Marine Engines, Diesel vs Petrol reliability

[Ianfs]

When I was a lad, big lorries/trucks in the UK and Europe, used diesel engines because of their long life, torque and diesel was cheaper. At this time, I really didn't know why they lasted longer, until I found myself in the Motor Trade. I was told that Diesel engines rev lower than petrol engines and had lots of gears to compensate, therefore they would go for several hundred thousand miles before a major rebuild. Whereas a car engine (petrol) would rev to say 4,000 rpm against a big Diesel at 2,000 rpm . Maybe its true, but it seems logical.

Marine Diesel and petrol engines seem to be basically lorry/truck engines which were marinised.

I've noticed over the years that Diesel engines have developed to quieter and even more economical units than petrol derivatives, probably because more and more car manufacturers are developing them. Except that in the last 2 years 2013-2015, some petrol engines, seem to have out striped diesel economy.

In the rush to keep up over the last 25 years, manufacturers have added items to their reliable engines to make them more powerful and to help them rev, in order that a smaller unit might work. I've noticed that Volvo for example produced the KAD 32 with both a Supercharger AND a Turbo Charger. This engine has turned out to be burst proof, or have we not seen any of the MAJOR issues yet. However there is lots of chat around the world about other marine diesel engines including Volvo which once reaching their hours are to be thrown away.

In the larger boats, most diesel engines run at around 2,000 - 2,500 ish rpm and there is some boost from the Turbo unit , so I would not expect much power wear to happen.

In the US a lot of their boats at 40' etc, still have twin petrol V8's, which when petrol was cheaper was not an issue.

Therefore, with costs and repair. My view is that at this stage a lot depends upon the engine build, sustainability and what the boat will be used for, which will focus on the engine life? So my question is, do any of you think that Petrol engines now more reliable than diesels?

Does anyone else have any thoughts on this?

[_Ed_]

Diesel is a natural lubricant, one of the major factors in why they last so much longer due to much lower rate of bore / ring wear.

Petrol will never be as efficient. Petrol engines due to the nature of how petrol burns require a controlled air to fuel ratio of around 14:1, this means at light throttle, when the intake valves open there is mostly a vacuum in the cylinders. Imagine trying to pull a piston down a bore with a sealed top, its hard as air pressure on the lower side of the piston is trying to push it up. This is known as pumping losses.

I'd probably say however that petrol engines now live longer (generally) as fuel and oils are so much cleaner, and diesel engines with the very very high pressure common rail injection systems now last shorter (also due to higher loading on engine components) but I do think diesel still has the edge for reliability over all.

Modern diesel repairs are however now very expensive if injector pumps/injectors fail, where by comparison petrol repairs are pretty cheap.

[Ianfs]

Interesting Ed, thanks.

Modern diesel repairs are however now very expensive if injector pumps/injectors fail, where by comparison petrol repairs are pretty cheap.

Which would explain why we hear more horror stories about diesel engines. Plus turbos , I think you mentioned once when my turbo went on the Land Rover, they have a shelf life.

[_Ed_]

Absolutely. Turbos have to work much harder on a boat. They are always under load. If you get 120K miles from a road engine turbo your doing ok. Not sure how long you'd expect them to last on a boat.