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Valve
timing refers to the timing event of opening and closing for both intake and
exhaust valves. Valve timing is expressed in degrees of crank angle. Valve
timing consist of five different valve events which are Intake Valve Opening
Time (IVO), Intake Valve Closing Time (IVC), Exhaust Valve Opening Time (EVO),
Exhaust Valve Closing Time (EVC) and Valve Overlap. Valve timing are controled by the profiling of the
camshaft. Different cam profile resulting on different valve timing. Variation
of valve timing give an impact on the engine performance and fuel consumption 11, 12. Therefore, in different engine speeds,
the valve timing will not be the same in order to achieve the best power and
torque for a particular speed.

 

Initially,
variable valve timing is invented to improve emission,  increase fuel economy when driving at low
engine speed and optimizing engine performance at high speed. Variable valve
timing (VVT) allow the timing to be controlled to aciheve better engine
performance. By controlling the valve events and
timing, it provides better volumetric filling of the cylinder at all engine
speeds, imorove brake torque and power, improve fuel consumption and reduce
emission 10,
12. Various techologies
invented based on the concept of VVT and one of them is the Honda’s Valve
Timing and Lift Electronic Control (VTEC). This technology have made
improvement on maximum power output at high speeds and better ideling as well
as engine stability at low speed 13.

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The benefit of having
control of the valve timing is we can simultaneously control the valve overlap.
Valve overlap is the condition where both intake valve and exhaust valve are
open. Valve overlap allow a little bit of exhaust gas to flow back into
combustoion chamber due to back pressure. In the order hand, overlap variation
for high engine is speed is not effective as when low speed 11.

 

Figure 2.6: Variable
intake cam profile 12.

 

 

Based on the study
conducted by Cinar and Akgun (2007), by varying only one of the valve event,
the engine performance can be improved 12.
In the study, a special camshaft with different profiles as shown in Figure 2.6 is used to vary only the IVC timing, and the
IVO and valve lift is left constant. A single cylinder 4 stroke engine is used
for this experiment. The standard IVC time of the engine is 48° aBDC. The
profiling of the cam shaft allows the variation from 38° aBDC to 78° aBDC with
the interval of 10° CA. The results of the testing is recorded as shown in Figure
2.8.

 

Figure
2. 7: Variable
valve timing diagram 12.

 

Figure 2.8: Engine
speed and brake torque variation with variable valve timing 12.

    

Based
on Figure 2.8, it is proved that different valve timing
will result in different peak power at any engine speed. The study shows that
the brake torque experienced 5% increase at low engine speed and 4.6% increase
at high engine speed with the implementation of variable valve timing 12.

Another
study conducted by Khudur, Saleh and Chaican (2015), shows the effect of valve
overlap to the engine performance. By using a single cylinder 4 stroke PRODIT
engine, with bore and stroke of 90mm and 85mm respectively. It produces the
power of  42kW at 4800rpm and maximum
torque of 208Nm at 1600rpm. The IVC and IVO 54° bTDC and 22° bTDC while the IEC
and IEO are. The variation of valve timing is done by changing the clearance
distance between the rocker arm and the valve stem. The valve overlap duration
variation is 104°, 108° and 112°. The result from the study shows that the
increase of engine performation 11. Therefore, variable valve timing should
be implemented to ensure the engine achieved better peak power and improvement
of amission.

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