34 replies to this topic

[GSX-R]

Hello,

Turbo diesel have a better BSFC than diesel. Does this profit is more due to a better charge/surface ratio, less friction (RPM) or even to a different combustion (quicker or even else) ?

Regards

GS

[hydra]

all of the above (although not as much reduced friction as increased mechanical efficiency)

[GeorgeTheCar]

Well the frictin should be lower due to lower RPM and consequent piston speeds

[hydra]

Yes, but whoever said you had to lower RPMs when you add a turbo? If anything you can raise rpms slightly due to quicker combustion due to the higher temps and pressures

[McGuire]

The diesel's greater BSFC and thermal efficiency are mainly due to its higher compression ratio/expansion ratio. CR is generally a major factor in comparing the efficiency of gasoline engines as well.

Also... there is a problem when comparing the BSFC of engines that run on different fuels. Diesel fuel has a greater energy density than gasoline (contains more BTU's per pound or kilo). Even when equivalence factors are employed direct comparisons may be misleading as these are typically based on LHV (lower heating value aka net calorific value).

[hydra]

McGuire,
He's not comparing CI to SI engines, just turbocharged CI engines vs. naturally aspirated CI engines...

[J. Edlund]

Turbo diesels offer a lower specific fuel consumption than a naturally aspiranted diesel due to mainly three factors.

1. Friction is basically dependant on engine speed and piston mean velocity, even though combustion pressure has a small contribution due to increased piston ring pressure, a power increase by increased air density allow the increasement of power with only a small friction increase.

2. A power increase by air density allow smaller combustion chambers for equal power output and therefore there will be a smaller heat loss.

3. The third reason is essentially reason 1+2, but with a note that a leaner mixture can be used without the problems of 1+2. Leaner mixtures offers lower exhaust emissions and a higher efficiency (due to air excess).

When comparing diesels with gasoline engines the primary reason for the higher efficiency of the diesel is that it doesn't suffer from knock, hence a higher compression ratio can be used. High boost pressures can also be used.

Turbocharging a CI engine does also mean a smaller and lighter engine.

On a mass basis the energy content of diesel fuel is slightly lower than gasoline, no bigger difference though. Due to higher density energy content on a volume basis is slightly higher.

[GSX-R]

My concern is only for diesel please.

Originally posted by McGuire
The diesel's greater BSFC and thermal efficiency are mainly due to its higher compression ratio/expansion ratio. CR is generally a major factor in comparing the efficiency of gasoline engines as well.

Pure adiabatic efficiency is a function of V1/V2 (volumetric ratio into the cylinder), cut-off ratio, burning curve, burning efficiency, but not the total pressure at TDC or the even the turbocharger compressor pressure or something of this kind.

A turbo diesel does not have a higher compression ratio, generally a decreased one. You cannot add the turbocharger thermodynamic cycle into the figure of the thermal efficiency except in term of losses (turbocharger efficiency)

[GSX-R]

Originally posted by J. Edlund
Turbo diesels offer a lower specific fuel consumption than a naturally aspiranted diesel due to mainly three factors.

1. Friction is basically dependant on engine speed and piston mean velocity, even though combustion pressure has a small contribution due to increased piston ring pressure, a power increase by increased air density allow the increasement of power with only a small friction increase.

2. A power increase by air density allow smaller combustion chambers for equal power output and therefore there will be a smaller heat loss.

3. The third reason is essentially reason 1+2, but with a note that a leaner mixture can be used without the problems of 1+2. Leaner mixtures offers lower exhaust emissions and a higher efficiency (due to air excess).

...

Always thinking about diesel, how much part could we imagine for each of those 3 components ?

In last, which could be today the ideal admission pressure for a modern turbo diesel engine considering engine robustness, turbo efficiency etc, ... ?

[Stian1979]

More Air bether combustion.
Bether combustion bether efficensy.
More Air means also biger exspansion when heated up by combustion.

[McGuire]

Originally posted by GSX-R
Turbo diesel have a better BSFC than diesel.

not really true, all else being equal.

[hydra]

But all else is never equal McGuire, so in practice turbodiesels have lower BSFCs than diesels no?

[GSX-R]

Turbo diesels have a better BSFC than diesels. I assume.

[J. Edlund]

Originally posted by McGuire


not really true, all else being equal.

You can't design a turbodiesel where everything else is equal to a naturally aspiranted diesel.

