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Better mileage in turbos, how do they work?
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larso  
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PostPosted: Thu May 23, 2002 4:39 pm    Post subject: Reply with quote

Would it be best to drive a turbocharged car over a certain rpm at all times? I know with NA cars it seems that coasting 2000 RPM is a good limit for getting good mileage, but anything under 2000 RPM seems to lug and chug and put more wear on the engine. With turbo's and all their science and crap that goes on with the engine, is it best to rev the car at a higher rpm? whenever the boost starts to build up? would it be wise to drive at 3000 RPM in the 931, 2800 RPM, or 3500? Or if you are not accelerating at all, would it still be best to drive it a 2000 (highway), and in the city maybe 3000? I can't seem to come to a consensus since the turbocharger does aid the engine.
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kyhm  
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PostPosted: Fri May 24, 2002 9:33 am    Post subject: Reply with quote

I'm no expert, but it makes sense that running the engine at the lowest RPMs that don't lug it should give the best fuel economy. Most (economy, non-performance) automatic transmissions seem to do this, so it makes sense.

Talking about cruising speeds and fuel consumption, you get into aerodynamics. There's some good tutorials out there, which explain the relationships between speed, drag, and power, I recommend http://www.miata.net/sport/Physics/06-Speed.html. The horsepower required for a given car to cruise at a given speed can be calculated, though it's ugly. However, if I've got the math right, calculating the difference in horsepower between one speed and the next is pretty easy.

FWIW, on modern highways I've been known to cruise at about 3250 in 5th, which is about 145kmh, which is a pretty bad habit considering the highest speed limit in BC is 110... In town my revs vary between 2000 and 6500...

If anyone's interested, I'll dig out some numbers I've got stored away somewhere and post 'em.

_________________
Morgan Hughes
'79 924 (10:1 comp project)
'81 931 (car with attitude)

[ This Message was edited by: kyhm on 2002-05-24 08:33 ]
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numbers  
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PostPosted: Fri May 24, 2002 9:44 am    Post subject: Reply with quote

Turbo or not, the lower the revs, the lower the fuel consumption. It is a straight mechanical relationship. I simplest terms, every time a piston goes down on an intake stroke, it sucks in a given volumn of air and fuel. More strokes, more fuel.
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larso  
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PostPosted: Fri May 24, 2002 2:01 pm    Post subject: Reply with quote

THe way I'm thinking is this: the engine does not have to work as hard when it has the boost, causing the engine to require less fuel.

You can turn your car completely off (ignition off) and coast down a hill in gear, if you go up to 3000 RPM you will not be wasting ANY fuel, becuase ignition is off.


Just as driving up a hill at 2000 RPM will cause you too lose more fuel than 2000 RPM DOWN a hill.

SO THERE STILL must be some area in which you can save mileage. It's a little bit confusing, but just think it out...turn the car off at 3000 RPM, your engine is still sucking in lots of air, but does not need the fuel. Go down a hill at 3000 RPM, and your engine does not need as much fuel as going up a hill at 3000 RPM. It is not a direct relation ship if going up a hill at a constant RPM requires more fuel than goign down a hill at a constant RPM. The engine carries more momentum, and therefore does not "ask" for as much air and fuel on the next stroke.

The other factor I am worried about is compression. The less boost you have, the less compression you have. so if driving around a 2000 RPM causes me to have less compression, the engien is less effecient. All the jap cars these days run high compression to get better mileage.
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dwak  
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PostPosted: Fri May 24, 2002 10:15 pm    Post subject: Reply with quote

The question here that begs to be asked, then, is why the turbo gets better mileage? Especially if, turbo or not, the engines are just pumps that consume x amount of fuel to produce y amount of work at 3000RPM.
Over time, an unscientific observation of mine is that turbo'd engines don't seem to last as long, but I digress.
dwak
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Geddy T  
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PostPosted: Fri May 24, 2002 11:33 pm    Post subject: Reply with quote

