ENGG1010 Dynamics - Prac 1

Students will be divided into three groups with each group assigned to one of the three engines shown in the picture towards the back. They will perform the tasks as specified in Prac 1-Reference Sheet. The green engine closest to the camera will be dismantled and assembled again by the tutor three times in the session - once with each group.

The following is distributed to the students at the beginning of the session:

One graph paper for each group
Question sheet for each student
The table to note the crankshaft angle and piston and valve displacements for each group

The following is handed in by the students at the end of the session:

One for each student
- Question sheet
- Derivation of Equation (1)
One for the group
- Graph of measured piston and valve displacement against the crank angle

The Tutor will record the students who

attended the Tutorial;
and submitted one group and two individual handouts described above.

Engine Dismantling

Students in the session will be divided into three groups. The tutor will dismantle the engine and re-assemble it with each group. This should take about 30 minutes. The other two groups will be taking valve and piston displacement measurements in the meantime. The following notes describe the dismantling session (normally to be repeated thrice in each laboratory session).

This part of the prac involves the dismantling of a four-stroke Briggs & Stratton engine. The following includes instructions for the tutor and questions to be raised with the group during dismantling.

How many cylinders are there?
- One
What are the four strokes that make up one cycle?
- Intake - Air and fuel are drawn into the cylinder through the intake valve
- Compression - The air+fuel mixture is compressed in the combustion chamber (the volume above the piston head)
  - What provides the force to push the piston upwards?
    - In a single cylinder engine, this is provided by the inertia of the rotating components, especially the flywheel.
- Power - The air+fuel mixture is ignited and you get controlled combustion/explosion pushing the piston down
- Exhaust - The combustion products (CO₂, N₂, excess air, unburned fuel, etc) are pushed out through the exhaust valve


1. Intake stroke	2. Compression stroke	Fuel ignites	3. Power stroke	4. Exhaust stroke
Diagrams showing the piston and valve positions in each stroke (from wikipedia)

Start the dismantling by unscrewing and pulling off the air filter.

During the intake stroke, the air comes through the filter and the fuel comes from the fuel tank. The air filter is a simple filter. Its job is to reduce the number of particles in the air entering the engine.

What is the problem with particles entering the engine?
- The crankshaft rotates (and the piston goes up and down) at very high speeds up to 3000 RPM or 50 times a second. Abrasive particles in the air can cause wear very quickly at such speeds. The function of the air filter is to protect against this.


Air filter	Remove the fuel tank	The long white tube fills the fuel cup; the short one sucks the fuel into the carburettor	The front cover showing the white spline for the starter engagement	The flywheel. The electrical attachment at the top provides the signal for the spark plug driven by a magnetic segment on the flywheel.	The top cover is out. The exhaust muffler is still sticking out. This can also be unscrewed at this stage.	A closer look into the cylinder. There is no piston or connecting rod in this engine (they have been taken out to make the disassembly easier)	Peek into the cylinder and see the crank shaft.

Show where the fuel comes in from the fuel tank. Explain that the air and the fuel mix together in the carburettor. Then unscrew the two bolts to take off the fuel tank. You can then unscrew the muffler that is on the exhaust. The muffler contains a number of baffles that redirect the flow to help reduce noise and also catch particle emission.

There are small holes on the fuel cap. What is their function?
- They keep the tank at atmospheric pressure. A sealed tank would develop a vacuum as it is emptied.
- Why would the vacuum be undesirable?
  - Because it would reduce the fuel suction during the intake stroke

The fuel pump draws the fuel out of the bottom of the fuel tank and fills a fuel cup. The existence of the fuel cup maintains a constant fuel flow rate into the carburettor regardless of the actual fill level of the fuel tank. Above the cup, there is a venturi section.

What is a venturi?
- A venturi is a narrowing section. To pass through this narrow section, the fuel velocity increases. This is similar to what happens what you might have noticed about the wind speed between buildings. The wind always is faster between buildings compared to the open space around them. Similarly, the water current is always faster through the narrower river sections.
Why do we want the fuel to go faster? It is still the same mass flow rate, isn't it?
- Yes it is. But when the fuel flows faster, its pressure has to come down to conserve the energy (this is known as the Bernoulli principle). The lower pressure helps the fuel to evaporate faster. We of course want the fuel to evaporate so that it can mix with the air better.

You can now take the front cover off. But before you do that note the pull chord that is used to start the engine. This is a hand-started engine and the pull chord provides the initial momentum to get the engine going. When you pull it, it does not stay pulled-out, there is a torsional spring that pulls the chord back in.

After you take the front cover out, try to rotate the starter shaft by hand. In one direction, it engages the flywheel and the crankshaft and is hard to rotate. In the other direction, there is no engagement and you can rotate it easily. It is deliberately designed that way so that the starter chord can turn the flywheel but not the other way around. Why?

