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Engine Picture Engine
Dissection
Project

 

1. Introduction

The goal of the Engine Dissection Project is to discover how easily a complex device like an engine can be understood. We will investigate the theory, design, and operation of a four-cycle internal combustion engine by taking it apart and putting it back together. Specifically, we will try to discover how the engine produces power by disassembling it and examining how it works. Then we will verify the advertised horsepower rating of the engine by applying some simple thermodynamic principles to model the power-producing cycle. Finally, we will restore the engine to working order and prove that it still runs.

 

2. Deliverables

You and your team are responsible for delivering three things at the end of this project:
  1. The engine, fully reassembled and working
  2. A group report answering the questions posed during the disassembly process (See below).
  3. Engine report - (See your section website for details).
TIP: As you disassemble your engine, reason out the answers to the questions by examining the parts and discussing them with your team members.

 

3. Project Schedule

4. Procedure

4.1 Start the Engine

  1. Get an aluminum plate and mount the engine on it. Use at least two or three bolts. Don't mount it wrong! The starting cord must be over the excess plate surface -- and the shaft should be reaching out over open space.
  2. Note the manufacturer, model, and serial number of your engine.
  3. What is the idle RPM and rated horsepower for this engine?
  4. Start the engine:
    • Take the engine to the parking lot outside the main door of the building.
    • If necessary, put a small amount of fuel in the gas tank (about 1/4 cup).
    • If necessary, put about a 1/2 cup of oil into the oil filler hole.
    • Engage the choke about halfway and open the throttle all the way (to the "rabbit" position).
    • Two team members must hold the engine steady by standing on each side of the plate.
    • Then, a third member may pull the cord to 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. Drain the fuel back into the red fuel container, then drain the oil into the black oil container.
  5. Return the engine to the lab.

4.2 Dissect the Engine

Warnings, tips, and recommendations: Follow the list of steps below. During each step, discuss the questions posed (in italics) with your team members.

 

Disassembly

Reassembly

(start at bottom)

1

Flywheel Shroud (Blower Housing),
Side Shroud, and
Cylinder Shroud (Cooling Fin Cover)
Picture - Details
The blower housing is the largest piece, surrounding the flywheel and containing the starter cord and starter clutch.

(1) What is the purpose of the starter clutch? Explain from your examination how it does its job.

(2) Why is there a cylinder shroud? What specific function does it serve?

11

 Replace the side shroud, cylinder shroud, and flywheel shroud.
Details

Now Restart the Engine (instructions at bottom of page)

2

 Muffler, Air Vane, and Fuel System
Picture - Details
The muffler can be removed by loosening two bolts.

The air vane (or governor blade) is the plastic blade that is connected to the armature. Remove it by very carefully bending the metal tab that holds it in place.

The fuel system is a large assembly, held by only three bolts. Remove the entire system in one piece. Do not disassemble the carburetor!

(3) From reading the appropriate sections of the repair manual, describe in general how the carburetor works.

(4) What is the function of the choke and how does it work?

10

 Replace the muffler, then the air vane and the fuel system assembly.
Bend the metal tab very gently when reinstalling the air vane.
Details

3

 Valve Spring Cover
Picture
This part is on the side of engine and has a rubber tube attached.

9

 Replace the valve spring cover.
Just a little beyond finger tight will do.

4

 Spark Plug
Picture - Details
Use the special spark plug socket head.

8

 Replace the spark plug.
Details

5

 Cylinder Head
Picture - Details
After removing the cylinder head:
  • Rotate the flywheel by hand and 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).
  • Get the following measurements:
    • Measure the bore and the stroke of the cylinder.  Use a ruler or calipers.
    • Measure the clearance volume, which is the volume in the cylinder when the piston is at "top dead center" (this includes the volume contained in the cylinder head itself, which you can measure by filling it with a known volume of water, or by measuring the difference in weight after filling it with water).

(5) Describe in general how fuel manages to get ignited in your engine (i.e. the ignition system).

(6) At what point(s) in the four-stroke cycle does a spark occur? How often does the spark plug spark? Does every spark result in combustion?

(7) Locate the intake and exhaust valves. Which one is larger? Why?

7

 Replace the cylinder head (140 in-lbs per bolt).
When you put the cylinder head back on, the top shroud (a metal plate) and the spark plug grounder must be in place first, or you will need to redo it.

BE CAREFUL ON THIS NEXT STEP!  IF DONE INCORRECTLY, THE BOLTS MAY BREAK (which are very difficult to fix).  Ask the instructor how to use the torque wrench.  If at any time you are having to pull with some effort on the torque wrench - STOP - loosen the bolt and start over.

Use a torque wrench to replace the bolts on the cylinder head (140 in-lbs). Follow the proper tightening pattern, as shown in the manual.
Details

6

 Crankcase Cover
Picture - Details
Drain oil first if you have not already and beware of the sharp edges.

After removing the cover, turn the flywheel some more and look at what goes on in the crankcase, particularly how the plastic camshaft interacts with the other parts.

