Utilizing low airflow strategies, including cylinder deactivation, to improve fuel efficiency and aftertreatment thermal management

Abstract

Approximately 30% of the fuel consumed during typical heavy-duty vehicle operation occurs at elevated speeds with low-to-moderate loads below 6.5 bar brake mean effective pressure. The fuel economy and aftertreatment thermal management of the engine at these conditions can be improved using conventional means as well as cylinder deactivation and intake valve closure modulation. Airflow reductions result in higher exhaust gas temperatures, which are beneficial for aftertreatment thermal management, and reduced pumping work, which improves fuel efficiency. Airflow reductions can be achieved through a reduction of displaced cylinder volume by using cylinder deactivation and through reduction of volumetric efficiency by using intake valve closure modulation. This paper shows that, depending on load, cylinder deactivation and intake valve closure modulation can be used to reduce the fuel consumption between 5% and 25%, increase the rate of warm-up of aftertreatment, maintain higher temperatures, or achieve active diesel particulate filter regeneration without requiring dosing of the diesel oxidation catalyst.

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International J of Engine Research
2017, Vol. 18(10) 1005–1016
ÓIMechE 2017
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DOI: 10.1177/1468087417695897
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Utilizing low airflow strategies,
including cylinder deactivation, to
improve fuel efficiency and
aftertreatment thermal management
Aswin K Ramesh
1
,GregoryMShaver
1
, Cody M Allen
1
, Soumya Nayyar
2
,
Dheeraj B Gosala
1
,DinaCaicedoParra
2
, Edward Koeberlein
2
,James
McCarthy
3
and Doug Nielsen
3
Abstract
Approximately 30% of the fuel consumed during typical heavy-duty vehicle operation occurs at elevated speeds with
low-to-moderate loads below 6.5 bar brake mean effective pressure. The fuel economy and aftertreatment thermal man-
agement of the engine at these conditions can be improved using conventional means as well as cylinder deactivation
and intake valve closure modulation. Airflow reductions result in higher exhaust gas temperatures, which are beneficial
for aftertreatment thermal management, and reduced pumping work, which improves fuel efficiency. Airflow reductions
can be achieved through a reduction of displaced cylinder volume by using cylinder deactivation and through reduction
of volumetric efficiency by using intake valve closure modulation. This paper shows that, depending on load, cylinder
deactivation and intake valve closure modulation can be used to reduce the fuel consumption between 5% and 25%,
increase the rate of warm-up of aftertreatment, maintain higher temperatures, or achieve active diesel particulate filter
regeneration without requiring dosing of the diesel oxidation catalyst.
Keywords
Heavy-duty federal test procedure, fuel efficiency, cylinder deactivation, variable valve actuation, aftertreatment thermal
management
Date received: 11 September 2016; accepted: 24 January 2017
Introduction
Heavy-duty vehicle fuel consumption is expected to
double by 2050.
1
The majority of heavy-duty vehicles
incorporate diesel engines, given their efficiency bene-
fits compared with spark-ignited engines. However, die-
sel engines emit air pollutants, including particulate
matter, unburnt hydrocarbons, and oxides of nitrogen
(NOx). To reduce the production of these harmful
gases, several on-engine strategies, including multiple
fuel injection, high injection pressure, late fuel injec-
tion, and exhaust gas recirculation,
2
have been incorpo-
rated in modern diesel engines. These strategies are
unable to meet the regulations at all operating points.
As a result, complex aftertreatment systems are coupled
with the diesel engine to meet tailpipe emission regula-
tion limits.
Typical aftertreatment systems incorporate selective
catalytic reduction to reduce NOx emissions, a diesel
oxidation catalyst to reduce hydrocarbons and carbon
monoxide, and a diesel particulate filter to reduce parti-
culate matter emissions. Selective catalytic reduction is
effective in converting NOx into N
2
and H
2
O,
3
pro-
vided the catalyst temperature is between approxi-
mately 250 °C and 450 °C.
4–6
Urea is injected upstream
of the selective catalytic reduction system, decomposing
into ammonia and carbon dioxide. This process
requires selective catalytic reduction inlet temperatures
above approximately 200 °C to avoid build up of solid
deposits on the catalyst bed.
1
The selective catalytic
reduction efficiency is maximized for selective catalytic
1
Purdue University, USA
2
Cummins, Inc., USA
3
Eaton Valvetrain Engineering, USA
Corresponding author:
Gregory M Shaver, Ray W. Herrick Laboratories, Purdue University, 177
S Russel Street, West Lafayette, IN 47906, USA.
Email: gshaver@purdue.edu

reduction catalyst bed temperatures from 300 °Cto
450 °C. To keep the aftertreatment warm in a robust
manner, turbine outlet temperatures between 350 °C
and 500 °C are desirable. The diesel particulate filter is
regularly regenerated in a passive manner by oxidizing
the collected particulate matter, provided the turbine
outlet temperature is approximately 250 °Cto300°Cin
the presence of NO
2
.
