Hitachi Review Vol. 58 (2009), No. 7
319
Variable Valve Actuation Systems for Environmentally
Friendly Engines
Seinosuke Hara
Seiji Suga
Satoru Watanabe
Makoto Nakamura
OVERVIEW: Use of variable valve actuation in engines has become
more common in recent years as an effective technique for improving fuel
economy and exhaust emissions as well as engine output, and it is growing
in importance as a promising technique for reducing CO2 in response to
environmental problems in particular. Hitachi commercialized VTC in
the mid-1980s and commenced mass production of VEL in 2007. VTC is
becoming a standard feature in gasoline engines and the fuel economy
benefits of VEL when used in combination with VTC have seen it adopted
increasingly wider. Hitachi is working on further enhancements to variable
valve actuation which has a key role as a technology for complying with
fuel economy regulations.
INTRODUCTION
VEHICLE engines have made remarkable progress
since they first appeared and the operating range of
engines in terms of rpm (revolutions per minute)
and torque have expanded significantly. However,
optimum engine design becomes difficult if the
operation timing of the engine valves is fixed. For
this reason, variable valve actuation has been used to
optimize the valve timing in response to the operating
Trends in fuel
economy
regulations
Europe CO2
USA CAFE
conditions since the 1980s. Although the technique was
initially used in a limited way for applications such as
simultaneously achieving both higher output and stable
combustion when idling, its effectiveness has grown
as fuel economy and exhaust emission regulations
have tightened and it has been adopted as a means of
complying with these regulations.
Hitachi was ahead of its Japanese competitors in
commercializing use of VTC (valve timing control) to
185 g/km (1995, actual data)
27.5 mpg (1985)
120 g/km (2012*)
35 mpg (2020)
140 g/km (2008)
*Including tires, etc.
Adoption of energy management
Trends in
engine
Improved engine
Port injectors
technology efficiency
Enhancements
to variable
valve
actuation
Idling stop
Direct injection
Cam
Lifter
Valve
Hybrids
Downsizing, supercharging
Exhaust Intake
Cam switching (VVL)
VEL (+ continuous VTC)
From 1997
Two-stage VVL
Variable phase
(VTC)
On/off VTC
From 1986
(commercialization)
Continuous
operation
2000
HCCI combustion
• Faster response
• Wider variable range
• Electric operation
From 2007
Enhancement • Integration with
other systems, etc.
VEL
Improvement
Waste heat utilization
From 1998
Continuous
operation
Combination technologies
Continuous VTC
Both for intake
and exhaust
2005
Year
Electrically
operated
2010
VTC: valve timing control VVL: variable valve lift VEL: variable valve event and lift control HCCI: homogeneous charge compression ignition
CAFE: Corporate Average Fuel Economy mpg: miles per gallon
Fig. 1—Enhancements to Variable Valve Actuation.
Variable valve actuation is growing in importance as a technology that benefits both vehicle performance and the environment. The
technology will be enhanced further in its role as a key technology for reducing CO2 (carbon dioxide) emissions from future vehicles.
320
Variable Valve Actuation Systems for Environmentally Friendly Engines
Performance items
• Torque
output
Charging
efficiency
Charging
response
Indicated
• Fuel
consumption thermal
(CO2)
efficiency
(net thermal
efficiency)
Mechanical
efficiency
• Exhaust
(HC, NOx)
Engine
outlet
Valve TDC
overlap
Main improvement
factors
Valve timing
Valve lift
Throttle-less
Compression
(expansion) ratio
Gas motion
Internal EGR
Fuel atomization
Lower residual gas
Pumping loss
Low valve lift,
deactivation
Fuel atomization
Suppression of
fuel adhesion
Internal EGR (NOx)
Catalytic
Catalytic converter
converter outlet warm-up
Exhaust
close
Shortening of intake stroke
Open throttle valve
100
Atmospheric Reduced negative
pressure
intake pressure
Effective intake stroke
Early close
Late close
Standard
10
10
(8.2%)*
Intake
TDC
late
−21
close 99
(5.7%)*
Standard
0
DOHC: double overhead camshaft rpm: revolutions per minute
Fig. 3—Pressure Change in Cylinder and Reduction in Pumping
Loss Achieved by Valve Timing.
