diesel-turbo needed or not

[eastern270]

a planing or semi planing boat which requires about 50HP to reach a top speed of about 18mph. Alot of diesel engines in this HP range seem not to be turbocharged until you start to get up around 80HP and above. Does the diesel engine need the turbo to be able to push the boat to this speed or will it just take longer to reach the speed. I ask because it seems very difficult to find new marine gas engines any lower than probably about 200HP and why buy the wasted power.

[Dan McCosh]

Turboharging will produce more power from a given size engine. That means that for a give horsepower, the engine will be smaller and lighter. This could be important in a planing hull--other than that, horsepower and torque are not directly related to whether an engine is torbocharged or not.

[nedL]

I know that this doen't really answer your question, but it might be useful for you. I believe a turbocharger does little for an engine that is in light duty service. The turbocharger is pretty much 'going along for the ride' so to speak & not contributing much to the hp in the lower RPM range under light load. When the engine is put under a load in the upper RPM ranges (& the exhaust gasses increase exponentially), then the turbocharger becomes effective. So saying, a boat that is easily moved along & used at slower speeds will not benefit as much from a turbocharger. The benefit will be seen in higher speed boats and boats that load their enignes down (commercial).

Oh yes, I suspect one reason for not seeing small diesels with turbochargers is that they are pretty expensive (smaller ones wouldn't be much cheaper - same # of parts), and it would add disproportionally to the price tag. (Add something like $1500 (guessing) to a $20K engine & it isn't so bad, but add the same $1500 to a $5000 engine and it makes you thing.

[ 02-02-2005, 12:43 PM: Message edited by: nedL ]

[eastern270]

what i am trying to find out is that if a design that calls for about 50HP gas to get a speed of about 18mph will be able to perform the same way with a 50HP diesel without a turbocharger. Here's an example of what i fear. My brother repowered a 24 ft searay with a 230HP turbocharged yanmar diesel. starting out from idle the boat will move along a couple of knots without the need of the turbo. Now to give it some gas and get up on plane the boat slowly gains speed for a few seconds and then the exhaust gets the turbo going and the boat will then take off like a bat out hell. It seems to me that without the turbo it would either take a very long time to get on plane if it would get on plane at all. So that was my question for anyone to answer if they know if the turbo is needed to reach the designed speed or will it just take much longer without it. I hope this explains the point i'm trying to get at.
Thanks

[Krueg]

A 50 hp diesel is going to be a lot heavier than a 50 hp gas engine. I've got a Perkins 4108 in my boat, right around 50 hp (less at normal RPMs). This engine weighs about as much as a small block V8. Though, I guess the new yanmars and such are probably considerably lighter.

[Dan Lindberg]

What Ned said. (mostly)
Turbo contribution is (mainly) a function of load (heat)(and turbo matching).

Part of what your seeing with your brothers boat is turbo lag, and part (my assumption) torque curve/load matching.

Turbo lag is the time it takes a turbo to get up to speed and generate pressure as/after the exhaust gas gets hot. Turbo matching is a trade-off between lag and when pressure is developed vs needed. Spin it too quick and too much pressure is developed, to slow and there is lag. Larger turbos have more lag but move more air, smaller turbos spin quicker but have to spin faster to push the same air. Wastegated turbos are one way to generate pressure quicker without too much pressure (some air is dumped, either exhaust or compressed air).

An engine w/o a turbo obviously won't have any turbo lag, but still could have a delay getting on plane if the torque curve is pooring matched to the boat/prop.

Dan

[Noah]

I really can't imagine that Turbo Lag plays any part of this...In an auto engine you are constantly changing the amount of throttle, and the rpm of the engine. True it can take a bit to spool up a turbo (I run a turbo making 21 PSI of boost in my auto) but you aren't going to have this spool up lag time in a marine setting. Lag time in an auto of more than 1-2 seconds is noticeable (and rarely is it longer than that), but in a boat I just can't see it making a difference. Furthermore you aren't going to have the same boost control issues. In a car you need a bypass or blow-off valve because when you slam the throttle shut when making full boost it creates backpressure that can damage the vanes of the turbo. Because marine engines aren't seeing the same very rapid throttle changes they have less of a problem with this. I would imagine that they have boost control valves, which simply change the amount of boost that the engine is seeing given different amounts of throttle. I don't know how sophisticated these systems are on marine diesels...

Every turbo and matching engine are going to have an RPM band where they put out full power. It could be that the turbo is designed to make full power at 2000-3000 rpm. Below that you are probably going to get below the rated horsepower and torque of the engine. Simply put the power band on a Turbo diesel may be smaller and peakier than with a non-aspirated diesel. 50 hp should be the same in a turbo engine and a non-turbo engine. The difference is the non-turbo is making this power from 1000-3000 rpm, while the turbo is doing it from only 1500-3000. Each engine will be different. I do want to say that turbos do work at low RPM. In my car engine it's putting out full boost at 1700 rpm (low for a car) and the engine is making roughly 270lbft of torque at that rpm.

One other advantage of turbos is that when they/you are off the gas they are actually pulling a vacuum making the engine more efficient. (Though I don't know if the same holds true for diesels).

So anyway...

I'm sure that you can get torque and horsepower curves for both the diesel and non diesel engines and compare. My guess is that for 50hp, the diesel will weigh less and use less fuel in average running conditions.

And your brother's boat has that great feeling of power because the amount of difference a turbo can make at full boost can be pretty fun.

[Dan Lindberg]

Noah,

"I run a turbo making 21 PSI of boost in my auto"
This sounds like fun. smile.gif

Back in the "old days" the turbo'ed cars from the manufacturers ran about 7 psi, and on the diesel engine I worked we ran about 15 psi, 'course that was a few years ago but...

Yes, I didn't mention anything about backpressure control, but I suspect you're right, probably not needed in a boat.

The wastegated turbos we had (Holset and AiResearch) had a valve that dumped exhaust gas to control manifold pressure. The T04 and T25 at this link are what we ran. http://www.majesticturbo.com/garrett.html

As for the lag, I suspect that diesels will have more lag then you're used to in your car, just cause they tend to use larger turbos, but yes, the lag shouldn't last longer then "seconds", though I think it could be more then 2-3, especially if the engine is cold.

As for the torque curve, again, on the engine I worked on (a Cummins model no longer available) the torque curve was wider and higher with the turbo then w/o it.

Dan