=Paper=
{{Paper
|id=Vol-452/paper-2
|storemode=property
|title=Studies of DI Diesel engine cold start combustion in an optical engine
|pdfUrl=https://ceur-ws.org/Vol-452/paper2.pdf
|volume=Vol-452
}}
==Studies of DI Diesel engine cold start combustion in an optical engine==
https://ceur-ws.org/Vol-452/paper2.pdf
Studies of DI Diesel Engine Cold Start Combustion in an Optical Engine
J.M. Desantes, J.V. Pastor * , J.M García-Oliver, J.G. Ramírez-Hernández
CMT-Motores Térmicos
Universidad Politécnica de Valencia, Spain
A study on ignition and early combustion development has been performed in an optical engine at in-
cylinder thermodynamic conditions representative of those occurring during the cold start of a modern DI
Diesel engine for passenger cars at an ambient temperature of -20ºC. A general description of the
process has been derived from in-cylinder pressure analysis and high speed imaging, and the effect of
most relevant engine parameters has been analysed providing guidelines for cold start optimisation.
Introduction peak temperature under motored, but respect-
Cold start at low temperatures in current D.I. di- ing distance between piston and cylinder head
rect engines is a problem which has not been at TDC so that most flow patterns are not se-
properly solved yet and becomes particularly criti- verely modified.
cal with current trend to reduce engine compres- • Reduction of engine speed to the lowest con-
sion ratio [1]. To promote fuel ignition a heating trollable value (250 rpm) so that engine dynam-
plug is usually installed in the cylinder head of ics is reproduced as closely as possible.
passenger car DI diesel engines. • External supercharging to achieve peak pres-
Although it is clear that there are some key fac- sure values of the real engine during cold start.
tors whose control lead to a proper cold start • Control of temperature of all engine fluids (wa-
process, their individual relevance and relation- ter, oil and fuel) to the minimum value achieva-
ships are not clearly understood [2-6]. Thus, efforts ble to avoid moisture condensation problems on
on optimisation of the cold start process are mainly engine surfaces which allows long-duration sys-
based on a trial-and-error procedure in climatic tematic experiments to be performed.
chambers at low ambient temperature, with serious • External operation of the injection system to
limitations in terms of measurement reliability dur- allow skip-fire mode to avoid temperature tran-
ing such a transient process, low repeatability and sients, to reduce instability problems near the
experimental cost [7-9]. operation limits of the injection system itself, to
This paper presents a novel approach for the avoid any engine ECU corrections, to provide
study of the first injection cycle of a light duty en- full flexibility to change any injection parameter,
gine at -20ºC during the starting process, combin- and to suppress the influence of residuals upon
ing visualization tests in an optical engine and heat ignition.
release analysis for individual cycles. Systematic • Measurement of instantaneous intake and ex-
studies, in the borderline between ignition success haust mass flow rate, so that blow-by can be
and misfiring are performed so that the relevance measured indirectly. Such a parameter, which
of engine parameters on ignition success can be is not usually controlled in optical engines but
assessed. On the basis of these studies, a descrip- can be severely influenced by piston-rings
tion of the combustion process under cold start wear, is crucial for the proper estimation of
conditions has been derived. mass trapped in the cylinder and, consequently,
the real bulk gas temperature.
Experimental facility and methodology • External control of glow plug power to control
Target in-cylinder conditions to be reproduced heat transfer.
in the optical engine around top dead center have • Flexibility for orientation between sprays and
been estimated from experiments in climatic glow plug.
chamber similar to those made by Payri et al [7].
Peak in-cylinder pressure is between 25 and 30 Preliminary tests showed that the ignition
bar, and peak in-cylinder temperature is between process under cold-start conditions can be ex-
335 – 350ºC. tremely variable. Thus, the analysis of consecutive
A conventional single-cylinder research engine combustion cycles (with 20 motored cycles be-
with optical access through the piston bowl, in a tween them) have shown different combustion
fully equipped test cell has been modified for the patterns appearing randomly, as shown in figure 1:
study of cold start in the following aspects [10]: in some cycles fuel does not ignite, in some others
ignition takes place properly and in some others
• Reduction of compression ratio and control of ignition occurs too late to be considered as suc-
intake air temperature to achieve in-cylinder cessful ignition for cold start.
