Directly actuated piezo injector for advanced injection strategies towards cleaner
diesel engines
O. Kastner*, F. Atzler, C. Juvenelle, R. Rotondi, A. Weigand,
Continental Automotive GmbH
Siemensstrasse 12, 93055 Regensburg, Germany
Low fuel consumption is one of the greatest merits of the Diesel engine, emission of nitrous oxides, NOx, and particu-
late matter, PM, are its most prominent disadvantages. For modern passenger car diesel engines, to achieve low fuel
consumption targets, i.e. CO2 emissions, and to simultaneously fulfil the ambitious EU6 emissions legislation, a com-
bination of intelligent injection strategies and high EGR rates with appropriate boost pressures is required.
Generally, NOx emissions can efficiently be decreased via exhaust gas recirculation, EGR. An unwanted conse-
quence of this is an increase in smoke, HC and CO emissions. This can be alleviated by a suitable application of
injection strategies. Injections strategies are also employed to curb excessive emissions of unburnt hydro-carbons,
HC, and carbon monoxide, CO, particularly in conditions of cold start. These strategies may also be used to facilitate
fast light-off of the oxidation catalyst.
In the current work the novel directly actuated piezo injector of Continental Automotive GmbH was used to evaluate
the emissions potential not only of multiple injection patterns, but also of advanced rate shaping strategies.
The application of this wealth of technology puts further cost onto the already expensive Diesel engine. Nevertheless,
there is a need to explore the benefits and the limits of such technology, since the fuel consumption and CO2 emis-
sion target of 120g/km across the European passenger car fleet can only be achieved with a sufficiently high share of
Diesel vehicles, also in the most dominant market segment of small and medium sized cars.
Cost in this respect needs to be considered from an overall engine system point of view. Higher cost for measures to
reduce engine out emissions may be well invested if exhaust gas aftertreatment can be avoided or at least be mini-
mised.
Introduction mogeneous charge compression ignition", HCCI,
The advent of the EU5 and EU6 [1] legislation the combustion initiation still is controlled by injec-
marks the revision of ideas and methods to tion timing. This means, that at a desired time with
achieve ever lower emissions targets. Where hith- respect to engine top dead centre at least the
erto the decrease of the "regulated four" - NOx, minimum critical mass of fuel, necessary to create
PM, CO and HC – was of prime importance, the loci of ignition, is introduced. Nevertheless, both
request to lower CO2 emissions – i.e. fuel con- techniques share to some extent a problem, also
sumption – slowly gains equal significance. Among well known from premixed gasoline engine: the
the most prominent means to reduce fuel con- presence of premixed fuel and air close to the
sumption is the decrease of engine size, generally combustion chamber walls, where wall quenching
termed "downsizing" [2], to reduce the level of leaves a layer of unburnt mixture. If additionally the
friction at part load. Part load is the dominant op- mixture is overly lean, the problem of hydrocarbons
erating condition of passenger car engines and leaving the engine unused is aggravated by flame
plays a large role in all of the currently applied quenching in mid air. This can be overcome to
homologation cycles. The other simple but effec- some extent by stratifying the in-cylinder charge by
tive way to reduce fuel consumption is the reduc- means of smart injection strategies [7, 8, 9]. These
tion of the overall level of engine speed. This is strategies have to respect both boundary condi-
termed "downspeeding" and denominates the ap- tions, that of the location of the fuel in the combus-
plication of very long gearing in six and even seven tion chamber and the available time for mixture
speed transmissions. preparation before the onset of the main ignition.
Both, downsizing and downspeeding, require Once ignition has occurred, the feeding of more
the increase of engine torque to provide the de- fuel to the combustion has to be done such, that
sired power for pleasurable driving. As a conse- excessive peak temperatures are avoided, since
quence engine boosting has to be increased mas- this helps to reduce NOx.
sively and, to limit the resulting NOx production, At the end of the combustion process, in-
simultaneously EGR rates have to be raised [3, 4, cylinder pressure and temperature drop, and
5]. charge motion decays. There, a post injection in-
The presence of large portions of more or less troduces some additional momentum for mixing
inert gas in the combustion chamber converts the and some thermal energy for further oxidation of
conventional Diesel combustion process such that carbonaceous species [10].
prolonged ignition delays allow for a more ex-
tended mixture preparation period before ignition
[6]. In contrast to the predominantly premixed "ho-
* Corresponding author: Oliver.Kastner@continental-corporation.com
Towards Clean Diesel Engines, TCDE 2009
Conclusively, it can be summarised, that injec- linearly with increasing engine speed, and the igni-
tion strategies serve the following purposes: tion delay is determined by the pressure and tem-
perature rise during the compression phase of the
1. appropriate pilot injections facilitate an op- engine. Also, at higher engine speeds the heat
timal mixture preparation before ignition of loss to the walls will be smaller, i.e. the gas tem-
the main injection perature for a given crank angle position will be
2. the injection rate allows to some extent higher, which shortens the ignition delay. Addition-
the control of cylinder temperature during ally, when the engine is boosted at higher loads,
combustion there will be a higher gas mass present in the cyl-
3. an appropriate post injection enhances inder. This increases compression pressure and,
the oxidation of carbon based pollutants hence, the gas temperature. Therefore, there will
to CO2 be a limit in terms of engine speed and load up to
which pre-main injection strategies can sensibly be
State-of-the-art injection systems allow for the applied. Figure 2 provides an overview of the
application of multiple injections per working cycle boundary conditions for mixture formation and
of the engine. Beside conventional injection strate- combustion at three engine operating points.
