Do the charged engines reach higher compression and why ? - The Technical Forum Archive (2024)

alpa

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Hello,

This is my first post here.
I've been following posts for several days now, looks like there is a good level of knowledge (experience) here .

Firstly, I don't talk about the compression ratio like 10:1, I'm talking about the effective compression.
Now the question:
Lets define the EC (effective compression) as the ratio between the volume of air at ambient temperature that entered the cylinder, and the chamber volume. Note that this ratio is different from the compression ratio BDC/TDC.
I'd like to compare the max EC of NA and super/turbo-charged engines, for instance on MPI indirect injected engines.
Looking at some engine numbers I have a feeling that charged engines reach higher EC levels, and that this is due to their compression process which is staged (compressor -> cooling -> piston compression). Such a staged compression keeps the final air T cooler and consequently farther from the knocking, which also allows a more optimal ignition timing.
I wonder whether there have been some measures/simulations of the max EC the engines reach when they produce their max torque.

Alexei

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McGuire

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Possible point of clarification: supercharging will produce higher cylinder pressure, to be sure, but not a higher compression rate or ratio. If the compression ratio is say 8:1, then the air in the cylinder will be compressed 8 times...more or less regardless of its ambient pressure going in.

Some terms as they are commonly understood in the industry:

Static compression ratio: the ratio of the cylinder's volume at BDC to its clearance volume.

Clearance volume: the cylinder's volume at TDC.

Effective compression ratio: the ratio of the cylinder's volume at the point of IVC (intake valve closing) versus its clearance volume. The intake valve typically closes at some point after BDC, which results in some pressure loss on the compression stroke which static CR does not take into account. Since it determines the piston's precise location at the point of IVC, rod/stroke ratio is also a factor in effective CR. Effective CR is also referred to as "dynamic compression ratio."

Equivalent compression ratio: a term often found in the aftermarket turbo-tuner business, and unfortunately often confused with effective or dynamic compression ratio. Equivalent CR is pretty simple: static compression ratio times manifold pressure in atmospheres. For example, an 8:1 compression ratio and 1.5 atm manifold pressure results in an "equivalent compression ratio" of 12:1, which is supposed to give some indication of the power and/or detonation properties of the engine. But in reality this is not a terribly reliable indicator of anything, as there are several other signifcant factors in play. Equivalent CR is a VERY rough rule of thumb; many believe it has little or no worth at all...myself included.

Mean effective pressure: the concept we are looking for here, I think, and yes, it can be considerably higher in boosted engines. This is not a pressure ratio but simply the effective pressure produced in the cylinder, as the term itself implies. Also, this is mean combustion pressure, not mean "compression pressure." There is a simple formula that illustrates the relationship of MEP, torque, and displacement:

MEP = 150.8 x torque / displacement

(Watt system...here torque is in lb ft, MEP is in PSI, displacement is in cubic inches, and 150.8 is simply 48*pi.)

If you plug the brake or indicated torque figures into the above fomula it will give you back the effective pressure, in brake or indicated form respectively, that was produced in the cylinder over one operating cycle, at mean, which resulted in work being performed.

Note this does not take compression ratio, boost, valve timing, rpm or anything else into account: only the amount of pressure captured within a given volume that was translated into a given quantity of torque at the crankshaft. However, it does indeed reflect any and all factors which serve to increase combustion pressure. With high boost levels some engines can achieve MEP of 500 PSI, even more, while normally aspirated engines are generally limited to around 250 PSI.

alpa

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Originally posted by McGuire

Note this does not take compression ratio, boost, valve timing, rpm or anything else into account: only the amount of pressure captured within a given volume that was translated into a given quantity of torque at the crankshaft. However, it does indeed reflect any and all factors which serve to increase combustion pressure. With high boost levels some engines can achieve MEP of 500 PSI, even more, while normally aspirated engines are generally limited to around 250 PSI.

My understanding is that MEP (like BSMEP) does not allow easy comparison as it doesn't depend on the Clearance volume but on the torque, which could result from a bigger bore or other parameter. MEP only allows us to note that we can get more torque from a charged engine, but does not show by which mean. The mean could be for example lower combustion efficiency with more mixture.

I'm trying to understand is there any 'fundamental' difference between NA and boosted engines from the combustion point of view. Is the difference due to a 'staged' compression, or are both concepts finally identical except the way to push more air into cylinders.
Let's imagine 3 very similar engines all having the same bore, same Clearance, but different strokes, cam timings and the rest.
- NA engine
- boosted without air cooling
- boosted with air cooling
Which of them could produce more Max Combustion Pressure, and which one less. And WHY. Don't think of torque, as torque will depend of plenty other parameters like R/L, crankshaft offset, ...

