Technology Close-up

* HYPER VTEC as of 2004

HYPER VTEC

Development of "Direct actuated" Type Valve Control System for the Engines of Sports Type Motorcycles.

Overview

With the growth of interest in global environmental protection, measures are being actively taken also in the development of motorcycles, such as the improvement of fuel consumption rate, cleaning exhaust gas and the reduction of driving noise. The reconciliation of the measures for environmental protection and engine output is a technically difficult problem. Honda has developed a "direct-actuated" type valve control system, "HYPER VTEC" (Variable valve timing and lift electronic control system), which will reconcile engine output and environmental protection measures for sports type motorcycle engines, for which high rotation and high power are required.

Honda has thus far developed variable valve control systems, such as the REV mechanism developed for motorcycles in 1983, VTEC mechanism having further developed the REV mechanism and recognized worldwide for application to automobiles and various VTEC mechanisms controlling variable valves by incorporating hydraulic circuit in locker-arms.
For sports type motorcycle engines, however, the mainstream is a "direct-push dynamic" valve system, in which cams directly drive valves, from the need of pursuing high revolution and high power. Sports type motorcycle engines requiring a high revolution zone of over 10,000rpm is commonly used at times. To reconcile such high revolution and high power with environmental protection measures, an entirely new and compact "direct-actuated" valve type variable valve control system with built-in hydraulic mechanism in valve lifters, HYPER VTEC, was developed. The HYPER VTEC is at present the only VTEC system without passage through the locker arms, including those used for automobiles.

*HYPER VTEC is a trademark of Honda Motors Co., Ltd.

Direct-actuated type HYPER VTEC Reconciling High Revolution and High Power and Environmental Performance

In sports type motorcycle engines, especially in 4-stroke engines of medium displacement, the maximum revolution reaches to a level as high as 13,500rpm. To cope with such high revolution, Asaka R&D Center has developed a compact and simple "direct-actuated" type variable valve control system, HYPER VTEC. This is an epoch-making system as switchover is made from two-valves to 4-valves per cylinder in the low and medium speed range and the high speed range is completed in the valve lifter without using the locker-arms, so the increase of mass is checked of the kinetic valve system and the ease of operation is secured in high revolution range.

Points of HYPER VTEC development

HYPER VTEC is a valve pausing system, further advancing VTEC technology through hydraulic control unique of Honda, in pursuit of the improvement in the suction, exhaust efficiency and combustion efficiency of 4-cycle engines. A direct-actuated type is adopted, in which cams will directly push up the valves through lifters. The system enabled to switch over the number of valve operations per cylinder between low and medium speed revolution range and high speed revolution range, and by setting cam profiles to give optimum valve timing to the valves on working side and on paused side, power output characteristics were demonstrated in the low and medium speed revolution range and in the high speed revolution range for further improvement in fuel consumption rate.

In January 1999, valve control system for 4-stroke engines, HYPER VTEC, was announced.
It was put on sale, mounting on the new sports bike, CB400 SUPER FOUR.

Further Evolution to HYPER VTEC SPEC II and III

In January 2002, HYPER VTEC evolved into Spec II aiming at letting everybody actually feel the power drive over the entire ranges. As one of the aims of Spec II development, it can be pointed out that the CB400 SUPER FOUR, the model on which the system was installed, is the one having a wide user class. There were voices to point out that beginners, in particular, were unable to fully utilize the four-valve stage from above the revolution range of 6,700. The development team then developed HYPER VTEC SPEC II anew to enable many more users to enjoy the four-valve stage of HYPER VTEC.
SPEC II achieved increasing torque in the middle speed range and smoother power by refining the valve timing, inteke and exhaust systems, and the valve switchover timing revolution from two to four valves was changed from 6,750rpm to 6,300rpm. It has resulted in powerful and sporty engines, enabling one to actually feel VTEC in all driving ranges from low to top. In December 2003, SPEC III was introduced. The switchover timing for a 6 speed only was changed from 6,300rpm to 6,750rpm to improve fuel consumption in cruising over high speed roads without spoiling the cheerful drive.

HYPER VTEC keeps on evolving, without slackening.

From REV to VTEC. Variable Valve Control System ever evolving

It can be said that VTEC, the original Honda variable valve control system originated from REV, revolution responding type valve pausing mechanism, announced in 1983. Receiving the demand for high power for sports bike engines at that time, prior study was started to reconcile high power in high revolution range and excellent drivability and high efficiency from idling throughout all ranges. An epoch-making variable valve system came into being, which enables to switch over from two valves to four valves. This technology later developed into the variable valve timing mechanism, VTEC (variable valve timing and lift electronic control system), which forms the nucleus of automobile engines.

