Diesel Marine Engines - Everything You Need To Know

Marine diesel engines are very different from their counterparts. This article covers the most common questions about marine engines.

How do marine diesel engines work?

Marine diesel engines are exclusively designed for use on the water – there are two types of marine engine:

Inboard – the engine is enclosed in the hull (we will insert the picture below)

Outboard – the engine is mounted outside the hull (we will insert the picture below)

Marine diesel engines are available in 2-stroke or 4-stroke.

2-Stroke Engine

The 2-stroke is a low speed engine, generally used for powering larger vessels such as ocean going ships, locomotives and standby power plants. The piston’s initial position is at bottom dead centre, air then enters through ports in the cylinder walls (the air can either come from a mechanically driven blower or a turbo compressor) charging the cylinder (scavenging describes how air is used in the process of forcing out any remaining combustion from the last power stroke) the piston rises and fuel is injected.

Exhaust and intake – Compression (1st stroke) – Power output (2nd stroke)

4-Stroke Engine

4-stroke engines are primarily used in cars. In the 4 stroke engine, the piston completes 4 separate strokes whilst turning the crankshaft.

Intake (1st stroke) – Compression (2nd stroke) – Combustion (3rd stroke) – Exhaust (4th stroke)

What makes a marine diesel engine different from a car diesel engine?

Price

The first noticeable difference is in price, the automotive diesel engine is considerably cheaper, there are however good reasons for this and a car engine should never be used in the place of a marine engine.

The overall duty cycle of a marine diesel engine is very different to that of a car. Marine conditions are much harsher than those on land – boats are surrounded by water and subjected to salt corrosion, leakages as well as strong vibrations and shocks in rough weather. There are a number of important modifications between the two.

Components

Many key components of the marine diesel engine such as core or freeze plugs (as they are also known as), the camshaft and drive shaft are made from corrosion resistant metals – an example being bronze or brass. Gaskets are also modified with better seals and are of a superior quality to help prevent water damage.

Starters, alternators and distributors have to withstand higher RPMs than the traditional car engine; an automotive engine reduces to lower RPM at top speeds using less of its horsepower to sustain its speed, whereas the marine engine works with sustained speeds and is always under load. The torque speed curve of a car engine will not meet the needs of a boat.

Starters, alternators and distributors also have to be ignition protected – fitted with special screens to protect against internal sparks. In a car, sparks do not cause problems due to the constant draft under the hood. The hull of a boat however, is more confined and flammable vapours can accumulate.

Valve Overlap

The valve overlap time is much shorter than that of an automotive engine, this is to help prevent water from entering the combustion chamber.

Usage

Marine engines can also spend longer periods of time inactive, whereas a car engine is generally used more regularly – keeping things moving helps prevent problems.

How Long Does A Marine Diesel Engine Last?

Engines in general like to run long and steady, and as previously noted, marine engines work much harder and under more stressful conditions than automotive engines – leaving them more prone to damage from wear and tear. Commercial boats can rack up hours upon hours of log time and suffer more complications as a result of this; whereas recreational boats may not endure the heavy usage, they will inevitably suffer from being stored away for extended periods of time.

How do I maintain a marine engine?

Regular and thorough maintenance is essential for all marine engines – it is the key to extended life expectancy, helping your boat to last considerably longer. Repairs are not only inconvenient and time consuming, but also extremely costly.

Marine engines are constantly fighting against the effects of corrosion. Salt water crystals form easily causing a number of problems. Engines should be visually inspected regularly – and everything wiped down.

Clean the Fuel System

The fuel system must be kept clean, this includes cleaning the filters – all of which must be checked regularly and cleaned to remove any unwanted sediments and salt crystals – and replaced where necessary. Fuel injectors must also be cleaned, read our guide here.

The fuel filter is especially important, it can be drained by loosening a plug at the bottom – both the primary and secondary fuel filter should be checked frequently.

Oil

Oil must be checked on a daily basis – any changes in colour and consistency may indicate contamination. Using high quality oil is an essential part of maintenance.

Cooling System

All aspects of the cooling system including the impeller, raw water pump, exhaust pump etc. should be inspected regularly for any signs of debris or corrosion.

Air Filters

Clean air is essential to the combustion chamber, marine engineers stipulate that engine compartments should be supplied by plenty of cool fresh air – lack of fresh air in the engine compartment is a common problem for marine diesel engines.

Transmission Fluid

This should also be checked regularly, a change in colour usually indicates a problem.

For more information on marine engine maintenance see our marine engine maintenance guide here.