May be the right question would be: is the global efficiency of boosted engines higher than that of NA engines at the same torque level. If I had 1L of petrol, would I get more work from a charged or NA engine. But here we face the turbo/blower questions, I would prefer discuss that in a separate thread.

alpa

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Originally posted by Engineguy
In a general sense, yes, the RPM that results in the highest number of oxygen atoms being trapped in the cylinder upon intake valve seating has the potential to be the RPM of max torque. I emphasize potential because bang-reducing compromises may have to be made if the resultant TDC compressed mixture temp is too toasty, as you alluded to.

I'd like to compare these numbers for NA and charged engines.

McGuire

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In making pressure, the NA engine is limited to whatever mechanical compression ratio can be arranged inside...the clearance volume is also the combustion space so there is always a compromise involved. (In a current F1 engine, with its comically large bore and pentroof chamber, the combustion space is shaped like a paper-thin wafer bent in half at the middle.)

However, the higher the compression ratio the more efficient the engine, and the reasons are not all to be found on the indicator card. Imagine a compression ratio of only 2:1. On the expansion (power) stroke, when the piston has traveled halfway down the bore the cylinder's volume has expanded only 50%. Before a large portion of the combustion pressure can be harvested it's already time to open the exhaust valve. The higher the CR, the later the intake valve can be closed and the earlier the exhaust valve can be opened, increasing volumetric efficiency.

But on a boosted engine, once the manifold pressure is raised to a signifcantly useful level the compression ratio must also be reduced in proportion, in order to control detonation. As I stated earlier, supercharging increases cylinder pressure but does not change the engine's compression rate. What it does is increase the engine's volumetric capacity. At two atmospheres of manifold pressure we have essentially two pounds of air in a one pound bag. But whatever the ambient pressure the rate of compression in the cylinder for a given CR remains the same.

And the trouble with that is: whether driven by the crankshaft or exhaust gas, superchargers require a significant amount of work from the engine in order to drive them, while in the NA engine this work is performed by atmosperic pressure.

So all else being equal, while the supercharged or turbocharged engine can produce more output per unit of displacement or per unit of engine weight, it will invariably produce less output per unit of fuel consumed. By that measure the boosted engine is less efficient, no getting around it. When we turbocharge an engine it will make more power, but it will consume another .05 to .10 lbs per hp per hour in fuel doing it. For a crank-driven supercharger it's somewhat more than that.

J. Edlund

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Originally posted by McGuire
In making pressure, the NA engine is limited to whatever mechanical compression ratio can be arranged inside...the clearance volume is also the combustion space so there is always a compromise involved. (In a current F1 engine, with its comically large bore and pentroof chamber, the combustion space is shaped like a paper-thin wafer bent in half at the middle.)

However, the higher the compression ratio the more efficient the engine, and the reasons are not all to be found on the indicator card. Imagine a compression ratio of only 2:1. On the expansion (power) stroke, when the piston has traveled halfway down the bore the cylinder's volume has expanded only 50%. Before a large portion of the combustion pressure can be harvested it's already time to open the exhaust valve. The higher the CR, the later the intake valve can be closed and the earlier the exhaust valve can be opened, increasing volumetric efficiency.

But on a boosted engine, once the manifold pressure is raised to a signifcantly useful level the compression ratio must also be reduced in proportion, in order to control detonation. As I stated earlier, supercharging increases cylinder pressure but does not change the engine's compression rate. What it does is increase the engine's volumetric capacity. At two atmospheres of manifold pressure we have essentially two pounds of air in a one pound bag. But whatever the ambient pressure the rate of compression in the cylinder for a given CR remains the same.

And the trouble with that is: whether driven by the crankshaft or exhaust gas, superchargers require a significant amount of work from the engine in order to drive them, while in the NA engine this work is performed by atmosperic pressure.

So all else being equal, while the supercharged or turbocharged engine can produce more output per unit of displacement or per unit of engine weight, it will invariably produce less output per unit of fuel consumed. By that measure the boosted engine is less efficient, no getting around it. When we turbocharge an engine it will make more power, but it will consume another .05 to .10 lbs per hp per hour in fuel doing it. For a crank-driven supercharger it's somewhat more than that.

It's more complicated than that, especially if one takes the throttleing effect into account. Also, an engine that is efficient at part thottle can be inefficient at full throttle or the other way around.

The work added or lost during exhaust and intake phase is also quite more complicated than you have mentioned. In a naturally aspiranted engine you normally have losses during both (due to pressure loss), all work is not performed by the atmospheric pressure. In the case of a supercharged engine the boost pressure will perform positive work during the intake phase while the exhaust will remain as for a NA engine. In the case of a turbosupercharger there will be negative work done during the exhaust phase and a positive work done by the boost pressure so any total loss or gain in pumping will depend on which work is the highest.

Another interresting thing is that for example Honda noted that when they increased the boost pressure in their turbocharged F1 engine BSFC was reduced (directly opposite to your theories). Going from 2.2 bar to 2.5 bar the BSFC was about 2.5% higher at 2.2 bar than at 2.5 bar. Acording to Honda; "in proportion to the rise in boost, charging efficiency increases and provides higher indicated horse power while the engine's mechanical losses remains almost the same, resulting in an improvement of brake thermal efficiency". Power at 2.2 bar was about 13% lower than at 2.5 bar.