7,8
Active regeneration of the diesel
particulate filter requires temperatures in excess of 450
°C. The diesel oxidation catalyst must reach approxi-
mately 200 °C to perform effective oxidation of carbon
monoxide and hydrocarbons. Once activated, the diesel
oxidation catalyst oxidizes hydrocarbons, resulting in
increased aftertreatment temperatures, while converting
NO to NO
2
, both of which enable passive reactions in
the diesel particulate filter at temperatures above
250 °C.
9,10
In short, the aforementioned aftertreatment
components work effectively in reducing emissions
when operated at the correct temperatures; however,
during cold start and low-load engine operation, the
exhaust gas temperature is too low to keep the after-
treatment catalysts at effective operating temperatures.
Thermal management strategies designed to increase
aftertreatment component temperatures are needed for
efficient aftertreatment operation over a wide range of
engine operating conditions.
11,12
Research continues on methods for improving after-
treatment thermal management by means of increased
diesel engine exhaust temperatures.
13,14
One technology
currently being studied to accomplish this goal is vari-
able valve actuation.
15–19
To assess the impact of advanced engine system stra-
tegies on aftertreatment thermal management, it is
helpful to compare strategies during operation of stan-
dardized test procedures. The heavy-duty federal test
procedure (HD-FTP) is used for regulatory emission
testing of heavy-duty, on-road engines in the USA.
This test was developed by taking actual operating data
over a variety of heavy-duty truck and bus driving pat-
terns on roads and expressways.
1
Figure 1 shows the
speed and normalized brake mean effective pressure
(BMEP) profile through time for the HD-FTP.
Figure 2 shows the fuel consumption over a HD-FTP
mapped around eight steady-state engine operating
regions. The number displayed next to a bubble sig-
nifies the percentage of fuel consumed at these operat-
ing conditions. In this study, the benefits of using
variable valve actuation for improving fuel efficiency
and aftertreatment thermal management are studied
for the cross-hatched bubbles at an engine speed of
2200 r/min, for two reasons.
1. More than 30% of the fuel used during the HD-
FTP is consumed in these operating regions.
2. As will be shown, there is an opportunity to
increase fuel efficiency and exhaust temperatures
at these conditions by reducing airflow via both
conventional engine actuators and variable valve
actuation.
During these high-speed low-load conditions, the
air-to-fuel ratio is elevated, given the reduced fueling
required at low loads and the engine ‘‘over breathing’’
at low loads, and to prepare for a sudden increase in
fueling resulting from a commanded increase in desired
torque and power. There are several possible strategies
for improving the engine torque response to acceptable
levels when air-to-fuel ratios are reduced at high-speed,
low-load operating conditions, including:
(a) early exhaust valve opening;
20
(b) internal exhaust gas recirculation via combustion
gas trapping or re-induction;
(c) turbocharger electrification;
21
(d) supercharging;
22,23
Figure 1. Speed and brake mean effective pressure versus time
for the heavy-duty federal test procedure. Highlighted sections
shows high-speed, low-load conditions, as cross-hatched in
Figure 2.
BMEP: brake mean effective pressure; LAFY: Los Angeles Freeway;
LANF: Los Angeles Non Freeway; NYNF: New York Non Freeway.
Figure 2. Fuel consumption distribution over heavy-duty
federal test procedure, mapped around eight operating
conditions.
BMEP: brake mean effective pressure; HDFTP: heavy-duty federal test
procedure.
1006 International J of Engine Research 18(10)

(e) powertrain hybridization;
24,25
(f) availability of look-ahead information,
26
through
vehicle data connectivity with other vehicles or
the Cloud, to allow anticipation of an upcoming
transient.
These methods are not the subject matter discussed in
detail in this paper. Instead, this paper outlines strate-
gies for achieving and the benefits of low air-to-fuel
ratio operation at high-speed, low-load conditions. Air-
to-fuel ratio reduction strategies considered in this
paper include:
(a) ‘‘opening up’’ the variable geometry turbine turbo-
charger (VGT);
(b) reducing the displaced volume through cylinder
deactivation;
(c) reducing volumetric efficiency via late intake valve
closure (IVC);
(d) some combination of these strategies.
Previous research by several of the authors has
demonstrated that cylinder deactivation is an effective
method for increasing exhaust temperatures and fuel
efficiency when the diesel engine is idle,
19
and for
enabling active diesel particulate filter regeneration
without requiring diesel oxidation catalyst fuel dosing
during highway cruise conditions.
17
The effect of early
and late IVC, including a notable increase in the
exhaust gas temperature is outlined by Ojeda
27
and
Lombard and le Forestier.
28
Prior efforts have also
shown that late IVC can be used to increase exhaust
temperature.
