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Development of the Variable Valve Timing and
Lift (VTEC) Engine for the Honda NSX
Takefumi Hosaka and Minoru Hamazaki
Honda R&D Co., Ltd.
Enginesrs, Inc.
ABSTRACT
The Honda variable valve timing and lift
electronic control system (VTEC) is incorporated in the
engine of the NSX sports car that is scheduled for
sales in Europe this year.
In the process of advancement of Honda's
engine technology, VTEC was developed for much
higher output and higher efficiency.
This is actually the first system in the world that
can simultaneously switch the timing and lift of the
intake and exhaust valves. This system has made
improvements in maximum output at high rpm, and
also improved the low rpm range, such as idling
stability and starting capability.
emphasis is placed on low speed performance, the
high speed performance then suffers.
In order to obtain smooth engine performance
over the entire rpm range, "making low-speed and
high-speed performance consistent" became the most
important objective. With this in mind, the double overhead cam (DOHC), variable valve timing and lift
technology (VTEC) was developed by H ~ n d a . ( l ) ( ~ ) ( ~ ) *
The DOHC VTEC engine is able to maintain a high
output throughout the entire rpm range, thus enabling
the high speed performance of a racing engine without
sacrificing low and medium speed performance.
Low fuel consumption and sharp throttle
response are made possible by staying with a
naturally aspirated system rather than adopting a turbo
or super-charged system.
INTRODUCTION
Most people define a sports car as a car with
high performance capabilities which can be turned
and stopped precisely, enabling the driver to feel as
one with the car.,
In order to satisfy these requirements, Honda
has developed an entirely new sports car named the
"NSX". The NSX is based on advanced technology
with emphasis placed on ergonomics which enables
anyone to enjoy driving this car.
The engine which is the heart of this car is
described here in the report.
DEVOLOPMENT AIMS
The main objectives in developing this engine
were high output performance throughout the rpm
range, sharp throttle response, and low fuel
consumption. in order to avoid being penalized by
restrictions imposed on high fuel consumption cars
(i.e. Gas Guzzler Tax). When high output performance
is attempted at a high rpm the conventional engine is
unstable at idle and there is a reduction of output
performance in the low rpm range. Conversely, when
* Numbers in parenthesis designate reference at end
of paper.
Fig 1
27
Complete cut-away of Engine
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ENGINE SPECIFICATIONS
Fig.1 shows a cut-away view of the engine, and
specifications are listed in Table 1. The C30A engine
is a small V6 with a die-cast aluminum block. The 90'
bank-angle cylinder arrangement has a total
displacement of 2977cm3(cc). The cylinder head has a
pent-roof shaped combustion chamber with a center
plug system and a compression ratio of 10.2. It is a
short stroke engine (cylinder bore diameter X stroke9Ox78mm) well suited for high rpm performance. At
8000rpm the maximum piston speed is 20.8mlsec.
The VTEC system is hydraulically controlled to
vary the valve timing and the lift according to
operational conditions. This is incorporated in a cross
flow 4-valve DOHC system.
The connecting rods are made of light weight
titanium in order to achieve higher rprn and reduce
vibration.
The fuel system is an electronically controlled
sequential injection system (PGM-F1:Programmed Fuel
Injection) with 6 injectors located in the intake
manifold.
To increase the torque in the medium speed
range, a Variable Volume Induction System (VVIS)
with a resonance chamber has been adopted for the
intake manifold.
The throttle body is a large diameter single-valve
side draft type.
The engine has a dual exhaust system with
reduced pressure in order to lower exhaust noise.
The direct ignition system (no distributor) is more
reliable and stable. Also there is a knock control
system with 2-knock sensors in each bank of three
cylinders.
MANUAL TRANSMISSION
2 0 1 kW 7100RPM
284.4N.m 5300RPM
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The exhaust gas control system is composed of
an EGR system, a 3-way catalyser, a air-fuel ratio
control system incorporating an 0 2 sensor with a
heater at each bank.
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Engine Type
Cvlinder No.
VTEC3.00 ENG
C30A
v-6
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VTEC (Variable VT)
Bore XStroke
Comoression Ratio
1
- -
Open (BTDC)
Close(ABDC)
1
I
201KW/7lOOrpm
274PS
284.4N.m/5300rpm
2 9 . 0 ke.m
7.5'
12.5'
25~
1
25'
Open (ABDC)
Close (ATDC)
j
1
5~
45~
Maximum Power
Maximum Torque
I
EX
I
~ift
9.Omm
Table 1
Engine Specification
ENGINE PERFORMANCE
The C30A engine output performance data is
shown in Fig.2.
