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R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 43
Special Review
Review
Research History of High-speed, Direct-injection Diesel Engine Combustion
Systems for Passenger Cars
Kiyomi Nakakita
Report received on Jun. 15, 2014
High-speed direct-injection diesel engines applied to passenger cars since the second
half of 1980’s have been making great strides through several stages in all of power, exhaust emissions,
and noise, vibration and harshness under keeping high thermal efficiency, especially since appearances of
common-rail (CR) injection system and high-boost-pressure supercharging systems in 1990’s. At each stage,
our laboratories played some key roles in clarifying in-cylinder phenomena and indicating directions and
measures for further improvement, such as clarifying high-pressure fuel injection effect, multiple-injection
effect and its optimization, development direction of the modern combustion system consisting of the
CR injection system and shallow-dish-type combustion chamber, effect of combining high-boost-pressure,
high exhaust-gas-recirculation rate and extremely high injection pressure, alternative combustion systems
of “dual-fuel stratified PCCI” and “quiescent” combustions, and so on. In this review, the evolution history
of diesel-engine combustion systems for passenger cars is outlined, and then results of the above-listed
subjects together with a useful numerical-simulation tool for engine planning and control-parameters
adjusting are all introduced.
Direct-injection Diesel Engine, Combustion System, Passenger Car,
In-cylinder Analysis, Optically Accessible Single-cylinder Engine,
Numerical Simulation
1. Introduction various originally-developed analysis methods, and
in indicating the direction and measures for further
Diesel engines originally have the highest thermal improvement. Hereinafter, the evolution history of
efficiency among internal combustion engines and diesel engine combustion systems for passenger cars is
also high torque at medium and low engine speeds, outlined and then the major research results obtained
and therefore have been widely used in trucks and in our laboratories are introduced.
buses worldwide since 1940’s. On the other hand, the
number of diesel-powered passenger cars was limited 2. Evolution History of Diesel Engine Combustion
until the middle of 1990’s, mainly due to both exhaust Systems for Passenger Cars
emissions, especially black smoke, and large noise,
vibration and harshness (NVH). 2. 1 First-stage Diesel Engines for Passenger Cars
Since the second half of 1990’s, however, not only
the performance but also the exhaust emissions and The indirect injection (IDI) combustion system
NVH of high-speed diesel engines for passenger cars with a sub combustion chamber was originally used
have been greatly improved through several stages, for passenger-car diesel engines, because good
due primarily to the appearance of common-rail fuel-air mixing was achieved by very high-speed
injection systems, high-efficiency aftertreatment swirl flow generated in the sub chamber so that
systems, and the remarkable advance of turbochargers high-speed operation was possible even with a simple
and electronic control systems. At each stage of the fuel injection equipment, and also somewhat low level
above-mentioned evolution of high-speed diesel of exhaust emission was realized. On the other hand,
engines, our laboratories played some important key heat loss from in-cylinder gas to combustion chamber
roles both in clarifying in-cylinder phenomena on wall was large, leading to relatively low thermal
the exhaust emissions, NVH and fuel economy with efficiency and limitation in increase of power density
© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/
44 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56
due to the high thermal load. In addition, further under actual driving conditions was excellent. Thus,
reduction of exhaust emissions corresponding to the at this stage, diesel-powered passenger cars widely
next generation, stringent emission regulation was spread in all the classes including premium cars.
essentially difficult in the IDI diesel engine.
2. 4 Remarkable Increase in Power and Torque
2. 2 Shift in Combustion Systems from IDI to DI Densities
In order to solve the above-mentioned problems, in In the middle of 2000’s, performance of CR-HSDI
the second half of 1980’s, shift in combustion systems diesel engines was remarkably improved mainly
from the IDI type to a direct injection (DI) type began owing to advanced supercharging technology such
in passenger-car diesel engines, which opened a door as a two-stage turbocharger system. In these engines,
of the age of high-performance and good fuel-economy power and torque densities reached 65 kW/L and
diesel engines. At the first stage, however, the 185 Nm/L, respectively. In addition, peak-torque
high-speed DI (HSDI) diesel engines were equipped range was furthermore expanded from 1,500 rpm to
with a jerk-type injection system and a large about 3,000 rpm, and also torque under higher engine
hole-diameter nozzle, leading to large NVH and speeds were kept to be high value such as 70 to 80% of
insufficiently low exhaust emissions. Therefore, at this peak torque at 4,500 rpm and still 60% at 5,000 rpm.
stage, diesel-powered passenger cars did not spread These characteristics were sufficient for emotionally
widely. sporty driving.
