Engine Quiz Show - University of Washington



Why is the diameter of the cam gear twice the diameter of the crank gear? What does it mean when someone says that the “timing of the engine” is off? Why won’t an engine run if the timing is off?

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Answer:

The crank gear is connected to the connecting rod, which is connected to the piston. The cam gear is on the camshaft, which is connected to the flywheel. The piston needs to go up and down TWICE (2 rev. of the crankshaft) for every rev. of the flywheel thus the diameter is twice as great.

The timing is off if the point where the 2 gears meet does not correspond to the timing marks. These timing marks ensure that the piston will be in the correct position with respect to the position of the flywheel (for example). If it wasn’t in the right position, the spark could occur at the wrong time in the cycle.

Why is this engine designed so that the spark plug sparks twice during the 4-strokes (also how do you know it sparks twice)? How could it be changed so that it only sparked once during the 4-strokes? Would this be a better design?

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Answer:

The spark plug sparks twice during the 4-strokes because the magnet on the flywheel passes under the ignition armature twice. The first time it sparks, it creates combustion which initiates the power stroke. The second time it sparks is right at the end of the exhaust stage where the exhaust is being pushed out the exhaust valve by the piston. Because the gas/air mixture (charge) has already been burned, the spark doesn’t do anything at this point.

One re-design would be to make the diameter of the flywheel twice as big, so it would only make one revolution during the 4 strokes (instead of 2). But then you would have to change the connecting rod so the piston still went up and down twice. So needless to say this would complicate the engine design unnecessarily and it would increase the size of the engine, neither which are improvements. So instead the current design is considered a very “elegant” solution to the problem – good engineering!

Hold a light strip of paper in your fingers right below your lower lip. Blow over the paper. What happens? How is this same principal used in the design of the carburetor (also as a side question – explain why the choke and throttle are important)?

What is the purpose of the air vane and how does it work? What are the multiple “jobs” of the flywheel?

Air vane

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In physics, WORK is defined as FORCE x DISTANCE (through which the force acts). POWER (like horsepower) is defined as the WORK / TIME. Identify what the “forces” are in the engine, what the “distance” is and what the “time” is. Explain why you have to subtract two quantities of work to calculate the net horsepower of the engine.

What is the compression release, why is it needed, and how does it work?

Compression release Exhaust Tappet

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Explain how a 2-stroke engine works compared to a 4-stroke.

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Your instructor gives you several poorly assembled ENGR 100 engines that don’t work. The problems are listed below. What do you think is wrong with each engine?

a. You go to pull the starter cord and it won’t budge.

b. You put your hand next the muffler and you don’t feel any air rush out as you pull the starter cord.

c. You feel air rushing out of the muffler but when you smell your hand, there is no smell of gasoline.

d. As you pull the starter cord, the engine sounds like it is turning over correctly but it just won’t start.

9. Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. It occurs when the piston nears the TDC, between the exhaust event and the intake event. Explain why it is used.

The intake valve is opened during the exhaust event just before TDC, initiating flow of new fuel/air mixture into the combustion chamber. As the exhaust gases are evacuated from the combustion chamber, small low-pressure area is created on the surface of the piston head.

By opening the intake valve earlier than TDC, the fuel/air mixture begins to fill this low-pressure area while exhaust gases exit. The low-pressure area helps to fill the combustion chamber to its maximum capacity. This is most useful at higher speeds and increases the available power from the engine.

10. Explain how the engine that you dissected is lubricated. What is the purpose of the oil ring on the piston?

The engine that we dissected uses splash lubrication system that is oil is directed to moving parts by a splashing motion. A dipper is an engine component attached to the connecting rod, which directs oil from the oil reservoir to bearing surfaces. The dipper enters and exits the oil reservoir as the piston travels to and from BDC to splash and distribute oil through the crankcase.

One of the pistons ring is the oil ring. It controls oil consumption. The oil ring has two thin rails or running surfaces. There’re holes cut into the radial center of the ring that allow the flow of excess oil back to the oil reservoir.

