Turbocharging a diesel engine?

Don't have experience with marine diesels, and not a diesel guru. But...

I grew up in a farming culture in the middle of the USA. Pretty much every truck my family had was a diesel, and lots and lots of other ones around too. The current one my dad has (I'm a computer programmer, not a farmer so a diesel truck would be a perversion) is a Cummins turbodiesel that's been running pretty hard for about 20 years. That truck is designed similar to the bigger 18-wheel rigs. They're set up to only work with a turbo, and that turbo spins up even when running empty. Dad's truck you can hear the turbo spool up at about 25 or 30 mph, empty.

While you said you're not interested in cars and trucks, I think my experience has more to do with industrial applications than with basic transportation. Which would mean it has more to do with boats. My truck experience involves a lot of overloaded trailers for short hauls and heavily loaded trailers for long hauls. I think that would correspond with a "medium" load on a larger boat.

If you're running half throttle then you are well within the range of having turbo help. Our trucks ran in that range a whole lot, and never saw any problems from it. Full throttle was WAY past what most non-farm people would try. That includes older mechanical fuel injection trucks. We generally ran the trucks till the truck fell apart, and the engine was still going strong. Replace the injectors every 150,000 - 200,000 miles or so and that's about everything engine-wise.

We never turbocharged a diesel that had been normally aspirated, but heard of some who did. You have a waste gate on the turbocharger that prevents over-pressurization of the intake manifold. That guarantees maximum torque at any RPM above a certain point, but AFAIK that won't help your boat a bit. You're already turning a high torque engine and it's gonna spin that prop at whatever rate your transmission will turn it turbo or not. Maybe you could put on a more aggressive prop? The most common place I've heard of that is on aircraft engines. Obviously not diesel, but they gained altitude that way without gaining much rated power. They ran at max rated intake manifold pressure anyway, so all the turbo did was move the throttle lever down a bit, and give you better altitude. Maybe if you drive your boat above 15,000 feet you would need this ?

I THINK the turbo will improve efficiency in the space where it is functional, and probably not hurt much below that.

The bit about the turbo driving the intake stroke is definitely false. It violates a pesky little rule called thermodynamics. The turbo compressor (intake side) is driven by the turbine (exhaust side) which makes it a circular system, and any system loses energy overall. Combustion causes the crank to turn and the exhaust gas to expand. The turbo causes the combustion to be more effective, but there is no case where the turbine will drive the engine.

I haven't looked at recent systems, but the way they got around turbo lag in cars as of a few years back, they had 2 turbos of different sizes. The smaller one would spin up faster so the turbo lag wouldn't be quite so bad. No difference at all to a truck or boat IMO but makes a lot of difference if you're a punk rich kid trying to get to the next stop light before the guy one lane over.

Sorry if I've repeated anything. I've tried to say when I THINK something as opposed to KNOW something. Most of my experience is with using pre-existing systems, not designing them. Hope it helps.

 

Are you saying that the boost pressure can somehow scavenge the exhaust gas even though the exhaust manifold pressure will be higher than the boost pressure in most applications?

A turbo must be thought of differently to a supercharger which presents no restriction to the exhaust flow. The supercharger gets its power via a mechanical drive. A turbo gets its drive via a gas turbine. This conversion of energy does not come for free either. It comes in the form of additional exhaust backpressure. It is much more efficient than a supercharger though.

Do not misinterpret what I said though. I am not saying gas flows backwards from the exhaust into the intake. I am just saying its the pumping of the engine which creates the gas flow through the engine and turbine, and this pumping is actually made more difficult because of the turbine. However the gains realized from the compressed intake charge massively outweigh this loss in both efficiency and power.

Even how stuff works agrees.

Too bad you are out Tom.

 

Denis RB,
Sub Tom,
Ron L,
Sorry about the confusion. My wording wasn't the best. I think all turbo engines have a balance issue. Lets say you loose 20hp from increased back pressure and get a 40hp increase in power. It's obvious that under reasonable loads a turbo engine will have a gain exceeding the losses but at slow speeds and light loads (like 20-35%) does one still have a positive investment. One invests looses and gains more than the looses. But the BMEP becomes so high on the hard working turbo engine that numerical compression ratio's must be reduced. So I suspect there must be a point where the loading is causing the BMEP of the turbo engine to be so low that the power output of the engine is lower and the fuel efficiency is also lower than the same non-turbo'd engine. This non-optimal balance of compression, back pressure and BMEP should (in my opinion) cause the turbo engine to be less efficient than a NA engine at low speeds and loads. Is this true? And if one adds aftercooling the decrease in BMEP becomes so great (because of cooling in the heat engine) that combustion temperatures drop so low that serious incomplete combustion causes sooting and probably other problems.
Any of this close to reality?

 

I agree with you easy rider. I think you will loose out in efficiency by using a turbocharged engine that is run under loads so light that it never sees positive pressure.

