ME 433 ME Elective: Automotive Engineering

ME 433 ME Elective: Automotive Engineering
HOW THE ENGINE PRODUCES POWER

THE ENGINE is the powerhouse of
the car. It converts the heat produced by
burning fuel into mechanical energy to
turn the wheels. The fuel, normally a
mixture of petrol and air, is burnt in
closed cylinders inside the engine- hence
the term “internal combustion “.
Petrol and air are mixed in the
carburetor and drawn into a combustion
chamber at the top of each cylinder.
Pistons, inside the cylinders, compress
the mixture, which is then ignited by a
spark- plug. As the mixture burns it
expands, forcing the piston down on its
power stroke.
The up-and –down movement of
the pistons is transformed into rotary
movement to drive the crankshaft,
which in turn transmits power to the
wheels through the clutch, gearbox and
final drive. Connecting rod links the
piston to crankshaft.
A camshaft, driven by the
crankshaft, controls inlet and exhaust
valves at the top of each cylinder. Initial impetus to set the engine in motion comes from the starter motor. This is connected
to a starter ring which is fitted around the edge of the flywheel – a heavy disc bolted to the end of the crankshaft. The starter
motor, which is geared to the flywheel, is operated electrically and turns the flywheel and crankshaft, which starts the pistons
and connecting rods moving up and down. The flywheel smooth’s out the power impulses of the pistons and gives a relatively
smooth rotation of the crankshaft.Because of the heat produced by an internal combustion engine, the metal parts would
seize without a cooling system. In most cars, water is circulated through channels in the engine called a water-jacket. The hot
water then passes through a radiator where the heat is dispersed to the atmosphere. The dispersal of heat is speeded by a
fan, which draws cooling air through the radiator. Other cars, mostly those with engines at the rear, are cooled by air which
circulates over fins cast in the cylinder head. These fins increase the contact area with the atmosphere.
The Valve-Operating Gear is
designed so that it opens and closes each
valve at the right point in the 4-stroke
cycle, gives an adequate opening period to
allow free gas flow, and operates quietly
and reliably over a wide range of engine
speeds.
There are several ways of meeting

these requirements. The most widely used
method is with push-rods and rockers
operated by camshaft placed inside the
cylinder block. The camshaft is driven by a
chain (or set of gears) from the adjacent
crankshaft, at half engine speed.
As the camshaft rotates, each cam in

turn lifts a tappet and push-rod, causing
the corresponding rocker to pivot and
push the valve down. The valve is closed
by a spring when further rotation of the
cam allows the tappet to descend. Some
engines have two springs for each valve.
For efficient operation, the valves must
return fully to their seats. To ensure this, a
gap known as a tappet clearance is allowed between the closed valve and its rocker. This allows the valve gear to expand
when it is hot. Tappet clearances vary considerably on different makes of engine, but it is important that they should be
adjusted to the precise dimension, as recommended by the manufacturer. Too great a clearance results in noise. As the
ignition system must provide a spark at each spark-plug at the right time in relation to the valve operation, the distributor,
which supplies high-voltage current to the plugs, is usually driven by gears from the camshaft or by the crankshaft.
Camshaft. This shaft is of either forged steel or cast iron, machined and hardened to give maximum resistance to wear
of the flanks of the cams. The cams are spaced at intervals to match the firing order.
Operating the Valves. A cam operates an overhead valve through a tappet, push-rod and rocker arm. As the tappet
and push-rod are lifted, the rocker pivots and depresses the valve. Further rotation of the cam allows the tappet and push-
rod to descend, allowing the valve to be closed by a spring.
CYLINDER HEAD AND ROCKER GEAR

ME 433 ME Elective: Automotive Engineering

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ME 433 ME Elective: Automotive Engineering

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ME 433 ME Elective: Automotive Engineering

HOW THE ENGINE PRODUCES POWER

There are several ways of meeting

As the camshaft rotates, each cam in

CYLINDER HEAD AND ROCKER GEAR

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