Engr 100 Sec. B Engine Dissection Project

Engine Dissection Project

1. Purpose

The purpose of the Engine Dissection Project is to show you that complex devices such as engines can be understood rather easily. We will investigate the theory, design, and operation of a four-cycle internal combustion engine by taking it apart and putting it back together. We will also verify the advertised horsepower of the engine by taking some simple measurements while the engine is apart and applying some simple thermodynamic principles.

2. Deliverables

Assembled, operating engine
group report from each team member, covering the questions posed below (questions should be divided equally among team members).

3. Engine Dissection Project Schedule

Day 1: Introduction, select and run your engine, begin disassembly
Day 2: Continue disassembly.
Day 3: Continue disassembly; take measurements for performance calculations; begin reassembly.
Day 4: Complete reassembly and restart the engine.

4. Procedure

Note: This project is not a race! There is a lot of time to do the work. Take your time to address the issues listed below as you progress in your dissection. Your ultimate goal is the individual report, not the assembled engine, so use as much time as you need to address the pertinent issues.

Recommendations: During disassembly, choose two of your team members who will be the only ones allowed to touch the engine. This pair should take turns with the various steps to ensure an even distribution of work. The remaining team member(s) should be noting observations and answering the written questions. Then, during reassembly, the remaining team members may take turns reassembling.

4.1 Repair Manual

Be aware of the following sections of the Briggs and Stratton Repair and Service Manual:

Section 1: General Information
Section 13: Tools

4.2 Preparation

Mount the engine on the aluminum plate provided. Pay attention to orientation: Mount it so the starting cord is over the excess plate surface (i.e., the shaft should be over open space).
Note the manufacturer, model, and serial number of your engine.
What is the idle RPM and rated horsepower for this engine?
Check that there is no fuel in the engine.
Determine if the engine has compression. This is done by pulling the starting rope and observing if there is a significant resistance during part of the cycle. If the engine spins freely with no resistance, you have no compression (but don't worry, you can fix it.)
Start the engine:
- Take the engine to the parking lot outside the main door of the building.
- Put a small amount of fuel in the gas tank (about half a cup).
- Engage the choke and open the throttle all the way (to the "rabbit" position).
- With two team members standing on either side of the support plate, have a third member pull the cord and start the engine
  - keep pants cuffs clear of the rotating shaft!
  - Allow the engine to run only a few seconds or it will get too hot to handle
- Stop the engine and drain the fuel (be careful of the muffler, which will be burning hot).
Return the engine to the lab.

4.3 Engine Dissection

Warnings, tips, and recommendations:

Print a copy of the Engine Dissection Tips and refer to it every day before getting started.
If you are wearing a watch, remove it and store it somewhere. The strong magnetic fields around the armature can damage a watch mechanism.
Make notes and sketches to ensure that you can reassemble all of the parts easily. Record the exact method you used to disassemble your engine (i.e., the order in which the parts were removed, etc).
Use nut drivers or sockets to remove all bolts. Only use combination wrenches if absolutely necessary! (These tend to wear down the hex-shaped bolt heads and make them impossible to remove. The only place where these are really needed is for removing the carburetor assembly.)
When you remove a bolt, instead of just throwing it in your parts bin, thread it back into the place where it came from (after the mating part has been removed!). For example, after you remove the carburetor, screw the two mounting bolts back into the engine block.
Be extremely careful with the gaskets: We may be able to reuse them and save some money (although if we were to use the engine to mow a lawn, it would not be a very good idea).
Drain oil first to avoid a flood!
Before removing the cam shaft, make sure you pay attention to the timing marks. Check the manual [1] to figure this out. If you are not sure, ASK!