By using late close or early close for the intake valve, the
effective intake stroke can be shortened and the throttle opened
accordingly. This reduces the negative intake pressure and
reduces pumping loss.
Phase
Intake
close
Crank
angle
: Possible
Variable valve
actuation functions
Type
• Intake valve close
(Early close, late close,
effective compression ratio)
Valve
timing
control
(VTC)
• Intake valve open
(Gas motion by late open)
• Exhaust valve close
(Residual gas sealed in)
• Exhaust valve open
(Effective expansion ratio)
• Valve lift
(High lift and event, very low lift)
• Valve overlap
(Increased residual gas,
scavenging)
• Valve deactivation
(Cylinder deactivation,
single valve deactivation)
BDC: bottom dead center TDC: top dead center HC: hydrocarbon
EGR: exhaust gas recirculation
Fig. 2—Relationships between Engine Performance
Improvement Factors and Variable Valve Actuation Functions.
Variable valve actuation has extensive benefits for fuel economy
and exhaust emissions as well as engine output.
TDC
BDC
TDC
58
100
1,000
Stroke volume V(cm3)
* Improvement in fuel economy due to lower pumping loss (standard ratio)
Intake valve
BDC
Effective intake stroke
−64
Pumping losses
Lift
BDC
Intake
open
Exhaust
Intake
With cam
Exhaust valve
Exhaust
open
(Valve timing)
• 4-cylinder, in-line 2.4 L, DOHC • 1,200 rpm−39.2 Nm Intake
TDC
early 28
Reduced negative intake pressure
close
TDC
BDC
1,000
: Partially possible
: Not possible
Event
Engine
ContiValve lift
Deacti- nuous perform- Install- Cost
characteristics Phase Lift Event vation control
ance ation
Low Good Low
Cam
switching
(VVL)
Variable
valve event
and
lift control
(VEL)
Without cam
Valve lift
FUNCTIONS AND PERFORMANCE
BENEFITS OF VARIABLE VALVE ACTUATION
Traditionally, the requirements for valve actuation
mechanisms were higher lift and faster operation. These
are also key issues for variable valve actuation where
the mechanism is more complex. On top of this, there
is a need to use the variable mechanism to improve
combustion and obtain better thermal efficiency
in order to get better fuel economy and emission
performance. Fig. 2 shows the relationship between
variable valve actuation functions and the factors that
improve engine performance.
The benefits of variable valve actuation are not
limited to engine output and include fuel economy
and exhaust emissions. In terms of thermal efficiency,
variable valve actuation has major benefits that include
reducing the pumping loss associated with the timing
while the intake valve is closed as well as achieving
better combustion by better gas motion and fuel
atomization through lower valve lift.
Fig. 3 shows an example of the reduction in pumping
loss achieved by variable valve actuation.
Fuel consumption is improved by approximately
Cylinder pressure
P(kPa)
change the cam phase in the mid-1980s. Hitachi started
developing variable lift in the 1990s, first introducing
VVL (variable valve lift) and later VEL (variable
valve event and lift control) which was selected for
the Infinity G37 (V6, 3.7 L) from Nissan Motor Co.,
Ltd. and other vehicles and entered mass production
in 2007 (see Fig. 1).
This article describes the performance benefits of
variable valve actuation systems which are an effective
means of making engines more environmentally
friendly, and gives an overview of VTC and VEL
which are typical examples of variable valve actuation
systems.
Hydraulic
or
electromagnetic
drive
High Poor High
* Additional functions made possible by combining with valve timing control.
Fig. 4—Comparison of Types and Functions of Variable Valve
Actuation.
Enhancing the functionality improves engine performance but
brings significant problems of cost and installation.
Hitachi Review Vol. 58 (2009), No. 7
8% under partial load. In terms of exhaust emissions, a
significant reduction is achieved in the HC (hydrocarbon)
and NOx (nitrogen oxide) toxic components. With
advances in catalytic technology, the level of HC
emissions before the catalytic converter warms up
when the engine is started from cold remains a current
problem but significant benefits can be obtained in
this area by using techniques such as low lift for
fuel atomization and overlap control to suppress fuel
adhesion to walls. Fig. 4 shows the classification
of the specific mechanisms used for variable valve
actuation.
