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Minnewaska 05-01-2012 06:33 PM

Turbo charged diesel
 
Why do they bother doing this to a sailboat engine?

I understand the principal of packing more air into a cylinder, and therefore more fuel, to create more power from the same engine. However, it seems we don't really need all that much power. For goodness sakes, my Harley has a much horsepower as my boat. Why can't we get enough HPs out of a normally aspirated engine? My boat motor must be 5 times bigger than the V-twin on the Harley.

I could also swear I read that my turbo doesn't even kick in until 2300 rpm, which I rarely run her at. 2200 rpm is pushing hull speed. The max rpm on my Volvo TAMD31 is 3600 or 3800, can't recall.

So what gives on the need for turbo? Is it the Marine Mechanic Full Employment Act? The stinking thing does suck boat units. I just replaced some burnt cooling fans on a turbo that I can't understand why I have.

klem 05-01-2012 07:19 PM

Re: Turbo charged diesel
 
The basic answer is that with a turbo, you can have a much smaller engine which is advantageous for many reasons. You are correct that turbos force more air into the engine. This then means that you can put more fuel in too. So for the same sized engine, you have more power.

Engines are positive displacement, volumetric devices so without a turbo, they will suck in a known volume of air which has a mass which is determined by the volume and the pressure drop associated with sucking the air in. With a turbo, the volume is the same but the pressure is much greater so the mass flow is greater.

If you consider a positive displacement engine, the closer you expand to atmosphere, the more power you will extract from the gas charge. This is a large portion of the reason why a diesel is more efficient than a gas engine because it has a higher compression ratio. Because of practical design concerns, you can't actually expand to atmosphere so a lot of energy goes out your exhaust. Turbos are thermodynamic devices which use this energy to do something useful.

Turbo sizing is a real science and a turbo will have an optimal operating point. This means that your turbo will spool up more as it gets close to the designed operating point. A lot of people talk about your boost pressure as a way to describe how much a turbo is doing. This is really on a measure of restriction in the intake and how much heat is going out the exhaust but it gives a decent indication of what your turbo is doing. You won't see it in your boat because it is very hard to change the engine load at a constant rpm but if you go in a turbocharged car or truck, mash the pedal and watch the boost climb while the rpm stays the same because you are putting more energy out through the exhaust. Just because a turbo isn't singing doesn't mean that it isn't doing anything. There is no point in having too much air.

The other reason for all of this is emissions. It is very hard to pass emissions without one these days.

Personally, I really like turbocharged diesels. The turbos are amazingly reliable considering the abuse that they take and they have huge benefits. It is not uncommon to have a turbocharged engine have a horsepower rating of 3X compared to the same engine in a naturally aspirated version. If your engine is simply too big, that is a different problem and has nothing to do with the turbo, someone sized it wrong.

Minnewaska 05-01-2012 09:34 PM

Re: Turbo charged diesel
 
Quote:

Originally Posted by klem (Post 865965)
The basic answer is that with a turbo, you can have a much smaller engine which is advantageous for many reasons.

Therein lies my point. It's 100 horsepower and multiple times the physical size of other 100 horsepower engines, like my Harley engine. I'm not getting where the economy was realized. The thing is physically larger than my car's engine, which has more than twice that horsepower. I get the science, but I don't get the practical application. To get 100 horsepower out of a diesel should not take a large motor. That's just not a very powerful engine in the grand scheme of things, despite its relative power in the sailboat world.

Quote:

It is not uncommon to have a turbocharged engine have a horsepower rating of 3X compared to the same engine in a naturally aspirated version. If your engine is simply too big, that is a different problem and has nothing to do with the turbo, someone sized it wrong.
There is no way that my engine is only the physical size of a typical 30 or even 50 horse diesel. It's a monster. I didn't mean the horsepower was too much, only that I'm not seeing the physical size advantage.

shogan50 05-01-2012 09:58 PM

Re: Turbo charged diesel
 
In short reliability and fuel consumption. Try running your Harley at 80% WOT for 10K hours. It would be lucky to make it 500 hours. A properly maintained diesel will commonly go 10K hours, 20 times as long. Fuel burn is the other reason. Your Harley runs at perhaps 20% WOT while cruising and would exhibit about .6-.7 BSFC at that setting. At near WOT it would come down to about .45 but you can't maintain that power setting. Your diesel fuel burn is probably closer to .3-.35 BSFC and can maintain near WOT for thousands of hours. IOW at the same (high) power setting, the diesel is 20-30% more efficient and more like twice as efficient at comparative cruise settings. The turbo increases carnot efficiency and thus decreases BSFC in most cases.

