More on VVT-i and VSC

More on VVT-i and VSC

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With this generation of the Toyota Corolla Altis comes two significant new technologies that enhance performance and safety. The first is variable valve timing – which, in the Toyota engine, is known as VVT-i and available in both the 1.6-litre and 1.8 litre engines – while the second is Vehicle Stability Control (VSC) with Traction Control.

VVT-i (Variable Valve Timing with Intelligence)
The concept of varying valve timing is not new. It first appeared in volume-produced engines in the mid-1980s in an Alfa Romeo model. The idea of varying the opening and closing times of the valves, usually the intake ones, is to achieve optimum performance throughout the rev range. In an ideal operation, the intake valve should open as the piston begins moving downwards on its intake stroke. It should shut when the piston reaches its lowest point in the combustion chamber sealing in the air and fuel mixture during the combustion stroke that creates power. Such a straightforward process works if the engine runs slowly – like 10 ~ 20 rpm.

When you increase the rpms, such a sequence does not work well. When the engine is running at just 4000 rpm, the valves will open and close 2,000 times every minute… that’s almost 4 times every second. This tremendous speed means that the flow of the air-fuel mixture cannot be completed within the fraction of a second the valve opens. Therefore, as the engine speed rises, the intake valve has to be made to open just prior to the intake phase so that by the time the piston starts moving down in the intake stroke the valve is open and air moves into the cylinder during the entire intake stroke.

From this, it is clear that maximum engine performance at low engine speeds will require the valves to open and close at different times from higher engine speeds. In conventional engine designs, engineers select a certain camshaft profile (which influences the valve timing) according to the type of car the engine will power. For example, a 1.3-litre car used for normal commuting may have a cam profile which optimises acceleration at lower speeds but does not provide good performance at high speeds. However, for high-performance models, low-end performance may be not as important as efficient breathing at high speeds, so a different type of cam profile is chosen.

Thus in most engines, the camshaft profile is a compromise and that is why when people modify a car for better performance, they usually look for different cams. However, this leads to a change in the characteristics of the engine and they may get stronger acceleration in the upper rpm range – fine for racing – but poorer torque at the low end, making the car unpleasant to drive in town (a lot of revving may be needed).

Variable valve timing makes such a compromise unnecessary and as one engineer put it, “it’s like having a tuner in your engine”. The opening time of the intake valve is automatically altered at low speeds and high speeds to match the demands and output is consistently high in any speed range.

Initially, the variable valve timing systems were ‘cam-changing’ mechanisms and operated in distinct rpm range. Honda’s VTEC mechanism, which first appeared in 1989, is the one many people are familiar with and it focusses on improving top-end power rather than torque by varying valve lift as well as timing. The change in valve timing occurs up to a maximum of three stages (Porsche’s VarioCam Plus system also works on a similar principle).

Compared to Honda’s VTEC, Toyota’s VVT-i mechanism, supplied by Denso (part of the Toyota Group), alters intake valve opening and closing continuously, a method called ‘cam-phasing’. This approach improves torque throughout almost the entire rev range and according to claims, the changes can be made within a thousandth of a second. An on-board microprocessor in the engine management system constantly monitors the engine operation (throttle opening, rpms, etc) to determine the optimum valve opening time.

Apart from improvements in straightline performance, VVT-i also increases fuel efficiency as combustion is optimised and little fuel is wasted. The efficient combustion also means that less toxic gases are generated in the exhaust fumes, making for even cleaner emissions.

First seen in Lexus engines, VVT-i is conceptually similar to the latest VANOS system in BMWs and well as the systems in some Volvo and Jaguar engines. Lighter, less complex and probably less expensive than VTEC, it is certainly a wonderful solution to the requirements of the go-fast enthusiast as well as the A~B housewife.

Vehicle Stability Control (VSC)
VSC began appearing is mass production models in the mid-1990s and the first Toyota model to have it was the Crown Majesta. The system was developed in collaboration with Aisin Seiki (a leading components manufacturer in the Toyota Group) which holds the fundamental patent for VSC in Japan and the USA. Its patent application in Germany is still pending as Bosch and Continental Teves also hold patents for a similar type of stability control system. Some other companies such as Nissan, Honda, Delphi, ITT and Hyundai also have such systems but each works on slightly different principles.

Aisin Seiki makes various VSC systems to suit different types of cars and that in the Corolla Altis is the latest version known as C-VSC (for Compact VSC) and includes a traction control system. Using a brake modulator with 12 solenoid valves and a vacuum booster with pre-charging function, C-VSC differs from the systems for the bigger models, mainly in the pump circuit structure. Brake control pressure is generated by the conventional pump of the ABS modulator and pre-charging vacuum booster and the brake fluid is supplied through the Master Cylinder chamber where the suction line is connected. In the systems for bigger and heavier cars, the brake control pressure is generated by a self-feeding pump or in SUVs like the Lexus LX470, there’s a separate hydraulic booster.

The development of VSC was a natural progression from the yaw control systems that began appearing in some cars’ suspension designs in the 1980s. With ABS installed, it became easier to add additional programming to maintain grip at crucial moments by decreasing rotating speeds using the brakes.

VSC helps control skids on slippery or dry road surfaces by detecting and correcting understeer and oversteer conditions. The VSC system electronically monitors vehicle speed and direction, and compares the vehicle’s direction of travel with the driver’s steering, acceleration, and braking inputs. The system integrates traction control capabilities to limit spinning of the drive wheels on slippery surfaces.

Apart from neutralising oversteer or understeer in corners, VSC can also stabilise the car during sudden lane-change movements which occur at speed. Such sudden directional changes cause significant instability to occur and the car can spin out of control if the surface is slippery. With VSC, the dangers of such a situation can be reduced substantially.

VSC uses some components shared with the ABS and an electronically-controlled engine throttle, as well as a dedicated computer and sensors. The sensors include a yaw rate sensor (to detect changes in the car’s rotation in a left or right direction), g-force sensor (to determine if the car is decelerating or accelerating), and a steering angle sensor (to evaluate the direction and rate of change in steering wheel movement).

A high-speed computer constantly compares the driver’s intentions – as indicated by steering wheel, throttle and braking activity – with the car’s actual motions measured by the various sensors. If they do not match, the VSC computer selectively applies individual wheel brakes and/or momentarily reduces engine power as necessary to help the car stabilise. In some cases, the system may react faster than the driver can realise that stability is lost.
For example, if the car continues straight rather than responding to the driver’s right turn of the steering wheel, VSC would typically reduce engine power to shift the vehicle’s weight to the front wheels to help provide more traction for steering. At the same time, the system would apply the right front brake momentarily to help the car turn to the right more quickly. Once proper vehicle attitude is restored, VSC returns to a standby state immediately.

Superior it may be and it will certainly reduce accidents, VSC does not mean that you can corner any faster than normal. It cannot improve tyre traction nor defy the laws of physics but it can help provide a measure of control in unexpected situations faced by even the most careful drivers.

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