The complete story about the new Honda CVT transmission
Date: June 30, 2002 06:49
Here's the complete story about the new Honda CVT trasmission in english. I also posted the link to the original Honda June newsletter
HONDA JAZZ CONTINUOUSLY VARIABLE TRANSMISSION (CVT)
Offers excellent fuel economy plus the fun of steering wheel-mounted manual shift
Since its launch earlier this year, the Honda Jazz's clever packaging and highly efficient engine technology have received wide acclaim for bringing a fresh approach to a crowded small car market. Now customers can opt for a CVT (continuously variable transmission) complete with 7-speed semi-automatic shift controlled by steering wheel-mounted buttons. It also has an intermediate "fully automatic" setting.
It means Jazz drivers can enjoy the convenience of automatic gear changing (either in CVT mode or automatic mode) or, when the mood takes them, the fun of fingertip shifting of gears. Why should supercar drivers have all the fun? And such is the advanced nature of this CVT design that there is little penalty to be paid in terms of either fuel economy or performance. The Jazz 1.4 SE/LS CVT can reach 100 km/h from standstill in 12.0 secs just as the 5-speed manual transmission equipped model does; while the move up to SE Sport/ES specification adds only 0.3 seconds.
Thanks to a design that maximises fuel efficiency, consumption figures are some of the best within the CVT/automatic equipped small car sector. Combined figures for the SE/LS and SE Sport/ES are 5.8 and 5.9 l/100 km/h respectively, while at cruising speeds, results for the CVT and manual cars are remarkably similar. Fuel efficiency is reflected in low CO2 emissions: 137 and 139 g/km.
Effortless motoring or shift-it-yourself fun
The advantages of CVTs are of course their ease of use, efficiency and the complete absence of any sensation of shift changes. Honda's latest CVT design offers an added level of flexibility that perfectly complements the Jazz's driving dynamics. Many drivers will undoubtedly be happy to drive their CVT in 'conventional' mode but equally, many, if only occasionally, will enjoy the greater sensation of control offered by the option of manual shifting which involves changing up or down through the seven pre-set ratios by means of buttons mounted on the steering wheel.
The compact, lightweight CVT has been engineered to provide crisp responses to driver input and it is also remarkably adaptable in a number of ways. Firstly, in normal CVT operation, the driver has the choice of either Drive (D) or Sports (S) modes (providing sharper acceleration). In turn, within each of these programs, the intelligent electronics assess the nature of the road from the driver's movement of the accelerator pedal in order to switch between different gear ratio mapping, thus matching engine performance to the prevailing conditions. Even the degree of slow speed creep is now determined by the ECU so that assistance is only provided when it is really needed, such as in slow moving traffic.
The principles of CVT
In simple terms, CVT consists of a drive pulley (input) and a driven pulley (output) connected by a steel belt, which transmit torque from the engine to the final drive. The steel belt is a ring of numerous steel element plates that are joined with two-layered ring bundles. One side of each pulley is fixed, while the other side can be hydraulically moved in and out, in effect altering the diameter of each pulley; those changes in pulley diameter are synchronised and as a result the belt rides lower or higher on the walls of each pulley to provide continuously variable changes to the gearing of the car. CVT therefore does not change gear in a series of steps, but instead delivers smooth, seamless performance from standstill to full speed operation.
From the driver's point of view the system is therefore effortless. The advent of sophisticated electronics and complex mapping has allowed CVT to exploit peak engine efficiency and torque to the full, and by using its continuously variable nature to match engine performance with road requirements, the transmission is able to maximise fuel economy. Compared to a conventional automatic transmission, there are further benefits: torque converter slippage is eliminated and power losses reduced, a wider spread of ratios is possible while there is, of course, no shift shock.
Of course, just as electronic control can be used to optimise efficiency, so too can it be used to provide more sporty performance.
Driving the new Jazz CVT
When operating the CVT conventionally, the driver uses the floor-mounted gearshift in a single plane, moving through a choice of Park, Reverse, Neutral, Drive, Sport, and Low. Drive (D) and Sport (S) options allow a choice of shifting regime that suits the driving conditions or the driver's own mood. For relaxed and economical motoring the ratios in D mode are mapped to ensure engine speed fully maximises engine torque. Alternatively, if the driver wants to use the Jazz's performance to the full, S mode provides maximum acceleration by allowing the engine to reach speeds at which it is providing peak power, holding it their as the gearing is progressively increased.
A high level of adaptability provides added sophistication. Within both D and S modes are subtly different programs designed to cope with different driving conditions which the system identifies from the frequency of use of the accelerator pedal and the extent to which it is opened beyond that necessary for cruising. So, within D mode, the system switches between D1 (conventional) and D2 which optimises ratios for the type of conditions typically encountered around town - more responsive acceleration is designed to cope with the stop/start and sudden acceleration requirements of urban congestion.