Although greater functionality leads to better
performance, it also brings significant problems such
as cost and installation(1).
VTC SYSTEM
As VTC uses the actuator on the sprocket at the
end of the camshaft to perform phase conversion as
shown in Fig. 5, the system is simple and practical.
Hitachi initially produced the system using hydraulic
on/off control with a helical gear mechanism. Although
the gear noise resulting from the alternating torque on
the camshaft is an issue for VTC, this was resolved
by a backlash elimination mechanism devised by
Hitachi. Subsequently at the end of the 1990s, Hitachi
commercialized a vane-type system with fast response
and large conversion angle using continuous control.
As continuous control optimizes the valve timing based
on the operating conditions, it requires a high degree
of precision. Specifically, crank angle, cam angle, and
other sensor signals drive a solenoid valve that adjusts
the oil pressure in the VTC hydraulic chamber. This
321
continuous control mechanism significantly improves
fuel economy, emissions, and other parameters and
has become a standard feature on gasoline engines.
Although the technique is mainly used for the intake
valves, use on the exhaust valves is becoming more
common. In particular, it is used to reduce turbo lag
in direct-injection gasoline engines by improving the
scavenging efficiency and other factors using valve
overlap in combination with turbocharging. It is
anticipated that future needs will include demand for
electrically operated systems that can optimize valve
timing from the moment the engine is started.
VEL SYSTEM(2), (3)
The concept of using variable lift to change the
lift and valve opening duration (events) has been a
subject of research since the 1960s. Hitachi’s first such
product was VVL, but because this involved switching
between two settings it was unable to provide sufficient
fuel economy benefits and therefore Hitachi went on
to develop VEL. The development of VEL drew on
Hitachi’s experience from prototype development for
SOHC (single overhead camshaft) engines in the early
1980s(4).
Basic Design and Principles of Operation
Fig. 6 shows the configuration of a VEL system.
The main development objectives were to: (1) ensure
that the system could be added to existing engines with
minimal modifications, (2) provide a wide range of lift
and event variation, (3) allow high engine rpm, (4) low
drive friction, and (5) highly responsive control. To
Actuator drive current
VTC actuator
Initial helical type
(on/off)
Backlash
elimination mechanism
Current vane type
(continuous)
Cam angle
Solenoid valve sensor
VTC
Valve lift
VEL
control
shaft angle
VEL
VTC
Engine and
vehicle data
Lock pin
Exhaust Intake
Angle sensor
Actuator
VEL
ECM
Vane
Hydraulic
Crank angle sensor
chamber Sprocket
Crank angle
Exhaust Intake
12.5 mm
Target VEL angle
(Maximum)
VEL
0.7 mm
BDC
(Minimum)
TDC VTC BDC
VTC control
ECM
Actual VEL angle
CAN VEL-ECU
ECM: engine control module
Fig. 5—VTC System.
The hydraulic pressure to the VTC is controlled based on the
crank angle, cam angle, and other sensor signals.
CAN: controller area network ECU: electronic control unit
Fig. 6—VEL System.
VEL lift control and VTC phase control operate in tandem.
Variable Valve Actuation Systems for Environmentally Friendly Engines
Valve lift adjustment
Rocker
Control
mechanism
arm
cam
Control
High valve lift
Low valve lift
shaft
Oscillation
Link A
pivot
Drive
cam
High lift
Link B
Oscillating
cam
Ball screw
Link A
Drive
cam
Actuator arm
Lifter
Valve
Fig. 7—VEL Principle of Operation.
The oscillation position of the oscillating cam can be modified
to change the valve lift.
VEL assembly and component parts
DC motor
VEL assembly
Actuator
Control shaft
Link
Low lift
Drive shaft
322
VTC/solenoid valve
Control shaft
Drive shaft
Angle sensor
VEL cylinder head (V6)
Rocker arm
Valve lift
Oscillating cam
adjustment Link B
mechanism
VEL-ECU
Valve lifter
(DLC coating)
DC: direct current DLC: diamond-like carbon
Fig. 8—VEL Assembly and Component Parts.
As the drive shaft is located where the conventional camshaft
used to be, the VEL assembly is compactly fitted in the cylinder
head.
ensure practicality, the variable mechanism was located
in the empty space between cylinders without changing
the camshaft position in existing engines.