AUTOMATIC TRANSMISSION
188kW 6600RPM
286N.m 5300RPM
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1000 2000 3000 4000 5000 600070008000
Fig 2
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,
10002000300040005000 60007000 8000
Engine Speed (rpm)
Engine Speed (rpm)
Engine
I
DOHC Swing Arm Type
Valve System
In
1
Performance C u r v e Line
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The C30A engine has high output and high rpm
similar to that of a racing engine. By using a naturally
aspirated engine, the throttle response is sharp. The
maximum output of the engine is 201kwn1OOrpm, the
maximum torque is 284.4n.m/5300rpm, and the
maximum engine rpm is set at 8000rpm (The
specifications for the Japanese domestic model: The
output
performance using premium unleaded
RON100 is 206kwn300rpm and 294n.ml5400rpm).
This engine outperforms conventional DOHC
engines in output performance. In Fig.3, a comparison
among naturally aspirated engines is shown in torque
per liter as well as output per liter.
105 -
10.5-
100a
n
+O
-
*
95 -
E
• 90P - 9.0- 5
:
v
.'C
858.5-
V)
8.0-
+ + NSX
(MT-J)
INTEGRA
*NSX *NSX
(AT-US] (MT-US]
9.5-9
g ?I-
n
NSX
(AT-JI
10.0-
*
conventional engine. A racing engine requires wider
valve overlap, and increased valve lift to obtain higher
performance at high engine speed. However in a
conventional engine, the valve timing and degree of
valve lift are set for higher performance at low and
medium engine speeds.
By designing a higher valve lift, wider valvetiming, and larger valve diameter, we are able to
obtain a higher volumetric efficiency to cope with
higher output engine speeds. Also by tuning the intake
and exhaust ports, an intake resonance is created
causing a super-charging effect. Improvement is also
made with the volumetric efficiency by reducing the
exhaust pressure.
With VTEC, the valve timing and lift can be
adjusted at low engine rotation to increase torque and
prevent air from being forced back through the intake.
In Fig.5 & 6 is an example of VTEC being used to
improve volumetric efficiency from low engine speed
to high engine speed. Using premium unleaded
(RON100) a wider torque band can be achieved up to
8000rpm. A maximum torque of 98.8n.m:lO.l kgmlliter
and an output of 206kw are possible.
0
0
0
m0
PC7O
000
cl 0
801
40
0
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50
60
70
Specific Power Output (PS/i?)
Fig 3
Relationship between Specific Power and Torque
Engine Speed (rpm)
VTEC PERFORMANCE
To develop a high performance engine it is
necessary to increase the volumetric efficiency and
reduce friction.
Fig.4 shows a comparison between lift and valve
timing in a racing engine VTEC engine and a
Fig 5
Racing Engine
DOHC VTEC Engine
Volumetilc Efficiency
Mass Produced Engine
Hlgh-Speed
Valve Timing
:
2
I s e l
5
1m
Crank Angle
Crank Angle
Valve Lift
lm
Crank Ang e
~ood
Good
Poor
Low RPM Torque
Poor
Good
Good
Idle
Poor
Good
Good
Max. Power
Fig 4
Lift and Valve-Timing Comparison between Enginesfor Racing and for General Use
29
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Low speed High Speed
A
1
- 1 -
VTEC Engine
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/
/
*
1
Switch-iver Point
Fig 6
Engine-Speed
(r~m)
The cam has 3 different profiles located at the
intake and exhaust of each cylinder. The center cam is
used exclusively for high speed and the 2 outside
cams for low speed.
The rocker arm assembly is composed of a mid
rocker arm with primary and secondary rocker arms on
each side. Inside the rocker arms are 2 hydraulic
pistons, a stopper pin, and a return spring, which make
up the change over mechanism.
The mid rocker arm has a lost motion spring so
that the valve operates smoothly at high speeds and
also stops the arm at low speeds.
The whole system is operated by a hydraulic
actuator which is controlled by the Engine-Control Unit
(ECU).
Output and Switch-over of Low speed and High speed
VTEC MECHANICAL OPERATION
1) VTEC LAYOUT
In Fig.7 the structure of the VTEC is shown. The
C30A engine has one extra cam profile and rocker
arm (mid rocker arm) for high engine speed.