Even if you start with two engines of the same type and then turbocharge one of them something will change. If you increase fuel flow with the turbocharged engine power output will increase, this increase is larger than the friction increase and BSFC goes down.
If you do not increase fuel flow on the turbocharged engine this engine will run leaner and with that BSFC goes down.

That's also why almost all diesels today are turbocharged. Efficiency can be increased further with the use of intercooling and turbocompound.

[McGuire]

Totally beside the point. Of course the fuel volume must also be increased if the air supply is increased. That really is too obvious to state.

I dispute this premise: that in engines in equal levels of development, the turbodiesel will pull measurably better BSFC numbers than the NA diesel. BSFC is not driving the current trend for turbodiesels. The issue is power per unit of weight. NA diesels are horribly heavy, especially for sedan-sized vehicles.

The piston engines with the best BSFC are the big NA two-stroke Sulzer diesels, at around .26 lb hp hr.

[McGuire]

Originally posted by hydra
But all else is never equal McGuire, so in practice turbodiesels have lower BSFCs than diesels no?

possibly, since all the cutting-edge passenger car development is now in turbodiesels rather than in NA. But there is nothing about the turbodiesel in principle that provides better BSFC. The most efficient diesels are big two-stroke NA stationary engines.

[GSX-R]

Originally posted by McGuire

The piston engines with the best BSFC are the big NA two-stroke Sulzer diesels, at around .26 lb hp hr.

Uh, do you want to talk about the well known 2 strokes TURBOCHARGED Wartsila Sulzer engines ?

http://www.wartsila....figures_96c.pdf

[hydra]

Aah yes I thought there was something odd about them being NA... :

[GSX-R]

Please take a look on the little giant snails that are stucked on the ctlinder head and tell me what it can be...
Or even take a look on page 5 and ask the girl if this engine is naturally aspired..

http://www.wartsila....ure_demands.pdf

[Calorus]

Expansion is incomplete at exhaust phase, meaning that the turbine charges not only from the expulsion of gas, but its continued expansion.

This allied to the proportional increase in effective CR & and capacity - without a change in operating friction give you your increase in efficiency.

[J. Edlund]

Originally posted by McGuire
Totally beside the point. Of course the fuel volume must also be increased if the air supply is increased. That really is too obvious to state.

I dispute this premise: that in engines in equal levels of development, the turbodiesel will pull measurably better BSFC numbers than the NA diesel. BSFC is not driving the current trend for turbodiesels. The issue is power per unit of weight. NA diesels are horribly heavy, especially for sedan-sized vehicles.

The piston engines with the best BSFC are the big NA two-stroke Sulzer diesels, at around .26 lb hp hr.

Fuel flow must NOT be increased just because airflow is in a diesel. You can boost the engine, use the same fuel flow and run the engine leaner if you want to.

Turbocharging a diesel will decrease BSFC and exhaust emissions while the engine becomes lighter/smaller for a given output. If turbocharging is used to run the engine leaner you usually get the largest gain in BSFC and emissions while increased fuel flow increases mainly power output in relation to size and weight but it also improves the BSFC.

The ship diesels that you mentioned are turbocharged. Usually they use quite high boost pressures, up to 5 bar absolute with single stage turbocharging. The turbochargers used are also very efficient, above 50% can be reached. Constant pressure charging is used.
Turbocharging of these two stroke engines have been used since about 1954.

[Calorus]

Originally posted by J. Edlund


Fuel flow must NOT be increased just because airflow is in a diesel. You can boost the engine, use the same fuel flow and run the engine leaner if you want to.

Turbocharging a diesel will decrease BSFC and exhaust emissions while the engine becomes lighter/smaller for a given output. If turbocharging is used to run the engine leaner you usually get the largest gain in BSFC and emissions while increased fuel flow increases mainly power output in relation to size and weight but it also improves the BSFC.

The ship diesels that you mentioned are turbocharged. Usually they use quite high boost pressures, up to 5 bar absolute with single stage turbocharging. The turbochargers used are also very efficient, above 50% can be reached. Constant pressure charging is used.
Turbocharging of these two stroke engines have been used since about 1954.

But then you wouldn't be comparing like for like as the A/F would be far higher - you might just run the NA plant at that A/R... Aspiration doesn't change the fuel properties. Or mixture entropy.

[GSX-R]

Originally posted by Calorus


This allied to the proportional increase in effective CR & and capacity - without a change in operating friction give you your increase in efficiency.

I don't agree with you.

