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CVT transmission

CVT transmission 1
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CVT Transmissions – Development Technologies – Author’s Practical Article

CVT – Development Technologies

Since the release of the first CVT (or CVT 1) for engines with a volume of 2-2.5 liters in 1997, the development of these types of transmissions has continued. The line of varieties CVT 1 or Hyper CVT, Hyper CVT M6 which was based on the modification RE0F06A – RE0F06V has been replaced by CVT2 or RE0F10A / 11A. CVT 2 designation was given by manufacturers as the second generation of midsize front wheel drive cars.

Chased several tasks:
– improved fuel economy
– acceleration dynamics improvement
– weight reduction
– downsizing

Consider Table 1, which lists the general characteristics of CVT 1 and CVT 2..

CVT transmission 2

First of all, it is noticeable in the Pully ratio line – the range of gear ratios has been expanded.
In other words, from virtual 6 CVT 1 gears to virtual 7. The larger the RATIO range, the lower the speed at cruising speed, the higher fuel economy, the lower exhaust emissions and noise levels.

In manual mode, 6 gears were left – but due to the extended range of gear ratios. The main refinement for expanding the ratio coverage touched the leading pulley – its inner diameter of the cone working surface near the shaft was reduced by 5 percent.

If we examine CVT 2 in detail, then we can say that this is not a modified CVT 1, but a completely redesigned transmission, since the changes affected absolutely everything. Only transmission principle left.

Since the operation of the CVT transmission is based on the friction effect, the operation of the latter leads to a large heat release – and therefore to losses.

The developers decided to seriously reduce losses in CVT 2, for this:
– new materials were used in the manufacture of pulleys, higher strength steel, which allowed to reduce the wall thickness of the pulleys. Pulleys become lighter, less moment of inertia.
– the surface cleanliness of the pulleys, including the internal cavities, was significantly increased, this allowed to reduce the gaps in the movable pulley joints. To do this, a thin high-strength coating layer is sprayed onto the working surface in a vacuum chamber.
– new friction pairs in o-rings and couplings were selected, together with increased accuracy of mating parts. This led to the creation of a single piston drive pulley..

Photo 1. CVT-1 twin piston drive pulley.

CVT transmission 3

In RE0F06A, the drive pulley had two piston designs. Photo 1 shows the internal piston and O-ring. An external piston was implemented in the outer pulley drum. Given the low precision of surface treatment, there were large losses due to leaks in the pistons. Accounted for
increase pump performance, although the area of ​​the two pistons was larger and with the same internal pressure on the two-piston pulley, the compression force of the belt segments was higher compared to the single-piston pulley.

– the design of the oil pump has been completely changed (Fig. 2) – it has become a vane and, together with the pressure reducing valve, has been transferred to the sump. This allowed to reduce its size, and placement in the pallet – to reduce pumping losses thereby increasing its efficiency. For this, the channel of the pressure reducing valve is combined with the receiving channel.

This design reduces pump noise, increases efficiency at high speeds. The overall transmission length is reduced. A separate circuit is used to drive the pump. Photo.3

Photo 2. CVT-1 pump

CVT transmission 4

Photo 3. CVT-2 pump

CVT transmission 5

Fig. 2 CVT-1 pump on the left, CVT-2 on the right

CVT transmission 6

During CVT operation, large volumes of hydraulic fluid are pumped into the pulley very quickly and must also drain quickly from the cavities. In the process of frequent acceleration – braking, these are unnecessary pumping and kinetic losses. The high performance at low revs needed to quickly fill the pulleys is not needed at high revs or in the static mode of movement when the volumes are balanced. Consequently, a high-performance pump most of the time simply runs idle, creating unnecessary losses and heating the working fluid, but a low-performance pump will not provide the initial dynamics of pressure build-up at low revs. CVT 2 pump design selected for optimum performance.