18
The first results section of this paper focuses on eval-
uating the fuel economy benefits of the specified strate-
gies at elevated speed (2200 r/min) and low-load
operating conditions (1.3–6.3 bar BMEP). The second
results section focuses on which strategy works best
when the main goal is to keep the aftertreatment tem-
perature elevated. The third results section focuses on
which strategies allow the fastest warm-up of the after-
treatment system. Finally, the fourth results section
focuses on which strategy is preferable for enabling
active diesel particulate filter regeneration.
Experimental setup
The system under observation in this study is a camless
six-cylinder Cummins diesel engine outfitted with an
electro-hydraulic variable valve actuation system, high-
pressure cooled exhaust gas recirculation, a sliding
nozzle-type VGT, an air-to-water charge air cooler,
and a common rail fuel injection system. A schematic
of the engine architecture is presented in Figure 3. The
fresh intake air flows through the laminar flow element
into the compressor and is then cooled in the charge air
cooler. The exhaust flows either through the turbine of
the VGT to the exhaust pipe or into the exhaust gas
recirculation system. The exhaust temperature is mea-
sured at the outlet of the turbine and is referred to in
this paper as the turbine outlet temperature. Two
Kistler 6067 and four AVL QC34C in-cylinder pressure
transducers are used in tandem with an AVL 365C
crankshaft position encoder together with an AVL 621
Indicom module for high-speed in-cylinder pressure
data acquisition. Laboratory-grade fuel flow measure-
ment is used to measure the fuel consumption.
Both the intake and exhaust valve pairs for each of
the six cylinders are actuated by the variable valve
actuation system, such that it has a total of twelve
actuators. The actuators use position feedback for
closed-loop control, enabling the desired cylinder-inde-
pendent, cycle-to-cycle operation of the system.
Figure 4 presents a schematic of the variable valve
actuation system. The valve profiles are generated in
Simulink and the dSPACE hardware is used to transmit
Figure 3. Camless Cummins multicylinder engine testbed at
Purdue University.
CAC: charge air cooler; EGR: exhaust gas circulation; LFE: laminar flow
element; VGT: variable geometry turbine turbocharger.
Figure 4. Purdue variable valve actuation system.
LVDT: linear variable differential transformer.
Ramesh et al. 1007

voltage feedback to the servo valves via the controller
and amplifier. The servo valves shuttle high-pressure
hydraulic oil to one side of the valve actuators. These
actuators push on the valve pairs through a valve
bridge to open them. The return forces from the valve
springs close the valves as the actuators retract.
The valve profiles have six key features that describe
the valve lift shape:
(a) Intake valve opening (IVO);
(b) Peak intake valve lift (IVL);
(c) Intake valve closure (IVC);
(d) Exhaust valve opening (EVO);
(e) Peak exhaust valve lift (EVL);
(f) Exhaust valve closure (EVC).
Using these inputs, valve strategies including IVC
modulation (early IVC or late IVC) can be realized on
the engine. Figure 5 illustrates the late IVC valve strat-
egy, wherein the IVC timing is delayed from the nom-
inal position.
Cylinder deactivation is achieved by deactivating the
valve motions and fuel injections for either two or three
cylinders, resulting in four-cylinder or three-cylinder
modes, respectively.
A butterfly valve is used in the exhaust pipe to simu-
late the back pressure that would be caused by a typical
aftertreatment system. The aforementioned equipment,
full access to adjustment of parameters in the engine
control module, and additional temperature and pres-
sure sensors are integrated using a dSPACE system.
The dSPACE system simultaneously controls the vari-
able valve actuation system, sends and receives data
with the engine control module, and samples all of the
external measurement channels.
Methodology and nomenclature
A total of five loads (1.3, 2.6, 3.8, 5.1, and 6.4 bar
BMEP) were selected to study the fuel economy and
aftertreatment thermal management benefits of reduced
air flow conditions at 2200 r/min. All experimental data
shown and discussed in the following sections were sub-
ject to strict emissions and mechanical constraints. The
mechanical constraints are shown in Table 1. The parti-
culate matter, NOx, and unburnt hydrocarbon limits
for each operating load are the same as the engine
achieves with stock calibration and conventional valve
motions. For the ‘‘open VGT’’ and ‘‘open VGT +
IVC modulation’’ strategies, the start of injection, and
rail pressure were modulated to screen for operating
strategies to achieve improved efficiency or thermal
management responses. During cylinder deactivation,
the VGT position, start of injection, and rail pressure
were modulated to achieve improved efficiency or ther-
mal management.
The brake thermal efficiency of the engine includes
contributions from the closed-cycle efficiency, open-
cycle efficiency, and mechanical efficiency, as
BTE = hclosed cycle 3hopen cycle 3hmechanical ð1Þ
Closed-cycle efficiency is affected by combustion com-
pleteness, piston expansion work, and in-cylinder heat
transfer. The open-cycle efficiency quantifies the effec-
tiveness of the gas exchange and is affected by the tur-
bine and compressor efficiency, and pressure
differences between the intake and exhaust manifold.