2) VTEC OPERATION
Fig.8 shows the VTEC mechanism while
operating at low engine speeds. In the low-speed
mode the 3 rocker arms are separated and use cams
A & B only. At this time the mid rocker arm is in contact
with the high speed cam due to the spring force in the
lost motion mechanism. It is separated from the
primary and secondary rocker arm and thus is not
actuating the valves. Fig.9 shows the VTEC
mechanism while operating in the high speed mode.
During high speed engine operation the 3 rocker arms
are connected and move together due to the 2
hydraulic pistons which have moved over due to
increased hydraulic pressure.
Cam Lobe for Low S ~ e e d
Primary Rocker
Fig 8
R~~~~~~~~
1
.
-
Secondary Rocker Arm
Rocker Arm Operation (Low speed Range)
Cam Lobe for High Speed
A
a Camshaft
a Hydraulic Piston A
a Cam Lobe for Low Speed Range @ Hydraulic Piston B
a Cam Lobe for High Speed Range @ Stopper Pin
@ Primary Rocker Arm
3 Mid Rocker Arm
6 Secondary
Rocker Arm
Fig 7
@ Lost-motion Spring
Exhaust Valve
@ Intake Valve
VTEC System Construction
Fig 9
Rocker Arm Operation (High speed Range)
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The unified rocker arms are operating from the
central cam profile that has a wider valve timing and
higher lift designed specifically for high speed.
When changing from high to low speed, the
hydraulic pressure is released and the pistons are
pushed back by the return spring housed in the
secondary rocker arm. The 3 rocker arms are
separated and return to low speed operation.
4) SETTING THE VALVE CHANGE-OVER POINT
Fig.11 shows the change-over point of the valvetiming and valve lift. In order to eliminate unnecessary
load on the engine, the change point is where the low
and high speed outputs intersect.
With the C30A engine, the change over point
does not change when engine load is varied. The
hysteresis between accelerating and decelerating time
is k100rpm (5800-6000rpm).Therefore the changeover rpm is at 5900rpm.
3) VTEC CONTROL SYSTEM
Fig.10 shows the control system of the VTEC.
When the control system detects a change in engine
rpm, car speed, or water temperature, it transmits this
information to the ECU. When preset conditions are
met, the ECU activates the solenoid valve which
causes the spool valve to turn on or off the hydraulic
pressure that controls the pistons in the rocker arm .
This hydraulic signal system is synchronized
with the fuel-injection system and electronic ignition
system. So that there is a proper air to fuel ratio and
ignition timing for either high or low speed operation.
In order to maintain smooth engine operation,
the control system must make changes from moment
to moment, adjusting valve timing, valve lift, air to fuel
ratio and ignition timing to meet existing conditions.
~ i g hspeed
P a r t i a l Loaded Condition
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b
Engine Speed
Fig 11 Switch-over Point (NSX Engine)
&
.. ......................................
....+,
Durlng Fall-safe Actuat~on
Sw~tch-over
r
I
Fixed at low-speed
Ftxed at IOW-speed
range
A ~ r ~ f u Program
el
Valve Tlmlng
l g n ~ t ~ oProgram
n
Valve L ~ f t
Or Use of Fall-safe
Program
v
Hydraulic Piston
Valve T ~ m l n g
-
Valve L ~ f t
T--i
1i
............................................
I
Total Performance
Fig 10 VTEC Control Diagram
31
'
1
-
6
Order of A c t u a t i o n
Hydraulic Path
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5)FACTORS EFFECTING CHANGE-OVER
The factors that effect the change-over
characteristics are the dimensional accuracy of the
variable valve system, the dimensional accuracy of the
constituent parts, the rigidity of the system, and the
composition of the hydraulic circuit. These different
factors have exact settings for various conditions of
operational use. Even under deteriorating conditions
due to long use, the change-over characteristics can
be kept within the set specifications. Fig.12 shows the
response time when switching to high and back to low
engine speed quickly. The graph compares the
hydraulic change-over signal, the change in valve lift,
and the change in hydraulic pressure inside the rocker
shaft. When changing from low to high speed
operation, the hydraulic pressure is increased in the
rocker shaft by the signal from the ECU. After the
response time TI it is changed to the high speed cam
profile. Then changing back to low speed the hydraulic
pressure is released in the rocker shaft and the return
spring releases the rocker arm assembly so that it
switches to the low speed cam profile. This response
time is T2. The response time is very fast and is
completed within one revolution of the cam shaft.
40
50
60
70
80
90
100
110
120
Oil Temperature ("C)
Fig 13 Relationship between Response Time and
Oil Temperature
60 r
~
Oil Temp 80°C
ON
Switch-over Signal
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0.2 0.3
\
=Response Time for Swbtch-over (ON)
iT2 =Response
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0.6
0.7
0.8
0.9
Return Spring Set Load
m'
o i l pressure
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0.4 0.5
!