As for the diesel a turbocharger allows you to go much leaner without making a very large and heavy engine, which offers both reduced fuel consumption and lower exhaust emissions.

McGuire

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Originally posted by J. Edlund
It's more complicated than that, especially if one takes the throttleing effect into account. Also, an engine that is efficient at part thottle can be inefficient at full throttle or the other way around.

Well sure. It's also more complicated than you have described. Are we trying to answer the poster's question, or are we trying to see how complicated we can make it?

Originally posted by J. Edlund
Another interresting thing is that for example Honda noted that when they increased the boost pressure in their turbocharged F1 engine BSFC was reduced (directly opposite to your theories). Going from 2.2 bar to 2.5 bar the BSFC was about 2.5% higher at 2.2 bar than at 2.5 bar. Acording to Honda; "in proportion to the rise in boost, charging efficiency increases and provides higher indicated horse power while the engine's mechanical losses remains almost the same, resulting in an improvement of brake thermal efficiency". Power at 2.2 bar was about 13% lower than at 2.5 bar.

Of course. No trick at all for a turbo engine to pull better BSFC at 2.5 bar than at 2.2 bar. All that says is this particular turbo setup could be made more efficient at the higher MAP. Any turbo system will have an optimal boost level, owing to the turbine and compressor maps and other factors of the calibration. If you take a system optimized for 4 bar and run it at 2 bar it will almost certainly be less efficient eh. But until someone develops a turbocharger of greater than 100% efficiency, it will always be less efficient than NA.

Originally posted by J. Edlund
As for the diesel a turbocharger allows you to go much leaner without making a very large and heavy engine, which offers both reduced fuel consumption and lower exhaust emissions.

Diesel, turbocompound and turbine engines are very interesting topics for discussion...but are not truly relevant to the specific question.

kup

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F1 Diesel Power
is it possible ?
i`m a novice in tech area of F1 and i`d like to ask profs of Diesel Engines
were they in F-1 someday or is it possible to be ?
how about lower costs etc, that FIA fights for ?
Thanx.

btw. GearBox
what type(s) of GearBoxes are in common use in F-1
in 2005
in 2000
in 1995
in 1990
Where can i find info of main gearbox changes since 1990 ?
Thanx again.

GearBox - SemiAutomat, oke i know this, but i`d like to know more details
what does "along" or "transversale" means
or "opposite" - is it about GearBox or Engine ?

Maybe you know good site with easy tech explains ?

kup

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common rail technology is a blessing for diesel engines

i just wondering "diesel" as part of FIA policy in slowering cars =)
afaik - diesel has less Revs, Fuel is cheaper ... maybe other "plusses"
and maybe "minuses" too =)

F-1 Rules - does Diesel engines prohibited ?
why ? since what year ?
what are main achivements of diesels in F-1 if any ?

McGuire

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Originally posted by kup
F1 Diesel Power
is it possible ?

No, not unless they make diesels mandatory.

But there are some rather serious diesel efforts pointed at LeMans. There it makes a lot more sense with respect to the design mission. (The Audi R8 is a kind of a "diesel" if you know what I mean, he remarked, strictly tongue-in-cheek.) As diesels advance they will continue to make progressively more sense, especially if they get a decent break in the rules.

Not that I am for it. I despise diesels, unless the vehicle has fewer than one or more than four wheels.

hydra

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McGuire

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Originally posted by hydra
Oh come on... That's pretty harsh McGuire!
I'm not exactly the biggest fan of diesels myself, but I would like to think that a 2-stroke DI-diesel with 4 poppet valves and high-pressure turbocharging (with a small supercharger responsible for scavenging at low piston speeds) would present a very interesting engine concept... DI Diesels are limited to 4000-4500rpm anyway, which is around where the valvatrain limit would be for a 2-stroke. And 2-strokes aren't very efficient above piston speeds of around 2500fpm anyway. so the whole concept seems to work really well. I'm not saying this would be an ideal solution for a displacement limited race car, but it would make great sense for a (sports-oriented) road car, with a B/S just over 1, and a cylinder volume of 500cc running 1bar+ boost. I think a BMEP of 200x2 wouldn't be unreasonable, giving ~7.5bhp/in^2 of piston area, or 400+bhp for a 3.0L 6cyl DIESEL with a redline of 4500rpm and returning 45+mpg! Sounds pretty exciting don't you think?

My objections to diesels are entirely personal and subjective. In my opinion, they sound perfectly awful, vibrate and smell bad. I have never driven a diesel car of which I truly approved.... with the possible exception of a Mercedes-Benz AMG C-class I tried in Europe (not available here). To me diesels are perfectly fine for working boats, locomotives and road trucks. After all, that is what they are for.