The mechanical efficiency captures losses from friction
and parasitic loads. For a more detailed explanation of
the cycle efficiency analysis strategy refer to Stanton.
1
Experimental results
The benefits of the low air flow strategies described in
the following four sections include:
(a) improving fuel efficiency, enabled through
increased open-cycle efficiency via reduced pump-
ing work;
(b) maintaining elevated aftertreatment component
temperatures through elevated engine exhaust tem-
perature as a result of reduced airflow operation;
Figure 5. Late intake valve closure modulation valve strategy
compared with nominal valve positions. The black line
represents the nominal intake valve position.
IVC: intake valve closure; LIVC: late intake valve closure; TDC: top dead
center.
Table 1. Mechanical constraints.
Mechanical parameter Unit Limit
Turbine inlet temperature °C 760
Compressor outlet temperature °C 230
Turbo speed kr/min 193
Peak cylinder pressure bar 172
Exhaust manifold pressure kPa 500
In-cylinder pressure rise rate bar/ms 100
1008 International J of Engine Research 18(10)

(c) increasing the rate of warm-up of the aftertreat-
ment components through elevated engine exhaust
gas temperatures;
(d) enabling active diesel particulate filter regenera-
tion through elevated engine exhaust gas
temperatures.
Fuel efficiency
Figure 6 summarizes the fuel efficiency benefits, over
the BMEP range, of using cylinder deactivation, IVC
modulation, or opening up the VGT. The results are
normalized with respect to the stock calibration, and
show that fuel savings between 5% and 30% are possi-
ble, depending on load (e.g., BMEP). Cylinder deacti-
vation leads to a significant (25%) fuel consumption
reduction at loads less than 2.5 bar. This is primarily a
result of a 35% increase in the open-cycle efficiency,
shown in Figure 7, achieved through a reduction in
pumping work.
Pumping work is lower because airflow is lower, as
shown in Figure 8. Airflow is lower as a result of a
reduction in displaced volume via cylinder deactivation.
Airflow is lower for the ‘‘open VGT’’ strategy, owing
to the reduction is boost pressure via VGT opening. In
general, the three-cylinder mode is more fuel-efficient
than the four-cylinder mode, when it is feasible to deac-
tivate three cylinders. The fuel efficiency benefit relative
to the stock calibration decreases as the load increases,
owing to reductions in the open-cycle efficiency
benefits.
At 2.5 bar BMEP, the increase in open-cycle effi-
ciency for cylinder deactivation compared with ‘‘open
VGT’’ operation is negated by a decrease in closed-
cycle efficiency during cylinder deactivation, as shown
in Figure 9. As a result, the brake-specific fuel con-
sumption is similar for the cylinder deactivation and
‘‘open VGT’’ strategies.
At loads greater than, and equal to, 2.5 bar, cylinder
deactivation has a slightly higher fuel consumption than
‘‘open VGT’’ operation as a result of decrease in closed-
cycle efficiency via:
(a) higher in-cylinder heat transfer, as shown in
Figure 10, and longer heat release caused by
higher per-cylinder fueling in active cylinders, as
shown in Figure 11;
(b) later injection timings required to maintain
engine-out NOx levels, as shown in Figure 11.
Cylinder deactivation was not implemented at loads
above 5.1 bar BMEP, as there is not enough oxygen
Figure 7. Comparison of open-cycle efficiency versus brake
mean effective pressure at 2200 r/min. The open-cycle efficiency
is highest for the three-cylinder mode at these low-load
conditions. Open-cycle efficiency quantifies the efficiency of the
gas exchange, which is impacted by turbine and compressor
efficiency and pressure differences between the intake and
exhaust manifolds. The benefit seen in open-cycle efficiency
decreases as the load increases, as there is a lower reduction in
airflow with higher loads.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Figure 8. Comparison of normalized air flow versus load at
2200 r/min. There is a significant decrease in air flow when
cylinders are deactivated or when the variable geometry turbine
turbocharger is opened. There is a minor decrease in air flow
when intake valve closure modulation is used, as the volumetric
efficiency is reduced.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Figure 6. Comparison of brake-specific fuel consumption
versus brake mean effective pressure at 2200 r/min. The lower
the brake-specific fuel consumption, the higher the brake
thermal efficiency.
BMEP: brake mean effective pressure; BSFC: brake-specific fuel
consumption; IVC: intake valve closure; VGT: variable geometry turbine
turbocharger.
Ramesh et al. 1009

available in cylinder deactivation mode to keep particu-
late matter within the constraints.
The ‘‘open VGT + IVC modulation’’ strategy has
a slightly lower fuel consumption than the ‘‘open
VGT’’ strategy, given better open-cycle efficiency and
closed-cycle efficiency at 2.5 bar BMEP. The open-
cycle efficiency increases for the ‘‘open VGT + IVC
modulation’’ strategy, as the air flow is reduced and
pumping work is reduced, resulting in an increase in
open-cycle efficiency (as shown in Figures 8 and 7).