I commute about 40 highway and freeway miles to work and back every day. Half of this is at 70-80mph (my speedometer doesn't even work, I'm just taking that from the charts in the back of the manual, so let's just talk rpm--so just under to at 3000 rpm), the other half is right at the bottom of 5th gear at 2000 to 2200 rpm. You would think that this would yield the best gas mileage, but not so. I consistently pick up 21 miles to the gallon doing this. Then one day I found myself cruising through Eastern Washington on I-90 at about three in the morning throroughly enjoying the deserted road (and, more importantly, deserted cop hideouts) at 4300 rpm all the way from Spokane (pretty much Idaho) all the way over the mountains to Western Washington. The mileage? 25. This was in an '82 turbo. Somebody mentioned that the best efficiency is to be had right at the lowest rpm in the highest gear without lugging (generally accepted to be 55 mph--which was why this happened to be the speed limit for so long in the states). It had always been my theory that with a turbo, the highest efficiency would be found at higher revs than with an N/A. A previous poster was correct: compression ratio directly affects volumetric efficiency which directly affects fuel efficiency. During every engine rotation, 75% of the fuel's energy is wasted to friction and heat. If you can get more fuel to go to work per crank rotation, these losses are slightly minimized. Therefore, it was my assumption that a turbocharged car would get the best mileage at the lowest rpm AFTER the turbo has reached full boost in the highest gear. It's just my theory, and my mileage on that trip confirmed the theory in practice to me. Maybe it was a freak thing, though. Who knows.
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Paul  
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PostPosted: Sat May 25, 2002 3:28 pm    Post subject: Reply with quote

Dwak,

If all a turbo did was to raise the compression ratio, they could never be any more efficient than a NA motor with the same compression ratio of the turbo motor at full boost.

They provide greater efficiency by turning the heat in the escaping exhaust into work that drives the compressor. In any motor, the more heat you convert into work, the more power you get.

[ This Message was edited by: Paul on 2002-05-25 14:31 ]
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Pat  
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PostPosted: Sat May 25, 2002 4:45 pm    Post subject: Reply with quote

Actually from my understanding of a turbo it uses the exhaust gasses not the exhaust heat, They also increase volumetric effeciency for the given engine ( not compression which is related to volume of the cylinder at TDC and BDC ). As for fuel economy, drive like scrooge
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Paul  
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PostPosted: Sat May 25, 2002 11:49 pm    Post subject: Reply with quote

Pat,

Yes, a turbo gives the hot gasses another place to expand and release heat, thus increasing the volume of the process of converting heat into work.

Here's a good explaination of how turbo's increase power:

http://www.dsm.org/archives/1997/06/19970618.txt/28.html


Or the whole site at:

http://www.dsm.org/menu.epl?item=246



[ This Message was edited by: Paul on 2002-05-25 22:54 ]

[ This Message was edited by: Paul on 2002-05-25 22:58 ]

[ This Message was edited by: Paul on 2002-05-25 23:26 ]
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larso  
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PostPosted: Sun May 26, 2002 10:34 am    Post subject: Reply with quote


Thanks Geddy T that's what I was aiming at trying to figure out...I've also heard about torque having a BIG relation to mileage...The 931 torque Peak is at about 3000-3800 RPM. I heard that you should always shift right around the Peak torque to get best mileage. It makes sense. SO for accelerating, going to 4000 RPM in the 931 is probably a good idea. Now that torque curve was for the EURO 931, I'm not sure of the US 931. In the haynes it says the peak torque for the 931 is 3000 RPM, I'm not sure if that is right. Anyone have a US spec owners manual? I know the torque curve was in my manual but I know longer have it.

I've read places that you are supposed to look at the torque curve, and find where it "steeps". Right after the steep point where the torque curve starts flattening is a good place to be driving, anywhere before the end of the up steep, or after the down steep is not good for mileage. Diesel engines have torque at really LOW rpm, and that is why driving one of those golf diesels would in fact get you better mileage if you cruse at 1300-1800 RPM. The best car to have for mileage, is one that turnstorque at low rpm: then you get the advantage of low rpm (less air being sucked in) also. With the 931 you get the disadvantage driving at 3500 RPM of higher rpm (more air being sucked in) but to keep the car up to speed, this is the rpm that will require less torque to keep the car at a constant speed. I dunno I guess I'll have to just watch my mileage and do some tests. The acceleration part is easy to do, but cruising RPM is a little bit harder to figure out with all the variables...such as maybe going to a little lower RPM to sacrifice some torque (right below the peak on the torque curve), but gain the advantage of less air being sucked in. A boost gauge would also be nice since boost takes tons of load off the engine. That's why cruising speed is complex, because you don't get that much boost at any rpm if you are cruising, boost only happens in big amounts when your accelerating. Complicated complicated, a mileage meter and a piece of notepad would be an even better solution, maybe it's time to make one of those oxygen meters up that hook up to the OX sensor.

I was always wondering why RICK MAC got so good mileage, all his 32 MPG claims...Because I know for SURE that RICK MAC doesn't piddle around at 1800 RPM.

