An internal combustion engine runs on fuels like petrol,
diesel, ethanol, propane and many more. Let me focus on a 4-stroke naturally
aspirated diesel engine in which obviously diesel is used as a fuel. In the
chamber, the 4 stages or strokes help this mixture to ignite and produce power
enough to push the car forward. The by-products produced in doing so are the
green house gases which are emitted out of the exhaust pipe. This process is
the key to bring any car in motion. But in a case of two cars, it was observed
that the two cars even with similar engines gave different performance outputs.
Second car was superior in performance with less produced emissions. No, don’t
think of turbocharged or supercharged engines because I have already mentioned
them as naturally aspirated engines. No, not even different types of variable
valve technologies. The answer lies in the design of the cylinder. This is
where I bring in swirl, tumble and squish into the picture. But before that let us have a quick recall about how a combustion engine works.
There are four strokes:-
🚦 Intake stroke - Piston moves down (bottom dead centre or
BDC) and sucks in air.
🚦Compression stroke – Piston travels up (top dead centre or
TDC) which compress the air raising its temperature. Fuel injector injects
diesel inside.
🚦Power stroke – Diesel and air mixture explodes forcing the
piston down and hence power is produced.
🚦Exhaust stroke – Piston again moves up and exhales the
burnt residuals i.e. gases like CO2, CO, NO2 etc out of the cylinder.
Of course, this is not a detailed version of the process and lot more is happening simultaneously but this is just a rough
idea of what goes on inside a running cylinder. It will help you to
understand the terms better. When fuel is entering into the cylinder through an
intake valve, it will flow in a specific manner. The manner of flow decides how well air
and fuel would mix together. This type of engine was first developed by Sir Harry
Ralph Ricardo from England.
Swirl – Motion
of the mixture is such that when it is pushed into the cylinder, it takes up a
swirl or helical turbulence motion. A type of turbulence that is in an oriented
manner with its axis parallel to the axis of cylinder. Just have a look at the helical or spiral design of bulb. This is exactly how swirl motion of the
air looks like.
How it
is done?
Design of intake system is modified from straight to helical. This adds a tangential component to the flow and makes it to swirl inside the cylinder. Redesigning intake system includes intake manifold, piston face, valve port.
Why it is done?
1) To result in proper air-fuel blending
and give a homogeneous mixture in shortest time.
2) To improve frame front expansion.
Squish – When piston reaches TDC, the space between piston
crown and cylinder head is minimum. The outer area of piston crown called
squish band is such that it gives a radial inward motion to the mixture
and forces it to flow rapidly to the centre of piston crown at the end of the
compression stroke.
How it
is done?
Redesigning the piston
crown with groove or air pockets on the peripheral area giving it
a bowl shape followed by modifying engine head and block design.
Why it
is done?
1) It helps
in proper blending of the mixture.
2) Reduces excessive
heating of cylinder side wall.
3) It also
reduces carbon formation called soot.
Tumble – Tumble also known as barrel swirl is a motion in
such a way that the axis of rotation is perpendicular to the axis of cylinder. It is a secondary flow produced by squish motion when the piston reaches TDC.
Why it is done?
1) To improve proper blending of the fresh air-fuel mixture (fresh
charge) entering the cylinder.
2) To reduce emission.
3) Quick blending.
4) To nullify knocking effect.
5) It helps in stabilizing combustion.
Engine with such an overall design possesses swirl, squish and tumble therefore, multiplying ignition process along with burning every molecule of the mixture. This helped the engine to reach higher rpm ultimately increasing power. Higher engine rpm means that the driver could now get full power output in each and every gear with less emission output and better fuel efficiency. This is the reason why 2nd car was better than the first car even after having same engines.
