Purpose: To learn about the theory, design, and operation of the four-cycle internal combustion engine, and to apply simple thermodynamics to estimate its output horsepower based on measurements obtained during the dissection.

Acknowledgment: This module uses as its inspiration materials from ME/IE 497: Product Dissection, a one semester course at Pennsylvania State University developed by John Lamancusa under the Manufacturing Engineering Education Partnership.

Background and reference materials: Briggs and Stratton Service and Repair Instructions, Briggs and Stratton Corporation, Milwaukee, WI, 1992, 1994, or 1997.

Activities: Dissection, sketching, calculations (engine displacement, power output, etc.)

Required Tools: basic tool box, parts bin, torque wrench, specialty tools (valve spring compressor, piston ring expander, flywheel holder, piston ring compressor, flywheel remover)

Deliverables:

1. Assembled, operating engine

2. Detailed journal record of all activities:

• Record of all procedures followed

• Answers to all questions posed below

• Sketches of part and assemblies

• All other pertinent information

3. Group Engine Report - details available

Approximate Schedule:

• Session 1: Introduction to engine operation. Overview of module requirements. Tool inventory
• Session 2: Run engine. External observations on engine. Start disassembly
• Sessions 3-6: Complete disassembly, gather data on engine, do performance calculations, reassembly, and test
• Session 7: Continue assembly, or work as a group on ideas for your reports.

Procedure

Readings: Briggs and Stratton Repair and Service Manual (select one team member to do the readings during the first session and to act as the manual resource person)

• Section 1 - General Information
• Section 13 - Tools

A note on procedure: First, this isn’t a race...there is lots of time to do the work. Take your time to address the issues listed below as you are going. Your ultimate goal is your report, not the assembled engine, so use as much time as you need to address the issues while the engine is apart. 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 manual resource person and the remaining team member(s) should be noting observations and answering the written questions below. During reassembly, the remaining team members are the only ones allowed to touch the engine, and they should also take turns with each task.

A. Tool Inventory

1. Count and record all tools in your tool box (ask if you don’t know what a tool is). Compare this with the tool list you are given.

2. Turn in your tool inventory to your instructor. This can be a copy of the inventory list you are given with check marks.

3. On a separate sheet, list shortages and overages, and give this to your instructor.

 

B. Preparation

1. Mount the engine on the plate provided. Mount it so the starting cord is over the excess plate surface (i.e., the shaft should be over open space).

2. Note the manufacturer, model, and serial number of your engine.

3. What is the maximum RPM and horsepower for this engine?

4. Check that there is no fuel in the engine.

5. 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. (This condition can be repaired during your rebuild.)

6. Explain each of the four steps in the engine cycle in your journal (use sketches in your description).

7. Take the engine to the parking lot outside the main door of the building. Put a small amount of fuel in the gas tank. 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 to avoid too much heat buildup. Stop the engine and drain the fuel (be careful of the muffler, which will now be burning hot). Return the engine to the lab.

C. Engine Dissection

1. Read the repair manual before you remove a part (e.g. the flywheel).

2. Remove and store watches. The strong magnetic fields around the armature can damage watch mechanisms.

3. 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.)

4. Use nut drivers or sockets to remove all bolts. Only use combination wrenches if absolutely necessary! (These tend to round the hex-shaped bolt heads. Note that the only place where these are really needed is for removing the carburetor.)

5. 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.

6. Be extremely careful with the gaskets - we may be able to reuse them (although this is not generally a very good idea).

7. Drain oil first to avoid a flood!

8. Make sure you note where the timing mark meshes with the gear on the cam shaft. Check the manual to figure this out. If you are not sure, ASK!

9. The following disassembly order has been successful in the past: flywheel shroud; air vane bracket (plastic piece and 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); armature wires to ground and spark plug; armature; carburetor assembly (do not disassemble carburetor!); muffler (use channel lock available from the lab instructor); side shroud; cylinder shroud; flywheel nut (you will need to use the special brace to hold the flywheel stationary during this step; 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); crank case cover - drain oil first if you didn’t already and beware of sharp edges; spark plug - use special spark plug socket head; valve spring cover (on side of engine with vertical tube attached), cylinder head.

After removing the head:

a. Reassemble the flywheel (just hand-tighten the flywheel nut) and rotate the flywheel.

b. 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).

c.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 want to get it right before you put the engine back together.

10. Continue disassembly: Remove the flywheel; valve springs (use spring compressor...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). Prying will ruin the valves.); valves; cam shaft; connecting rod; piston (do not remove the rings from the piston).

 

D. Understanding How the Engine Works

As you disassemble your engine, complete the following steps and questions. This should be included in your report appendix.

Ignition

1. Identify 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. What type of air cleaner is used on your engine? (You can remove this from the carburetor assembly, but don’t take anything else off.) Why is the air cleaner needed?

7. From reading the appropriate sections of the repair manual, have one of your team members identify in general how the carburetor works.

8. What is the function of the choke?

 

Engine Block, Valves, Crankshaft, Camshaft, Pistons, etc.

9. Locate the intake and exhaust valves. Which one is larger? Why?

10. Which valve has a special valve guide? Why?

11. Explain what happens in the valve spring compartment.

12. Why do you think the cam shaft made of nylon?

13. What is the significance of cam timing, and how is it set?

14. 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?

15. Sketch the camshaft assembly.

16. What are tappets, and what is their function?

17. 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.

18. Sketch the piston assembly. What is the purpose of the rings?

19. What type of bearings does the crankshaft use? How do these bearings avoid becoming worn out?

 

E. Completely reassemble the engine

Notes: 1. Use small amounts of engine oil to lubricate mating parts (bearings, valves, etc.), 2. The service manual must be followed during reassembly, 3. Before you reassemble your engine, tell the lab instructor if you have any broken parts or torn gaskets.

Do not overtighten the bolts. Many of the smaller bolts (e.g., crankcase, breather, or 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 require the use of the torque wrench: (1) connecting rod bolts, (2) head bolts, which must be tightened in a specific order, (3) crankcase cover bolts, (4) spark plug, and (5) flywheel bolt.

1. Reassemble your engine. Install parts in the reverse order which you disassembled them (this is a test of your memory, journal completeness, and common sense). Note that when you put the head back on, the top shroud and the spark plug grounder should be in place, or you will need to redo the head installation later. Also, the windvane bracket must be installed on the armature when the armature is gapped, or you will need to redo the armature installation.

2. Be sure to use a torque wrench when it is recommended - i.e. rod cover, crank case cover, connecting rod bolts, flywheel and head. · Torque all bolts to the factory specification. Is a 100 Lb-in force a large force? Try each torque wrench on an exterior bolt before you use it for real to see how it works. Finger tighten bolts before using the torque wrench. · Do not over-tighten any of the bolts - remember, the engine block is aluminum, and the threads can get stripped very easily.

3. Use the ring compressor to place the piston back in the cylinder.

4. Install the head and remaining parts.

5. Why is it important to torque the bolts to a certain value?

6. Why are the bolts torqued in a certain pattern?

7. What is the purpose of the head gasket?

8. The air gap between the flywheel and the armature must be set according to the specification in the manual. 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.

 

F. Run your engine

1. Fill the engine with oil to the bottom of the fill hole, and then take it outside.

2. Fill the engine with gas. Start the engine using the same procedure you used on day two.

3. Did your engine operate smoothly?

4. Let your engine cool.

5. Drain the fuel.

 

G. Final checkout

1. Clean your tools

2. Inventory your tools

• If you know of any missing tools, find them.

• If you have extra tools, find out where they belong.

• Organize the sockets in your box.