IMO If this is occurring its because the engine is more powerful than it needs to be. So the loss is not the fault of the the turbo but the fault of using too powerful of an engine. However if you need large amounts of power sometimes, and this need for large power must be met, you will still be better off using turbocharging to get that extra power rather than more engine capacity.

On the engine I had, the sequential system used 2 turbos of the same size. It worked by shutting off one turbo at lower RPM then at higher RPM the second one joined in. These twin turbos were only good for 400 or so HP so I changed them for a larger single turbo which was good for over 600. The drawback was much worse lag/boost threshold.

Also there is confusion about what people consider lag to be. The actual definition of turbo lag is the amount of time it takes for the turbo to boost again after you lift the throttle. So in reality lag in its true sense is fairly irrelevant for a boat. But most people these days view lag as the period of the lower RPM range where the turbo is unable to make positive boost pressure, even though this is more accurately called the boost threshold. This is more relevant to boats.

Both lag and boost threshold will be reduced by using a smaller turbo (or smaller turbine housing) at the expense of maximum possible power production. So yes you can almost eliminate lag and reduce the boost threshold to almost nothing, but you would need to select a turbo so small that it would severely limit maximum power production. (not including advanced turbo charging techniques)

 

DennisRB,
I got quite a bit out of that post but I still don't see how anyone could be concerned about turbo lag on a boat. One rolls the throttle on slowly on a boat so how could it be an issue?
"loads so light that it never sees positive pressure."
You mean over atmospheric? If you had less than atmospheric in the intake manifold would you have the in rushing air driving the compressor turbine and that would smack of what I was trying to express several posts ago. The air rushing past the compressor turbine (CT) would tend to cause a vacuum and the CT would need to work just a little bit to overcome that and have atmospheric pressure on both sides. But it would seem to me a turbo'd engine would always have over atmospheric pressure in the intake manifold .... but how much does it take to make up for the loss of numeric compression ratio?

 

Don't lose sight of the freewheeling effect, as long as any exhaust gas is flowing a small force is being applied to the spinning of the impellers, unless it has locked due to mechanical failure. Even a locked unit would not prevent flow of intake and exhaust.
Ron

 

Sorry bout the comment on your cam lobes, and as stated to Easy Rider, I'm not expert on turbo's.
I did find mention of intake pressures as high as 200 PSI on tractor pull engines, I just don't think exhaust pressures will be as high as I think you are indicating.
I have all the books I need to find the answers, but my motivation at the moment is Thermal related conversion, so I best follow Tom.

Ron

 

In my post I said lag is irrelevant for boats. So I was agreeing. But I said the boost threshold is a bit more relevant. But in reality, its still not that important either because there should be enough slip in the prop for the engine to get past this point. Boost threshold is what many people mistake for lag. It is the period of the lower RPM range where the turbo can not make positive pressure, and therefore the engine will have much less torque. Depending on what sort of boat/gearing/prop you have this area of low torque may make it difficult to get on the plane.

The compressor wheel (which is the correct term for the cold air side) will create some restriction but when measured, this restriction will not noticeably create a vacuum below atmospheric when measured with a normal vacuum/boost gauge like this.



So it won't be reducing power much during the boost threshold period of the RPM range, but there must be some loss. For illustration purposes I will explain what happens on my petrol car. Imagine you are looking at the gauge. When opening and closing the throttle below the boost threshold (3000rpm in my car) the gauge goes between vacuum and zero. So if I floor it, the gauge goes from negative figures due to the throttle plate to zero (until I allow the RPM to climb over 3000, which is when I will see positive pressure build) If there was a significant intake restriction from the compressor wheel and compressor housing the gauge would not reach zero immediately. It would stay in the vacuum section until the boost threshold RPM is passed and the turbo is spinning hard enough to produce boost. (boost = positive intake pressure over atmospheric).

There is no throttle plate on a diesel, so I can therefore say the vacuum will just be pretty close to zero the whole time before the turbo can make boost. Even on diesels there will not always be positive pressure on the intake. Diesels still follow the laws of turbocharging so the turbine still needs a certain amount of exhaust energy to spin the compressor fast enough to produce boost.

The turbine wheel and turbine housing (which is the correct term for the hot side) will still be choking the exhaust somewhat but it will present more of a restriction depending on how much exhaust gas is leaving the engine. The turbine wheel never directly increases power on its own. It only reduces power by causing restriction. So up until the point where the turbo is spinning fast enough to produce boost, the turbo is creating a power loss. The amount of loss this causes, I am not sure of. It might be 3% or 15%. I'm not certain and the loss will depend on engine load. But I am certain there will be a loss.

Therefore I do believe that if you are operating a turbo engine below the loads and/or RPMs that create boost you are running less efficiently that the NA version of that engine. Specially when you add the fact that the engine will have lower CR than the equivalent NA engine. But like I said in a previous post, you might still be more efficient overall by cruising an engine in this state if you require large increases in power periodically, such as getting on a plane etc, out running a storm.