My prejudice is probably the result of early imprinting. When I was a young sprout one of my responsibilities was the maintenance of a fleet of vans powered by Detroit 3-71's and Perkins. Both fine engines really, wore like anvils, but these had gotten unbelievably grimy over the years and hundreds of thousands of miles, and of course with diesels the odor of #2 fuel permeates everything. When I would get home in the morning after working all night my wife (at the time) would make me undress on the back porch and go straight to the shower so I didn't bring the foul stench into the house.

soubriquet

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Originally posted by McGuire

My objections to diesels are entirely personal and subjective. In my opinion, they sound perfectly awful, vibrate and smell bad.

When you are working out in the Great Sandy Desert, or the Eyre Basin, the relentless, remorseless mechanical clatter of a Nissan or Toyota six is one of the sweetest sounds you can hear. Diesel doesn't smell so bad when you need 150 litres on the roof as well as 120 in the tank to get the job done.

McGuire

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Originally posted by soubriquet

When you are working out in the Great Sandy Desert, or the Eyre Basin, the relentless, remorseless mechanical clatter of a Nissan or Toyota six is one of the sweetest sounds you can hear. Diesel doesn't smell so bad when you need 150 litres on the roof as well as 120 in the tank to get the job done.

Then I will try to do my best not to end up there.

It is my understanding that in long periods of solitary confinement, prisoners eventually reach a state where they develop an attachment to the movement and voices of their guards each day.

hydra

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alpa

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Originally posted by hydra
What is it about diesel fuel that makes it smell so diesely anyway? Surely something can be done to minimize the stench... Coconut oil burns similarly, yet smells nothing like it... You didn't tell me what you thought of my idea - from a tech perspective. The idea makes great sense on paper, and doesn't really use any untested technology, so why doesn't anybody go for it?

The idea of diesel 2 stroke turbo engines was extensively used in desert truck races. I believe Kamaz trucks had 5L over 500hp engines.

McGuire

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Originally posted by hydra
What is it about diesel fuel that makes it smell so diesely anyway? Surely something can be done to minimize the stench... Coconut oil burns similarly, yet smells nothing like it...

Sulfur, mercaptans and microorganisms make diesel fuel stink.

Yes, biodiesel is one answer...though it has the same problem with production volume as ethanol. One of my favorite stories I ever worked....5-6 years ago I went out in the Ohio countryside to visit a 16-year old who independently figured it out. A neighbor lady gave him an old VW diesel...he went to the library, did the basic research and then on his own put together his home-cracking process, using methyl alcohol to split the triglycerides out of the hydrocarbons. Working in fast food gave him the idea to cut deals with all the area restaurants to drain their fryers for them in exchange for the old oil. Voila, "free" fuel.

And yes, when his car ran it smelled just like french fries! Now the cool part about this story is this kid was not a brainiac, nor did he have a research chemist in the family and a lab in the garage. He was just a normal but naturally curious, also clever and resourceful 16-year old, who did it all on his own.

Originally posted by hydra
You didn't tell me what you thought of my idea - from a tech perspective. The idea makes great sense on paper, and doesn't really use any untested technology, so why doesn't anybody go for it?

Traditionally, the big problem with diesel two-strokes is emissions. That's what put the previous generation out of business. But I don't know, perhaps there are some new developments on that front.

hydra

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McGuire

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Originally posted by hydra
The traditional 2-stroke emissions problems do not apply here, as we are running DI and uniflow (poppet valve) scavenging, which means that no raw HCs are dumped out the exhaust.

The original GMC Detroit Diesels (Series 71, 53, and 92; 1938 to 1980-something) were direct-injected, uniflow two-strokes. At one time virtually every city bus in America was powered by a 6-71. But high emissions took them off the market in the early-mid '80s. In its time the Series 71 was possibly the dirtiest engine on the road.

...lots of folks have heard of the Detroit Diesel, originally a division of GM...well, there was a Cleveland Diesel division too. They made the big stuff...like the 16-278 diesel, 4448 CID, 1800 hp. (In GMC diesels, the number before the hyphen is no. of cylinders, while the number after the hyphen is the displacement per cylinder.) These were also uniflow two-strokes essentially identical to the Detroit, just bigger. Cleveland Diesel had its origins in the Winton automobile.

ciaoduc1

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I think the biggest problem with putting a diesel on the market is that Americans fear change. We especially fear a change back to a technology that was previously such a failure.

The only working knowledge of diesels most Americans have is the black smoke billowing dump truck that gets in the way during their drive to the post office and/or those loud, dirty, smelly and most importantly SLOW GM passenger car diesels of the 70's...which were nothing more than your standard 350ci Chevy small block with different heads and an injection pump in place of the distributor.

Personally, I would love to see a diesel hybrid car enter the market. A small 2 stroke 3cyl turbo charged DI engine making 60-70hp would fit nicely in place of the current 1.0L gas engine in the Honda Insight. I imagine with this combo, 80-90mpg wouldn't be out of the question.

Why hasn't this been done in more open minded countries?