The IVC modulation also allows for a reduction in
effective compression ratios, which decrease NOx
through reductions in in-cylinder charge temperatures
prior to, during, and following combustion. This
enables earlier injection timing for the ‘‘open VGT +
IVC’’ strategy at 2.5 bar, as shown in Figure 11, thereby
providing a small closed-cycle efficiency improvement,
as shown in Figure 9.
The ‘‘open VGT’’ strategy and ‘‘open VGT + IVC
modulation’’ strategies have similar fuel consumption
benefits (Figure 6), and lower airflow (Figure 8), when
compared with the stock calibration for loads greater
than 2.5 bar BMEP. However, a shortcoming of these
low air flow strategies is that their turbo speeds are
lower when compared with the stock calibration
(Figure 12), as the exhaust flow through the turbine is
lower when cylinders are deactivated, IVC modulation
is used, or VGT is opened.
Lower pre-acceleration airflows and turbo speeds
increase turbo-lag in turbocharged diesel engines
Figure 11. Comparison of apparent heat release rate of an
active cylinder in cylinder deactivation and six-cylinder open
variable geometry turbine turbocharger strategy at 2200 r/min,
2.5 bar brake mean effective pressure. Cylinder deactivation has
higher per-cylinder fueling in an active cylinder to maintain the
demanded torque. This leads to a larger and more spread-out
heat release rate when compared with six-cylinder strategies.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Figure 10. Comparison of in-cylinder heat rejection of an
active cylinder in cylinder deactivation and six-cylinder open
VGT strategy at 2200 r/min, 2.5 bar brake mean effective
pressure. The in-cylinder heat rejection is calculated by
performing a first-law analysis with the boundary around the
cylinder. The fuel energy is distributed as brake work, exhaust
stream energy, and in-cylinder heat rejection or loss.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Figure 9. Comparison of closed-cycle efficiency versus brake
mean effective pressure at 2200 r/min. Closed-cycle efficiency
quantifies combustion completeness, piston compression and
expansion work, in-cylinder energy release during combustion,
and in-cylinder heat transfer. The closed-cycle efficiency
decreases for cylinder deactivation, owing to increased in-
cylinder heat loss during combustion.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Figure 12. Turbocharger speed versus brake mean effective
pressure at 2200 r/min for different strategies. All the strategies
have a lower turbocharger speed when compared with stock
calibration. All these strategies have similar turbocharger shaft
speeds despite the variation in air flow.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
1010 International J of Engine Research 18(10)

during transients.
20
In addition, as shown in Figure 13,
the pressure differential across the exhaust gas recircu-
lation loop is decreased for these strategies, as com-
pared with the stock calibration, owing to the
reduction in airflow (as shown in Figure 8). Lower
pre-acceleration exhaust gas recirculation pressure dif-
ferentials decrease the potential to drive exhaust gas
recirculation flow during transients, increasing transi-
ent NOx emissions.
The implication of these phenomena for transient
performance at these conditions requires further study.
As mentioned in the introduction, early exhaust valve
opening, internal exhaust gas recirculation, turbochar-
ger electrification, supercharging, and powertrain
hybridization are all potential solutions, but are beyond
the scope of this paper. Availability of look-ahead
information, through vehicle data connectivity with
other vehicles or the cloud, to allow anticipation of
upcoming transients would also allow the use of one of
the strategies outlined in this paper when an upcoming
transient is not immediately pending.
Maintaining elevated aftertreatment component
temperatures
The selective catalytic reduction system operates most
efficiently when temperatures are between approxi-
mately 300 °C and 450 °C. Once the aftertreatment has
reached these temperatures, it is preferable to maintain
turbine outlet temperature at the upper end of this
range so that diesel oxidation catalyst fuel dosing
(which still requires temperatures above 250 °C) is not
required to keep the selective catalytic reduction system
temperatures elevated.
Reducing the air-to-fuel ratio is the most direct way
to increase turbine outlet temperature. Figure 14 specif-
ically shows a decreasing relationship between turbine
outlet temperature and air-to-fuel ratio, regardless of
operating strategy or load. As shown, and expected, a
reduction in air-to-fuel ratio results in an increase in
turbine outlet temperature. The most fuel-efficient way
to reduce the air-to-fuel ratio is by reducing the airflow
(as opposed to increasing the amount of fuel required).
As discussed in the previous section, cylinder deactiva-
tion and IVC modulation are fuel-efficient ways to
reduce engine airflow, and as such, are also effective
strategies for maintaining exhaust aftertreatment com-
ponent temperatures. More specifically, at a given load,
lower air-to-fuel ratios are possible via cylinder deacti-
vation and IVC modulation, as shown in Figure 15,
resulting in higher turbine outlet temperature, as shown
in Figure 16.
