Spark Plug Thread Pitch Diameter Variation

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Solution Overview

Problem

Longer spark plugs tend to loosen due to decreased surface pressure per unit area as a result of increased contact area with the engine head, leading to vibration-induced instability.

Innovation Solution

A spark plug design featuring a tubular metallic shell with an attachment screw portion having a pitch diameter maximum portion and a local maximum portion, where the local maximum portion is located on the forward end side, enhancing thermal expansion and increasing surface pressure to prevent wobbling and loosening, and optionally including an insulator and center electrode for improved heat dissipation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the length of the attachment screw portion is increased to improve the degree of freedom in determining the water jacket location, then the contact area between the screw portion and engine head increases, but the surface pressure per unit area decreases causing the spark plug to loosen easily under vibration

Engineering Contradiction:

Improvedegree of freedom in determining water jacket locationVSAvoidresistance to loosening under vibration

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The pitch diameter of the external thread is made non-uniform along the axial direction, creating local variations in contact pressure distribution. Specifically, the pitch diameter decreases in the forward end portion compared to the rear end portion, concentrating contact pressure at critical locations to prevent loosening while maintaining overall long contact area for water jacket flexibility.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The pitch diameter parameter of the external thread is changed along the axial direction rather than maintaining a uniform value. This parameter variation creates different contact pressures at different axial positions, with the smaller pitch diameter at the forward end generating higher contact pressure to counteract loosening forces from vibration.

Inventive Principle:

Principle #35Parameter changes

2Ease of manufacture

If the pitch diameter is made uniform to simplify manufacturing, then the structure is easier to manufacture, but the surface pressure distribution is insufficient to prevent wobbling and loosening under thermal expansion

Engineering Contradiction:

Improvethread pitch uniformityVSAvoidresistance to wobbling and loosening

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

Instead of uniform pitch diameter, the invention applies local quality variation by making the pitch diameter smaller in the forward end portion than in the rear end portion. This local differentiation creates higher contact pressure at the forward end where thermal expansion occurs, preventing wobbling and loosening while remaining manufacturable.

Inventive Principle:

Principle #3Local quality

3Temperature

If the forward-end-side portion of the metallic shell receives more heat causing greater thermal expansion, then heat dissipation is improved, but the increased expansion causes wobbling and loosening of the spark plug

Engineering Contradiction:

Improveheat dissipation efficiencyVSAvoidstability against wobbling and loosening

Core Design Contradiction:

TemperatureVSReliability

Solution Approach 1:

The invention converts the harmful effect of thermal expansion into a beneficial force. By designing the pitch diameter to be smaller at the forward end, the thermal expansion of the heated portion generates increased contact pressure that actively prevents loosening, transforming the expansion problem into a locking mechanism.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The design effectively prevents spark plug loosening and enhances heat dissipation by optimizing thermal expansion and contact points, ensuring secure attachment and efficient heat release.

Implementation Method 1

the forward-end-side portion of the metallic shell receives a larger amount of heat from the interior of an engine as compared with a rear-end-side portion of the metallic shell. Thus, the forward-end-side portion of the metallic shell tends to thermally expand in a greater amount as compared with the rear-end-side portion of the metallic shell

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

since heat from the interior of the engine is efficiently released from the metallic shell through the pitch diameter local maximum portion, heat dissipation is enhanced

Methodology Applied

Scientific EffectHeat conduction: Conduction (thermal)

Data Source

PatentUS20200313402A1Spark plug

Publication Date: 2020.10.01 NITERRA CO LTD

AI summary

A spark plug includes a tubular metallic shell having an attachment screw portion having an external thread. The attachment screw portion has a pitch diameter maximum portion at which the external thread has a maximum pitch diameter, the pitch diameter maximum portion being located on a rear end side of a center of the attachment screw portion in a direction along an axial line of the metallic shell, and a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter, the pitch diameter local maximum portion being located on a forward end side of the pitch diameter maximum portion in the direction along the axial line.