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• A shallow angle pent-roof type combustion chamber (Fig. 5.2b) is a good compromise as it allows use of 4 valves of optimum size and positioning. A higher volumetric efficiency and tumble air motion are obtained in these combustion chambers resulting in higher burning rates. These compact combustion chambers have lower heat transfer losses. Thus, quench layer thickness is minimized lowering HC emissions.

Pentroof combustion chamber being shallow compared to hemispherical combustion chamber it has somewhat higher  surface to volume ratio. However the pentroof combustion chamber is a good compromise between compactness of the combustion chamber and use of multiple valves. A shallow angle pentroof type combustion chamber allows optimum valve size and their positioning in multiple-valve engine configuration. Pentroof combustion chambers having 2 and 3 intake valves and total of 3 to 6 valves per cylinder are in use. The 4-valve combustion chambers are the most common The inclination of the intake and exhaust valves to each other tilts the pair of valve heads such that they resemble to an arch and take the shape of a pentroof, hence the name. The spark plug is located close to centre. Mixture from the intake port flows across the cylinder to the walls from where it is deflected downwards direction and rolls perpendicular to the axis of the cylinder in a tumbling motion. This motion is called air ‘tumble’.