HOW DOES IT WORK? Mechanical Mysteries explained by Portland Bill

After the success of a basic mechanics night organised by the Chapter in conjunction with our sponsoring Dealers Riders of Bridgwater, a few further questions arose concerning how the actual motorcycle works. So, I was sort of volunteered in my capacity as a bit of a gobs worth to explain what I knew about what is going on when you press the magic button and make all that noise.

Let’s start with some basic components of the bike, the most obvious of these is of course the engine. Now we shall use a Harley Sportster as our demo vehicle as it is a fairly simple design with most of the main bits clearly on show. The engine is made of some critical parts, namely the carburettor, or in newer bikes, the injection module, the cylinder heads, cylinders and the crankcase.

On the side of the bike, by your right knee if you’re sitting on it, is the air cleaner, or filter housing. It’s a large metal tin, chrome polished usually, that contains a filter element. This does just what it’s name describes, it filters the air being drawn into the engine.

To produce power we need to burn air and petrol together in a controlled fashion. After the air has been filtered, or cleaned, it enters the carburettor. Now this device is a small complicated bucket with some precisely drilled holes and a form of volume control mechanism hidden inside. Here the petrol, having passed from the tank above enters the carburettor and is sucked up through those precisely drilled holes to mix with the air and form a mixture that is ready to be burnt to produce energy.

If you have an injection engine, just the air is drawn through the air cleaner and into the body of the injector module (a small air filled chamber) and the injector, a small electrically operated nozzle opens for a fraction of a second and an atomised amount of petrol is allowed into the airflow as a gas, not a squirt. It is under pressure from the pump that is hidden inside your tank, forcing the petrol through to the injectors at a fairly high pressure.

The mixture is then drawn in through an open valve at the top of the cylinder, in the cylinder head, which we will come back to later, this is known as the inlet valve, and closes at a timed interval to seal the mixture inside the cylinder.

Now the cylinders, are the big finned things that make up the famous V-twin of the engine. Inside here there are large round stoppers, called pistons, and these slide up and down to create the suction that draws the mixture in. As the piston reaches the bottom of it’s up and down stroke, it has drawn in the amount of mixture needed for combustion. This is known as the induction stroke.

The inlet valve now closes and the piston rises again to compress the mixture. This is known as the compression stroke. In doing this it increases the heat and volatility of the mixture. At the top of the piston’s stroke the spark plug fires, that is what is on the end of those thick wires attached to the left hand side of the cylinders. With that vital spark, combustion occurs.

The mixture is ignited and this creates and small, controlled explosion which sends the piston back down to begin another stroke. This is the ignition or power stroke. With the mixture now burnt, the exhaust valve opens in the cylinder head, and as the piston begins to rise again, it expels the burnt mixture out through the exhaust into the outside air. This is known as the exhaust stroke.

So now you can ‘see’ the four strokes of the engine. This is a fairly important process to grasp, to enable you to understand what is going on in there to produce power from air and petrol.

Now, those pistons. What keeps them sliding up and down? At the bottom of the piston there is a connecting rod, which is connected, as the name suggests, to the crank shaft. This is a U shaped round shaft that lies in the bottom of the engine, in the appropriately named crankcase. The top of the U shape sits either side of the engine in bearings where it rotates, providing some motion that we will come back to. The bottom of the U shape is where the connecting rod is joined. This transforms the up and down motion of the pistons into a rotary motion.

at the end of the crankshaft further engine parts are attached. These are primarily the cams and other important parts like the oil pump. Cams, I know what you are thinking, a new word. These are tear-drop shaped parts that rotate on a short shaft set centrally in the large or bulbous part of the tear-drop shape. The action of these is to rotate, and as the pointy part of the tear-drop shape comes around it pushes up against a lifter, a small oil filled bucket with a lid to keep the oil in, which in turn ‘lifts’ the push rod.

You will have seen the tubes that surround these push rods at the right hand side of your engine, under the air cleaner and tucked in tight against this side of the cylinders. At the top of the push rods, in the cylinder heads (coming back to the heads) there are rocker arms, I know, more new words! These are exactly what the name suggests, they ‘rock’ on a central pivot, much like a see saw, and press down on the stem of the valve, remember those? The valves are kept closed by a strong spring attached to each of them, which allows the valve to open and then return to the closed position.

It’s all a matter of timing, when the engine turns it enacts all of these changes as a complex mechanical action. A bit like an elaborate clockwork device.

The electrical parts are a bit more complicated, as they perform several roles. The main one is the ignition circuit, this is provided with power from the battery to a coil, a small electrical storage device, which discharges all of its stored electricity at a precisely timed moment to create a spark at the spark plug. In a very basic sense, a big jolt of electricity is sent down those thick cables we mentioned earlier and as it reaches the end (the spark plug) there is a gap where the electricity 'jumps' said gap, forming a spark which ignites the petrol and air mixture. The electricity is then earthed through the cylinder head completing the circuit.

Secondary to this is the charging circuit. On the left hand side of the bike is one end of the crank shaft and here in a casing is an alternator, a form of electrical generator. This makes alternating current, the wrong type for what we need incidentally, so it passes through a regulator rectifier (that strange black finned thing at the front of the bike) which converts it to direct current which in turn re-charges the battery. From here of course there is a myriad of wiring and fuses that powers everything from your headlight to the horn.

There, I hope that has answered a few questions?

Rambling on......................Portland Bill.