Figure 16 and Table 2 show that:
1. three-cylinder operation is preferred for loads
below 2.5 bar;
Figure 13. Comparison of delta pressure across the exhaust
gas recirculation versus brake mean effective pressure at 2200 r/
min for different strategies. All the strategies have a lower
exhaust gas recirculation delta pressure, compared with stock
calibration. The main reasons for the lower delta pressures are
the reduction in boost pressures for ‘‘open VGT’’ cases and
displaced volume for the cylinder deactivation strategy.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Figure 14. Direct correlation between air fuel ratio and
turbine outlet temperature at an engine speed of 2200 r/min for
different operating strategies. The shaded region indicates the
turbine outlet temperature range consistent with maintaining
selective catalytic reduction temperature between 300 °C and
450 °C. This temperature range is typically the aftertreatment
sweet spot wherein the conversion efficiency is maximum.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Figure 15. Air-to-fuel ratio versus brake mean effective
pressure at 2200 r/min.
BMEP: brake mean effective pressure; IVC: intake valve closure; VGT:
variable geometry turbine turbocharger.
Ramesh et al. 1011

2. four-cylinder operation is preferred for loads between
2.5 and 4 bar, and six-cylinder operation with an
open VGT is preferred for loads above 4 bar.
Also, note from Figure 16 that below ;6.5 bar BMEP,
using stock engine calibration and valve profiles, it is
not possible to meet desirable turbine outlet tempera-
tures for maintaining already elevated exhaust after-
treatment component temperatures. Conversely, the
combination of opening the VGT and valvetrain flexi-
bility (IVC modulation or cylinder deactivation) allows
desirable temperatures to be reached for loads greater
than ;1.5 bar.
Reaching desired aftertreatment component
temperatures quickly
During cold-start conditions, the aftertreatment system
would ideally warm-up as quickly as possible. This
section compares the catalyst warm-up characteristics
of various strategies at different loads. As a first
approximation, the heat transfer rate between the
exhaust gas and an aftertreatment catalyst depends on
the turbine outlet temperature, exhaust flow rate, and
instantaneous catalyst bed temperature. As an approxi-
mation, consider the heat transfer rate between an
incoming gas and the wall within a round pipe,
19
as
given by
q=C3_
m4=53(TTurbine Outlet TCatalyst)ð2Þ
where _
mis the experimental mass flow rate of the
exhaust gas going through the catalyst, TCatalyst is the
temperature of the catalyst, and Cis a constant that
depends on the geometry and material of the catalyst.
For each TCatalyst, this simple model yields a pre-
dicted heat transfer rate from the exhaust gas to the
catalyst by using the experimentally measured exhaust
mass flow rate and the turbine outlet temperature for
each load for different strategies. A positive heat trans-
fer rate corresponds to catalyst warm-up, as heat is
transferred from the exhaust gas to the catalyst.
Negative heat transfer rate corresponds to catalyst
cooling down, as the heat is transferred from the cata-
lyst to the exhaust gas.
For each load, predicted heat transfer rates are nor-
malized by the ‘‘six-cylinder stock calibration’’ case at a
catalyst bed temperature of 0 °C for that particular
load. The result allows assessment of the relative warm-
up characteristic of each strategy, at various loads. A
higher heat transfer rate is preferred during the catalyst
warm-up phase and is achieved using the optimal com-
bination of exhaust flow and turbine outlet temperature
for a particular catalyst bed temperature.
Figure 17 illustrates the catalyst warm-up character-
istics of the stock calibration, ‘‘open VGT,’’ ‘‘open
VGT + IVC modulation,’’ and cylinder deactivation
strategies at 1.3 bar BMEP. The six-cylinder stock cali-
bration mode has the highest heat transfer rate when
Table 2. Summary of experimental results illustrating ideal operating modes for fuel efficiency, aftertreatment warm-up,
aftertreatment temperature maintenance (e.g, ‘‘stay warm’’), and active diesel particulate filter regeneration. The top row shows
percentage fuel savings when compared with stock calibration. The bottom two rows show the turbine outlet temperature for the
strategy. Results show significant fuel consumption and aftertreatment thermal management benefits for cylinder deactivation and
intake valve closing modulation within the load range 1.3–6.3 bar BMEP.
Operating load, bar 1.3 2.5 3.8 5.1 6.3
Fuel efficiency 3 cylinder IVC modulation IVC modulation Open VGT Open VGT
(25.1%) (28.7%) (17.7%) (12%) (5.6%)
Aftertreatment warm-up Stock calibration Stock calibration Stock calibration 4 cylinder IVC modulation
(T\1008C)
Aftertreatment warm-up 3 cylinder 3 cylinder 4 cylinder 4 cylinder IVC modulation
(T.1008C)
Aftertreatment 3 cylinder 4 cylinder IVC modulation Open VGT Open VGT
maintain temperature (314 °C) (371 °C) (377 °C) (397 °C) (417 °C)
Active diesel particulate 3 cylinder 3 cylinder 4 cylinder 4 cylinder IVC modulation
filter regeneration (314 °C) (473 °C) (484 °C) (537 °C) (440 °C)
BMEP: brake mean effective pressure; IVC: intake valve closing; VGT: variable geometry turbine.