In addition to the power increase, good fuel economy
2. 3 Appearance of DI Diesel Engines with was kept by reducing losses of friction and heat flux to
Common-rail Injection System walls, and NVH level was further improved mainly by
the evolution of CR FIE such as increased number of
In the same age, the second half of 1980’s, a multiple injections and higher peak injection pressure
common-rail (CR) injection system was first developed up to 180 MPa. Exhaust emission levels were also
by Denso Corporation. In the conventional jerk-type further reduced with the evolved CR FIE, thermal
injection system, fuel can be injected only at about management and high efficient aftertreatment systems
compression top dead center (TDC) and injection of diesel particulate filter (DPF) and advanced catalysts
pressure depends on engine speed and load (i.e. injected such as a nitrogen oxides (NOx) storage reduction
fuel quantity). On the other hand, in the CR injection (NSR) catalyst, a selective catalytic reduction catalyst
system, multiple fuel injections at any timing during with reducing agent of urea (Urea-SCR), and so on.
compression and expansion strokes are possible and
injection pressure can be set independently of engine 2. 5 Two Main Streams of Diesel Engine
speed and load. In addition, the CR injection system Development
had higher peak injection pressure of 145 MPa.
Responding to this innovation of fuel injection Since the last stage of 2000’s, diesel engines newly
equipment (FIE), in the second half of 1990’s, HSDI developed were classified into two categories. One
diesel engines with the CR injection system (CR-HSDI is the downsized diesel engine with remarkably
diesel engines) were developed and used for passenger high power and torque densities, and the other is
cars. These engines, assisted by sophisticated the cost-effective diesel engine with both necessary
turbochargers such as a variable-nozzle-turbine (VNT) performance for practical use and good fuel economy.
turbocharger, had high power and torque densities of Both types are outlined as follows.
about 50 kW/L and 145 Nm/L, respectively, which
were sufficient for normal driving in passenger cars. 2. 5. 1 Engine Downsizing Based on Further
In addition, with the multiple fuel injections, the NVH Increase in Power and Torque Densities
levels decreased to levels comparable with those of
gasoline-powered cars, and also exhaust emissions Since around 2010, power and torque densities of
reduced drastically. Furthermore, the maximum brake CR-HSDI diesel engines have been increasing up
thermal efficiency reached 42 to 43% and fuel economy to 93 kW/L and 247 Nm/L, respectively, owing to
© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/
R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 45
further advance in supercharging technology such described in Section 2. 2, usage of high pressure fuel
as two-stage supercharging systems with double or injection was the most important key item for realizing
triple turbochargers and decreased loss of air the HSDI diesel engines applicable to passenger cars.
path systems, and also further evolution in CR FIE Concretely speaking, what should be understood were
such as furthermore increased number of multiple fuel injection pressure effects on combustion and
injections and peak injection pressure up to 250 MPa. exhaust emissions and also mechanisms of the effects,
In addition, a new combustion technique, that and then desired specifications of the FIE and main
is, combination of high boost-pressure charging, components of the combustion system.
high exhaust-gas-recirculation (EGR) rate and the Responding to this requirement, researches on the
very high injection pressure led very low engine-out effect of fuel injection pressure and also injection rate
exhaust emissions even under medium and high load were conducted by using a conventional jerk-type
conditions. This technique enabled drastic engine FIE and a pioneering prototype CR FIE supplied by
downsizing such as a shift from 8-cylinder engines Denso. In these studies,(1-4) a special single-cylinder
with displacement volume of 4.0 L to 6-cylinder with diesel engine with wide observation area shown in
3.0 L. The engine downsizing really contributed to Fig. 1 and diesel fuel with an additive of copper oleate
reduction in engine weight and size and improvement for visualizing non-luminous flames were both used,
in car-based fuel economy. and high-speed direct photography and two-color
(5) were applied for analyzing the
pyrometry method
2. 5. 2 Cost-effective and High-efficiency Diesel in-cylinder phenomena such as fuel spray behavior,
Engines flame development process, flame temperature, and so
on. As a result, the following points were clarified.