BSFC is Brake Specific Fuel Consumption.

klem 05-01-2012 10:06 PM

Re: Turbo charged diesel
 
I am not familiar with the specific engine in your boat but it sounds like it is a monster physically. There are only 2 ways to get the horsepower up on an engine, spin it faster or put more fuel in. Things like sport bike engines have phenomenal power to size ratios but they also spin over 10,000 rpm. It is possible that you have a very small turbo designed to give you a bit of extra top end or keep within emissions ratings on the top end, this would result in a very large engine with a turbo. Sorry, I just don't know the specific engine.

One of the most popular diesel engines in different applications is the Cummins 6bt of which they offered both naturally aspirated and turbocharged models. The standard naturally aspirated marine version is 120 hp while the turbocharged versions are 250-375 hp (the newer ones with common rail are up to 425). Another example of this might be the Detroit 671 (okay it had a blower but it was really low pressure and it helps with power anyways) which was up to ~190 hp without a turbo and up to ~450 hp with a turbo depending on the year. It really depends on how big of a turbo they put on it and how good of a job they do of heat management and emissions but huge gains are possible.

tdw 05-01-2012 10:11 PM

Re: Turbo charged diesel
 
I could be way off track but my thinking is that for getting you to the starting line or a quick dash down harbour a lightweight turbo engine would be the bees knees but for a cruising boat I really want relative simplicity and low down grunt so for me non turbo is the go.

Minnewaska 05-01-2012 10:16 PM

Re: Turbo charged diesel
 
Thanks guys. Mostly just a curiosity. They did change to a 100 hp Yanmar a couple of years after my model year. I will have to research whether it was turbo as well.

steel 05-01-2012 10:35 PM

Re: Turbo charged diesel
 
Quote:

Try running your Harley at 80% WOT for 10K hours. It would be lucky to make it 500 hours.
It might at 800 or 1200 RPM. You can do 100% throttle on a gasoline spark ignition engine no problem (with sufficient cooling) it's the high RPM that'll wear it out. Diesels don't like being over 80% throttle (with black smoke coming out.) I think it's something to do with the combustion temperature getting too high.

Turbo chargers are bad. They increase the intake pressure which means the engine has to be built stronger to handle it or the compression ratio has to be lowered. They end up doing really low like 16:1 compression which kills your efficiency and you're not much better off from a gasoline 10:1 compression.

shogan50 05-01-2012 11:11 PM

Re: Turbo charged diesel
 
Steel, you have no idea what you are talking about.

If your sailboat smokes at 80% throttle, something is amiss, likely the prop pitch. The smoke only happens on a properly setup diesel when it can't reach governed RPM.

Good luck making hull speed with a Harley at 1200 RPM.

Why do you think that nearly all over the road trucks in the world use turbo diesels. - because they hate efficiency? Turbos increase effective compression ratio. They generally demonstrate better efficiency than NA engines, not worse(but probably not at 8-1200 RPM).

steel 05-02-2012 12:13 AM

Re: Turbo charged diesel
 
Quote:

Originally Posted by shogan50 (Post 866093)
If your sailboat smokes at 80% throttle, something is amiss, likely the prop pitch. The smoke only happens on a properly setup diesel when it can't reach governed RPM.

You mean 80% speed on the governor which is the "throttle" on the boat. It might govern to something like 50 or 60% of actual engine throttle or 90% if your boat is tied up and can't move.

I meant over 80% throttle, or about when it starts putting out black smoke. Diesels are not intended to be used continuously like this.

Quote:

Good luck making hull speed with a Harley at 1200 RPM.
If you get an engine that is twice as big, you can run it at half the RPMs and it will last MORE than twice as many hours. That's why only cheap generators for temporary emergency use run at 3600 RPM. They are shot after 1500 - 3000 hours. An 1800 RPM one might do 6000 to 10,000 hours.

Quote:

Why do you think that nearly all over the road trucks in the world use turbo diesels. - because they hate efficiency? Turbos increase effective compression ratio. They generally demonstrate better efficiency than NA engines, not worse(but probably not at 8-1200 RPM).
American engine makers don't care about efficiency (it's a political economic thing.) That's why you can do a software mod on some GM diesels to boost the MPG by like 6 (it probably has to do with injecting the fuel at the proper time when the piston is as close to T.D.Center as possible.) Trucks want to have a lot of power in a small package and having a turbo is great for this.

Compression ratio is compression ratio. The only way to change it is to change the pistons or head. Effective compression ratio is well "effective" which I assume is referring to how much power the engine puts out and has nothing to do with efficiency.

Increasing the intake pressure increases the amount of air in the cylinder ("effective compression ratio"), and the amount of fuel can be increased by the same amount, which will result in the cylinder pressure at combustion and power output being increased by that amount. Increasing the actual compression ratio means that the same amount of air is going in the cylinder, the same amount of fuel is going in the cylinder, but the cylinder pressure at combustion is increased and power output is increased with the same fuel use!

Are you sure you can boost the efficiency on the SAME MODEL of engine by putting a turbo charger on it?


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