Overview

Topic 1 - Combustion chamber shapes to improve mixing and homogeneity

The combustion chamber of reciprocating piston IC engine is the volume made up by the top of the piston (which forms the bottom of the combustion chamber), the bottom of the head (which forms the top) and the portion of the wall of the cylinder which is exposed when the piston is at top dead center.   The shape of this combustion chamber volume can vary greatly.  Here are a few examples (click to enlarge).

The variations in shape effect the amount of mixing of the fuel-air mixture that occurs prior to combustion.  In general, more mixing is a positive effect, because a well-mixed fuel-air mixture is more homogeneous and therefore more likely to combust evenly, thoroughly and completely.  Turbulence in the Combustion Chamber increases the mixing of fuel and air and also helps spread flame front, both of which aid combustion.

There are three mixing effects that occur.  The first of these is Swirl, and the swirling effect is shown in the following figure:

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The swirl effect is created as the piston moves downward on the intake stroke, pulling fresh air into the cylinder.  Proper design of the intake ports can aid in this mixing, by ensuring that the air flow into the cylinder experiences a rotating motion about the center of the cylinder during the piston’s downward motion.  To create this swirling motion, the valve port would not be directly vertical, but angled, or curved, as shown in this example:

Wikimedia / CC BY-SA 3.0

The Swirl Ratio is a dimensionless parameter used to quantify the rotational motion of the gasses within a cylinder.  There are two ways to define the ratio.

${\text{(SR)}}_1=\text{(angular speed)/(engine speed) = ω/N}$ 

${\text{(SR)}}_2=\text{(swirl tangential speed)/(average piston speed)}={\text{u}}_{\text{t}}/{\text{U}}_{\text{piston-average}}$ 

The second mixing effect is Squish, which occurs as the piston reaches TDC on its upwards motion, and the air trapped between the piston and head is forcibly squished out.  This is shown graphically in the following:

Reverse squish occurs as the piston moves downward from TDC, and the sudden low pressure region in the squish zones pulls the flame front toward the outer edges, spreading combustion.

The third mixing effect is Tumble, which is another rotational motion within the cylinder, but different from Swirl.  Tumble occurs when the gasses pushed inward by Squish begin a rotational motion. If the piston has a relieved top surface, it facilitates the mixing provided by Tumble, as the rotational tumble effect can roll into the void.

Wikimedia / CC BY-SA 3.0

Pistons can be designed to maximize Tumble.  The tumble Ratio is a dimensionless parameter TR = (angular speed of tumble)/engine speed = ω_t/N

Topic 1 - Application

Which combustion chamber would have better “Squish” when used with a flat-top piston?

The 'squish area' is the part of the combustion chamber that is made to very nearly come into contact with the piston when the piston is at top dead center. What these areas do is cause the combusting gasses to rapidly travel towards the center of the combustion chamber, which increases turbulence, thus increasing mixing or combustion rate.

The one on the left has more squish areas around the sides of the valves. The right version is quite round by comparison and will have less squish.

Which piston would have better “Tumble” when used with a flat-top combustion chamber?

While both pistons shown would increase tumble over a flat-top piston, the one on the right would create more mixing.

The advance of computer modelling, and specifically Computational Fluid Dynamics (CFD) has made it possible to create computer generated flow models that aid in the optimising the design of combustion chambers.

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Topic 2 - Combustion chamber shapes to create non-homogeneity

In general, it is preferable that the fuel-air mixture which fills our combustion chamber just prior to the spark is a homogeneous mixture, i.e. the fuel and air are equally distributed throughout the volume.  This would be considered to aid in thoroughly combusting the mixture, which is the best way to maximize available power.

Stratified charge engines violate this principle, however, in that the mixture is intentionally different throughout the combustion chamber.  This creates varying combustion characteristics as the flame front moves through the chamber.  A rich mixture is combusted in a pre-combustion chamber, and the flame explodes into the main combustion chamber as jet, enhancing combustion there.  The main mixture can now be lean, because the stratified charge will facilitate the initial combustion and start of flame propagation.

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Topic 2 - Application

Once again, advanced computer analysis can be utilized to model the flame propagation across the combustion chamber, aiding in optimization of the combustion chamber shape.

Flame propagation and computer modelling

This can also be used to predict knock locations, that is, the areas within the combustion chamber where the fuel-air mixture may self-combust prior to the spark, under the increasing temperatures of compression (shown by the arrow).

Summary

Advances in computer analysis has improved the ability to model and predict many aspects of the engine combustion process:

• Airflow into the cylinder, allowing for optimisation of the intake manifold, intake ports and passages in the head, and valves
• Mixing of the fuel-air mixture, allowing for optimisation of the shape of the combustion chamber
• Flame propagation during combustion, allowing for optimisation of combustion chamber shape to achieve full combustion of the fuel-air mixture
• Self-ignition sites, allowing for changes to be made to improve mixing and pre-combustion temperatures
• Flow of exhaust gasses out of the cylinder, allowing for optimisation of the exhaust manifold, exhaust ports and passages in the head, and cylinders

Reference and bibliography

Heywood, J. (2018). Fundamentals of Internal Combustion Engines. New York, USA: McGraw-Hill.

Pulkrabek, W. (2015). Engineering Fundamentals of the Internal Combustion Engine.  New York, USA: Pearson.