* Corresponding author: jpastor@mot.upv.es
Towards Clean Diesel Engines, TCDE2009
Time ASOE: 1992 us 2656 us 3320 us 4980 us
8
DHRL[J/CAD]
1x10
80 Icumul[-]
400
In-cylinder pressure [bar]
1x107
60 300
1x106
40 200 9960 us
20 100 1x105
0 0 1x104
-5 0 5 10 15 20 25 30
Crank angle [CAD]
Fig. 2. Image sequence of the pilot flame combustion
with cycle-resolved evolution of pressure and luminosity.
Fig.1. Combustion patterns for individual cycles for a
single test. Since energy required for fuel evaporation
stems not exclusively from the heating plug but
Thus, an analysis methodology was defined for also from the surrounding air, this provokes a de-
this particular kind of studies combining different crease of local gas temperature which could cause
sources of information: flame quenching and slow down or prevent proper
• Conventional heat release analysis derived flame propagation.
from in cylinder pressure traces, but applied to So, the rest of the fuel of this and the other
individual cycles and adapted in some few as- sprays further away from the glow plug will be
pects, burnt later, only if a second injection (main) is per-
• Light radiation registered with photodiodes and formed.
photomultipliers using or not interference filters If main injection is introduced into the chamber
to track different radicals, in the period where pilot mass autoignition occurs,
• High speed imaging with a CMOS camera at ignition delay for main injection is within the injec-
6000 frames per second under different optical tion event duration. A very steep heat release is
configurations and obtained, usually with a single peak. Images have
• Other complementary sources of information shown that the intense heat release period corres-
such as injection rate and spray momentum ponds to the time interval where a reaction front is
flux measurements or outputs from modeling. travelling throughout the combustion chamber (see
OH images of fig. 4). This front burns the prepared
Results mixture. After the peak, injection is usually over.
In order to improve knowledge about the Due to the higher temperature, liquid fuel from the
process, only pilot injection test cases (figure 2) piston wall can evaporate and burn and a residual
have been considered in addition to single+main combustion process can be observed during the
injection cases (figure 3). A general description of rest of the combustion process. In spite of the low
the combustion process has been derived, for intensity of this late combustion period, the amount
conditions in which combustion succeeds: of energy may reach around 20-30% of the total
The injected fuel mixes with air, but due to the energy release. However, this phase is really diffi-
low air and engine temperatures evaporation be- cult to control.
fore the start of combustion is really poor. Accord-
ing to calculations, only around 10-20% of the in-
jected mass is evaporated, and most of the evapo-
ration proceeds during the injection event. The rest
of the injected mass is deposited on the piston
surface in liquid state. If heat release starts, some
additional evaporation occurs due to combustion-
induced heating. Limitations in evaporation are one
of the main hurdles for obtaining an acceptable
combustion process.
After pilot mass is evaporated, most of it under-
goes a long autoignition process. Fuel injected
close to the glow plug starts burning around 3 ms
after the start of injection. However, due to the low Fig. 3. - Image sequence of the pilot+main flame
amount of fuel vapour, reaction does not propa- combustion with cycle-resolved evolution of pressure,
gate to the rest of the chamber, and heat release is luminosity and rate of heat release.
too low to be detected.
Injection pressure plays a major role on the ig-
nition success and combustion stability. Results
show that low injection pressures improve largely
ignition probability, since flow velocities are low
and consequently, energy dissipation rates. How-
ever, if combustion progresses, higher injection
pressures lead to better mixing and higher imep is
obtained.
Other factors analyzed (e.g. injector-glow plug
orientation and distance, glow plug temperature,
in-cylinder pressure and temperature, or swirl in-
tensity) can modify or modulate the picture de-
scribed above on the combustion process. Howev-
er, further research is necessary to clarify their
influences
Acknowledgement
The authors acknowledge that part of this work has
been performed in the frame of the OPTICOMB
project (TRA2007-67961-C03-01) funded by the
Spanish Ministry of Education and Science.
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