Internal Memorandum

Date:	Today (use the current date in your own memos!)
To:	Engr100 Students
From:	Engr100 Instruction Team
Subject:	Engine Group Report

Group Assignment

Describe what you have learned about the theory, design, and operation of an internal combustion, (single-cylinder) four-cycle, spark-ignition engine via a technical report.

The technical report should give the reader

1. a general idea of how the engine operates,
2. a description of the design features (i.e., subsystems) of the engine, and
3. a prediction of the engine horsepower (extra credit).

A detailed description of each of these elements are given below.

Deliverable

As before, your technical report should be a professional document: You must use a word processor; the report should contain figures (drawings and plots), tables, and equations which will clarify the text where possible; references should be formally cited; etc.

Recall that the technical report follows the following formal structure:

FRONT MATTER

-	Cover page
-	Executive Summary
-	Table of Contents
-	List of Figures
-	List of Tables
-	Nomenclature

MAJOR SECTIONS

1.0 Introduction

This section briefly lays out the exercise to be performed. After reading this section, the reader should know in broad terms what you did, and your goal in doing this. (For example, you tore apart and rebuilt a small engine to understand how the design details support the overall operation of the engine.)

2.0 Procedure

This should be an overview of what you did, not a blow-by-blow description of each nut and bolt extracted. For example, this might describe how you ran the engine, the order of disassembly, the order of re-assembly, and how you ran the engine again. You should include anything noteworthy, such as points where the suggested procedure list was in error or inadequate, and details that were not in the list that you felt the reader should know about. In other words, you are writing a road map that somebody can follow to repeat what you did.

3.0 Description of Major Subsystems

This section should describe how the details of the design support the function of the various subsystems. Again, remember that everything you see in the engine was put there for a purpose. Use the notes you generated in your journal to describe the intent of the designer behind each of the features. You will probably see some features whose function you do not understand, especially associated with the electrical system (we certainly did during our dry run!). Do not worry about these, but note them.

• 3.1 Air/Fuel Subsystem

This includes the systems for handling the air, fuel, and exhaust products. Note that since we did not dissect the carburetor, the description of this system will be abbreviated.

• 3.2 Mechanical Drive Train

This should include the piston, connecting rod, crankshaft, fly wheel, starter clutch, and valve train. This section should include a description of the timing system, using the fact that the spark fires when the magnet is near the armature. This should be the largest section of the report

• 3.3 Lubrication Subsystem

How are the parts, which are in contact with one another and in motion with respect to each other, kept from wearing down?

• 3.4 Cooling Subsystem

Think about the purpose of the fins on the engine, the vanes on the flywheel, and the shrouds.

• 3.5 Electrical Subsystem

Again, you are not expected at this point to understand all the details of the electrical ignition system, but go as far as you can in explaining the key features, and identify those features that you do not understand. How does the system that turns the engine off work?

4.0 Conclusion

Bring out the major points you feel are worth emphasis.

BACK MATTER

References

Appendix A

Answer to the questions in the procedure

• 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?

Appendix B (extra credit)

ENGINE PERFORMANCE ESTIMATION

1. Using the procedure discussed in class and the measurements made on the engine, make a plot of distance from bottom dead center (x-axis) vs. the force on the piston (y-axis) for the compression stroke. Assume the valves are both closed at bottom dead center and the initial pressure is atmospheric. Estimate the work required to do the compression. (Note that work equals force times distance, but since the force varies with distance, you will need to break up the calculation into small slices as described in class.)

Use the temperature or pressure in the cylinder after combustion has occurred (given in class) to estimate the work generated during the power stroke. The work from this expansion is calculated in an analogous manner as the work from the compression stroke.

2.Use an engine speed of 2000 RPM to estimate the power output (in Horsepower) for your engine. Note that you get only one compression and power stroke pair for each two engine cycles. The other cycle involves moving the exhaust out of the cylinder and pulling the fresh fuel/air charge in.

(use the power calculation template)

Grading

The grading of the assignment will follow the evaluation table given below.

CATEGORY	POSSIBLE	SCORE
Correct report format	10
Quality of figures, tables, equations	15
Logic of report organization	10
Clarity of introduction	5
Relevant background	5
Description of specific design features	50
Quality/conciseness of summary	5
Engine performance calculation (extra credit)	25
TOTAL	100