Fig. 3 Placement and drive of the pump with CVT-2 chain

CVT transmission 7

– ball bearings are replaced as less noisy and minimize friction
– To improve the operation of the belt – pulley pair and reduce noise, the shaft bearings are spaced: one on the pulley, one in the housing. (in Hyper CVT – both bearings were on a pulley)

Photo 4. Placing the shaft bearing in the housing

CVT transmission 8

– the forward – reverse planetary gear mechanism has been reduced, and the total transmission length has been reduced due to pump transfer
– an oil separator is installed in the differential gear cavity. Friction losses, oil heating and foaming at high speeds are reduced. The cross-section of the separator is selected in such a way that with an increase in speed, the oil level decreases in the region of the working surface of the differential gear, but not lower than the specified minimum.
– in general, the measures applied allowed, according to the developers, to reduce friction losses by 30% compared to CVT 1.

PRESSURE OPTIMIZATION

In connection with the transition to a single-piston drive pulley, it was necessary to increase the pressure in it, and this led to an increase in losses. Since the linear pressure in RE0F06A was not controlled in any way, which led to unnecessary kinetic losses, a rigid pressure feedback was introduced in CVT 2, in addition, the pressure in the pulleys began to be regulated independently. To implement this, a linear solenoid was added to the channel of the secondary pulley, the pressure in which is controlled independently of the linear one. High precision electronic pressure sensors allow you to independently monitor the pressure in the pulleys and lower it to the minimum required without the risk of belt slippage on the pulleys. Pressure reduction leads to lower CVT losses. Numerous bench and road tests based on the analysis of various traffic conditions allowed to reduce the pressure to optimal values.

Photo 5. Hydraulic control unit.

CVT transmission 9

EXTENSION OF THE HYDRAULIC TRANSFORMER LOCK RANGE

One effective way to save fuel is by locking the torque converter. As mentioned earlier – in the torque converter mode, its efficiency does not exceed 80%, when the torque converter (GT) goes into the fluid coupling mode – then the efficiency tends to 95%, only when the GT is completely blocked its efficiency will be close to 99
% The difference is kinetic, pumping and heat losses. GT blocking can significantly reduce them, especially if it occurs as early as possible. But at low speeds, blocking the GT can lead to shocks and noise due to uneven engine operation at low speeds. This was mainly complained by the owners of CVT 1 with SR20 motors with a mechanical throttle drive and QR20 with an electronic throttle, whose speed is not adapted. CVT 2 introduces a two-stage locking GT – slip and full. Two solenoids control these modes. The blocking pressure is controlled by the level of minimum necessary slippage in accordance with the torque from the engine. This improves the response of the lock in the event that the driver brakes sharply, as it extends the lock range and the engine fuel cut-off range. The one-stage complete blocking of the CVT 1 led to a noticeable push when the GT was opened at a speed of 20 km / h and uneven deceleration. On a motor with a mechanical throttle, this was exacerbated by early cut-off and the reaction of the engine idle system.

IMPROVEMENT OF ACCELERATING CHARACTERISTICS

By reducing the volume of oil in the pulleys (inner chambers) and increasing the efficiency of the pump, as well as altering the hydraulic circuit, control, etc., the overall reaction is significantly improved. Accuracy of pressure maintenance and its stability allow switching quickly and stably without loss of engine torque.

Improving the response time has made it possible to improve the acceleration characteristic and increase the sense of acceleration. These, it would seem, two identical points are considered by developers in different ways. A survey of drivers showed that they biasedly evaluate the acceleration of the car, since in CVT 1 at the initial moment
The engine speed was low. During acceleration, the engine speed is fixed at a certain level, and the speed is gained by changing the CVT gear ratios. The owners had a feeling of insufficient acceleration due to the habit with speed automatic transmissions, where the engine speed is proportionally increased by the speed of the car. In other words, the owners did not have enough engine noise during acceleration, but not fixed but proportional to speed. But initially the development of CVT has other goals, and in the first place is the environment, efficiency, noise reduction. This does not fit in with high engine speeds. Therefore, a large number of test trips were conducted and the switching algorithm was set so that the driver retained a sense of high acceleration. This is achieved by reducing the delay in the reaction of the car to the accelerator pedal from low speeds by changing the control program. A more drastic change in gear ratios required a change in the cross section of the channels and plungers, for example, on CVT -1, the shape of the plunger was chosen to smoothly relieve pressure in the drive pulley due to conical edges.

The shape of the CVT-2 plunger prevents leakage in the channel and instantly opens the channel when depressurizing it..

Photo 6. Servo links for CVT -2 and CVT – 1 gear ratios

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