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is attached to the bottom of the cylinder block underneath the
crankcase. The functions of the sump are
(a) to store the engine's lubrication oil for circulation within the lubrication system;
(b) to collect the oil draining from the sides of the crankcase walls and if ejected directly
from the journal bearings;
(c) to provide a centralized storage area for any contaminants like liquid fuel, water,
combustion products blown past the piston ring, and worn metal particles ;
(d) to provide a short recovery period for the hot churned-up and possibly aerated oil before
it is re-circulated in the lubrication system; and
(e) to provide some inter-cooling between the hot oil inside and the air steam outside.
The sump (Fig. 3.18) may be made from a single sheet-steel pressing or it may be an
aluminium-alloy casting with cooling fins and strengthening ribs. Both the constructions have
a flanged joint face, which matches with a corresponding joint face on the underside of the
crankcase. A soft flexible gasket is used in between to seal the joint and is tightened down by
set-screws. The sump generally has a shallow downward slope at one end, which changes into
a relatively deep but narrow-walled reservoir at the other end. The incoming oil flows towards
the deep end, where it submerges the pick-up pipe and strainer of the lubricating system. A
drain plug is located at the lowest level in the sump for easy drainage of used oil. Generally the
sump is not designed to add to crankcase rigidity, except in some transverse front-wheel-drive
engines.
The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge.

When the piston of an engine compresses the charge, an increase in compressive force supplied by work being done by the piston causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition.

The increase in fuel vaporization occurs as small droplets of fuel become vaporized more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area allows gasoline to release more vapor rather than remaining a liquid.

The more the charge vapor molecules are compressed, the more energy obtained from the combustion process. The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion.

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