(Kgf)
Fig14 Relationship between Response Time
Return Spring Set Load
Time tor Swrtch-over (OFF)
Tim0
Fig 12 Response Time for Switch-over
Fig.13 shows the relationship between engineoil temperature and the change over response time.
As oil temperature increases, the high-speed change
over response time becomes longer and the lowspeed change over response time becomes shorter.
This is due to changes in oil viscosity as temperature
increases.
Fig.14 shows the relationship between the
rocker arm piston return spring load and the changeover response time. The lower the set load is, the
shorter the high speed change-over time becomes,
and the longer low-speed change-over time becomes.
This is due to the characteristic forces between the
hydraulic pressure and spring load. The factors
effecting the change-over point have been analyzed
individually, and specifications have been set
accordingly.
HIGH R.P.M RELIABILITY
Parts with enhanced reliability and durability are
necessary in an engine designed to function at high
revolutions. The following parts enable the VTEC
C30A engine to be utilized to its fullest potential.
1) TITANIUM CONNECTING RODS
The reciprocating mass of moving parts must be
low in a high speed engine. Light weight connecting
rods are important in a high-speed engine. To reduce
the weight of the connecting rods, titanium was used.
Titanium has a specific gravity of 4.48 and a
tensile strength of 65kg/mm2. This is superior to steel
which has a specific gravity of 7.85 and a tensile
strength of 50kg/mm2. However, due to cost, forgability,
formability, and machinability considerations,
titanium's use was limited to specific applications.
A new titanium material, 3A1.2 vanadium
sulphur (Ti-3A1-2V-S) was developed for use in the
C30A engine. This material's weight is 30% lighter
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than steel, allowing for an additional 700rpm to be
added to this engine. The big-end of the connecting
rod which is subjected to thrust is surface-treated with
chromium nitride and its resistance against burning
from contact with the crank shaft is double that of steel.
2) NEW VALVE DESIGN
The intake and exhaust valve must have a high
thermal strength, be light weight and durable in order
to function in a high speed engine. The exhaust valves
are made of a newly developed material combining
tungsten and molybdenum with a nickel base. This
improves the heat resistance of the exhaust valves by
30% over conventional materials. Also, in spite of a
10% increase in valve diameter, we were able to
reduce the stem to 5.5mm, thus reducing the valve
weight by 15%. The intake valve also has a thin stem
to reduce its weight.
3) PRECISION CRANK SHAFT
To reduce the amount of wear of the bearing
meta! of the crank shaft and to reduce the friction, a
new machining process was incorporated. By using a
super-micro grindstone lapping system we are able to
improve the surface roughness of the crank shaft
by more than 50% and the roundness by 40%.
4) IMPROVED OIL COOLER DESIGN
The oil cooler has been improved to cope with
the higher oil temperature at high rpm. By increasing
the density at the core of the oil cooler, it can be made
more compact and lighter in weight. A higher cooling
efficiency was achieved by changing fin direction and
shape so it is more suitable for oil flow.
5) LOWERING VIBRATIONS
By using computer analysis, engine vibrations
have been reduced. One example is the crank shaft
torsional damper. To increase the power plant rigidity,
double structures are used by joining the transmission
with the deep skirt of the cylinder block.
CONCLUSION
The new V6, DOHC VTEC engine developed by
Honda has achieved the following development aims.
1) It is possible to obtain smooth output characteristics
and excellent throttle response over the entire rpm
range.
2) The C30A engine has an output per liter and torque
per liter far above conventional engines.
3) The emission requirements and low fuel
consumption are met while maintaining high
performance.
With the C30A engine, it has been made
possible for Honda to produce the "NSX" sports car
that is high in performance, reliable, durable, and
allows smooth comfortable driving over the entire rpm
range.
REFERENCE
1) "A high power, wide torque range, efficient engine
with a newly developed variable valve lift and
timing mechanism",
K.lnove et.al.,SAE Paper 890675,1989.
2) "A high power,wide torqe range, efficient engine
with a new variable valve timing and lift
mechanism.; Part 1 Development of Variable
Valving System",
K.lnove et.al.,JSAE 891004,1989.
3) "A high power, wide torque range,efficient engine
with a new variable valve timing and lift
mechanism.; Part 2 Engine Performance and Car
Performance",
K.lnove et.al.,JSAE 891005,1989.