As for how turbo's work, I think it is the actual "heat" that turns the turbine moreso than just the exhaust gas "flow" coming out of the engine. There is some debate about this..but the heat may actually get inside the turbine and force it to move faster than the actual exhaust gas movement itself. If you have ever put your hand above the exhaust manifold in a 931, you can feel the air move...imagine this, but 1000 times stronger. I believe that it is more the heat that actually turns the turbine than the air being pumped out of the engine. When you put you hand behind a tail pipe, there's not that much movement coming out, it's more expansion of the heat that forces the turbine to move around and carry momentum. Without the heat, the turbo wouldn't work..imagine an engine that pumped out air at 80 Ferenheight, it wouldn't turn the turbine very much at all, it would maybe get you 0.00004 pounds of boost. Of course they say that turbos spool up faster and get better boost if they are coupled directly to the engine, because the heat flame is projected directly onto the turbo...but have a look at the 951, the turbo is not coupled anywhere near the engine and it still gets a hell of a lot of boost. I think the heat in the exhaust from turbocharged cars is so high that a turbo not coupled to the engine will not make a huge decrease in performance. Maybe only if the turbo is big enough or more effective such as a big diesel truck or formula car.
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dwak  
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PostPosted: Sun May 26, 2002 1:12 pm    Post subject: Reply with quote

I think the efficiency aspect of the turbo is due to the utilization of exhaust/air flow to drive a turbine which in turn drives the intake compressor which in turn increases flow into the engine...... but then if you're increasing flow, you're using more fuel....but then you're going faster....shit, I don't know...I guess it's an efficiency thing and a turbo produces more power for the same fuel.
The heat being positive...I'm not sure I buy that....especially when we need intercoolers to cool intake down.
dwa
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Geddy T  
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PostPosted: Sun May 26, 2002 2:38 pm    Post subject: Reply with quote

You guys are on the right track. Turbos increase volumetric efficiency in the cylinders by cramming in more air/fuel and getting more out of each rev. They do this for free by using waste heat for input energy, thus raising the overall efficiency of the engine. There was a little debate going on in the previous few posts that was unnecessary. Some thought that it's the exhaust pulse pressure that pushes the turbine, others thought that it's exhaust heat that pushes the turbine. These are the same thing. When the compressed mixture in the cylinder is ignited, it heats up rapidly and expands, doing work on the piston. Just because the piston reaches the bottom of its stroke doesn't mean that this expansion is finished. Hot exhaust is forced into the turbine housing where it still wants to expand. The heat of the exhaust causing the gas to expand is what drives the turbine. A turbocharger will always be less effective if it is located farther from the exhaust port unless you've got some super insulation or something. The hotter you can get that gas into there, the more of that heat energy will be converted into torque in the turbine. In simplest terms, a turbocharger is a means of converting waste heat into useful work.
As for the mileage of a car, there is one other aspect that I forgot and that is aerodynamic drag. This builds exponentially with speed, making it only a factor at high speeds, and along with friction are the only limiting factors to a car's top speed. Just as a turbo is free energy, thus increasing efficiency, aerodynamic drag is "free resistance," decreasing efficiency. Now, you have to be going pretty fast for drag to start really hitting efficiency, so it might not be a factor at the speeds close to where maximum efficiency is achieved. But who knows.
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dwak  
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PostPosted: Mon May 27, 2002 2:23 am    Post subject: Reply with quote

Well put Geddy, of course hot air expands, stupid of me. It also indicates that the 931 turbo placement is more efficient than, say, a 951's. Interesting. A double edged sword though due the negative effect of heat on intake gases. More efficient but certainly more of a pain in the ass for service accessability.
dwak
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Rick MacLaren  
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PostPosted: Mon May 27, 2002 2:38 am    Post subject: Reply with quote

Hey Lars,

You asked "I was always wondering why RICK MAC got so good mileage, all his 32 MPG claims...Because I know for SURE that RICK MAC doesn't piddle around at 1800 RPM."

Nearly one word answer: "Break-in period", used 2 increments of 4000 RPM MAX for 1000 KM, 4500 RPM MAX for 1000 KM.

City break in was first, highway break in second. On the highway, seldom went over 90 KPH just cause of pure fear of more expenses.

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Rick MacLaren  
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PostPosted: Mon May 27, 2002 2:40 am    Post subject: Reply with quote

Hey Lars,

You asked "I was always wondering why RICK MAC got so good mileage, all his 32 MPG claims...Because I know for SURE that RICK MAC doesn't piddle around at 1800 RPM."

Nearly one word answer: "Break-in period", used 2 increments of 4000 RPM MAX for 1000 KM, 4500 RPM MAX for 1000 KM.

City break in was first, highway break in second. On the highway, seldom went over 90 KPH just cause of pure fear of more expenses.

I like Geddy's answer, but I just thought that my mileage was high because of an exceptionally good balancing job on the engine (i.e., rods, pistons, crank, pully, clutch all turned) and my more gentle break- in driving habits along with a lower compression motor (8:1).

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