emissions on a 2.0L
Euro 6 Turbocharged GDI Engine
Blow-through causes the exhaust gas presented to the
aftertreatment to become overall lean, even though
combustion in the cylinder was close to stoichiometry. This
produces low PN (since the combustion lambda remains well
controlled) but can lead to NOx breakthrough at the tailpipe.
The phenomenon of blow-through (over-scavenging) has
been observed in other boosted GDI engines. During valve
overlap, and when significant boost pressure exists, intake
air may pass straight through the exhaust valve. True
combustion lambda and measured exhaust (aftertreatment)
lambda diverge. The level of blow-through depends on many
factors, including boost pr essure, engine speed and valve
timing (which is usually variable depending on load).
emissions legislation
requires ever lower
emissions levels and
since 2014 includes
a Particle Number
constraint which will
tighten further in 2017.
Effects of Blow-through on Tailpipe PN Emissions on a 2.0L
Euro 6 Turbocharged GDI Engine
Alternatively short-duration spikes of PN can appear during
hard accelerations and gear changes, despite a warm
engine. The exhaust lambda does not indicate particularly
rich conditions, but the fast CO analyzer identifies that
combustion lambda was rich. The difference between
combustion lambda and exhaust lambda is explained by
blow-through.
The use of fast analyzers to identify such problems and
to help assess calibration improvements is likely to be
indispensable as more transient drive cycles are adopted for
legislative purposes.
www.cambustion.com
Issue 17 PECM
17