Light to Medium Duty

[ian38_39]

Thought I would post this as new, rather than causing further drift on another thread. On the how old is your engine topic LDA and medium range engines have been mentioned and duty cycles of 25000 hrs to overhaul are being quoted.

Where does light duty end (not necessarily Automotive) and medium range cut in? Please feel free to correct as it is not first hand knowledge but aren't KAD 42a's purely AD41's with bits bolted on yet early versions had major issues due to higher stress levels. These are not LDA, as far as I am aware, but commercial vehicle engines, so why does 25000 hrs sound like a pipe dream and 3000 hrs seem to be doing well on even well maintained leisure boats?

Are there suitable engines for a say 40' fast planing boat that will give the duty cycles or are the all just too big and too heavy?

 

[PCUK]

Duty cycles are designed to ensure that engines have the same sort of life-span regardless of their use. So a heavy duty engine will be highly de-rated in output to provide maximum for 24 hour a day use.
Medium duty is de-rated less for say ten hours per day use and light duty is maximum output for no more than one hour in eight and say 250 hours use per year.

 

[ian38_39]

Makes sense I suppose, so what you are saying is they are made for 10 years life rather than 25000 hrs?

 

[MapisM]

Where does light duty end (not necessarily Automotive) and medium range cut in?

Light and medium are hardly engineering concepts.
I'm not aware of any standards on the matter of engine ratings, in fact each manufacturer has its own definitions.
But out of those I've seen, imho Caterpillar has the clearest ranking system - you can find it here, if you're interested to have a look.

 

[ian38_39]

Interesting reading and sort of blows away the common conception that diesel engines are better if they are run at their optimum load all the time.

 

[PCUK]

Well Ian,
The optimum load is going to vary depending on the duty cycle so it should work out that way, although in practice a heavy duty commercially rated will last many years and thousands of hours while a sports boat light duty engine will apparently not last very long. I think lack of use is probably another factor not generally considered in duty cycles.