What is an engine stuffing box?
In a crosshead type two-stroke marine diesel engine, the stuffing box provides a seal between an engine’s scavenge space and crankcase. Stuffing boxes are required to reduce the likelihood of a scavenge space fire, crankcase explosion, and/or oil contamination. Trunk type engines do not have a stuffing box.
A stuffing box has three main sections:
- Upper section – prevents cylinder lubricating oil and gases from entering the crankcase.
- Middle section – allows draining of oil if there is any carry-over from the upper or lower parts.
- Lower section – prevents crankcase lubricating oil entering the scavenge space.
The ‘upper section’ is also referred to as the ‘sealing section’. The ‘middle section’ is also referred to as the ‘drain section’. The ‘lower section’ is also referred to as the ‘scraping section’. Each name reflects upon the sections function. The sealing section forms a gas seal, the drain section allows for draining of oil, whilst the scraping section is responsible for scraping of crankcase lubricating oil.
Marine Two-Stroke Engine Stuffing Box
This article focuses on the stuffing boxes used within two-stroke slow speed marine diesel engines. These types of engine have a small bore-to-stroke ratio, which means that a crosshead must be used in order to limit the size of the crankcase. Without a crosshead, the crankcase would be uneconomically wide and sealing between the piston rings and cylinder liner would become difficult.
Good to know – the scavenge space is located beneath each engine piston, it allows fresh air to enter the engine cylinders, causing exhaust gases from the combustion space to be discharged. The engine crankcase is located directly below the stuffing box.
Good to know – trunk engines use a connecting rod (con rod) to connect the piston to the crankshaft. Crosshead engines connect the piston to the crankshaft via the piston rod, crosshead, and connecting rod.
Trunk and Crosshead Engines Compared
What is the purpose of an engine stuffing box?
The main functions of an engine stuffing box are, to:
- Prevent crankcase lubricating oil entering the scavenge space when the piston rod moves upwards.
- Prevent scavenge air from entering the crankcase when the piston moves upwards or downwards.
- Prevent cylinder lubricating oil entering the crankcase when the piston rod moves downwards.
- Prevent debris entering the crankcase when the piston rod moves downwards.
- Prevent exhaust gases entering the crankcase (if the piston rings are not sealing correctly) when the piston moves upwards or downwards.
Note that the piston rods direction of travel dictates what function the stuffing box is performing at any given time.
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What are the main parts of an engine stuffing box?
The main parts of a stuffing box are listed below.
Stuffing Box Housing
The stuffing box housing forms the outer casing that encloses the sealing rings and scraper rings. It is typically constructed from cast iron or steel and assembled from two parts.
Good to know – the ‘stuffing box housing’ is also called the ‘diaphragm’, ‘diaphragm casing’, or ‘diaphragm gland’.
Stuffing Box with Labelled Parts
Sealing Rings
Sealing rings are primarily responsible for preventing scavenge air entering the crankcase. If the piston rings should be passing (allowing gases to pass from the combustion space to the scavenge space), the sealing rings will also prevent these gases from entering the crankcase. Sealing rings fit into grooves within the stuffing box housing and are typically manufactured from bronze. As it is not possible to assemble the rings within the stuffing box as a single piece, each ring is formed by three or four different sections which connect together to form a circle.
Sealing Ring Split into Four Sections
Good to know – passing piston rings can cause crankcase explosions and/or scavenge space fires. For this reason, it is imperative that defective piston rings are replaced as soon as possible. Passing piston rings will often be discovered due to a drop in engine power within the affected cylinder.
Good to know – crankcase explosion valves are used to mitigate the effects of a crankcase explosion, should it occur.
Upper and Lower Scraper Rings
Scraper rings are located above and below the sealing rings. They exist to remove excess lubricating oil from the piston rod as it travels upwards, and, to remove excess cylinder lubricating oil and debris from the piston rod as it travels downwards. Similar to sealing rings, scraper rings fit into grooves within the stuffing box housing. In the past, the entire scraper ring was manufactured from a soft material like cast iron, or bronze, but more modern designs use thin lamellas whilst the non-wearing parts are manufactured from steel; this is because only the lamellas need replacing rather than the whole ring, thus waste and costs are reduced. Scraper rings consist of three or four individual sections that form a circle when assembled (similar to sealing rings).