Fig. 7 shows the principle of operation of VEL. The
cyclic motion generated by the rotation of the drive
cam is transferred to the oscillating cam by the multilink mechanism. Moving the rocker arm oscillation
pivot up or down changes the oscillation position of
the oscillating cam and therefore modifies the lift and
events in a continuously variable way. A feature of
VEL is that it uses the multi-link mechanism to forcibly
transfer the cyclic motion to the oscillating cam. This
reduces the drive friction in the mechanism because
no return spring is required to pull back the oscillating
cam. The forced drive via the multi-link mechanism
also facilitates higher engine speeds and the system
shown can operate at up to 7,500 rpm, the highest rpm
achieved by the mechanism of this type. VEL improves
fuel economy and engine output because it can control
the valve lift over a wide range (0.7 mm to 12.5 mm).
In particular, throttle-less operation can be used with
very low lift which significantly reduces the pumping
DC motor
(Rated output: 60W) Angle sensor
Low lift
High lift
Ball screw mechanism
Fig. 9—VEL Actuator.
Features such as the highly efficient ball screw reduction
mechanism allow a smaller electric motor to be used.
loss associated with the throttle. Also because the
valves are used for throttling, the acceleration response
is also improved due to better intake charge response.
Although cylinder to cylinder variation in intake was
an issue when using very low lift, this was resolved
using a unique lift adjustment mechanism.
Fig. 8 shows the VEL assembly and component
parts and Fig. 9 shows the VEL actuator. The actuator
consists of a brushed DC (direct current) motor, ball
screw reduction mechanism, and control shaft angle
sensor. Features such as the low friction of the VEL
mechanism and high transmission efficiency of the ball
screw mean highly responsive control can be achieved
using a small electric motor with a rated output of
60 W.
Control System
VEL drive control is performed by using the vehicle
and engine data to calculate the target values for VEL
in the ECM (engine control module) and then sending
these to the VEL-ECU (VEL electronic control unit)
via the CAN (controller area network) (see Fig. 6).
The VEL-ECU then performs actuator drive control
using the angle sensor signal from the control shaft
as feedback.
In addition to a high level of responsiveness and
stability, the functions required for VEL drive control
include: (1) ability to specify the frequency response
characteristics, (2) a high degree of robustness over
time and with respect to changes in ambient conditions,
and (3) low electric power consumption of the electric
motor. Model reference control with two degrees of
freedom was adopted to achieve (1) and (2). For (3),
a specially designed model reference filter circuit
was used. VEL generates an alternating torque in
the control shaft from the reaction force of the valve
Hitachi Review Vol. 58 (2009), No. 7
*1 P −1(S)R(S)=F −1(S)P −1(S)R(S)F(S)
Feedforward compensator*1
P −1(S)R(S)
Filter location after optimization
Reference Feedback
model
compensator Filter VEL
VEL angle signal
+
+ +
K(S)
F(S)
P(S)
R(S)F(S)
−
F(S)
Target
VEL
angle
High
Model reference control
(before optimization)
(Engine: 1,000 rpm)
80
Target VEL
angle
VEL angle
signal
Lift
60
40
Low
VEL angle signal (deg)
Filter location before optimization
20
0
1.5
2.5
3
3.5
220 ms*2
80
*2: 90% response time
60
Target VEL
angle
VEL angle
signal
40
4
20
0
4.5 1.5
180 ms*2
2
2.5
3
3.5
4
4.5
Amplitude of
Amplitude of
manipulated variable is high. anipulated variable is low.
20
10
0
−10
−20
1.5
Manipulated
variable (V)
2
Model reference control
(after optimization)
2
2.5
3
3.5
Time (s)
4
20
10
0
−10
−20
1.5
4.5
2
2.5
3
3.5
Time (s)
4
4.5
Fig. 10—Block Diagram of VEL Drive Control and Optimization
of Filter Location.