The above-mentioned highly-downsized diesel (1) As shown in Fig. 2, increase in injection pressure
engines require high-cost components such as the remarkably reduces smoke (i.e. particulate), but, in
high-boost-pressure supercharging system, high-class this case, the effect is saturated at injection pressure of
CR FIE, structural parts of high-class material about 100 MPa.
for sustaining the high in-cylinder
pressure and thermal load and so
on, leading to increase in engine
cost. Thus, there is the other trend Inj. Press.:
to develop cost-effective diesel 35 MPa
Inj. Timing:
- 5 ATDC
engines without downsizing which deg.
have sufficient power and torque NOx: 530 ppm deg. deg. deg.
Smoke: 3.2 BSU 1.0 ATDC 5.8 ATDC 15.4 ATDC
for normal driving and good fuel
economy under actual driving Peak Temp.:
Flame Temperature K 2804K
conditions. These engines are 1470 2070 2670
suitable to popular-edition cars. deg.
10.6 ATDC
3. Researches on Combustion
System for Each Generation Inj. Press.:
Diesel Engine Conducted in 95 MPa deg. deg. deg.
Our Laboratories Inj. Timing: 10.4 ATDC 15.2 ATDC 24.8 ATDC
5 deg. ATDC
3. 1 Effects of High-pressure NOx: 530 ppm Peak Temp.: 2830 K
Fuel Injection on Smoke: 0.3 BSU
deg.
Combustion and Exhaust 15.2 ATDC
Emissions Fig. 1 Effect of fuel injection pressure on flame development under the same
At the stage of the shift in diesel exhaust NOx condition.
combustion systems from IDI to DI Reprinted and modified from Proc. of JSAE Symp, New Aspects of Diesel Combustion
(in Japanese), No. 9303 (1993), pp. 40-48, © 1993 JSAE.
© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/
46 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56
(2) Increasing injection pressure leads to the following 3. 2 Optimization of Multiple Injection Patterns
phenomena. and Its Effects on Combustion and Exhaust
(a) Ignition delay is shortened (Fig. 2) and ignition Emissions
position shifts from the vicinity of nozzle tip to that
of piston cavity wall (Fig. 1). This shift prevents the At the stage of the development of CR-HSDI diesel
spray from being wrapped by flame and thus ensures engines described in Section 2. 3, clarifying the usage
air entrainment into the spray. of multiple fuel injections was the most important key
(b) Luminous flames (diffusion flames) are reduced item, because various multiple injection patterns tried
and non-luminous flames (premixed flames) shown at the first stage led to very puzzled results such as
as the green flame areas in Fig. 1 appear, which unexpected, worsened exhaust emissions, lubricating
corresponds to the accelerated mixture formation. oil dilution, and so on.
(c) Both combustion period and lifetime of the Responding to this problem, studies to clarify the
luminous flame are shortened, but those effects are effects of multiple fuel injections and the mechanisms
saturated also at about 100 MPa in this case (Fig. 2). were conducted by using two optically-accessible,
(3) Heightening injection pressure up to about 100 MPa single-cylinder engines; one is the same engine as
increases NOx emission in this case. The increase that used in Section 3. 1 and the other is a modern one
(7)
in NOx is caused not by increase in peak flame with four valves and a centrally-located injector.
(6,7)
temperature but by enlargement of high-temperature In these studies, a high-speed color shadowgraph
(3) photography were additionally applied for analyzing
(for example, over 2,170 K) flame region.
This study led to the above-described results was the in-cylinder phenomena. Some of the concrete
a pioneering work and indicated the development problems tackled in these studies and also obtained
direction. results and solutions are as follows.
3. 2. 1. Close-pilot Injections
A small-quantity fuel
injection just before main
Φ0.26mmx 4hole injection is called “close-pilot
Φ0.29 x 4 injection”, which effectively
reduced combustion noise,
fuel consumption and NOx
emission, but was apt to
remarkably increase exhaust
smoke in complicated
manners depending on pilot
fuel quantity, interval between
pilot and main injections,
main-injection timing, and so
on. Thus, it was very difficult
to properly utilize the close-
pilot injection. The analysis
results and solution from our
(6) are the followings.
study
(1) In the cases of high exhaust
smoke level shown in the
3 Φ left- and right-columns of
1800rpm, 35mm /st ( =0.42), Inj. Timing: TDC Fig. 3, pilot flames formed
Fig. 2 Effect of injection pressure on exhaust emissions and combustion characteristics. prior to main injection are
Reprinted and modified from JSAE Tech. Pap. Ser. (in Japanese), No. 901078 (1990), conveyed by swirl flow and
© 1990 JSAE.
© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/
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