McGuire

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Here it is, the direct-injection, uniflow two-stroke diesel as popularized by General Motors. This illustration is from the Operator's Handbook for the Cleveland Diesel Series 278, 1944. These were the first true high-speed diesels to find wide use...though it was not originally a GM innovation. GM got the basic design from Winton, buying out the company (which became the Cleveland Diesel division). But even it was not quite a new idea. There were uniflow steam engines in the 1820's.

McGuire

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Originally posted by ciaoduc1
I think the biggest problem with putting a diesel on the market is that Americans fear change. We especially fear a change back to a technology that was previously such a failure.

The only working knowledge of diesels most Americans have is the black smoke billowing dump truck that gets in the way during their drive to the post office and/or those loud, dirty, smelly and most importantly SLOW GM passenger car diesels of the 70's...which were nothing more than your standard 350ci Chevy small block with different heads and an injection pump in place of the distributor.

Currently the biggest obstacle to diesel automobiles among American consumers is the relatively low price of gasoline. Americans are not going to pay a premium for cars that rattle and smell bad.

Policy-wise the biggest obstacle is emissions. Diesels have an emissions problem, no getting around it. They require their own emissions standards, which is bad policy and trouble politically. There is also an infastructure problem in that high-quality diesel fuel is not in wide distribution here. There are indeed some impressive high-performance diesel autos available in Europe, but they won't run right on American 2D. This is why Mercedes-Benz USA sells only one diesel here, a 200 hp six in the E-class sedan. Don't be looking for the AMG diesels anytime soon.

True, the Olds diesel was no help in winning over consumers to the fuel-oil burners. That mess was based on Oldsmobile V8 architecture and while virtually all its components were upgraded, it was still totally inadequate. However, for as long as it could be kept running the performance was actually pretty decent, especially compared to the awful-performing gasoline engines of the early emissions era, which weren't much better. At that time, a typical V8 made around 150-160 hp, while the diesel V8 was rated at 125 hp, same as a typical V6. In 1978 all new cars were pigs.

That was the problem with the Olds diesel V8, really: gasoline engine architecture, and gasoline engine performance targets. Totally unrealistic. And the entire blunder was rather ironic, since a few years earler GM diesel technology had led the world. In the Olds diesel program, that entire knowledge base was ignored and instead this half-baked idea was launched.

Originally posted by ciaoduc1
Personally, I would love to see a diesel hybrid car enter the market. A small 2 stroke 3cyl turbo charged DI engine making 60-70hp would fit nicely in place of the current 1.0L gas engine in the Honda Insight. I imagine with this combo, 80-90mpg wouldn't be out of the question.

Do you mean you would love to buy it, or that you would love to see it?

alpa

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Originally posted by McGuire

Policy-wise the biggest obstacle is emissions.

This is not true. The emissions are very different.
Basic diesels make more particles and NO but much much less CO and HC. The NO is lowered down to the petrol level with EGR. All modern diesels are turbo engines, which make a good hp/L level.
There is a trend to introduce throttle into diesels in order to avoid the lean mode, the first reason of a high NO level.
So the only current problem are particles, basically due to DI. Fiat Multi-jet injectors have lowered the level. There are 2-3 exhaust side technologies that are being introduced to the market, kind of afterburners.
There has been a common report of GM/Ford and european car makers about diesel emission. Curriously they all agree on what I've described, even GM and Ford.

McGuire

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Originally posted by alpa

This is not true. The emissions are very different.
Basic diesels make more particles and NO but much much less CO and HC. The NO is lowered down to the petrol level with EGR. All modern diesels are turbo engines, which make a good hp/L level.
There is a trend to introduce throttle into diesels in order to avoid the lean mode, the first reason of a high NO level.
So the only current problem are particles, basically due to DI. Fiat Multi-jet injectors have lowered the level. There are 2-3 exhaust side technologies that are being introduced to the market, kind of afterburners.
There has been a common report of GM/Ford and european car makers about diesel emission. Curriously they all agree on what I've described, even GM and Ford.

It's totally true. You and I are saying exactly the same thing: diesels have a special problem with particulate emissions. They can't meet gasoline standards, so a political decision is made to construct parallel standards for them... with the hope that in the long run a greater mix of diesels in the vehicle fleet will do more good than harm.

J. Edlund

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Originally posted by McGuire
Of course. No trick at all for a turbo engine to pull better BSFC at 2.5 bar than at 2.2 bar. All that says is this particular turbo setup could be made more efficient at the higher MAP. Any turbo system will have an optimal boost level, owing to the turbine and compressor maps and other factors of the calibration. If you take a system optimized for 4 bar and run it at 2 bar it will almost certainly be less efficient eh. But until someone develops a turbocharger of greater than 100% efficiency, it will always be less efficient than NA.

When boost is increased the compressors power consumption increases, the small effciency gains possible during the conditions mentioned above would not affect that. Basicly this shows that increased power consumption of the compressor does not always decrease the efficiency of a turbocharged engine.

