In the previous automotive article, we learnt about
The 4 Stoke Cycle which involves the opening and closing of camshafts as the
cylinders move up and down.
But, did
you know the typical car engine running at more than 4000 RPM have valves
opening and closing for more than 2000 times per minute, or, 33 times per
second?
How do they
work and why is this so? Continue to read on to find out more.
As the camshaft spins, the lobes open and
close the intake and exhaust valves in time with the motion of the piston.
Before understanding the whole process, look at the instantaneous motion of the
piston in relative to the valves’ opening and closing from time to time.
When the
piston moves downward in the intake stroke, the intake valve opens. It opens to
enable more air to come in and fuel to be injected into the space as the piston
descends to the bottom. The intake valve would close right as the piston
bottoms out.
On the other
hand, just as the piston bottoms out, the exhaust valve would open to enable
the release of waste exhaust.
A Single
Overhead Cam Engine (SOHC) denotes an engine with one cam per head. For SOHC
engine with an inline 4-cylinder or inline 6-cylinder, it will have one cam.
Meanwhile, for V6 or V8 configurations, it will have two cams per head.
 |
| SOHC |
A Double Overhead Cam Engine (DOHC) has two
cams per head. So inline engines have two cams, and V engines have four. One of
the obvious reasons to use double overhead cams is to allow the usage of more
intake and exhaust valve. More valves denote the gases can flow more freely
with more opening for them to flow through. Hence, producing higher power
output.
 |
| DOHC |
As
mentioned earlier, DOHC engine allows the usage of more valves per cylinder. If
a SOHC engine could allow 4 valves per cylinder, let say, having a DOHC engine
would not be useful as it carries an additional weight.
In order to
promote effective and efficient combustion, DOHC engines are equipped with a
spark plug located in the middle of the combustion chamber.
On the
contrary, a SOHC engine has its camshaft placed in the middle of the head in
order to drive both the intake and exhaust valves.
As a
result, the spark plug could not be located at the optimum location as DOHCs
do. Below is the comparisons of the pros and cons of both types of engines.
SOHC
|
DOHC
|
Better starting torque
Lower peak torque and horsepower
|
Lower starting torque (Additional weight of valve
trains)
Greater peak torque and horsepower
Easier to implement variable valve timing
technologies.
|
A pushrod
engine has similar configurations to SOHC and DOHC engines. The valves are
located in the head, above the cylinder. The key difference is that the
camshaft is inside the engine block, rather than in the head.
We learned
a great deal about the operations of camshafts today. It is all how the
camshafts and crankshafts are synchronized by a connector known as the timing
belt.
Next, we
looked at how the SOHC and DOHC engines work by comparing their
characteristics. Lastly, we get to know the pushrod engine in frivolous manner.
In the next
automotive article, we will look at how a car’s suspensions work. The article
consists of two parts and the first edition of it will cover several topics
such as the suspensions parts, components, etc. Be sure not to miss one of the
most unmissable automotive article yet!
Source: www.auto.howstuffworks.com
www.paultan.org
By: Vincent Ti
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