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Engines
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VVT-i
(Variable Valve Timing with intelligence)
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Why
VVTI
In contrast to the fixed valve timing of
conventional engines, the VVT-I system is an computer controlled
mechanism
that continually varies the timing for opening and closing the intake
valves in accordance with the vehicle's operating conditions.
By determining the level of performance that is
demanded of the engine at a given time by way of the driver's accelerator
operation and the engine rpm, the VVT-I enables the overall performance of
the engine to be enhanced all the way from the low to the high rpm range.
Design
VVT-I is comprised of three major components
1.The electronic control unit(ECU), which calculates optimum intake valve
timing based on engine operating conditions.
2.The oil control valve (OCV), which controls hydraulic pressure under the
ECU's instruction and
3.The VVT pulley, which continuously changes the intake valve timing using
hydraulic pressure.
The VVT-I is an engine control system that has achieved high levels of the
seemingly opposing performances of Higher power out-put, Lower fuel
consumption, Cleaner exhaust gases.
D-4D High technology, clean
diesel
Originally introduced in Europe in the top selling Avensis range, D-4D is
widely recognised as one of the most advanced diesel technologies on the
market today. Toyota D-4D engines are available on select models of Yaris,
Yaris Verso, Corolla Verso, Innova, Avensis, Avensis Verso, RAV4, Land
Cruiser and Previa models.
Until recently, diesel engines relied on relatively simple technology with
a low-pressure mechanical injector delivering fuel to a pre-combustion
chamber in the cylinder head, where a single ignition - fired by the intense
heat of high compression - takes place.
The system was reasonably fuel efficient but lacked precision and
refinement, leading to vibration and noise. Fuel combustion can be
incomplete, allowing smoke and other emissions to pass into the exhaust.
D-4D components
Toyota's common rail system makes use of a high pressure pump and
intelligent, computer controlled injectors to deliver precisely the right
amount of fuel, at precisely the right time, directly into the centre of
the combustion chamber.
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How
VVT-I will help ?
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The VVT-I achieves "high output power".
"low fuel consumption", and "clean exhaust gases" by
using the operting condition of "Idling","normal
driving", and "uphill or passing acceleration" as VVT-I
controls examples.
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Car Operating Condition
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Normal Engine Operation
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VVT-I Operation
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VVT-I Advantage
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Idling:
During idling, the engine does not need to generate power to runthe
vehicle. Then, the performance that is demanded of the engine during idling
is to operate in a stable manner with minimal vibration and low fuel
consumption.
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Because of valve overlap the exhusts gases get
into the intake port and mix with the air-fuel mixture. When the air-fuel
mixture is contaminated in this manner, it does not produce reliable
combustion and results in an unstable engine rpm.
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VVT-I delays the timing at which the intake
valve opens to eliminate the valve overlap, thus preventing the exhaust
gases from flowing back into the intake port
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1.Stable combustion, which result into less
vibration.
2.Better fuel economy.
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Normal Driving:
During normal driving, when the accelerator pedals depressed somwhat with a
certain amount of force fuel economy and clean exhaust gases are demanded
of the engine.
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During normal driving, when the throttle valve
is not opened too widely, a vacuum is created in the intake port. Similar
to when the throttle valve is large, this vacuum causes the exhaust gases
to flow back into the intake.
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VVT-I utilize the backflow of the backflow of
the exhaust gases into the intake port to an advantage by:
1. By reducing the engine power loss.
2.By utilizing the unburnt fuel contained in the exhaust gases and also by
lowering the combustion temperature.
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1.Increased fuel economy
2.Cleaner exhaust gases
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High Speed or Sudden Acceleration :
During sudden acceleration, a large amount of torque is demanded of the
engine.
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During Sudden acceleration, when the piston
turns around at ists lowest position, the pressure in the cylinder and the
pressure in the intake port soon reach their equilibrium . Therefore,
unless the intake valvel is closed before the piston comes up, the air-fuel
mixture gets pushed back and thus reducing the efficiency.
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During the acceleration pedal continues to be
depressed, the engine rpm increases, resulting in alonger intake lag. Using
sensors to monitor the engine rpm, the VVT-I gradually delays the timing to
close the valve so that the maximum amount of air-fuel mixture can be
drawn-in according to the increase in the engine speed.
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Maximize the potentinal of engine which result
in:
1.Maximum power.
2.Optimum fuel consumption.
3.Cleaner exhaust gases.
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HOW AN ENGINE WORK (TOYOTA)
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