In general a turbocharger will only reduce efficiency (knocking related issues ignored) when it increases exhaust pressure more than air pressure, and you don't need a turbocharger with an efficiency greater than 100% to create higher intake pressures than exhaust pressures. On roadcars however, usually the turbocharger is undersized for maximum power to give high low speed torque which results in high exhaust pressure at high power outputs. That is usually not the case of a racing engine though. On a road car the turbocharged engine does however offer a reduced fuel consumption, but that's because of a smaller engine can be used for a given power output (which have been showed in statistics over fuel consumption between serial production NA and turbo engines in small cars).

When the exhaust valve of an engine is opening the pressure inside the cylinder can be above 10 bar and the temperature higher than 1000 degC. This means that by expandning the gases further by for example a turbine (or a second cylinder) it's possible to create almost free power (the only downside being increased pumping losses for the first cylinder). The pumping losses are however very small compared to the power extracted. From an efficiency point of view it would be better to add that energy to the output shaft of the engine (like done on compound engines), but if we used it to power a compressor we can basicly power a compressor without taking any power from the crankshaft.

Originally posted by McGuire
It's totally true. You and I are saying exactly the same thing: diesels have a special problem with particulate emissions. They can't meet gasoline standards, so a political decision is made to construct parallel standards for them... with the hope that in the long run a greater mix of diesels in the vehicle fleet will do more good than harm.

Today there are diesel engines availible with particle traps. Those engines have very low emissions together with a reduced fuel consumption (compared to gasoline engines) and therefore reduced emissions of CO2.

McGuire

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Originally posted by J. Edlund

When the exhaust valve of an engine is opening the pressure inside the cylinder can be above 10 bar and the temperature higher than 1000 degC. This means that by expandning the gases further by for example a turbine (or a second cylinder) it's possible to create almost free power (the only downside being increased pumping losses for the first cylinder).

The key word here being "almost."

We can create almost free power...which is another way of saying we can almost create free power eh.

J. Edlund

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Originally posted by McGuire

The key word here being "almost."

We can create almost free power...which is another way of saying we can almost create free power eh.

No, the keyword is "it depends on".

Most gasoline engines fitted with turbochargers use very small turbines/housings so boost should come early, this means that when the gas flow increase the turbine loses efficiency and choke, hence the exhaust pressure increase more than the boost and this increases pumping losses.

If a larger turbine and comrpessor is used, or if the engine is run at a lower speed, one can reach boost pressures which are higher than the exhaust pressure and the turbocharger then doesn't take any power from the engine crankshaft (only from the exhaust gases), it actually gives power to the engine in form of reduced pumping losses.

The mechanical efficiency of an engine can be said to depend on the friction and the brake mean effective pressure, as a turbocharger increase BMEP but doesn't significantly affect the friction of the engine the efficiency will increase with increaseing boost, which also have been showed by for example Sir Harry Ricardo.

With the gasoline engine, the problem with the turbocharger is that at full throttle they are usually knock limited, and the use of higher boost pressures, or turbocharging must come at the cost of decreased compression ratio, decreased spark advance and a richer fuel mixture, all these does reduce the BMEP while incerasing the fuel consumption. In the case with the Honda that engine was most likely not knock limited as in the case of a diesel and BSFC goes down when boost is increased, or a turbocharger is added.

As for the gasoline engine the typical effect of a turbocharger is that the power output is increased, and therefore a smaller turbocharged engine can be used in the place of a larger NA one. If one compare the BSFC of these engines it will be shown that the NA consumes less fuel at high loads while the turbocharged one consumes less on lower loads. For a typical road car it should therefore be possible to save fuel by using a smaller turbocharged engine instead of a larger NA engine, it has however been showed that isn't so easy to achieve. BSFC maps and explainations of that can be found in for example "Internal Combustion Engine Fundamentals" by John B. Heywood.

McGuire

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Originally posted by J. Edlund
The mechanical efficiency of an engine can be said to depend on the friction and the brake mean effective pressure, as a turbocharger increase BMEP but doesn't significantly affect the friction of the engine the efficiency will increase with increaseing boost, which also have been showed by for example Sir Harry Ricardo.

The turbo does indeed increase friction -- in the molecules of exhaust gas the engine is trying to pump out of the cylinders, and also in the molecules of inlet air the turbo is trying to compress.

I believe you and I have covered this same ground several times before, for example here:

http://forums.atlasf...y=&pagenumber=2

J. Edlund

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Originally posted by McGuire

The turbo does indeed increase friction -- in the molecules of exhaust gas the engine is trying to pump out of the cylinders, and also in the molecules of inlet air the turbo is trying to compress.

I believe you and I have covered this same ground several times before, for example here:

http://forums.atlasf...y=&pagenumber=2

The losses that you talk about are pumping losses and occur during the exhaust stroke. The difference in loss during the exhaust stroke can be equal to, smaller or larger than the difference in loss occuring during the intake phase. Basicly these pumping losses are only a problem when trying to make a turbocharger to produce boost in the low speed range, as one then needs to use a small turbine. Depending on if we will have an increase, or a decrease in pumping losses the efficiency will be affected thereafter. To produce boost pressures much higher than turbine inlet pressures is not a problem, at least not during a given condition.