Turbocharging is one of my hobbies. I would like to see a turbo on everything So I am in no way trying to talk it down. But there are some drawbacks if not implemented correctly.

 

Interesting that the truck engines (frequently our source of marinized engines) is re discovering the PRT.

A power recovery turbine is a turbo and gear set that simply runs the power usually lost in the exhaust directly to the engine.

This is really old hat in piston aircraft engines (pre WWII) where mechanical supercharging was common and long hours at cruise power is the norm.

FF

 

Fred. Check this bad boy out. The Napier Deltic http://en.wikipedia.org/wiki/Napier_Deltic

Its is a bizarre piston powered engine with what appears to be a turbo (with compressor and turbine) however the turbo is also mechanically directed to the engine. When a compressor is connected directly to an engine it is called a supercharger. But this has the turbine connected too. So I would say it is supercharged with the added benefit of regaining some lost energy in the system via the turbine.

 

7-10%, you are correct. There are no current production engines to do this comparison with but I dusted off some old Volvo spec sheets I have:

MD70C (Naturally aspirated)
TMD70C (Turbo charged)
TAMD70C (Turbo Charged and After Cooled)

All are from the mid 70's.
BSFC (g/kw/hr) and on the full load curve (Rated rpm the same for all but peak hp is different, comparison is OK for this purpose)

MD70C: 247 @ WOT, 229 @ 65% of WOT and 240 @ 40%.
TMD70C: 233 @ WOT, 215 @ 65% of WOT and 240 @ 40%
TAMD70C: 224 @ WOT, 215 @ 65% of WOT and 239 @ 40%

"...Why, or what exactly is taking place inside the engine to make the engine produce the same HP on less fuel? Really simple question when you think about it, but I am wondering if there is a simple answer..."

No, there is not a simple answer and there is, as others have stated, a lot of confusion in this tread as to how Turbo charging works.

Main reasons (IMO):
1. Better fill rate and more homogenous charge - More of the fuel injected actually used for combustion.

2. Less pumping losses (See data above, the Turbo charged engines are not more efficient at the load with little/no boost pressure)

It really is a matter of recovering the energy released by the fuel. Numbers pulled out of my rear: 35% of the energy in the fuel is used to rotate the crank, 35% is transferred away by the cooling system as heat, 2% is radiated heat, 2% is producing noise and vibrations and the balance escapes via the exhaust. The turbo recovers part of this energy and use it to rotate a turbine.

Case in point: Two identical engines; one with a wet turbo and one with a dry, heat shielded, turbo. Given that everything else is equal the dry turbo engine will be more efficient. That set-up recovers more of the energy in the exhaust gases instead of getting rid of it via the cooling system.

Karl2

 

If you do the maths on some of these engines you will find that the 'piston' part almost became a gasification unit for a turbine engine, some also added more fuel after the valve/port before the exhaust turbine.

 

Come on!

This thread is an attempt by the original poster to find out whether it's practical to turbocharge his diesel boat, with an engine he already has.

I don't believe for an instant that it's even remotely possible for Cheesy's scenario to happen, and even if it was practicality would have absolutely nothing to do with it.

The amount of misinformation on this thread is stunning. Most of this thread seems to have nothing to do with the intended topic. The math-heavy posts I have no idea about, but there are some basic principles of a turbocharger that have been badly described over and over. What somebody does in their lab or garage "just because he can get it to work" has nothing to do with running a boat reliably and efficiently for a long time.

A turbocharger as it is attached to an engine is NOT a power producer. It's a power consumer. It's an impeller and compressor on the same shaft, one driven by exhaust gases which powers another which is used as a compressor. The power comes from expanding gases due to burning fuel, which happens mostly in the combustion chamber of the engine.

No matter how you slice it, the amount of power consumed by the turbo is greater than the amount of power put out by it. The increased power of combustion in the engine is where the gain is.

The turbo does not need to induce positive pressure compared to ambient pressure in order to "break even." It only needs to increase pressure compared to the normally aspirated pressure of the intake manifold, plus a bit more to account for losses in the turbocharger. This can still be at considerably less than ambient pressure.

Turbochargers are not magical in any way. There are losses to running one that are hopefully gained back by improved combustion inside the engine.

I don't know what the OP thinks, but IMO we would be better off if those who have never even seen a turbocharged engine would lurk rather than post.

Sorry for being cranky.

 

Come on!

Thats like saying guns don't kill people, bullets do.

 

OK then, if it's a power producer then you can pull it off the engine, dump fuel into it and pull power off the shaft when you light it on fire.

Is your fuel injector pump a power producer? Is your head gasket a power producer? No to both. How about glow plugs? But you can't produce power at all if you don't have any of these.

A turbocharger is a relatively simple machine which takes mostly-wasted power and uses it to improve combustion. It's not an engine. It does not produce power.