Figure 16. Turbine outlet temperature versus brake mean
effective pressure at 2200 r/min. The shaded region indicates
the turbine outlet temperature range consistent with
maintaining selective catalytic reduction temperature between
300 °C and 450 °C. This temperature range is typically the
aftertreatment sweet spot wherein the conversion efficiency is
maximum. BMEP: brake mean effective pressure; IVC: intake
valve closure; VGT: variable geometry turbine turbocharger.
1012 International J of Engine Research 18(10)

the catalyst bed temperature is lower than 100 °C . This
is because the positive impact of elevated exhaust mass
flow is more important than elevated turbine outlet
temperature at lower catalyst bed temperatures.
As the catalyst bed temperature increases, higher
exhaust gas temperatures are required to maintain heat
flow from gas to bed (equation (2)). As a result (see
Figure 17), the three-cylinder mode outperforms all
others above bed temperatures of 100 °C. The catalyst
can only reach a temperature of 200 °C if the engine
only operates in the six-cylinder stock calibration mode,
whereas the three-cylinder mode enables catalyst tem-
peratures up to 300 °C. Figure 17 illustrates that the
fastest way to warm up the catalyst bed at 1.3 bar is to
operate in the stock six-cylinder mode until the bed
reaches 100 °C, at which point a mode switch to three-
cylinder operation should be made. Combination of
Figures 6 and 17 suggests that the six-cylinder ‘‘open
VGT’’ mode could be used instead of the stock six-
cylinder mode, if fuel economy is paramount during the
warm-up.
Figure 18 illustrates the catalyst warm-up character-
istics of the stock calibration, ‘‘open VGT,’’ cylinder
deactivation, and ‘‘open VGT + IVC modulation’’ at
2.5 bar BMEP. Like the 1.3 bar load condition (Figure
17), operation in the stock six-cylinder stock mode will
result in the highest gas-to-bed heat transfer rates when
the bed temperature is below 100 °C, while the three-
cylinder mode is preferred above this temperature.
Significantly, the three-cylinder strategy can heat the
catalyst to temperatures in excess of 470 °C.
Figure 19 shows the catalyst warm-up characteristics
of the stock calibration, ‘‘open VGT,’’ ‘‘open VGT +
IVC modulation,’’ and four-cylinder strategies at 3.8
bar BMEP. Again, until the catalyst bed temperature
reaches 100 °C, the six-cylinder stock calibration has
the highest heat transfer rate, while at this load four-
cylinder mode is preferred above 100 °C. This is a
result of the four-cylinder mode having a turbine outlet
temperature about 200 °C higher than the other two
modes, as shown in Figure 16.
Figure 20 illustrates the catalyst warm-up character-
istics of stock calibration, ‘‘open VGT,’’ ‘‘open VGT
+ IVC modulation,’’ and the four-cylinder strategy at
5.1 bar BMEP torque. The four-cylinder strategy has a
higher heat transfer rate than the other three cases at
all catalyst bed temperatures. The four-cylinder mode
Figure 19. Catalyst warm-up characteristics of different
strategies at 2200 r/min, 3.8 bar brake mean effective pressure.
Predicted heat transfer rates are normalized using the heat
transfer rate of the ‘‘six-cylinder stock calibration’’ case at a
catalyst bed temperature of 0 °C.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Figure 18. Catalyst warm-up characteristics of different
strategies at 2200 r/min, 2.5 bar brake mean effective pressure.
Predicted heat transfer rates are normalized using the heat
transfer rate of the ‘‘six-cylinder stock calibration’’ case at a
catalyst bed temperature of 0 °C.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Figure 17. Catalyst warm-up characteristics of different
strategies at 2200 r/min, 1.3 bar brake mean effective pressure.
Predicted heat transfer rates are normalized using the heat
transfer rate of the ‘‘six-cylinder stock calibration’’ case at a
catalyst bed temperature of 0 °C.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Ramesh et al. 1013

at this load can also heat a catalyst up to temperatures
in excess of 520 °C.
Figure 21 illustrates the catalyst warm-up character-
istics of the stock calibration, ‘‘open VGT,’’ and ‘‘open
VGT + IVC modulation’’ strategy at 6.31 bar BMEP.
The open VGT + IVC strategy has the highest heat
transfer rate, and can heat a catalyst to temperatures in
excess of 450 °C.