Disassembly		Reassembly (start from bottom)
1	Flywheel Shroud This is the big outer casing of the engine that surrounds the flywheel and contains the starter cord and starter clutch.	17	Replace the flywheel shroud.
2	Air Vane Bracket This is the plastic piece and its support, mounted on the side of the armature. You will need to carefully disconnect the wire linkage to the carburetor. Note which hole in the vane is used for the linkage -- remember, you will have to put it back together several days from now.	16	Armature The air gap between the flywheel and the armature must be set according to the specification in the manual [1]. Use two feeler gages to set the gap (borrow one from another group). This is most easily done with the magnet below the armature as this holds the armature tightly on the flywheel during tightening.
3	Armature First remove the wires to ground and spark plug, and then the armature itself.	15	Replace the wind vane bracket. It must be in place before you gap the armature because it shares the same bolts that the armature uses.
4	Carburetor Assembly This is a big conglomeration of parts that comes off with three bolts. Do not disassemble the carburetor!	14	Replace the carburetor assembly.
5	Side Shroud	13	Replace the side shroud.
6	Cylinder Shroud	12	Replace the cylinder shroud.
7	Flywheel Nut You will need to use the flywheel iron (ask the instructor) to hold the flywheel stationary during this step.	11	Replace the flywheel nut. Use the flywheel iron again to stabilize the flywheel while you tighten. Again, use a torque wrench set to the proper torque.
8	Flywheel You may need to use the gear puller to remove the flywheel. Your instructor will show you how. Watch for the little flywheel key that fits in the slot between the flywheel and the crank shaft...it may either drop out or stick to one of the parts. It is very easy to lose!	10	Replace the flywheel. Don't forget the key!
9	Crankcase Cover Drain oil first if you have not already and beware of the sharp edges.	9	Replace the crankcase cover, using a torque wrench set to the proper torque.
10	Spark Plug Use the special spark plug socket head	8	Replace the spark plug.
11	Valve Spring Cover This part is on the side of engine and has a rubber tube attached.	7	Replace the valve spring cover. Just a little beyond finger tight will do.
12	Cylinder Head After removing the cylinder head: Reassemble the flywheel (just hand-tighten the flywheel nut) and rotate the flywheel. Record your observations (carefully note the sequence of events...piston location, valve location, armature/flywheel magnet, etc. By repeating the process you should be able to understand exactly how the engine performs each of its major functions). Measure the bore and the stroke of the cylinder. Estimate as accurately as you can the volume in the cylinder when the piston is at top dead center (this includes the volume contained in the cylinder head, which you will get by filling the volume with water and getting a weight difference on a scale). Ask if you do not understand since you will need these volumes for your calculations. You need to get it right before you put the engine back together. When done, remove the flywheel again and proceed.	6	Cylinder Head Use a torque wrench to replace the bolts on the cylinder head. Look up the proper torque in the manual. When you put the cylinder head back on, the top shroud and the spark plug grounder should be in place first, or you will need to redo it.
13	Valve springs Use the valve spring compressor for this step. This is the single trickiest procedure in the entire dissection. Each of the two disassembly team members should work together on this. Do not at any time try prying the valves from their seats with a screwdriver or anything. This is not necessary (the valves will simply lift out when they are correctly disengaged) and will ruin the valves.)	5	Make sure that everything works by rotating the engine (using the flywheel) and checking the operation of the valves. Are the exhaust and intake valves opening and closing at the right time, with respect to the position of the piston? If not, check the timing marks on the gears again.
14	Exhaust and Intake Valves	4	Replace the valve springs and the exhaust and intake valves, using the spring compressor.
15	Plastic Gear (a.k.a. Camshaft) Before you remove the camshaft, try to find the timing marks on the plastic gear and the metal gear that it meshes with. You will need to realign these marks when reassembling! When you remove the plastic gear, the tappets will fall out.	3	Put the tappets back in and reinstall the plastic gear (camshaft). Pay close attention to the timing marks!
16	OPTIONAL Connecting Rod This step is optional. Feel free to stop here and begin reassembling, unless you are ahead of schedule. Remove the connecting rod from the crankshaft by removing the crank pin. Be careful in removing these bolts. They are very fragile.	2	Using a torque wrench, reconnect the connecting rod to the crankshaft. It is important to set the wrench to the proper torque or you will strip the bolts and ruin the engine. You know you have achieved the proper torque when the torque wrench 'clicks' once as you are tightening the bolt. At this point, stop tightening. Contrary to popular belief, the torque wrench will not 'ratchet' but only click quietly a single time.
17	OPTIONAL Piston The piston should now slide out by pushing it upward. Be careful to avoid cutting yourself on the sharp crankcase edges. Do not remove the rings from the piston.	1	Reinsert the piston by using the piston ring compressor which should be in your toolbox. It looks like a wide black cylinder of sheet metal. The ring compressor is used to force the piston rings into their grooves, allowing the piston to slide back into the cylinder.
STOP Before beginning reassembly... Tell the instructor or TA if you have any broken parts or torn gaskets. They can be replaced. When reassembling, do not over tighten the bolts. Many of the smaller bolts (crankcase, breather, and shroud bolts) are small and are easily broken off. Most bolts need only be tightened a little more than "finger tight". Note that the following bolts require the use of the torque wrench: connecting rod bolts; head bolts, which must be tightened in a specific order; crankcase cover bolts; flywheel bolt. Refer to the manual [1] for the pertinent specifications.

