Turbo Boost and FE

[itjstagame]

Gollum, I don't really know how you can say that increased efficiency doesn't neccisarily mean increased FE. Yes, I guess I can understand if you had a very lead footed driver or if the only way to get the increased efficiency was to operate some absurdly large engine that used more fuel to begin with.

But what we're talking about is the same exact engine only with increased volumetric and thermal efficiency. I understand your point that it's going to use more fuel just as a larger discplacement car would, but even if it uses the same amount of fuel as a 3L V6, if it's more efficient it will be putting out either more energy or using less fuel (that's the definition of efficiency).

Of course a 3L uses more than a 1.5L which is the point in proposing a 1L with a turbo that's more efficient that a 2L NA with similar power but with better FE.

Originally Posted by BEEF

mrjetskey,

VW is making a diesel-electric hybrid car that is supposed to get 70 or so MPG. it works on a similar concept that you were describing (I think). they are putting it in a golf or a rabbit. not sure if it will make it to the states. emissions suck (laws I mean). I think that is the plan though, for it to come to the states.

No electric involved, just plain old TDI: http://www.vwvortex.com/artman/publi...nter_319.shtml

[GasSavers_Hal9000]

Originally Posted by Gollum

Oh, and just because a turbo increases thermal efficiency doesn't mean better FE. Thermal Efficiency is about how much power an engine is making for the given heat generated by the engine. This has nothing to do with fuel usage under a cruise or around town condition.

I've got disagree with you here. If engine heat can be equated to wasted fuel energy (which it can), then increased thermal efficiency will mean that you're extracting more energy per lb of fuel burned and converting it to mechanical energy. Wether you use that to accelerate faster, or cruise a greater distance at low throttle openings is primarily up to the driver. Engine design can also affect this but for a given combination, you have a lot of choices (The force is within you.... you must use it wisely... )

What IS true is that a turbo may only increase the thermal efficiency of the engine at certain rpm/throttle openings. The goal is to get the increases in the area where they will be most useful for FE fans. That's a different goal than HP fans.

On another (although similar) note, I've had an idea rattling around in my head for years now.... It's for a true tunnel ram intake that might produce a small amount of positive pressure at highway speeds. It would have to incorporate a turbo style BOV to avoid pressure waves, and would need a long straight stretch to work (my original idea was for a rear-engined VW), but it would probably have the same effect on the highway as a Low pressure turbo would.... If there are any VW owners around who want to do some fabrication, I'll be happy to share my thoughts.

[GasSavers_Gollum]

Oh but the arena of engine power and how it relates to thermodynamics is complex!

For the sake of simplistic aregument, I'm going to use made up numbers to illistrate my point.

Say your air intake temp is at 90 degrees, and the exhaust temp is at 900 degrees and you're actually making 50hp. If we lower the intake temp to 70 degree, all other things being equal, we're now making MORE power.

If we raise the compression and the exhaust temp goes up to 950, we've now INCEASED power.

If we reduce the fuel and lean the engine out it's now making LESS power but it's raised the exhaust temp.

All of these change the thermal efficiency of the engine. So you see power and efficiency aren't always related to the temperature different of intake and exhaust.

A turbo is said to make an engine more thermodynamically efficient because it's converting heat to power, making use of the temperatures the engine is already creating. It will raise the exhaust temps considerably, but it raises the power MORE susbstantially, hence the increase in power.

What this is really doing is increasing the VE of the engine. Naturally aspirated engines are lucky to reach 95% from the factory, while naturally aspirated engines are doing something wrong if they can't hit 110% from the factory.

Wasted fuel energy DOES equate to heat in the exhaust, but there's many other factors that equare to heat changes of the engine both on the intake and exhaust. A turbo modifies both, and helps the engine become more efficient under load, hence extra power.

If a turbo trully improved wasted fuel energy THAT much (which is almost strictly about chamber temps, timing, compression, mixture, and quence) then it would see a DRAMATIC fuel efficiency increase... which I've never seen with documentation to prove it. The dramatice thermal dynamic improvements turbos are giving an engine result in an increase in air for the given volume and heat being generated, thus more fuel. If you can get the turbo to increase air volume without creating too much heat you WILL truly see a slight improvement of wasted heat energy, but it's because of the air temp/air volume ratio more than anything else.

[itjstagame]

I've thought about the ram air idea too. It definately has been known to add a couple HP to racing motorcycles, at the HP they were running it was about 5% increase and only from a fairly quick speed.

I'm sure I could look up the equations, but what kind of pressure could you really expect to see at 55MPH as most of us go. I agree it might help offset the losses associated with going at an increased speed a little, like if you got 65MPH you'll get 4x increase of pressure on the front of your car so now you'll have a bit more boost to your intake, but overall I'd be a loss.

Of course you could always completely seal the entire engine bay off and get a gigantic radiator openning and then all air would have to either be forced into the engine or slow you down as drag. Actually now that I think about that, the more pressure you have from a RAM intake setup the more drag you neccissarily have to be inducing on your car. The drag of course is worse for a giant radiator hole than for a small intake hole (based on openning area). Also would a smaller openning be better because of less drag, or would a larger openning that tappers waaay down to a small intake be better because it increased intake PSI?

[GasSavers_Gollum]

Regular classic ram air intakes like the shaker hoods on mustangs have been known to get as much as 3psi when going fast enough. At highway speeds .5 psi might be more reasonable. Still enough for a gain in efficiency if the fuel is still atomizing well enough.

[itjstagame]

darn you, you just reminded me I still have to experiment with 3 cycle engines using direct water injection. I thought I was making progress on my list of stuff to test but now I have to add that one.

