Expected Engine Life
How long will a marine diesel engine
The following information is extracted from a letter written to a commercial marine insurance company in response to the question "How long will a marine engine last?"
Before I try to answer this, I should say that it is difficult to provide even generalized answers. There are many variables involved.
Engine Design Life
Engines have a "design life". They typically are intended to provide a certain minimum number of hours in a given application. These applications range from heavy duty, continuous service to light duty automotive. Even within a single manufacturer's model line you find engine series with differing design lives. Some may be designed with light weight in mind for mobile or automotive uses. Often these engines will not have cylinder liners or other components found in engines designed for more demanding, longer term service.
In our fishing applications, both propulsion and auxiliary engines might be expected to operate for 20,000 hours without major overhaul in "typical" usage--if "typical" exists. Overloading or poor maintenance practices will radically reduce this service life. A good combination of loading and maintenance can increase the service life beyond this number. We have many John Deere and Kubota engines in service with in excess of 30,000 hours without major overhaul.
Some applications such as crabbing, prawning, trolling etc. tend to cause a lot of carbon build-up in modern engines. (Some naturally aspirated, low compression, low B.M.E.P. engines tolerate light loads very well.) These services may require engine work earlier just to remove carbon built up in the head(s) and on the piston crowns. We have worked on some vessels where even at full throttle, in gear and tied to a dock, we were unable to get the exhaust stack temperature up to a point where we could prevent carbon build-up. This is not too unusual and these vessels are lucky to get 15,000 hours between decarbonizing treatments.
As to what makes an engine last or not last, there are again a number of determining factors. In my mind, two conditions stand out. The first is that a diesel engine needs to be properly sized in the first place. Either too much or too little load will shorten the engine's life. How much it is shortened will depend on the degree of severity of the over or under sizing.
The other aspect that comes immediately to mind is maintenance. No engine will survive if it is not properly maintained. One instance of overheating can ruin an engine whether it has 30 or 30,000 hours. Failure to maintain the cooling system is probably a more common fault than failing to change oil and filters. Most of the engines that we repair in the field or rebuild in our shops require service as a result of neglect, abuse or "circumstance" such as poor quality fuel, broken piping, failed coolers etc.
Engine r.p.m. is another determining factor in engine life. Typically the higher the speed, the shorter the engine's life will be. Higher speed vessels such as planing and semi-planing hulls usually need a higher speed, lighter weight engine to get satisfactory hull performance. In these cases the engine manufacturer often requires that the engine r.p.m. be reduced once the vessel is on plane. If the speed is not reduced the engine's life will be shortened, often quite radically. My guess is that engine life in planing hulls is more like 12,000 to 15,000 hours given good operating practice, proper sizing and good maintenance. A lot of gillnetters fall into this category.
Overhauling an engine
An "overhaul" brought about by hours would consist of piston assemblies, liners, cylinder head repairs as required, usually main and rod bearings and the overhaul of all sub-assemblies such as turbocharger, pumps, cooler(s), electrics and injection. We would expect 2/3 to 3/4 of the original engine service life under the same operating conditions.
As you can see, there is no simple answer to this question.