In a conventional marine diesel engine the power is produced by hot compressed air igniting fuel sprayed under very high pressure into the cylinder head. A marine diesel engine does not use a carburetor to mix fuel and air or spark plugs to ignite the mixture. Instead it employs the pistons to compress the air to 3000 kPa which causes it to become extremely hot and the fuel is ignited as soon as it is injected into the cylinder.
Some marine diesel engines are fitted with a heater plug in the inlet manifold or a glow plug in the pre-combustion chamber of each cylinder to provide additional heat to the combustion air during starting.
Diesel engines are heavier and slower revving than petrol engines but they are also more reliable because they do not rely on external carburetion or an electrical spark for ignition.
Newer engines use an electronic fuel injection system whereby fuel and air are mixed more thoroughly in the pre-combustion chamber before entering the cylinder. This system maximizes power and fuel economy and is also less polluting.
Every boater should have an understanding of how their engine works so let's start by explaining the mechanical cycles.
Most reciprocating piston internal combustion engines work on one of two mechanical cycles?either the four-stroke cycle or the two-stroke cycle. These cycles designate, in correct sequence, the mechanical actions by which the fuel and air gain access to the engine cylinder, the gas pressure ? due to combustion ? is converted to power and, finally, the burnt gas is expelled from the engine cylinder.
The Basic Four-Stroke Diesel Engine
From its name, it is obvious there are four strokes in one complete engine cycle. A stroke is the movement of the piston through the full length of the cylinder and ? since one such movement causes the crankshaft to rotate half a turn ? it follows that there are two crankshaft revolutions in one complete engine cycle.
The four strokes in the order they occur are:
1. Inlet stroke. With the inlet valve open and the exhaust valve closed, the piston moves from top dead center (TDC) to bottom dead center (BDC), creating a low-pressure area in the cylinder. Clean, filtered air rushes through the open inlet valve to relieve this low-pressure area, and the cylinder fills with air.
2. Compression stroke. With both valves closed, the piston moves from BDC to TDC, compressing the air. During this stroke the air becomes heated to a temperature sufficiently high to ignite the fuel.
3. Power stroke. At approximately TDC, the fuel is injected, or sprayed, into the hot, compressed air, where it ignites, burns and expands. Both valves remain closed, and the pressure acts on the piston crown, forcing it down the cylinder from TDC to BDC.
4. Exhaust stroke. At approximately BDC the exhaust valve opens and the piston starts to move from BDC to TDC, driving the burnt gas out of the cylinder through the open exhaust valve.
The Two-Stroke Diesel Engine
The two-stroke engine uses two piston strokes to complete one power stroke and, therefore, fire twice as often as a four-stroke engine. A two-stroke engine is smaller and simpler with fewer moving parts. A two-stroke engine has the potential to produce twice as much power as a four-stroke engine of the same size, however, because of the extra fitting required in a two-stroke diesel engine, for example blowers and governors, they become more expensive to produce. There has been a shift towards four stroke diesel engines which have become more efficient and smaller.
Protect Your Marine Diesel Engine
Protect your engine by avoiding long periods (more than 10 minutes) of idling in a ?no-load? situation. This is often done to charge batteries or cool refrigeration but if done repeatedly it will glaze the bores of the engine and cause premature engine failure. If the vessel is in a berth the engine can be put in gear to create load at idle.
All boat owners should have an understanding of basic marine diesel engine maintenance to keep themselves and their families safe on the water.
Marine Diesel Engine Maintenance
It's hard being a boat engine! Unlike its automotive cousins, a boat engine is run at extremely high RPM's and under quite a load when in operation and it sits in storage a whole lot of the time. It's kind of the worst of both worlds. Today's marine engines are very well made and unlike their predecessors, really experience very few mechanical problems if they are properly maintained.