REV born from aiming at reconciliation of 200PS per liter and low revolution range performance

Faced with the demand for high power for sports bike engines, a preceding study started off on the assumption of 200PS performance per liter and the development of mass production engines with drivability from the same idling as before. The assignments for the project were to achieve the target power performance and come up with countermeasures against power drop in low speed revolution range and improper idling. It was revealed in the analysis of these issues that the large bore ports for achieving high revolution and high power and for reducing air inlet resistance and the intake and exhaust system with a pleural number of intake and exhaust valves hindered obtaining good idling and low and medium speed revolutions.
An experiment conducted by forcibly stopping several multiple valves made it possible to confirm the stability of idling and a dramatic improvement in the power of low and medium speed revolution range. This was the starting point for the development of valve pausing mechanism (REV: Revolution-modulated valve control), which was applied to the CBR400F in 1983.

Honda CBR400F equipped with REV, marketed in 1983

Starting point of VTEC.
Revolution responding type valve pausing mechanism "REV"

REV is a system to operate four valves in high speed revolution range in accordance with the revolution of engines and to reduce the number to two valves in low and medium speed revolution range. Pause the operation of valves on both inlet and exhaust sides at the rate of one each per cylinder. A sensor detects the revolution of engines. Four valve operation and two valve operation are automatically changed over, performing the separation and connection of locker-arms by the shift of hydraulic pistons built inside the locker-arms split into two. When four valves are in operation, high power is produced in high speed revolution range while two valves are put into operation, blow-by of fuel air mixture is reduced, so that flow velocity is improved. A high swirl effect and excellent filling efficiency will materialize power improvement in low and medium speed revolution range.

VTEC meeting with various requirements, such as practical use types and low fuel consumption types

VTEC engines started from the REV for motorcycles have undergone subsequently various evolutions. DOHC VTEC engines with switchover for valve timing and the amount of lift for intake and exhaust system aiming at high power and high torque. VTEC engines with a single cam shaft to drive intake valves, switching over low speed valve timing lift and high speed valve timing lift by attaching importance to the balance in the practical use range. Furthermore, VTEC-E engines, which attempted substantial improvement of fuel consumption by enabling lean burn, suspending one valve in low speed range, in addition to the driving by practical valve timing lift in medium and high-speed range. And, 3 STAGE VTEC engines were put out. Switchover was made literally into three stages, using three cams for low, medium and high-speed operations on air inlet side, suspending one valve at low speed operation and taking switchover motion to valve timing lift in medium speed and high-speed operations.

Intelligent new-generation engines, DOHC i-VTEC

Honda has further evolved VTEC and newly developed DOHC i-VTEC, reconciling low fuel consumption of world top class and clean exhaust gas and having high power and rich torque characteristics over the entire ranges. For this engine, highly intelligent valve timing lift mechanism, which combines Honda's original VTEC switching over valve timing and the amount of lift in accordance with the revolution ranges of engines with VTC (variable timing control) continually controlling the phase of air inlet valve timing in accordance with the load on engines.

Mechanism of direct-actuated type variable valve control system, HYPER VTEC

Aiming at reconciling engine power and environmental protection measures for sports type motorcycle engines of high revolution and high power, the Development Team of Asaka R&D Center challenged to the development of "direct-actuated type" variable valve control. As the result, an epoch-making variable valve system came into the world, which has a valve operation switchover system built inside valve lifters.

Goal of Development of Valve Control System for Sports Type Motorcycle Engines

A variety of technological goals were set for the development of HYPER VTEC. The first was to guarantee the operation of kinetic valve system up to the high revolution range of engines over 10,000rpm. The next was to minimize the increase of mass around the valves, the most serious impediment to high revolution engines. Moreover, to make the mechanism as compact and simple as possible, let it have high reliability and durability and enable to be mass-produce at reasonable costs. The development of "direct-actuated type" valve control system started, aiming at these goals.

Details of HYPER VTEC Mechanism

HYPER VTEC is designed for four valve engines. A cylinder is equipped with two intake valves and two exhaust valves. Set to each of them were normal valves working over the whole ranges from low speed revolution to high speed revolution, and paused valves which shut down in low and medium revolution range and are actuated at time of high revolution operation only. The system is made up of, in addition to them, a hydraulic control system including the valve operation switchover system and hydraulic circuit built inside the valve lifter of paused valves. The valve shutdown and actuation are controlled by the positions of the valve switchover pins built inside the valve lifters (hereinafter called the slide pins). Needless to say, cam profile giving the optimum valve timing is set on each normal use and shutdown valve.

State of Valve Shutdown

While the valves are in the state of shutdown, no hydraulic pressure works on the slide pins so that the slide pins are pushed against the stopper pins by the return springs. Since the valve stems then crawl into the through holes provided in the slide pins, the valves will remain in the state of shutdown even if the valve lifters and switchover mechanism are driven by the cams. The valve lifters and switchover mechanism follow the motion of the cam profiles with the outer valve springs.

