Introduction
A marine two-stroke engine exhaust valve allows the exhaust gasses to be expelled from the combustion chamber and is actuated by the engine’s camshaft via hydraulic means, or via mechanical means in older engines (in the form of a rocker assembly). The exhaust gas valve is usually controlled electronically in modern engines with the camshaft providing the timing and motive force required to actuate the valve. Exhaust valves are only found in uniflow scavenging two-stroke engines and are situated on top of their associated engine cylinder. Two stroke engines which use loop or cross flow scavenging do not require exhaust valves as the exhaust gasses are discharged via ports at the side of the cylinder, rather than via valves at the top of the cylinder.
Slow Speed Marine Two Stroke Engine (exhaust gas valve highlighted)
The exhaust valve must be able to withstand the high temperatures and pressures at which it operates, and it must be able to open and close quickly; this will allow for efficient engine operations whilst ensuring maximum reliability.
Marine Two Stroke Engine Exhaust Gas Valve Position
Main components and Construction
The main components of marine two stroke engine exhaust valves are listed below.
Valve Body
The valve body is typically made from stainless steel or other steel alloys. The valve body acts as the main housing; it has an internal channel to allow exhaust gases to flow from the combustion chamber to the exhaust manifold when the valve is open. The valve body also has, incorporated within the valve body, additional channels which allow for the flow of cooling water.
Marine Two Stroke Engine Exhaust Gas Valve Cross Section
Valve Guide
The valve guide is mounted inside the valve body. The valve guide seals the exhaust gas chamber from the valve actuation mechanism and keeps the valve stem oriented vertically. Typically made from ‘pearlite’ cast iron the air supplied to the air spring contains a small quantity of lubrication, which passes down through the exhaust guide to assist with cooling and lubrication, whilst also preventing exhaust gases passing up through the guide.
Valve Seat or Bottom Piece
The valve seat (bottom seat) sits at the base of the valve body. The valve seat presses against the valve body and provides a surface for the valve disc to seal against. It has a machined conical shape that has been optimised for sealing and heat transfer. Typically, it has a ‘Stellite’ coating (cobalt-chromium alloy) due to the seat being prone to corrosion and erosion.
Air Spring and Piston
Control air, typically at 7 bar pressure, is supplied to an air piston via a non-return valve. The control air is used to create an ‘air spring’. When the hydraulic pressure collapses/reduces due to the profile of the cam entering the ‘dwell period’, the pressure of the compressed air overcomes the hydraulic pressure causing the return of the exhaust valve to a closed position. Any excess lubricating oil that collects at the bottom of the air spring cylinder is drained to a collecting tank. The air spring design has replaced the use of a mechanical spring design in large two stroke marine diesel engines; it also provides a cushioning effect when the exhaust valve is actuated.
Slow Speed Marine Diesel Engine
Oil Actuator and Piston
A hydraulic oil pump, which is either operated by a camshaft or an electronic control system, provides the hydraulic pressure to a piston at the top of the valve stem. The pump controls the opening and closing of the valve which in turn compresses the air within the air spring.
Valve Spindle
The valve spindle is usually made from heat resistant, high-strength alloy steel (such as molybdenum chrome alloy), and is fitted with a valve rotator. Rotating the valve helps to distribute heat evenly and prevent deposits from accumulating on the valve seat. The valve rotator utilises the kinetic energy from the exhaust gases to rotate the valve spindle and disk i.e. as the gas flows over the rotator, a force is imparted on the rotator, which causes the spindle and disc to rotate.
Exhaust Gas Valve Rotator
Operation
The operation of an exhaust gas valve is described below.
Hydraulic Pressure Increases
The hydraulic pump increases the oil pressure to the operating piston; this is achieved due to the profile of the cam pressing against the pump’s follower as the camshaft rotates.
Marine Two Stroke Exhaust Gas Valve Control System
Valve Opening
The increasing hydraulic pressure results in the exhaust valve opening. At the same time, the air spring is compressed due the use of a non-return valve on the control air supply. Opening of the exhaust gas valve allows the high-pressure exhaust gases to be discharged from the combustion space to the channel within the valve body. The discharge of exhaust gases occurs at the start of the exhaust stage, or, approximately 110o after top dead centre (depending upon the engine design).
Valve Closing
To close the valve, the hydraulic pressure is reduced, which leads to pressure being exerted by the air spring instead. Pressure from the air spring results in the exhaust gas valve closing (the disc presses against the seat), which reseals the combustion chamber and provides a gas tight seal. Valve closure is assisted by the pressure in the engine cylinder as the piston travels towards top dead centre (TDC); this occurs at the end of the scavenging stage and the beginning of the compression stage i.e. at approximately 110o before top dead centre (depending upon the engine design). Fast closure prevents the fresh air charge from escaping the cylinder during the compression stage.
Maintenance and Failures
The aim of exhaust valve maintenance is to prevent the build-up of deposits and ensure the valve’s correct operation when in service. Poor maintenance of the exhaust gas valve can lead to several issues, including:
- Disrupting the flow of exhaust gases leaving the cylinder, thus reducing the air available for combustion on the next stroke, which reduces the engine’s power rating.
- Disrupting heat transfer in the valve body, thus creating hotspots.
- Increasing the build-up of particulate matter, which can increase corrosion and prevent valve closure and/or rotation.
A typical valve failure mode is cracking of the thinner tapered area of the valve head. To avoid these kinds of problems, scheduled maintenance and the removal of deposits should occur.
Prevention
The following measures can be undertaken to reduce the likelihood of exhaust gas valve failures:
- Perform running and regular low interval engine checks and maintenance.
- Check the sealing oil level indicator regularly.
- Check the valve lift and rotation indicator regularly.
- Inspect the tell-tale holes regularly.
- Listen for abnormal sounds/noises.
- Check for any excess vibration.
- Analyse exhaust gas temperature trends to identify anomalies.
Additional Resources:
https://www.marinesite.info/2021/06/exhaust-valve-in-marine-diesel-engine.html
https://www.marineinsight.com/videos/watch-ships-main-engine-exhaust-valve-working/