Furthermore, if the driver demands maximum acceleration while in D mode, then the system recognises the sudden throttle opening and switches to a further programme, RS, which helps to achieve that. Similarly in S mode, the S1 sporty mode is supplemented by S2 or 'winding' mode. The latter keeps engine revs generally higher, particularly above 50 km/h, for better responsiveness when exiting corners. Finally, L mode acts in exactly the same way as that in a conventional automatic, maintaining low gearing/high engine speeds for progress at slow road speeds.
Using the 7-speed semi-automatic shift
The new CVT used in the Jazz offers the choice of a semi-automatic shift, allowing the driver to move progressively through seven carefully chosen ratios, just as you would with a manual gearbox. As well as the fun element of steering wheel-mounted controls, this arrangement also means hands can be kept on the wheel for maximum control, especially useful on fast, winding roads. The only automated aspects of the system are an automatic upshift when the engine reaches 5700 rpm, and automatic downshifts below 1600 rpm until second gear is reached. The system is set to reach maximum speed in sixth gear.
Activating '7MT' mode while in either D or S modes is simply achieved by pressing the '7 Speed Mode' switch beneath the right hand spoke of the steering wheel. The driver can then effect gear changes by pressing the buttons mounted on either side of the steering wheel; for ease of operation each button provides both up (+) and down shifts (-).
So that the status of the system is absolutely clear to the driver at all times, a display is located alongside the fuel gauge. Illumination of the letters PRNDSL corresponds to selections made during conventional CVT operation, and alongside this, an LED displays the numbers 1 to 7 to indicate the gear selected when in 7MT, and beneath this the letter M to indicate manual mode.
If having pressed the '7 Speed Mode' button on the steering wheel the driver then chooses not to make any gear shifts via the +/- steering wheel controls, then the system acts like a conventional automatic 'box and will change gear based on the seven pre-set ratios without any further input required on the part of the driver (7AT mode); the dash display shows the gear selected (1 to 7), but the 'M' manual mode indicator is not illuminated. Pressing the +/- buttons will introduce 7MT mode.
To change back to conventional CVT mode is simply a matter of pressing the '7 Speed Mode' switch once more, when operation reverts to the PRNDSL shift lever. If the engine is switched off while in 7AT or 7MT, the system automatically defaults to CVT mode; like a conventional automatic transmission, the Jazz CVT will only start in P for safety reasons.
As well as a smoother take up, the Jazz's new CVT has been designed to feel altogether more responsive and with this aim in mind, there is now a more linear relationship between engine speed and accelerator pedal angle. Similarly, the system delivers smarter responses to throttle inputs by the driver. Thus when the accelerator is pressed slowly, the engine speed changes slowly and when the accelerator is pressed quickly, the car is far more responsive than in previous designs. This approach makes for more natural and logical progression.
The provision of a certain amount of creep with any automatic gearbox gives the driver an added degree of control, but what might be a satisfactory level at slow speeds can become inappropriate at other times. The Jazz's advanced CVT therefore delivers improved levels of control by providing strong creep at low speeds (when in traffic, for example) which is reduced when the brakes are applied (thus saving fuel). However, when stopping from higher speeds, much weaker levels are provided, only building up when the car begins to move again. Creep strength increases on steep inclines.
The creep performance is as good as that provided by a conventional automatic transmission. The levels provided are under the control of the ECU, which varies creep strength by altering the pressure of the departure clutch.
Advanced electronics play a large part, but behind the innovative features displayed by this CVT gearbox are also a number of significant mechanical design revisions. The flywheel for example is now single rather than dual mass to reduce weight and to give smoother engine revolution. Of particular importance is an increase in the area of the drive pulley (30 per cent larger than previous versions) which boosts the overall spread of ratios, so contributing worthwhile improvements in fuel economy. Using a direct driven torchoidal oil pump both saves weight and makes for more compact dimensions.
The side pressures applied to both drive and driven pulleys are an essential part of CVT power transfer and the changing of ratios. Compared to previous versions, the new system has two (rather than one) pressure control units, controlling the drive and driven pulleys independently with the appropriate minimum pressures. Further more, the drive and driven pulleys have different piston diameters (drive is approximately 15% larger than driven) which can give a lower operating pressure. These systems reduce oil pump load and increases transmission efficiency.
Finally, a new clutch material has been especially developed for the Jazz's CVT transmission and the addition of carbon fibre improves friction characteristics to make pulling away altogether smoother. It's also more durable and heat resistant.