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Lamellas (black) Installed Within Scraper Rings (grey)
Garter Spring
A garter spring is installed behind each sealing and scraping ring. The springs serve to press the rings against the piston rod, thus ensuring good sealing. If a spring’s tension is too high, excessive wear of the piston rod, sealing rings, and scraper rings may result. If a spring’s tension is too low, inadequate sealing will occur.
Drain
Between the upper sealing rings and lower scraper rings, is a drain. The drain allows any crankcase lubricating oil which passes the lower scraper rings to be returned to a used lubricating oil tank; this oil can then be treated and reused. The drain also allows any cylinder oil which passes the upper scraper ring to be returned to the same tank for processing.
How does an engine stuffing box work?
The stuffing box is bolted in place to the main engine A-frame. As the piston travels up and down with each stroke, the piston rod moves up and down through the stuffing box.
When the piston travels upwards…
Any lubricating oil from the crankcase that has come into contact with the piston rod is removed by the lower set of scraper rings; scraped oil drains back to the crankcase. Scraper rings are pressed against the piston rod by their associated garter springs.
When the piston travels downwards…
Any cylinder oil present on the piston rod is removed by the upper scraper ring as the piston moves downwards. Sealing rings prevent gases (air or exhaust gases) from the scavenge space entering the crankcase. The scraper ring and sealing rings are pressed against the piston rod by their associated garter springs.
Good to know – if the scraper rings or sealing rings are installed too tightly, wear of the rings and piston rod will occur (not desired).
Good to know – a small amount of lubricating oil is required to lubricate scraper rings and sealing rings. The rings should not come into physical contact with the piston rod; a thin film of oil separates the piston rod from the rings, thus reducing wear on all parts.
Good to know – exhaust gases will only be present if there is a failure or excess wear of the piston rings or cylinder liner; a cracked piston may also allow for exhaust gas leakage.
Are stuffing boxes suitable for all engine types?
Stuffing boxes are only suitable for crosshead two-stroke engines. There are two main types of two-stroke internal combustion engines: the trunk type and crosshead type.
Crosshead Engine
In a crosshead engine, a crosshead converts the rotary motion of the crankshaft into the linear motion of the piston. This means that the piston rod moves up and down linearly, whilst the crankshaft rotates around a centre axis of rotation.
Trunk Engine
A trunk two-stroke engine does not have a crosshead, meaning that the piston rod moves both vertically and horizontally on each stroke. These two degrees of freedom/motion mean that a stuffing box cannot be used to provide a seal on a trunk type two-stroke engine. The lack of sealing arrangement means that lubricating oil can enter the scavenge space of a trunk two-stroke engine; this is an inherent design weakness for this type of engine.
Good to know – four-stroke engines are not large enough to require a crosshead i.e. their bore-to-stroke ratio is too large, thus they cannot utilise a stuffing box.
Stuffing Box Features
The following features are why stuffing boxes are used on marine slow-speed two-stroke crosshead engines:
Simplicity and Reliability
Stuffing boxes are simple devices with high rates of reliability. They are relatively easy to maintain and repair, and they are less prone to failure than other types of seals.
Ability to Accommodate Large Gaps
The gap between the piston rod and the A-frame in a two-stroke marine engine can be quite large; stuffing boxes are able to accommodate this large gap whilst still obtaining a good seal.
Low Cost
Stuffing boxes are relatively inexpensive to manufacture and install; this makes them cost-effective.
Proven Design
Advancements in material science have allowed stuffing box designs to be continuously improved. Design improvements have led to reduced wear rates, better sealing, and longer intervals between maintenance interventions.
Additional Resources
https://marineinbox.com/marine-exams/stuffing-box/
https://journals.sagepub.com/doi/10.1177/1468087418796615