As you disassemble your engine, reason out the answers to the following questions by examining the parts and discussing them with your team members. Each team member should take responsibility for an equal portion of the questions, and answer those questions in their individual report. The set of reports submitted by the members of a given team must, together, cover all of the questions.

Ignition
- (1) Describe how the ignition system on your engine works.
- (2) What types of materials are used on the flywheel? Why does the flywheel have one section made from a different material?
- (3) What is the purpose of the flywheel? How much does it weigh? What would be the advantages and disadvantages of making it heavier? Why does it have fan-like vanes on it?
- (4) What is the purpose of the starter clutch? Explain from your examination how it does its job.
- (5) What determines at what point in the cycle the spark occurs?
Carburetor
- (6) From reading the appropriate sections of the repair manual [1], describe in general how the carburetor works.
- (7) What is the function of the choke?
Engine Block, Valves, Crankshaft, Camshaft, Pistons
- (8) Locate the intake and exhaust valves. Which one is larger? Why?
- (9) Which valve has a special valve guide? Why?
- (10) Why do you think the cam shaft made of nylon?
- (11) What is the significance of cam timing, and how is it set?
- (12) What is the relationship between the cam timing and the flywheel magnet? When in the cycle does the spark occur? Does every spark result in combustion, or is one spark each cycle wasted?
- (13) What are tappets, and what is their function?
- (14) On the camshaft there is a small metal piece, including a spring, that is connected where the gear meets the shaft. Looking at the relation between the tappets and cams, can you guess what purpose it might serve? Try putting the tappets and the camshaft back into the engine, rotate the cam through its motions, and see if you can figure it out.
- (15) What is the purpose of the rings on the piston and why is there more than one ring? (if you did not remove the piston, look at one of the disassembled pistons in the lab)
- (16) What type of bearings does the crankshaft use? How do these bearings avoid becoming worn out?

4.4 Engine Start

Take your engine outside to the TA who will be waiting to help you.
Find the oil fill hole, unscrew it, and fill it with oil to the bottom of the threads.
Put about a half cup of fuel in the fuel tank.
Start the engine using the same procedure you used before.
Did it start? Usually it will. If not, there are several things that could be wrong:
- Choke or throttle may not be set to proper position. Choke to halfway; throttle to "rabbit".
- Did you remember to add fuel?
- Did you gap the armature when re-installing it? If you don't know what this means, you probably didn't.
- Did you connect the armature wires properly?
- As a last resort, ask the TA to put a few drops of gasoline into the cylinder (via the spark plug hole). If it still doesn't start, you probably have no spark and so there is an ignition problem.
- Another way to tell if there is a spark is to remove the spark plug wire and have someone crank the engine. If there is a spark, you will see it or feel it. But be careful.
- Sometimes somebody in the group fiddles with the carburetor when they shouldn't have. This can also lead to failure.
Let your engine cool.
Drain the fuel into the fuel container.
Drain the oil into the oil container.

Final checkout:

Clean your tools;
Inventory your tools;
- If you know of any missing tools, find them.
- If you have extra tools, find out who they belong to and give them back.

Acknowledgements

This project is based on a module from ME/IE 497 Product Dissection, a one semester course at Pennsylvania State University developed by John Lamancusa under the Manufacturing Engineering Education Partnership.

Reference Materials

[1]	Briggs & Stratton Corporation, Repair Manual For Single Cylinder 4-Cycle Engines, Briggs & Stratton Corporation, Milwaukee, WI, 1992, 1996, 1997.