gies, "Boot" and "Ramp" injections are discussed Clearly visible is the drastic shortening of the igni-
in recent publications [9, 11, 12] as appropriate tion delay with increasing in-cylinder pressure and
means to control the injection rate. The purpose of temperature.
these strategies is to further reduce any of the
critical parameters, pollutant emissions, fuel con- N / rpm 1500 1500 2280
sumption and combustion noise. Figure 1 gives an IMEP / bar 4.2 6.5 10.5
overview of the injection strategies, currently feasi- pRail / bar 750 900 1650
ble on an industrial scale.
pBoost / bar 1.06 1.16 1.7
(a) Multiple injection (MI) Density_gas / kg/m³ 12.7 13.5 20
mF_pilot / ms 1.0 (0.5+0.5) 1.0 1.0
injection rate
Ignition delay 1.4 0.66 0.23
/ ms / °crk 13 6 3.2
TGas_Max / K 1760 1890 1990
Figure 2: Thermo-physical boundary conditions for mix-
pilot main post ture formation and chemical reaction at three engine
time operating points.
(b) Boot injection (c) Ramp injection
An additional limitation for the use of any injec-
injection rate
injection rate
tion strategy is engine power output. When high
boot
level power output is demanded, then thermodynamics
require the necessary amount of fuel to be injected
within a sensible time window, unless a loss in
boot length engine efficiency, i.e. the deterioration in fuel con-
time time sumption is accepted. Additionally the resulting
Figure 1: Overview of possible injection strategies extreme exhaust temperatures pose severe prob-
lems for engine and turbo charger.
Obviously there are boundaries to the applica-
tion of injection strategies, beyond which they do The current paper will elucidate basic mecha-
not work anymore. nisms of injection strategies, including multiple
Premixing before ignition is limited on the one injection, MI, and rate shaping. For this the directly
hand by the local extension of the cloud of gase- actuated Continental NG injector was applied in a
ous fuel, propagating from each injected jet into single cylinder research engine, based on a 2 liter
the combustion chamber. The proper distribution of 4 cylinder and 4 valve engine. The influence of
the fuel cloud requires a suitable combination of engine operating conditions on the use of injection
injection pressure, charge movement and charge strategies will be discussed.
density, as well as a suitable shape of the spray
plume. On the other hand injection timing is cru- Multiple injection
cial, since the formation and propagation of the air Pilot injections have different purposes depend-
fuel mixture is a function of time. ing on the operating point. At low load they serve
These temporal and spatial restrictions for mix- for the reduction of HC and CO emissions (see
ture preparation and ignition are dictated by the Figure 3). At low engine speed, small pilot injec-
operating condition of the engine. The time avail- tions have time to mix well in the combustion
able for propagation and mixing of fuel shrinks chamber. Because the general temperature level is
low at low load, the pre-ignition chemistry is slow. Legend
Therefore a relatively large amount of fuel can be 0.5 0.8 1.2
introduced into the combustion chamber. In order
NGB0.2-N13-0.5-x-0.8; p_rail [bar] = 1650
to prevent early ignition of the pre-main injected 12 6 250
NGB0.2-N13-0.8-x-0.8; p_rail [bar] = 1650
fuel, it has to be introduced in several
NGB0.2-N13-1.2-x-0.8; portions
p_rail [bar] = 1650 at
10 5 240
dp/dphi [bar/°crk]
sufficiently long timeNGB0.2-N13-0.3-0.3-x-0.8;
intervals. Otherwise p_rail the
[bar] =criti-
1650
ISFC [g/kWh]
cal fuel concentration for ignition will be exceeded. 8 4 230
PM [g/h]
If on the other hand the injection sequence starts
too early before ignition, wall and mid-air quench- 6 3 220
ing will be the result, which increases the HC and 4 2 210
CO emissions
6 massively. 40 350
pRail [bar] = 750 2 1 200
2+M; pRail [bar] = 750 Figure
5 3 depicts the effect of the number of 340 pilot
dp/dphi [bar/°crk]
2+P3+M; pRail [bar] = 750 330 0 0 190
injections on HC and CO 30 emissions. The curves
T_Exh [°C]
320 10 20 30 40 50 60 10 20 30 40 50 60 10 20 3
NOx [g/h]
4
show an EGR rate variation, with EGR rate310 in-
NOx [g/h] NOx [g/h] NOx
creases3 from right to left in20the diagram. PM emis- 300
sions were
2 12
on a very low level for all shown 280
290
set-
50 200
Figure 4: Effect of pilot injection fuel mass on pollutant 250
10 emissions and combustion noise at n = 2280rpm,
tings and1 far below EU6 engineering 10 target. 270
160 240
IMEP = 10.5 bar
ISFC [g/kWh]
260 40
0 00.5 0.5 0.58 250 230
NOx [g/h]
PM [g/h]
CO [g/h]
0 5 0.510 15 0.5200.5 0 5 10 15 20 0 5 10 15 20 120
NOx [g/h] NOx6[g/h] Boot
NOx injection
[g/h]
30 220
1 40 150 The application of the so-called
250 80 zero-dwell be-
-4
50% 210
35 240 tween pilot and main injection suggested, that an
0.8 120 20
ISFC [g/kWh]
30 - 30% 2 even closer coupling of the pre-main
40 fuel mass to 200
230
CO [g/h]
HC [g/h]
0.6 25 90 the main injection may offer further PM reductions.
