Valve Timing In I.C. Engines
TIMING (VT) is one of the most important aspects of consideration in the design
of an automobile engine. Simply defined, it is the timing, or regulation of the
opening and closing of the valves. In simpler terms, it is the way an engine 'breathes'.
an I.C.engine, usually the inlet valves open a few degrees (of crank angle) prior
to TDC, and close after BDC. Similarly, the exhaust valves open a few degrees
before BDC and close a few degrees after TDC. This is done to maximise:
Intake of air/air-fuel mixture; and
" Scavenging, i.e. the exhaust of
recently, most engines around the world utilised ordinary or static VT, where
the parameters of valve opening, lift, and closing (VO, VL and VC) were fixed.
This was satisfactory at normal engine speeds, but posed problems at high and
low speeds. Since the VT did not vary with speed, the additional requirements
that arose at the extreme speeds could not be met with static VT. For example,
at high speeds, the engine requires greater amounts of air. This implies that
the IV should remain open for a longer period of time. This, though beneficial
at high speeds, would be a menace at low speeds as it may lead to exhaust of unburnt
fuel, which results in fuel wastage, increased emissions and lower performance.
is where variable valve timing (VVT) comes into play. As the name suggests, the
timing of the valves is not fixed, but varies, as per the demands of the situations.
Therefore, the extra demands of the engine can be met, which in turn, results
in improved engine performance.
timing is the regulation of the points in the combustion cycle, at which the valves
are set to open and close. Since the valves require a finite period of time in
which to open or close without abruptness, a slight lead-time is always necessary
for proper operation. The design of the valve-operating cam provides for the smooth
transition from one position to the other, while the cam setting determines the
timing of the valve.
In a typical four-stroke engine, the inlet valve is set
to open before TDC (top dead centre), towards the end of the exhaust stroke and
close after BDC (bottom dead centre), at the start of the compression stroke.
Inlet Valve Timing
While the intake valve should open, theoretically at
TDC, most engines utilise an intake valve opening, which is timed to occur a few
degrees prior to the arrival of the piston at TDC on the exhaust stroke. This
is because by the time the valve becomes fully open, the piston would have travelled
considerably down the bore, and since the valve would have to be fully closed
before BDC, the actual time the valve would be fully open would be minimal.
the inertia of the incoming mixture plays a big role. Keeping the inlet valve
open after BDC forces more mixture to pack into the cylinder, in spite of the
fact that the piston is moving upwards.
Exhaust Valve Timing
The exhaust valve is set to open before BDC, towards
the end of the power stroke and close after TDC, at the beginning of the intake
stroke. The reason the exhaust valve is opened before BDC is to prevent the exhaust
gases from forming a high-pressure cushion, which would impede the movement of
the piston and rob the engine of power. This also ensures that the valve is fully
open at the start of the exhaust stroke. Keeping the exhaust valve open after
TDC ensures that the entire burnt mixture is thoroughly scavenged.
Valve overlap refers to the time when both the intake and
exhaust valves are open. It ensures that the exhaust gases rushing out of the
cylinder create suction, in order to draw in fresh mixture, and the fresh mixture
entering the cylinder pushes out the burnt fuel mixture.
Therefore, valve timing
of any engine depends on:
" The amount of valve overlap
and lead, i.e. the degrees that the crankshaft turns between valve opening and
TDC or BDC
" The intended usage of the engine.
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