Active diesel particulate filter regeneration
The engine needs to operate in such a way that the tur-
bine outlet temperature is above 450 °C in order to
enable an active diesel particulate filter regeneration
without dosing the diesel oxidation catalyst. The three-
cylinder mode yields the highest turbine outlet tempera-
ture when compared with the stock calibration and
open VGT strategy at 1.3 bar BMEP. Cylinder deacti-
vation (three-cylinder and four-cylinder) enables tur-
bine outlet temperature to reach 450 °C for loads
between 2.5 and 5.1 bar BMEP, which is enough to
realize active regeneration in the diesel particulate filter
without dosing the diesel oxidation catalyst. However,
cylinder deactivation is not possible at a load of 6.31
bar BMEP or higher, as the air flow is too low to sus-
tain smoke-free combustion. As such, at 6.31 bar,
BMEP the best way to operate would be to use the
‘‘open VGT + IVC’’ strategy, as it has a higher tur-
bine outlet temperature as well as a lower fuel con-
sumption when compared with the stock calibration.
Conclusions
This paper demonstrates the benefits of cylinder deacti-
vation and other low airflow strategies under elevated
speed, low-load operation when the goal is to:
(a) reduce fuel consumption;
(b) warm up the aftertreatment;
(c) maintain elevated aftertreatment temperatures; or
(d) enable active diesel particulate filter regeneration
without dosing the diesel oxidation catalyst.
A summary of experimental results is shown in
Table 2. The key conclusions are as follows.
1. The fuel savings of cylinder deactivation, ‘‘open
VGT,’’ and IVC modulation are between 5% and
25%, depending on the load. The fuel savings are
primarily due to a reduction in airflow leading to
an increased open-cycle efficiency. For loads less
than 2.54 bar BMEP, cylinder deactivation has
higher fuel efficiency than ‘‘open VGT’’ operation.
For loads greater than 2.54 bar BMEP, the ‘‘open
VGT’’ and IVC modulation strategy have better
fuel economy than cylinder deactivation, as there
is higher in-cylinder heat loss during cylinder deac-
tivation, which causes overall efficiency to decrease
for the cylinder deactivation strategy.
2. The VGT is squeezed under these low-load condi-
tions in order to prepare for a transient event.
Opening up the VGT decreases the delta pressure
across the engine, thereby increasing the fuel effi-
ciency. However, this might have detrimental
effects on transients; this needs to be understood.
3. At catalyst bed temperatures below 100 °C, the
stock calibration warms up the aftertreatment
faster for loads between 1.3 and 3.8 bar BMEP.
For loads between 3.8 and 5.1 bar BMEP, the
four-cylinder mode will warm up the aftertreat-
ment system more quickly.
Figure 20. Catalyst warm-up characteristics of different
strategies at 2200 r/min, 5.1 bar brake mean effective pressure.
The predicted heat transfer rates are normalized using the heat
transfer rate of the ‘‘six-cylinder stock calibration’’ case at a
catalyst bed temperature of 0°C.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
Figure 21. Catalyst warm-up characteristics of different
strategies at 2200 r/min, 6.31 bar brake mean effective pressure.
The predicted heat transfer rates are normalized using the heat
transfer rate of ‘‘six-cylinder stock calibration’’ case at a catalyst
bed temperature of 0 °C.
IVC: intake valve closure; VGT: variable geometry turbine turbocharger.
1014 International J of Engine Research 18(10)

4. At 6.31 bar BMEP, there is not enough oxygen
available in the cylinder to sustain smoke-free com-
bustion during cylinder deactivation. However, the
‘‘open VGT + IVC modulation’’ strategy can
help realize faster warm-up at all catalyst bed
temperatures.
5. When the aftertreatment is warm, cylinder deacti-
vation helps keep the engine and aftertreatment
system above 250 °C for loads below 2.54 bar
BMEP. For loads above 2.54 bar BMEP, the open
VGT and IVC modulation strategies help keep the
system warm in a fuel-efficient manner. Cylinder
deactivation on average gives rise to a turbine out-
let temperature increment of 150 °C–250 °C in the
1.3–5.1 bar BMEP load range.
6. Cylinder deactivation not only helps to warm up
the diesel particulate filter faster under certain con-
ditions but also enables diesel particulate filter gas
inlet temperatures required for active diesel parti-
culate filter regeneration without using a diesel oxi-
dation catalyst doser.
7. Strategies including turbocharger electrification,
supercharging, powertrain hybridization, and
availability of look-ahead information can be used
to improve the transient behavior of the engine.
Several variable valve actuation strategies, such as
zero valve overlap or early exhaust valve opening,
could also prove viable strategies to improve tran-
sients but are beyond the scope of this paper.
Acknowledgements
The heavy-duty engine was generously provided by
Cummins Inc. Technical assistance was provided by
both Cummins Inc. and Eaton for this work. The
authors would also like to thank the engines lab per-
sonnel at Ray W Herrick labs, particularly David
Meyer and Ron Evans, for their immense support
toward this work.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest
with respect to the research, authorship, and/or publi-
cation of this article.
Funding
The author(s) disclosed receipt of the following finan-
cial support for the research, authorship, and/or publi-
cation of this article: Funding for this project was
provided by Cummins Inc. and Eaton.
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