Water Pump Maintenance - Most marine engines are cooled by their pumping of lake or ocean water into the engine from a pickup in the lower unit of the outdrive or outboard engine. This water is circulated by a water pump that contains a rubber or plastic impeller or fan which pulls the water from the lake and pumps it up and through the water jacket of the engine to keep things cool. As you might expect, there are sometimes impurities in the water or the operator (somebody else, I'm sure) that runs the lower unit aground and the impeller picks up sand, dirt or other grit. These foreign substances wear on the impeller and sometimes cause it to shred into pieces and fail. Also, if the engine is stored for a period of many months, sometimes the rubber of the impeller gets brittle and cracks up. In any event, it is simply a good idea to proactively replace the impeller every 3-4 boating seasons. If the impeller fails while you are running and you fail to notice the temperature rising, your engine can easily and quickly overheat and self destruct.
Oil Change ? Marine engines are typically not run more than 60-80 hours per year and, therefore, do not require oil changes very frequently. Usually, it is a good idea to change the oil (and filter) once per year at the end of the season. If the old, dirty oil is in the crankcase when the engine is stored in the off season, it can turn acid and damage the internal engine components it is supposed to protect. Of course, 2 stroke outboards have no crankcase and therefore no oil to change. On these applications, it certainly does pay to stabilize any fuel remaining in the tank and to fog the engine with fogging oil before storage.
Fuel Injectors ? Most newer marine engines are fuel injected and, when fuel is permitted to age and thicken during storage, the fuel injectors can easily become clogged and may fail at the start of the season. To avoid this occurrence, it is a good idea to run some fuel injector cleaner mixed into the last tank of fuel before the engine is put up for storage.
Battery ? If you take care of your boat's battery, it will provide you with several years of good service. You should take care whenever you complete a voyage to ensure that all electrical components are turned off and, if you have a main battery switch, be certain that it is turned off. Whenever the boat is stored for any prolonged period of time, the battery cables should be disconnected.
Lower Unit Lubrication ? The lower part of your outdrive or outboard engine is filled with a lubricant fluid that keeps all of the moving parts properly lubricated and running smoothly. The reservoir should never contain any water in the fluid. The drive should be inspected at least annually to ensure that the drive is full of fluid and that no water is present. This is relatively simple and inexpensive to accomplish.
Electronic Control Module ? Most modern marine engines are controlled by a computer call an ?Electronic Control Module? (ECM) which regulates the flow of fuel and air as well as the timing of the ignition system. Another valuable function of the ECM is that it stores operational data while the engine is running. Certified marine mechanics have digital diagnostic tools which can be connected to the ECM to learn the running history of the engines as well as any problems.
Anodes> On the underwater section of every outdrive and outboard engine, there are one or more little metal attachments called ?anodes?. They are usually made of zinc and are designed to attract stray electrolysis. This occurs when stray voltage in the electric system of a boat is transmitted through the metal parts of the boat in search of a ground. The anodes are designed to be sacrificial and to absorb the stray current and gradually deteriorate. This process is magnified in salt water. At least once a year, you should check your anodes for decay and replace those that appear to have decayed greatly. Replacement anodes are not tremendously expensive and they serve to protect your boat from some serious decay of some very expensive metal marine parts.
If a marine engine is properly maintained, it should give you many years of trouble free operation. It should be important to you to know a qualified marine technician in your area. As with most things, ?An ounce of prevention is worth a pound of cure?. Have a great cruise!
Both Chris Miley & Don Seibert are contributors for EditorialToday. The above articles have been edited for relevancy and timeliness. All write-ups, reviews, tips and guides published by EditorialToday.com and its partners or affiliates are for informational purposes only. They should not be used for any legal or any other type of advice. We do not endorse any author, contributor, writer or article posted by our team.
Chris Miley has sinced written about articles on various topics from . Chris Miley is an online author and a keen boater. You can find out more about maintaining your marine diesel engine at his website Marine Diesel Secrets and the. Chris Miley's top article generates over 880 views. Bookmark Chris Miley to your Favourites.
Don Seibert has sinced written about articles on various topics from Yatch Insurance, Finances and Mortgage Insurance. Don Seibert is a veteran of the U S Coast Guard and is a retired marina owner and boat dealer, His websiteMore Free Boating Articles, has many more free. Don Seibert's top article generates over 5400 views. Bookmark Don Seibert to your Favourites.
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