How To Strip An Engine

Engineering Report

Engine Dismantle

And Rebuild

Travis Wight

TEN053B

Mr. Mulligan

Contents

Introduction p1

What is a Two Stroke Engine? P2

How a Two-Stroke Engine Works

Power Stroke p3

Fuel Intake p4

Compression stroke p5

Dismantling the Engine p

Engine Assessment p

Engine Rebuild p

Appendices

Introduction

The task at hand was to find a medium sized internal combustion engine. With this engine we had to dismantle it recording all the steps taken to pull it down. After the engine had been taken apart, all the functions of the engine had to be observed and recorded. After observing and recording all the information, it was then required to make any minor repairs, adjustments or replacements. Once the repairs had been carried out, the engine had to be rebuilt back to its original or better then working condition.

What is a Two-Stroke Engine?

A two-stroke engine is an engine that is mainly used in lower-power devices. Two stroke engines are used in devices such as jet skis, dirt bikes, lawn mowers and other petrol powered garden equipment such as whipper snippers, chainsaws and leaf blowers. Two-stroke engines are renowned and used for their simplistic design. Without having valves it reduces weight and reduces complexity of its design. The engine can function in any orientation. This means that it can be moved upside down, around in any direction without the engine stopping. This has a massive advantage over four stroke engines because they need to stay upright for the oil flow to remain constant.

Two stroke engines have the ability to create a lot of power because of the amount of power strokes per revolution, in addition with its small lightweight design; two stroke engines have a high power-to-weight ratio.

Fig 1) Cross section of a two-stroke engine's barrel and crank case.

How a Two-Stroke Engine Works

Power Stroke

Starting from the point where the spark plug fires. Previous to this, the piston has compressed the fuel/air mixture in the cylinder. When the spark plug fires, it creates an explosion. This explosion drives the piston downward which turns the crankshaft, which is attached to the shaft that drives the cutting deck on the lawn mower. As this is happening it is compressing the fuel/air mixture that has come from the carburetor, in the crank case. As the piston starts to go down, the exhaust port is uncovered, and the pressure in the cylinder forces most of the exhaust gases out of the chamber.

Fig 2) The power stroke in a cross section of a two-stroke engine.

Fuel Intake

Perhaps the smallest step in the combustion process, but one of the most important. As the piston reaches its most downward point, the piston has pressured the fuel/air mixture in the crankcase. When it reaches its most downward point the fuel intake port is uncovered and the pressurized fuel/air mixture rushes into the cylinder. As the fresh fuel mixture fills the cylinder, it expels the remaining exhaust gases left in the cylinder. Most two-stroke engines have specially designed piston heads, so that the fuel doesn't flow over the top of the piston and out the exhaust port.

Fig 3) Cross section of the crank case and cylinder during the intake step.

Compression Stroke

This is the most complex stroke of all in the combustion process of a two stroke engine. The momentum of the crankshaft starts to push the piston upwards to the spark plug for the power stroke once again. The fuel/air mixture gets sucked into the cylinder and this creates a vacuum within the crankcase. This vacuum causes the reed valve to open and more fuel/air to get sucked in from the carburetor. Once the crankshaft pushes the piston to the top again, the spark plug fires and the cycle is started over again

Fig 4) The compression stroke of a two stroke engine

Dismantling the Engine

The engine that was used to execute the report on was a small two stroke lawn mower engine. See Fig 5.

The first step in dismantling the engine was to detach the engine from the mower itself. This was achieved by un-doing one primary nut, which was attached from the shaft of the engine to the cutting deck of the mower. There was four smaller nuts the attached the engine to actual mower frame. These five bolts were undone with a medium-size shifter. The large bolt had to be "cracked" because it was on tight and tapping it with a hammer causes the nut to crack and let go of the bolt.

Upon removing

...