Gas torque vs Diesel torque

It would be interesting to know how much a long flexible propshaft absorbs/damps out these torsional vibrations.

 

That is roughly the direction that diesel engines have been evolving in the last decade or so; lower static compression and more external boost. Some of the very high-output engines, like some form MTU, for example, are as much "turbomachines" as they are a piston engine.

 

The flywheel-mounted flexible couplings used on modern high-speed diesels are quite elaborate and their elastic elements very carefully selected and taylored for each installation based on an end-to-end drive line torsional dynamics analysis. Getting that coupling right is at least as important for the health and longevity of the engine internals, like the crankshaft, as it is to minimize any external vibrations.

 

Depending on the masses and stiffness of the propshaft, transmission, etc the "flexibility" of the propshaft may decrease or magnify the torsional vibrations. As BMcF mentions the drivetrain from engine to propeller needs to be considered as a system.

 

Thanks, I suppose resonance is an issue. Come to think of it, I've seen some wear maps where an engine showed significantly increased wear at certain loads/rpms, even when that operation point was allowed by the manufacturer.

 

To get the boost (pressurized intake stroke) the engine must force exhaust gasses out of the cylinder at a pressure considerably above the exhaust pressure for a non-turbocharged engine. So in this case, the intake stroke can give some positive work to the cycle, but the exhaust stroke then becomes a much larger parasitic load. You cannot get that boosted inlet without spending somewhat more energy on the exhaust stroke to drive the turbo. So in the end you would still have the similar shuffling of work around in the four stroke cycles, and the Diesel torque variations are again larger than for a SI engine.

Diesels could have a lower compression ratio on the pistons because of the turbo boost, but then the cycle becomes less efficient. Turbo boosting generally just increases the mass flow of combustion air through the engine cycle, so a turbo engine can give more power with smaller piston displacement.

 

its a whole system
I have a buddy that does the measurement and analysis for a living, what he really does is fix cockups, seems to be a life long job and luckily ship yards keep doing the same mistakes over and over..money for old rope..
Just like the designers that designed the wobbly bridge in London, forgot to check what happens at 1 hz.......well thats walking and its a footbridge DOH
http://en.wikipedia.org/wiki/Millennium_Bridge,_London

 

It could very well be the uneven nature of the torque from a diesel. This could be mitigated somewhat with more flywheel inertia, but perhaps not practically.

It could also be the rule of thumb expectation that a gasoline engine is recreational and therefore gets fewer hours per year of ownership, and also runs far less often at maximum torque. As a rather extreme example- A gearbox in a bowrider rated for 500 ft-lb will have a life quite different from one in a lobster boat with 5x the displacement.