Conditions of Valve Operation

Hydraulic pressure passes through the hydraulic passage inside the cylinder head and acts on the slide pins through the outer peripheral orifices of the lifter from the hydraulic grooves inside the lifter hole, to slide the slide pins. Since the slide pins slide, when the valve lifters are driven, the flat section of slide pins will push the valve stems to move the valves and valve lifters in one body. Valves, valve retainers and cotters follow up with the motion of the cam profiles by inner springs.

Comparative Examination of Valve Shutdown Mechanism

With respect to the valve shutdown mechanism built inside the valve lifters, various mechanisms were examined in an effort to make it more compact and simple. What to do with the shim used for adjusting tappet clearance? How to regulate the movements of valve lifters and valve stems while the motion is idle, i.e., in the state of shutdown? We will introduce here four plans of valve shutdown mechanism, which were compared and examined at the stage of development.

The Plan A is characterized by the adjustment of tappet clearance with outer shims and slide pin installing holes were formed directly on the valve lifters. The plan grows large in the mass of mechanism because of outer shims. Furthermore, machining of slide pin installing holes on the valve lifters is difficult.

The plan B is featured with the adjustment of tappet clearance with inner shims, with the switchover mechanism being separated from the valve lifters and with projection provided in the middle of valve lifters, and the spring for idle motion of valve lifter was arranged between the valve lifter and switchover mechanism. The shutdown mechanism and valve lifter were placed on top of the conventional kinetic valve system. Detents were needed for the valve lifters and shutdown mechanism, and space was insufficient for the installation of springs for idle motion of valve lifters, so that spring load could not be set to follow up to high revolution range.

For the plan C, the slide pin was located downward to reduce the idle motion space for the valve lifters. In the plan, the mass of mechanism for outer shims grew large and the machining of slide pin installing holes on the valve lifters was difficult.

For the plan D, the tappet clearance was adjusted with inner shims and the switchover mechanism was made a separate body. The switchover mechanism could be manufactured by cold forging to make it lightweight.

As the result of comparison and examination of these plans, the plan D was adopted to produce very compact and lightweight valve shutdown mechanism suitable for HYPER VTEC.

Merits of Direct-Actuated Type Variable Valve Control System, HYPER VTEC

Direct-actuated type variable valve control system, HYPER VTEC, developed with the aim of reconciling high revolution and high power performance for sports type motorcycle engines and environmental performance. Ease of handling over the whole ranges from low to medium speed revolution to high-speed revolution and the feeling of sporty acceleration, improvement of fuel consumption rate and even the reduction of driving noise mainly of intake and exhaust sound, not to mention the improvement of power have been realized. Through the development of the HYPER VTEC, Honda realized simple and compact variable valve control system, which enabled high-speed revolution of over 10,000rpm and further evolved VTEC.

Engine Output Characteristic

Comparison of power characteristic of HYPER VTEC engines and the base engines having the conventional valve mechanism shows that power improvement is seen in medium and low speed range in the engines adopting HYPER VTEC and improvement is attempted also in high-speed range by designing intake and exhaust system suited for high-speed range. Power improvement in medium and low speed range enabled to materialized power characteristic easy to handle and sharp feeling of acceleration is obtained by the change in torque characteristic at switchover from two to four valves.

Reduction of Friction and Improvement of Fuel Consumption Rate

According to the results of friction measurement of kinetic valve system of HYPER VTEC engines, friction increases also in the range where four valves are put into operation. In the range where two valves are operated, the friction is reduced as compared with the base engines equipped with conventional valve mechanism, and thus contribute to the improvement of fuel consumption rate by approximately 5.5% in EC40 mode.

Effect of Reducing Drive Noise

HYPER VTEC has an effect of reducing the sound of intake and exhaust system in the range where two valves are working. According to an analysis of the driving noise frequency, a reduction of 2 to 5dB was seen as compared with base engines in each frequency zone mainly in the areas of intake and exhaust sound below 1kHz. Accelerated drive noise measured by the method specified by European legal control (EEC87/56) was reduced by 1.7 ~ 2.0dB both in the second gear and third gear despite an increase in the speed from approximately 10 to 15%. Concerning the mechanism of drive noise reduction, it was confirmed that the sound pressure level inside the induction and exhaust pipes was reduced with the shift to two valves, and at certain revolution range the blow-down pressure was reduced at the beginning of the opening of the exhaust valves and the spitting from the exhaust system to intake system decreased at the time of valve overlap. Those are considered attributable to the reduction of intake and exhaust sounds.

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