Why does the starter shaft engage the flywheel only in one direction and rotates freely in the opposite direction?
- To prevent injury and damage if the engine starts firing and the flywheel starts rotating under engine power while the starter chord is still out

The flywheel is heavy. It is a mechanical storage device. The momentum is stored in the rotation of the flywheel during the power stroke and used up in the compression stroke. This is how one gets continuous four-stroke cycle operation with only one cylinder.

The fins on the flywheel draws the air through the openings on the front cover. This air provides cooling for the engine. Compare this to the engine in your car, which is most likely to be cooled by water, which is circulated around the engine block by a water pump and is itself cooled in the radiator.

The flywheel was manufactured by casting. Casting is where molten metal is poured into a mold and left to solidify.

The flywheel is locate don the crankshaft with a soft metal key, e.g. aluminium. The key is deliberately designed to break first if there is a torque overload. Rather than breaking the shaft, the flywheel or other expensive components, the key shears off first uncoupling the flywheel from the shaft.

The flywheel is cast as one part except for a segment, which is a permanent magnet. This magnetic segment converts some of the mechanical energy of the flywheel into electrical energy to fire the spark plug. This is done through the interaction of the magneto placed above the flywheel (but not touching) and the flywheel magnet. The magneto consists of a primary and secondary circuit. These are wrapped around the same core in the same coil but electrically isolated from each other. The secondary circuit has lighter wires and there about 60 turns in the secondary for each turn in the primary winding. As the flywheel rotates, the permanent magnet is brought into proximity with the coil and, at the engine speed of 3000 RPM, this generates a voltage of 170 V in the primary winding and about 10,000 V (=170x60) in the secondary winding. The secondary winding fires the spark plug through the black cable shown inthe following image. Note that this way, the firing of the spark plug is linked to the position of the flywheel (and hence the position of the piston in the cylinder).

Now take off the top plate which is the cylinder head. There are eight bolts holding it in its place. These bolts are necessary to make sure that the top plate will not blow off during the power stroke when the fuel explodes inside the cylinder. The gasket is there to seal the gap between the cylinder head and the cylinder. It is difficult to obtain a complete seal between to metallic surfaces. A softer gasket when compresed by the bolts provide sufficient sealing.

Show the inlet and exhaust valves. Note that they are of diferent sizes and made froom different materials:

The exhaust valve head must be made of material that will resist the combustion temperatures as high as 650^oC. The inlet valve head, the cylinder head, the cylinder, and the top of the piston are all exposed to the same combustion heat. Why is temperature-resistance critical for the exhaust valve head but not for the others?
- The intake valve head is cooled in each cycle by the cool mixture of air + fuel coming in.
- The cylinder and the piston head are cooled by the air stream from the flywheel and also the oil in the crankcase.
Why is the inlet valve opening larger?
- A larger area means a smaller pressure drop or less resistance to flow. The intake has a larger area to minimise the pressure drop so as to maximise suction of fuel. The exhaust gases are driven by the high cylinder pressures and having a small extra pressure drop is not as important.

Point out the space that holds the oil down at the bottom of the engine block. The oil is splashed around by the rotation of the crankshaft. A dipper arm is attached to the crankshaft and rotates with the crankshaft splashing the oil inside the engine casing.

Show the connection rod. It has a big end fitting onto the crankshaft and a small end connecting to the piston. The gudgeon pin connects the conrod to the piston.

Let the students measure the connection rod length. They should measure it at least twice and use the average.

Engine Operation

The opening and closing of inlet and exhaust valves must be closely synchronised with the position of the piston for the four-stroke cycle to work. What provides this synchronisation?

What opens and closes the inlet and exhaust valves at the precise instant when air and fuel mixture has to be let into the cylider (open the inlet) or when the hot combustion products have to be let out (open the exhaust valve)?
- Both valves are pushed in and out by the rotation of the camshaft. The cams (lobes) on the camshaft are 270^o apart and they push on tappets pushing into the valves. The camshaft is geared to the crankshaft as shown in the image on the left. This way, the valve positions are synchronised withh the crankshaft angle (and hence the position of the piston).

The valves will open in one crankshaft revolution and should stay closed in the next one. Examine the stroke diagrams above to convince yourself that this is true. This means the camshaft has to do one revolution for every two revolutions of the crankshaft.

It is important to maintain partial vacuum conditions in the crankcase to prevent oil from being forced out of the engine, at the piston rings, oil seals, etc. It is the function of the breather to maintain this vacuum. The breather has a fibre disc valve that gives passage to air only on the way out. Air can flow out of the crankcase as the piston moves back and forth but is not let in. This way a partial vacuum condition is maintained inside the crankcase.