(8) Where are the timing marks on the camshaft and crankshaft gears? How can you tell if the timing is correct? Why is this important?

6

 Before proceeding:

Make sure that everything works by rotating the engine (by turning the flywheel) and checking the operation of the valves. Are the exhaust and intake valves opening and closing at the right times, with respect to the position of the piston? If not, check the alignment of the timing marks on the gears, and try again.

If all is well, then replace the crankcase cover. Do not overtighten (85 in-lbs)!
Details

7

Camshaft and Tappets
Picture - Details

*If you have a new engine, you need to use the flywheel puller and wrench (located in the brown toolbox) to remover the ball bearings FIRST. 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.

(9) Why is the cam shaft made of nylon? Refer to US Patent# 5,497,679 to find out.

(10) When was the nylon camshaft in your engine manufactured? How can you tell?

(11) What are tappets, and what is their function?

(12) 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. You can also refer to the patent.

5

 Put the tappets back in and reinstall the plastic gear (camshaft). Pay close attention to the timing marks!

Details

**New engines: Use the metal shaft and rubber mallet (located in brown toolbox) to put the ball bearings back in place on the crank shaft. 


 

8

 Flywheel Nut
Picture
You will need to use the flywheel iron (ask the instructor) to hold the flywheel stationary during this step.

4

 Replace the flywheel nut (50 ft-lbs).
Use the flywheel iron again to stabilize the flywheel while you tighten. It is important to use a torque wrench set to the proper torque, or you may strip the bolts and ruin the engine.
The repair manual lists required torque for the flywheel nut, cylinder head bolts, and connecting rod bolts (if applicable). 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.

9

Flywheel
Picture
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!

(13) What types of materials are used on the flywheel? Why does the flywheel have one section made from a different material?

(14) 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?

3

 Replace the flywheel. Don't forget the key!

STOP

Now, let's take a moment and see if we can compute the horsepower of this engine using the measurements we have taken.
  1. Get a copy of the Power Calculation Spreadsheet and enter your measurements into the appropriate cells.
  2. Ask the instructor for any values that you are not sure about.
  3. Does the computed horsepower come close to the advertised horsepower?
  4. If not, double-check your measurements, or ask the instructor for advice!! It should be pretty close to 3.5 HP.
    5.
  5. Print out the spreadsheet and turn in with the questions.  Also write a couple sentences explaining how horsepower is calculated.

Begin reassembly, but first...
  • Examine the connecting rod, crankshaft, and oil stirrer to look for any damage. If anything feels loose or looks bent or broken, please tell the instructor.
  • Tell the instructor or TA if you have any torn gaskets or other broken parts. 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 nuts and bolts require the use of the torque wrench. Refer to the manual [1] for the torque values:
    1. connecting rod bolts (if removed)
    2. cylinder head bolts, which must be tightened in a specific order
    3. flywheel nut
  • Instructions are provided below in case you need to make repairs to the connecting rod or piston. Otherwise, please do not remove them!
Begin reassembly with Step 3 above  (note questions 15 & 16 below)

10

 IF REQUESTED
Connecting Rod
Picture - Details
Remove the connecting rod from the crankshaft by removing the crank pin. Be careful in removing these bolts. They are very fragile.

(15) What type of bearings does the crankshaft use? How do these bearings avoid becoming worn out?

2

 Using a torque wrench (100 in-lbs), 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.
Details

11

IF REQUESTED
Piston
Picture - Details
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.

(16) 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)

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.
Details

 

4.3 Re-start the Engine

  1. Take your engine outside to the TA who will be waiting to help you.
  2. Find the oil fill hole, unscrew it, and fill it with oil to the bottom of the threads.
  3. Put about a half cup of fuel in the fuel tank.
  4. Start the engine using the same procedure you used before.
  5. 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 connect the armature wires properly?
    • See if the engine is getting fuel: put your hand loosely over the muffler opening while another member pulls the cord several times. If your hand smells like gasoline, then the fuel system is working fine.
    • See if the engine is getting a spark: 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.
    • Did you feel puffs of air leaving the muffler as the engine was cranked? If not, the engine has no compression.
    • As a last resort, remove the spark plug and ask the TA to put just a few drops of gasoline into the cylinder, then replace the spark plug. If it still doesn't start, you probably have no spark or an intermittent spark.
    • Sometimes somebody in the group fiddles with the carburetor when they shouldn't have. This can also lead to failure.
  6. Let your engine cool.
  7. Drain the fuel into the fuel container.
  8. Drain the oil into the oil container.

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 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.

The On-Line Manual (Details section) was prepared by Greg Dufour, Gabrielle Chifor, and Dr. P.R. Frise  from University of Windsor, Canada.  Permission granted to use the on-line manual.

Reference Materials

  1. Briggs & Stratton Corporation, Repair Manual For Single Cylinder 4-Cycle Engines, Briggs & Stratton Corporation, Milwaukee, WI, 1992, 1996, 1997.
  2. Small Engines, Radcliff, Bruce, American Technical Publishers, Inc., 1997 (Authorized by Briggs & Stratton).