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Cam Lobe

Cam Gear

Crankpin Journal

Crank Gear

Scavenging

and Intake

(ports open)

Air compressed in crankcase

Exhaust

(intake port closed)

Combustion

(ports closed)

Compression

(ports closed)

Air Taken Into

Crankcase

Choke

Venturi

Higher Pressure Outside Engine

Lower Pressure in Combustion Chamber

Fuel

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Throttle

Answer:

If you blow over the paper, it flies up. The reason is the faster moving air from when you blow causes the air pressure on the paper to drop so it isn’t “weighted down” as much. The same principal is in action when an air plane wing gets lift (the air on the curved top portion of the wing has to travel faster since it goes a greater distance. The faster the air speed, the more the pressure is reduced thus resulting in lift).

In a carburetor, a similar increase in airspeed happens when the air travels through the “Venturi”. Since the diameter is smaller, the air has to speed up to go through it, thus creating a drop in pressure that “sucks” the gas from the fuel tank into the carburetor. In physics, this is called the Bernoulli principle.

The choke “chokes” off the air supply so less air is in the gas/air mix and the mixture is more “fuel rich”. This is important when the engine is cold so it needs more gas to run. Once it is warm, the gas droplets vaporize more easily (which is important for combustion) and it takes less gas to keep the engine running so you need to remove the choke. In new lawn movers, a primer replaces the choke – the primer squirts gas directly into the carburetor to make a “fuel rich mixture.” The throttle controls the total amount of air/gas that enters the cylinder. If you idle a car, the throttle is closed. If you hit the accelerator, the throttle is open.

Answer:

The air vane “governs” the speed of the engine. It is connected by a spring to the throttle. If the engine starts running really fast, the wind generated by the turning flywheel causes the air vane to blow away from the flywheel (creates a pneumatic force on the air vane). As the air vane blows back, it pulls on the spring, which closes the throttle a little. As a result, the engine gets less gas/air and thus slows down a little. Thus there is a smaller wind force on the air vane and it floats back towards the flywheel a little, thus opening the throttle a little. This opening/closing, speeding up/ slowing down process allows the engine to run at essentially a constant speed – very clever engineering!

The flywheel is heavy so it has a lot of inertia to keep the engine running between power strokes. The mass also helps “dampen” the sudden increase in force from the power stroke so it also helps keep the engine running at a constant speed. The fan-like vanes on the flywheel generate wind to cool the engine (also clever engineering). Finally the magnet on the flywheel is what causes the spark to spark.

Answer:

The “force” is the force exerted on or by the piston. The “distance” is the stroke or distance the piston travels (TDC to BDC). Finally the “ time” is the time it takes to complete the 4 strokes.

The two situations where there is significant force on the piston are during the compression and combustion phase. During compression, the piston does “negative” work to compress the gas air mixture. During combustion the piston does positive work as it is forced down due to the explosion of gas and air.

To calculate “net” horsepower, think about the total amount of power output the engine provides to run a lawnmower. Since it takes some work to compress the mixture first, the net horsepower will be slightly less than if engines only had power strokes. So you have to subtract the compression work from the combustion work to get the net horsepower.

Answer:

The compression release is a small black metal piece on the cam gear. Its purpose is to make it easier to start the engine.

Basically it allows the exhaust valve to be cracked slightly when someone pulls on the engine starter cord. This is helpful because it prevents both valves being closed during the compression phase, which would require the person starting the engine to be really strong to pull the flywheel around and provide enough force for the piston to compress the gas/air mixture. With the valve cracked, it is easy to move the piston in the compression stroke.

When the engine starts, the compression release flies out of the way due to centripetal acceleration so that the exhaust valve makes a tight seal during engine operation. The valve works by causing the exhaust tappet to be raised slightly off the plastic shaft of the cam gear.

Answer:

A two-stroke engine works by combining intake and compression and combining power and exhaust. See the diagram above to note how air comes in through the crankcase.

Answer:

Engines don’t start for 3 main reasons – bad mixture (too much/too little gas or air in mixture), no compression - for some reason the mixture is escaping during compression, or no spark. Any reasonable answers will work for the questions above but some possible examples listed below:

a) The flywheel is locked for some reason – the oil splasher on the connecting rod was put on backwards and is hitting the inside of the crankcase, the flywheel key is damaged, the starter clutch is damaged, the connecting rod is on too tight.

b) The timing is off (valves open at the wrong time in the strokes) – no compression. Students didn’t line up marks on cam gear and crank gear.

c) Something is preventing the gas from getting in the mixture – the carburetor is bad, the throttle is stuck in the mostly closed position.

d) The spark plug is bad, the wire from the ignition armature was not connected.

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