MARCH-MAY -2012



MECHANICAL: THE ENGINE

by Felix Aringo.
We all move from one point to another at sometime. Apart from the wilder beast, man is one most mobile animal. We have legs but they can only take us so far, thanks to the constant advancement in transport engineering we have other modes to move. However, with these modes being ever more common, do we ever stop to look beyond the steering wheel past the dashboard? How many ever wonder where the acceleration pedals lead? Or what the steering wheel is connected to, better still how do the wheels get to rotate that fast? Yeah, you may be thinking, so long as you are able to get from home to school or work, and armed with your mechanic’s number you could leave the rest to the engineers.

Well, you could even be thinking, electrical energy powers a motor that rotates the wheel… Possible, yes, even Makerere University has had a short at electric cars but no. At least just not yet, electric cars are still at research and development stage a normal car has an internal combustion engine. The other available option is the hybrid of electric motor and an IC engine like the Prius .When you look into the engine bay, it may look like a big confusing jumble of metal, tubes and wires. The engine is the power horse whose work is to convert heat produced by burning petrol or diesel into mechanical energy that turns the wheels. Engines are classified as internal or external according to mode of combustion or application e.g. reciprocating, rotary etc. a car engine is an internal combustion engine, i.e. combustion takes place internally. A steam engine in old-fashioned train or steamboat is the best example of an external combustion engine. The fuel in a steam engine burns outside the engine to create steam under pressure, which in turn creates motion inside the engine. Internal combustion is a lot more efficient than external combustion, and an internal combustion engine is a lot smaller than an equivalent external combustion engine. This explains the propensity of General Motors to avoid steam engines.

Ignition process starts when the keys are turned; they complete an electrical starter motor circuit that turns the crankshaft using the power from the battery. As the crankshaft turns, the pistons begin to move up and down in a predetermined pattern by virtue of the shape of the crankshaft. As the crankshaft turns, it also turns a camshaft that via a pushrod opens the valve to allow fuel in through the inlet manifold. As the piston rises again, the valve closes the petrol/air mixture is compressed. Another camshaft connected to the distributor meanwhile, opens and closes electrical contacts called points, which create electrical impulses, which amplify the volts from the battery via the coil. This very high voltage passes through the rotor arm to electrical contacts in the distributor cap and by high-tension cables to the spark plug. The plug sparks at precisely the correct moment and ignites the fuel. The piston moves down and it turns the crankshaft with power from the fuel. The starter motor disengages completing the ignition process

Engines are operated on different kinds of fuels whose characteristics may have considerable influence on the design, output, efficiency, fuel consumption and even the reliability and durability of the engine. For the petrol engine, petrol and air get mixed in the carburetor then injected into the combustion chamber. Modern engines, however, use electronic fuel injection-EFI in place of carburetors. This helps reduce the number of wearing parts, prevent surging when tilted, provide constant petrol/air mixture to various cylinders and prevent loss of volumetric efficiency due to throttle valves, inlet pipe and bends. The petrol/air mixture is compressed by the piston for ignition by the spark plug. As the mixture burns, it expands (explodes) and the force of that explosion forces the piston downwards..The piston transmits the combustion generated gas pressure to the crank pins via the connecting rod.

As the piston moves up again, the camshaft, helps open the exhaust valve to force burnt fuel out of the chamber into the exhaust manifold and away down the exhaust pipe. The piston then moves down again sucking in fuel and the cycle begins again. For a four-stroke engine, four stages are involved. In the first (Induction) stroke, the piston sucks the fuel, second (compression) stroke, piston compresses it, third (power) stroke, it is ignited and the piston is forced down and in the fourth and final stroke (exhaust stroke), the piston pushes out the exhaust gases. The camshaft ensures the timely opening and closing of the valves during these strokes. The inlet valve opens as the piston sucks-induction stroke while fuel gushes in from the fuel injection system into the engine cylinder towards the end of the compression stroke just before the start of combustion.

The piston connects to a crankshaft so when it moves down it turns the crankshaft that rotates the flywheel from which the clutch transfers motion to the gearbox. Depending on the design and configuration of the vehicle, motion passes through the gear trains in the gearbox to the front or the back wheels for motion.

The crankshaft rotates at the rate of between 500 to 8,000 RPM and this causes valves to open, spark plug impulses and pistons motion. This leads to a lot of friction on these surfaces generating a lot of heat that could melt the engine. Thanks to the cooling system around it and the oil lubricants. Water circulates and absorbs the heat from the metal and returns to the radiator where it cools for recirculation. This is how the car is able to move by converting chemical energy from the fuel through combustion to rotary motion in the flywheel and finally linear motion on the road.

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