It was true, long ago, when diesel engines were large, heavy, slow turning, and low powered. The idea that diesel engine can have an expected life expectancy of many thousands hours is patently untrue today. The average life expectancy of a marine diesel engine in a pleasurecraft is somewhere around 1500 hours between major overhauls. Double that if used frequently or in a non-continuous use commercial application (like we would be doing). If this surprises you, it may surprise you even more when I tell you that gas engines average around 900 hours before overhauls.
Diesel engines gained the reputation for long service life early on in the history of the engines, and mainly from engines that were used in commercial operations. These were big, very slow turning engines, usually in the 600 to 1,000 RPM range. Obviously, not only is an engine running at that speed turning four times slower than an engine at 2400 RPM, but the heat generated and the strain is a great deal less.
Then we hear about how long truck engines last. Well, there's a whole lot of difference between a load that is rolling on wheels and a hull that is pushing tons and tons of water aside. Even though truck diesels turn faster, they have a lot less load on them, so they also last longer, mainly due to continuous operation. Then there are companies like Stuart-Stephenson who take stock diesel engines and micro tune them by essentially remanufacturing them to extraordinary tolerances, for use in such applications as power generators. These engines can have service lives as long as 60 - 80,000 hours. But that again is because they are operating continuously under constant loads.
The long service life of the diesel is not a myth when used in its proper application; its only a myth in boats where engines are operated intermittently, infrequently, at high and low speeds, and usually under very heavy loads and adverse conditions.
There is a direct relationship between service life and the weight of engine blocks and cylinder heads. The heavier, or thicker the castings, the longer they will last. That's one of the reasons why older engines just seem to go and go, while we often refer to the engines of recent years as "throwaways."
The problem with light castings is a problem of both strength and heat distribution. Diesel engines, with their 350-550 lb. internal cylinder compression, develop tremendous heat within the cylinders and heads. When castings heat up they expand, and when casting thickness are unequal, this can lead to cracking. It follows then, that the thinner the casting, the weaker it is, and therefore more prone to heat distortion and cracking.
This has been one of the major problems of trying to adapt light weight automotive engines to marine use. Because the loads are much greater, more heat is generated, and therefore more distortion of the castings occur. And when distortion occurs, the close tolerances of the moving internal parts such as crankshaft, bearings and journals, rods, pistons and cylinder walls goes out of whack. The end result is an early demise of the engine. Therefore the move to adapt high speed, light weight small truck engines to marine use results in an engine with a decidedly shorter service life. One of the most common problems that we see with light engines is the frequent cracking of cylinder heads, which is the first place that designers seek to reduce weight.
Another reason why high performance, light weight diesels don't last long is related to over-fueling. When you take an automotive engine that doesn't require as much power to push its load, and increase its power output, you do so by increasing the amount of fuel and air. This not only creates much more heat, but it has yet another side-effect: the increased fuel injected into the cylinder washes away the lubricating oil on the cylinder walls. This is true of both gas and diesel engines. The primary cause of all high performance engine failure is related to the pistons. This is closely followed by failures in the valve train, which is greatly stressed by increased heat and stress. To overcome these problems, these automotive engine systems must be completely redesigned. Unfortunately, they often are not.
Why not? You have to understand that the marine engine market is a rather limited market that doesn't generate the kind of revenues that the automotive market does. Over the years, this has been a universal problem for marine engines of all kinds, namely that the marine conversions simply don't go far enough to account for the differing service loads. And with the push to produce more efficient and clean-running diesels, the problem of marine conversions promises only to get worse, not better.
Internal displacement is the best measure of an engine's ability to deliver power efficiently. And the ratio of CID to horse power (divide CID by engine HP) is the simplest measure of how much service life can be expected. The inviolable rule for service life is that the more power is squeezed from an engine block, the shorter it's life span. A 350 CID block generating 260 HP is going to last a whole lot longer than the same block putting out 350 HP, whether its gas or diesel. That's why the old 6-71 Detroit Diesel will run darn near forever at 265 HP from 465 CID, but self-destruct in 6-800 hours at 450 HP. A ratio of 1:2 is about ideal for a marine engine, but at 80% to 90% at least yield reasonable service life of 2,000 hours if well maintained. At 1:1 and above it should be considered a high performance engine with a very short service life indeed. If you have an engine with an 0.70 CID/HP ratio, then you can expect 3000 hours engine life. Otherwise, you are a victim of the myth.
Yet other factors come into play, engine speed and weight. There is no escape from the fact that fast turning diesels have substantially shorter life spans. Slow speed diesels can be longer lived precisely because they do turn much slower. But when you soup them up, that advantage is lost, for a variety of reasons. Diesel engines running at 3200 to 3600 RPM are lightweight automotive engines for which good service life should not be expected in marine applications. A vessel in water and a vehicle on wheels are two entirely different load situations. The light weight diesel was not designed to push heavy vessel loads any more than the gas engine was.
Also, new EPA rules are having a major impact on diesel engines. The mandate to make them lighter, more fuel efficient and cleaner is going to translate into engines that are vastly less reliable. Why? Because they're going to start cutting out all that necessary extra cast iron, and in many areas start replacing it with cast aluminum. The marine engine industry tried cast aluminum once before back in the late 1960's; it didn't work then and won't work now, never mind all the smoke they'll put out about "technological advances." The German transmission manufacturer Z-F has tried making cast aluminum gear boxes, a foray into the future that has blown up in their faces along with their gearboxes. Aluminum is simply too weak to handle these kinds of loads, too heat sensitive, and unresisting to corrosion to be an acceptable substitute. Now that these new rules are going into effect, I predict a healthy future for remanufacturing old engines.