20 0 220 10 shows the application 0 of boot injection 190
0.4 15 60 10 20 30 40 50 Figure105 20 30 40 50 10 20 30 40 50 10 20
210
NOx [g/h] strategies NOx
with [g/h]
different boot lengths
NOx and
[g/h] levels NOx
0.2 10 30 200 (see Figure 1). The pre-main fuel mass was app.
5
0 0 0 190 the same for all shown injection patterns.
0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20
NOx [g/h] NOx [g/h] NOx [g/h] NOx [g/h]
boot-low-
Figure 3: Minimising HC and CO emissions by means of + MI
post
multiple injections at n = 1500 rpm, IMEP = 4.2 bar boot boot
-low- -high- +EGR
At higher part load PM and NOx emissions are
of main concern. There, the local temperature is
high enough for good oxidation of HC, but also is
favourable for NOx generation. The equivalence
ratio is locally rich, promoting soot production. -20%
Shown in Figure 4 is an EGR trade off. PM emis-
sion and combustion noise are plotted over NOx
emission. It is clearly demonstrated, that the in- Figure 5: Effect of different boot settings on PM emis-
crease of the pilot fuel mass leads to a significant sion. Reference is a Multiple injection pattern. The or-
ange box defines the EU6 engineering targets
PM increase, but also to a reduction in combustion
noise. Therefore, the trade off at this operating
Both boot patterns without post injection
point is between PM and combustion noise. Both
achieve about the same emissions level as the
are a strong function of the gas temperature in the
optimised MI pattern. However, the MI pattern also
combustion chamber before the onset of the main
included a post injection. If a post injection is com-
combustion, Tbm [9]. In [9] it was also found - by
bined with the better boot strategy, the PM-NOx-
means of 3D combustion simulation - that very
trade off improves beyond that achievable with MI.
small dwells between pilot and main injection
would reduce PM emission further.
Ramp injection
A further option to modify the injection rate is
the application of a ramp injection. This is the con-
tinuous increase of the initial injection ramp, rather
then the stepped shape used in the boot injection.
Shown in Figure 6 is the comparison of a slow
injection rate increase (blue with circles) versus a
fast one (black with triangles).
__PMPo__EGY110; p_rail [bar] = 1650
__PMPo__EGY60; p_rail [bar] = 1650 16 fast 20 [10]250
Pischinger., Becker., Rohs., Grünefeld., Greis.,
14 19
rate increase Wieske., REDUKTIONSPOTENZIAL FÜR RUß
slow18 240
12 UND KOHLENMONOXID, MTZ 11/2004
ISFC [g/kWh]
rate increase
HR50 [°crk]
17 [11] U. Gärtner, T. Koch, G. König, Analysis Of Diesel
230
PM [g/h]
10 16 Engine Combustion Processes To Assess The Po-
8 15 220
14 tential Of Flexible Injection Rate Shaping, Interna-
6 tionales Symposium für Verbrennungsdiagnostik,
210
13
4 Baden-Baden, pp 6 – 19, 15.-16. June 2004
12 200
2 11 [12] F. Atzler, O. Kastner, R. Rotondi, A. Weigand,
0 10 Multiple injection and rate shaping. Part 1: Emis-
190
0 10 20 30 40 0 10 20 30 40 0 10
sions 20 30 in40
reduction passenger car Diesel engines,
NOx [g/h] NOx [g/h] NOx
4 [g/h]
ICE2009,
x 10 9th International Conference on Engines
6 Figure 6: Effect 20 100
of initial injection rate gradient ("opening &3Vehicles, Capri, Naples (Italy), 2009
ramp") on the PM-NOx-trade off at n 90 = 2280rpm,
5
dp/dphi [bar/°crk]
80 2.8
15 IMEP = 10.5 bar 70
4
CO2 [g/h]
CO [g/h]
HC [g/h]
60 2.6
3 At higher part
10 load the "fast ramp"50offers an ad-
vantage in the PM-NOx-trade off. 40 However, this 2.4
2 effect is also dependant 30 point and
on the operating
5
1 further work is required to fully exploit20its benefits. 2.2
10
0 0 0 2
0 10 References
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