However, due to the increased cylinder pressures the piston ring to liner friction will increase somewhat just like when one increases the compression ratio.

On large two stroke engines the turbocharger is often used as a spool pump and can thereby offer large reductions in pumping work done by the engine

Powersteer

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Kill me for such a late post (sorry). I used the search option.

If the mechanical aspect of naturally aspirated compression ratio makes it 8:1 at TDC. That should double on the mechanical aspect of a charged engine boost producing twice that of the atmospheric engine. Put a supercharger producing twice the amount of air and the compression ratio goes up?

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That thread was most enjoyable.....I did notice this one little tidbit.

When the waste gate opens the exhaust pressure at the turbine inlet drops and the turbine immediately slows down. But the EGT will remain constant until the mixture or combustion are affected by the drop in intake pressure on the other side of the turbo.

I wonder if when the waste gate first begins to open, the turbine inlet pressure in fact drops and the turbine slows down immediately ....or OTOH, as the vehicle accelerates and the engine mass air flow requirements increase, the opening wastegate vents only enough pressure via a metered/controlled exhaust leak to maintain turbine inlet pressure to not lower or exceed the pre-set inlet manifold pressure as RPM's increase.......so in this context, it would seem that the turbine inlet pressure and/or turbine speed would not actually decrease as engine speed/mass flow requirements increased.....if it did, especially at full throttle, it would be kinda like hitting the brakes when the wastegate opened!

I once attended a pep talk on an integrated manifold/carb/turbo setup at a local speedshop and the "instructor" stated that the turbine is powered predominately by heat generated by the combustion process of the engine.

Heat.....predominately.....interesting! I then asked him if I would notice any difference in turbine rotational speed if I baked the compressor/turbine assembly in an oven at 350 F as opposed to 500 F......glare! I then asked him if there was anything else he wanted to add to the heat deal?

John

McGuire

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Originally posted by Powersteer
Kill me for such a late post (sorry). I used the search option.

If the mechanical aspect of naturally aspirated compression ratio makes it 8:1 at TDC. That should double on the mechanical aspect of a charged engine boost producing twice that of the atmospheric engine. Put a supercharger producing twice the amount of air and the compression ratio goes up?

No, the compression ratio remains constant regardless of manifold pressure. Compression ratio is simply the difference in the cylinder's volume at BDC vs. TDC. If the CR is 8:1 and you stick one atmosphere in the cylinder, it will be compressed 8 times. If you stick two atmospheres in the cylinder, it will be compressed 8 times. The cylinder pressure will increase, obviously, but the compression ratio itself remains the same.

McGuire

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Originally posted by NTSOS
I once attended a pep talk on an integrated manifold/carb/turbo setup at a local speedshop and the "instructor" stated that the turbine is powered predominately by heat generated by the combustion process of the engine.

Heat.....predominately.....interesting! I then asked him if I would notice any difference in turbine rotational speed if I baked the compressor/turbine assembly in an oven at 350 F as opposed to 500 F......glare! I then asked him if there was anything else he wanted to add to the heat deal?

John

That is a heretical view in this forum. Around here, turbochargers are magical thermal energy converters. You and I both know turbos do not turn heat into kinetic energy, they turn hot air into kinetic energy. But that only pisses people off around here.

Powersteer

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I see, the cylinder compressing ratio sticks at 8:1. How far from detonation is a naturally aspirated engine if the compression ratio is say 11:1 on low octane fuel?

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NTSOS

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You and I both know turbos do not turn heat into kinetic energy, they turn hot air into kinetic energy.

Mac,

Yes, and here's a guy that seems to understand the above concept and doesn't spend a whole lot of time fretting over EGT's.

He was ridiculed a few years back by his lack of "understanding".......at present, not many people are laughing and his "rotary muffler" business has become quite successful and covered extensively by the industry.

http://www.ststurbo.com/

C5 Vette

John

Powersteer

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After thinking about it, I find my question embarrasing. I guess Shaun was refering to dynamic compression. However, my question was based on the rule of thumb sort of question that on a certain amount of octane will only get a cetain limit of compression ratio or on what level of compression would tuners be sensitive to on a given octane level.

Sort of like asking would different levels of octane change the air/fuel ratio?

EDIT: Squires turbo system says it has no additional turbo lag. What do you think?

McGuire

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Originally posted by McGuire

The original GMC Detroit Diesels (Series 71, 53, and 92; 1938 to 1980-something) were direct-injected, uniflow two-strokes.

Getting back to the GM two-stroke diesel...I drove past this relic this morning (somewhere just west of Medina, OH) and just had to stop and take a picture. This is an example of the machine that changed, saved, or ruined (depending on your POV) the American rail industry forever, the F Series diesel-electric locomotive, built by the Electromotive Division of General Motors. It put the steam locomotive out of business virtually overnight. I believe this one is an F7 model but I am no rail expert.