If the crankshaft would be linked to the turbine, yes it would. But in this case this is just mid-price energy for compressor. You do not directly increase the efficiency in the main thermodynamic cycle but benefits are indirect lie J Edlund said : Lean mixture, less friction and less heat losses. We cannot integrate compressor compression and turbine expansion in the main efficiency calculation.

Compound, yes uses that.

[McGuire]

Originally posted by GSX-R
Turbo diesels have a better BSFC than diesels. I assume.

Why?

[McGuire]

Originally posted by GSX-R


Uh, do you want to talk about the well known 2 strokes TURBOCHARGED Wartsila Sulzer engines ?

http://www.wartsila....figures_96c.pdf

Sulzer makes many kinds of engines, but the most efficient are straight uniflow two-strokes.

[McGuire]

Originally posted by J. Edlund
Fuel flow must NOT be increased just because airflow is in a diesel. You can boost the engine, use the same fuel flow and run the engine leaner if you want to.

Then what point is there in turbocharging the engine? Of course fuel-air ratio can be altered in a diesel. That is the only way to modulate the output, since they have no throttle. Sometimes I think you just like to argue.

[GSX-R]

Originally posted by McGuire


Why?

A clue ?

Get a glance on the title. And then read the head of the post.

[GSX-R]

Originally posted by McGuire


Sulzer makes many kinds of engines, but the most efficient are straight uniflow two-strokes.

If you can provide me the Natural Aspirated engine production reference that does better BSFC than RT96, i'm interested to get that reference. RTA96 is 52,8 % of effectiveness.

[McGuire]

Originally posted by GSX-R


A clue ?

Get a glance on the title. And then read the head of the post.

If in principle turbodiesels really do deliver better specific fuel consumption than NA I would assume there is some reason that is susceptible to explanation.

[GSX-R]

you do it purposely?

[Greg Locock]

The way it was explained to me is that the mechanical losses associated with increasing the expansion ratio of the piston assembly exceed the energy to be gained from further expansion of the gas.

Adding a turbo allows additional work to be extracted from the end gas, which can then be used to reduce the pumping losses, in the case of a turbo, or to provide addiotnal crank power, in the case of an exhaust turbine that is geared to the crank.

This does not explain why on an SI engine adding a turbo generally increases the fuel consumption for a given power output, so it is doesn't seem rock solid to me.

[hydra]

As we all know, SI engines are detonation limited, so you end up having to lower compression ratio when you add boost, significantly offseting some of the theoretical gains of turbocharging. With a diesel, the only things holding you back are mechanical/durability requirements.

Also in practice, you have to dump a lot of extra fuel in (around 10-20% more) for durability's sake, and this results in a similar decrease in efficiency. Furthermore, production SI turbos are usually under-sized , resulting in pretty high backpressure and pumping losses. This effect is more prevalent in SIs than CIs due to their wider operating range (700-7000 rpm as opposed to 700-4500rpm)

[GSX-R]

I totally agree with my friend Hydra.

Reduced volumetric ratio seems for me the main reason why S.I don't take too much benefits from turbocharging (reduced CR reduced thermodynamic effectiveness).

We can add, the reduced gaz flow at low charge that add latency time, the extra fuel ratio to reduce the turbine temp, etc..

Turbocharging means higher pressures, higher temperatures. Gasoline don't support them as good as gasoil does.

But we can add a 4th point thank to you.

Probably like you said Greg, less pumping loss at inlet for turbocharged engines.

1 Solid Frictions (FMEP)
2 Walls heat transfer
3 Pumping losses (gaz frictions) (PMEP)
4 Easy lean mixture fitting (limited unburned HC and limited residual temp)

But now, how to arrange them ? Difficult task.

GS

[J. Edlund]

Originally posted by McGuire


Then what point is there in turbocharging the engine? Of course fuel-air ratio can be altered in a diesel. That is the only way to modulate the output, since they have no throttle. Sometimes I think you just like to argue.

If you turbocharge and run the engine leaner emissions and fuel consumption goes down (with equal power output). The same can of course be acheved with a larger displacement or higher engine speeds, but with increased friction and/or heat losses.

[J. Edlund]

Originally posted by GSX-R


If you can provide me the Natural Aspirated engine production reference that does better BSFC than RT96, i'm interested to get that reference. RTA96 is 52,8 % of effectiveness.

To my knowledge the turbocharged Wärtsilä-Sulzer RTA96-C is the most efficient production diesel, it consumes 0.26 lb/hp/hr when used for max efficiency (85% load).