The transfer characteristics were optimized by placing a filter in
the output stage of the drive control to prevent the value of the
manipulated variable from getting too large.
springs and other sources. This alternating torque
can interfere with the actuators and, if the value of
the manipulated variable used to suppress this torque
becomes large, the result is an increase in the power
consumed by the electric motor. Although filtering the
sensor signals is one way to prevent this increase in
the value of the manipulated variable, this leads to a
loss in responsiveness. To resolve this problem, a filter
was placed in the drive control output stage as shown
in Fig. 10 and the control system was redesigned to
include a reference model by assuming the transfer
characteristics obtained when this filter is treated
as part of the frequency characteristics of the VEL
mechanism. The resulting control system successfully
suppresses the amplitude of the manipulated variable
without loss of responsiveness or stability. The electric
power consumption of the VEL system in actual use is
2.2 W which is less than the power used by the VTC
solenoid (2.8 W).
Thanks to the above mechanisms and control
developments, the performance benefits provided
by VEL when deployed in an actual vehicle include
fuel (CO2: carbon dioxide) savings of approximately
10%, a reduction of approximately 50% in exhaust
(HC) emissions when cold starting, and an increase of
323
approximately 10% in engine output.
Future issues for VEL include further improving
responsiveness, expanding the variable range, reducing
costs, and making the mechanism more compact.
CONCLUSIONS
This article has described the performance benefits of
variable valve actuation systems which are an effective
means of making engines more environmentally
friendly, and given an overview of VTC and VEL
which are typical examples of variable valve actuation
systems.
The more sophisticated variable valve actuation
systems become, the greater the number of control
parameters involved and the more advanced the system
development capabilities that are needed. With VEL,
an optimum system was able to be commercialized
by developing the mechanism and control systems
in tandem. Although there is an ongoing shift to
electrically driven power trains in response to strict
environmental regulation, it is likely that the internal
combustion engine will remain the mainstay for the time
being and it is anticipated that variable valve actuation
will have an increasingly important role in extracting
the maximum efficiency from these engines.
Hitachi intends to utilize its system development
capabilities and various underlying technologies to
enhance the functions of variable valve actuation as far
as possible at a reasonable cost. Hitachi’s strategy is
to help reduce CO2 emissions from vehicle engines by
pursuing a combination of technologies that includes
direct injection, turbocharging, HCCI (homogeneous
charge compression ignition) combustion, and hybrid
systems.
REFERENCES
(1) S. Hara, “Variable Valve Actuation System,” Specific
Technologies for Vehicle Engines (I), All Advancing
Technologies, pp.165-181, Sankaido Publishing Co., Ltd.
(June 2005) in Japanese.
(2) M. Nakamura et al., “A Continuous Variable Valve Event
and Lift Control Device (VEL) for Automotive Engines,”
SAE 2001-01-0244.
(3) Y. Yamada et al., “Development of Continuous Variable
Valve Event and Lift Control System for SI Engine (VEL),”
SAE 2008-01-1348.
(4) S. Hara et al., “Application of a Valve Lift and Timing
Control System to an Automotive Engine,” SAE 890681.
Variable Valve Actuation Systems for Environmentally Friendly Engines
324
ABOUT THE AUTHORS
Seinosuke Hara
Seiji Suga
Joined Hitachi Unisia Automotive, Ltd. in 1995, and
formerly worked at the Engine Component Design
Department, Design & Development Division, Engine
Components Division, Hitachi Automotive Systems,
Ltd. Mr. Hara is a member of the Society of Automotive
Engineers of Japan (JSAE), and the SAE International.
Joined Hitachi Unisia Automotive, Ltd. in 1978,
and now works at the Engine Component Design
Department, Engine Components & Control Brake
Division, Engine Components Division, Hitachi
Automotive Systems, Ltd. He is currently involved in
design and development of engine components.
Satoru Watanabe
Makoto Nakamura
Joined Hitachi Unisia Automotive, Ltd. in 1985, and
now works at the Control System Design Department,
Powertrain Design Division, Powertrain & Electronic
Control Systems Division, Hitachi Automotive
Systems, Ltd. He is currently involved in design and
development of control systems. Mr. Watanabe is a
member of the JSAE.
Joined Hitachi Unisia Automotive, Ltd. in 2001,
and now works at the Engine Component Design
Department, Engine Components & Control Brake
Division, Engine Components Division, Hitachi
Automotive Systems, Ltd. He is currently involved in
design and development of variable valve actuation.
Mr. Nakamura is a member of the JSAE.