[GasSavers_Gollum]

Doing some map calculations, looks like a T04E 46 trim would get me to about a 2000rpm minimum boost.... Those are cheap enough turbos. Maybe I'll find one and rebuild it for experimentaing sake. Should support my engine up to about 325 hp too, about what I'm shooting for anyways (don't plane on going over 300 crank/250 wheel HP).

It all kinda hinges on the job situation though. I'm about to be in limbo between old and new jobs next week... and I don't have a new job yet. This is what happens when you go back to school. Grrr.

Oh, water injection... Yea. I want to do that eventually once I get megasquirt up and running. Has some amazing possabilities for power AND economy.

[GasSavers_Hal9000]

Originally Posted by Gollum

Oh but the arena of engine power and how it relates to thermodynamics is complex!

Now that we can agree on! But for all the complexity you want to quote, efficiency is efficiency. A net improvement in efficiency of an engine (after taking all the interactions into account) will yield a net increase in the amount of fuel energy converted to mechanical motion. You're not going to get an improvement in efficiency and a decrease in the amount of energy extracted... period. You can't argue that a turbo increases the efficiency of the engine on one hand, then claim that you're wasting as much or more energy than before on the other.... Now it's just a matter of using that increase in a judicious manner.

Originally Posted by Gollum

What this is really doing is increasing the VE of the engine. Naturally aspirated engines are lucky to reach 95% from the factory, while naturally aspirated engines are doing something wrong if they can't hit 110% from the factory.

I don't necessarily agree with your percentages (they seem awfully optimistic to me, but it really doesn't matter for the discussion at hand) but I'd argue that an improvement in the VE of an engine can be translated into better Fuel effiency. In a global sense, the increased power output of engines over the past generation has allowed greater hp from smaller engines than ever before. Those increases are the direct result of increased VE. I dare you to compare the fuel efficiency of a 175 hp V8 from 1988 and a 175 hp 4 cylinder from 2008. I haven't done such a comparison myself, but I'd bet my house that the new 4 cyl has better FE..... Granted, it's not exactly apples to apples, but it demonstrates the point. You can see similar changes through generational changes in small engines. Look at VVT vs non-VVT engines of the same type (D series Honda, or Nissan K series for example) and I bet you'll find a similar result.

Originally Posted by Gollum

If a turbo trully improved wasted fuel energy THAT much (which is almost strictly about chamber temps, timing, compression, mixture, and quence) then it would see a DRAMATIC fuel efficiency increase... which I've never seen with documentation to prove it. The dramatice thermal dynamic improvements turbos are giving an engine result in an increase in air for the given volume and heat being generated, thus more fuel. If you can get the turbo to increase air volume without creating too much heat you WILL truly see a slight improvement of wasted heat energy, but it's because of the air temp/air volume ratio more than anything else.

Nobody ever claimed that the improvement was dramatic. I believe that a few percent improvement is what we've been talking about. Couple that with the fact that people who buy turbo cars or add turbochargers themselves want more useable power and drive more aggressively.... Do you really expect to find documentation to prove it? I think that it'll be up to people like us to provide that documentation.

[GasSavers_Gollum]

The VE figures I mention aren't based on the power potential of fuel. On that rating systems engines are lucky to ever reach 30% That's where fuel efficiency comes from honestly. How much power is it making for a given air volume?

The VE I'm talking about is the same VE rating you'd use to turn a MAP based ECU or see in a Dyno simulation software. IIRC 100% is 100 pounds of torque for every liter of displacement. This is what a turbo is REALLY doing, it's increasing the ability of the engine to make power in relation to it's displacement. It's not making the actual combustion more efficient. That's the REALL efficiency improvement people talk about in regards to adding a turbo. Sure, there might be a small percentage of combustion improvement, but it's small like we both agree on. Worth spending the time on? Maybe. Worth talking to everyone about? I don't think so. Worth dabating about? Hell yes.

If we don't sit here and argue back and forth it's unlikely and testing will be done, and if anyone goes into testing this too optimisted, the real results might be lost in a cloud of dissapointment.


Oh, and I bet a 175hp V8 from the 60's is a lot less efficient than a 175hp modern 4 cylinder... but apples and oranges man. Let's compare a pushrod 4 cylinder engine from the 60's, or the DOHC V8 ford made in the 60's. That's a bit more apples to apples.

Most of the economy improvements over the last 50 years hasn't come from any single factor. Less friction coatings, lighter allows, computer controlled fuel and ignition, and chamber shape are all huge factors.

For a comparison, people were getting over 1000hp out of the Nissan L engine back in the 80's. People are considered lucky nodays to get 800hp out of them. Why? All the research that was done on reshaping the combustion chamber and ports has been lost. Those people have moved onto working with other engines. Even on the most modern engines, 1000hp out of 3 liters is impressive. Even that old SOHC Datsun engine could make some increadible power. It comes down to many other factors than just how many valves you have, and how many amazing technologies your engine employs.

[GasSavers_Gollum]

Oh, I was just thinking about something... On top gear, at the begining of the new season Jeremy did a test. He followed a prius around a track following in a BMW M3. The prius was driven all out, and Jeremy just had to keep up in the BMW.

The BMW averaged 2mpg higher than the Prius.

Why so??? The BMW is more in it's range off efficency being pushed that hard. While the prius was going well beyond it's peak torque to squeeze out the most HP possible, the BMW was just reaching peak torque, and just loafing along easily for that engine.

MPG is about so much more than how efficient an engine is.

As I stated earlier in this thread. The only way I could see a turbo having a significant improvement on FE would be if it's actually IN it's efficiency range while at cruising speeds, which doesn't leave much room if you actually put your foot into it. The turbo will reach the end of it's efficiency and overspool badly. Is it possible to really find a balance? It's worth experimenting with.