The F Series was of course a diesel-electric hybrid using EMD 16-567 engines...16 cylinders, 567 CID per cylinder = 9072 CID. All these General Motors diesels (Detroit, GMC, Cleveland, EMD) were uniflow two-strokes with Roots blowers, from the 2-71 Detroit on up to the very biggest...I don't know how big that would be, but the biggest I ever saw was a 20-645. Seen one, seen them all. They are all essentially identical except for displacement and number of cylinders, though in later years turbocharging was added. Some marine versions produce over 6000 hp.

shaun979

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Originally posted by NTSOS

Mac,

Yes, and here's a guy that seems to understand the above concept and doesn't spend a whole lot of time fretting over EGT's.

He was ridiculed a few years back by his lack of "understanding".......at present, not many people are laughing and his "rotary muffler" business has become quite successful and covered extensively by the industry.

John

Hi John, while the system may sell well and work ok or better than satisfactory, it is definitely a compromised system in terms of throttle response and work harnessed from the exhaust gasses (heat/pressure loss). This is especially so considering the average street car RPM and volumetric flow. Degree of the compromise is arguable without ubiased tests run or very accurate model of the system created but the compromise is definitely evident.

NTSOS

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Hi Shaun,

I guess if you believe everything you read, lag is not a big problem. That FAQ states that since the units are mounted in the rear, the turbine section uses a smaller A/R housing that is sized appropriately for the resultant cooler/denser exhaust flow and depending on what RPM you hit the throttle, the long compressor discharge tubes supposedly take .05 seconds to pressurize.

Did you happen to read the FAQ, he makes some interesting arguments!

Anyway....I agree, it's not exactly what I would call the ultimate in an optimized turbo-supercharged sytem......simply illustrating the value of kinetic energy.........but what will be interesting is following the progress of EFI-1's thread on digital corvettes. Apparently that's his Vette in the photos and it is being converted from a Magnuson supercharger to the twin STS system.......a nice A-B comparison.

http://www.digitalco...0&highlight=STS

John

McGuire

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Originally posted by Powersteer
After thinking about it, I find my question embarrasing. I guess Shaun was refering to dynamic compression. However, my question was based on the rule of thumb sort of question that on a certain amount of octane will only get a cetain limit of compression ratio or on what level of compression would tuners be sensitive to on a given octane level.

As a VERY rough rule of thumb the current limit for 87 octane fuel is somewhere in the neighborhood of the ballpark of around 10:1 to 11:1, depending. There are production engines with advanced chamber designs and control systems (including knock detection) that operate in this range as well.

McGuire

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The Squires remote system is clever and works because the intake column is not really much longer than many conventional setups with intercooler...with EFI fuel inertia and fallout are not an issue and the remote intake plumbing will have charge cooling properties of its own. And the standard package operates at only 6 PSI boost anyway.

Such a layout would not be optimal for maximum output for racing but that is not what this system is for. For real world street performance with minimum headaches and problems this is a very clever approach. As I see it the main point of consumer resistance is all the hardware is hidden under the car, instead of on display in the engine compartment for maximum wow factor. Some people lke to show off. On the other hand, some folks like a clean, cool engine compartment.

Powersteer

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Thanks McGuire. If you don't mind me asking again, you have any clue as the octane ratio goes up how much would the compression ratio follow?

Greg Loco*ck

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Do you know how octane is measured? that's a bit like asking for the relationship between a ruler and the length of a piece of string.

Figure 7 in http://naca.larc.nas...naca-tn-647.pdf probably helps.

McGuire

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Originally posted by McGuire

All these General Motors diesels (Detroit, GMC, Cleveland, EMD) were uniflow two-strokes with Roots blowers, from the 2-71 Detroit on up to the very biggest...I don't know how big that would be, but the biggest I ever saw was a 20-645. Seen one, seen them all. They are all essentially identical except for displacement and number of cylinders, though in later years turbocharging was added.

First, the littlest of the GM two-stroke diesel family, the GMC 2-71...that's two cylinders of 71 CID each, that is, an inline twin of 142 CID, rated from 65 to 75 bhp. They were used in generators, reefers, boats in various capacities etc, farm tractors and even some small package trucks.

While only 142 CID and 75 bhp, the 2-71 engine weighs around 975 lbs. Hey, I got curious once so I weighed one. In other words it is built for extreme duty. I fully believe that if you start up one of these puppies, adjust the governor for 75% load, make arrangements for fuel and fluids to be added periodically, and walk away, you could come back in 200 years and it would still be running fine. It requires only diesel fuel and lubricating oil. The injector system and governor are 100% mechanical. There is no electrical system.

Now here is among the biggest of the family, the 20-675 (13,500 CID) which has been used in a few large locomotives, harbor tugs etc. This one is configured as a generator set to electrify a fair-sized village:

Again, from the 2-71 to the 20-675 and beyond, these engines were essentially all the same, differing really only in displacement per cylinder and number of cylinders.