so if i say i have 2 identical cars, except one has a turbo and one does not. Who'll get the better FE if driven identically?
I'd say it totally depends on the driver. The downside of turbochargers from an FE standpoint is that it makes it much more tempting to have fun... More fun to go zipping away from lights, accelerate around slow traffic, or just get on the gas to hear the sound of the turbo or the beautiful exhaust note. If you're like 99% of the population, even if the turbo engine would do better when driven conservatively, you'll end up giving in to temptation and driving harder than with the NA car (ruining your mileage in the process). The human factor is probably the biggie here.
The only way I can see to solve that problem would be to limit the amount of boost the turbo produces (thus limiting the power on tap) to a very low number.
If the engines are identical, the non-turbo one will probably do better. The whole reason turbos are associated with fuel economy is because you can use a smaller engine.
It lets a small engine provide a drivable car. I used to have a VW Jetta TDI (1.9L turbo diesel). Operating correctly, it had about 95hp and about 140lb-ft of torque. It drove very nicely, good acceleration, capable of flying on the highway (at least 110MPH... um, no officer, I never tried that, I swear).
For a while, I had a fault that disabled the turbo. The car was a slug without it. It was drivable, but felt like a late 70's econobox. Definitely not the image VW was going for. Putting a turbo on increases the power more than it decreases the efficiency, so a small (say, 2.0L) engine with a turbo that performs like a 3.5L engine but uses less fuel. I wish they would bring back ideas like the old Chevy Sprint... it came (optionally) with a 1.0L Turbocharged engine that got nearly 50MPG and was still fun to drive.
EDIT: I'm not necessarily saying that the Chevy Sprint turbo was a fantastic car, but I think that a modern econobox (Honda Fit, Toyota Yaris) with a modern very small turbocharged engine (say 1.2L) could be really awesome.
EDIT 2: Wow, two posts between mine and the one I saw as most recent. I must be a real slow-poke today. Oh yeah, I had to answer the phone while I was writing.
I happen to drive a car that provides a great comparison; 1990 Plymouth Laser, which is the same as the Mitsubishi Eclipse and Eagle Talon.
From 1990 through 1994 they made this car with a turbo 2.0L and non turbo 2.0. There were only a few differences between the NT and turbo versions;
NT had slightly milder cams and different compression (9.0:1 vs 7.8:1).
Here's the gov't rating for the 1990 Laser turbo. http://fueleconomy.gov/feg/calculato...lumn=1&id=6573
19mpg city, 26mpg highway, 22 combined.
and here's the gov't rating for the 1990 Laser 2.0 NT. http://fueleconomy.gov/feg/calculato...lumn=1&id=6574
The two cars were also very similar in weight with the NT being slightly lighter at 2650 lbs. NT came with 135HP and 125 ft.lbs at 5k rpm, while the turbo model has 195HP and 203 ft.lbs at 3k rpm. Notice the low rpm torque advantage?
A significant difference also shows up in their transmission gearing. vfaq.com/mods/Trannies.html
The NT was given lower gearing and lower final drive.
My theory is that the NT needed lower gearing because it didn't make much torque down low, so gearing had to make up for it. The turbo car makes excellent torque at low rpm ( similar to VW's turbo 1.8 and turbo 2.0 engines, which have their torque peak at 1800 rpm.)
which makes it easy to shift early and saves gas.
My friend has a turbo supra (1987) and has seen over 35mpg highway on more than one occasion and his car is pretty worked over. Will make nearly 380 hp at the rear wheels (I've witnessed it make 371 and now has more tuning done).
I just bought a 96 Miata w/ a DIY turbo install halfway done, when I finish the install and begin driving it, I'll have something to report of my own as far as turbo and FE is concerned.
I think the note about the higher compression engine making more hp at any given throttle setting (with boost factored out) is a bust because to my knowledge, there isn't a single turbo production car on the road that uses the SAME engine as its non-turbo counterpart. Like DRW has said, the NT to Turbo models had variations in cams, as well as timing, fuel injectors, and in many cases valves per cylinder. It's almost illogical to think two engines could be identical aside from a turbo added to one because it's just not that simple.
My Miata for example, yes, the engine was originally non turbo, and now a turbo has been added, but the exhaust system is totally different now, the injectors will be nearly double in cc's, and MUCH tuning will need to be done.
another interesting experiment would be to take a stock engine with pretty good FE and shave the heads to see if there is any merit to the whole increased compression ratio getting better FE.
I understand that it isn't that easy to do. you could also just get a thinner head gasket to prove/disprove it. I know that guys have been doing that for years for more power. I think if you had a dyno sheet of before and after, you could determine if there would be FE gains by the torque slope. if the torque went up (especially in the lower rpms) then you would see FE gains.
like I said though, it wouldn't be cheap to do ether and possibly not worth the gains depending on how much they are
Be the change you wish to see in the world
Increasing compression should help- The 1984-87 CRX 1.5 HF engines had a few more tenths higher compression ratio than the non-HF 1.5 engines made during the same years.
The only issue with shaving the head on an OHC engine is that you end up with slightly retarded valve timing from. The gains from increased compression ratio would probably outweigh any losses from retarding valve timing a tiny amount.
Valve timing is an easy item to fix. Adjustable timing gears are a dime a dozen nowadays. I'd even go so far as to suggest that it would be a good addition to a stock engine to really tune things for max FE. Most of the time, manufacturers adjust their engines to meet parameters other than FE and power. Noise, emissions (especially cold start emissions), and driveability are usually the most important factors. I've seen examples of otherwise stock engines that gain more than 30 hp just through adjustment of the valve timing.
Being a mechanic and working on small engines in the 65 to 240 hp range,Watercraft.Higher compression will give better throttle response,a bit more acceleration and actually slightly less to speed on two identical engines.The only problem with this formula is it is two stroke engines I am talking about.I would think the optimal car would be an electric driven, powered by a small turbo diesel running at its optimum rpm and power output, charging the battery ,and being off when not charging or running at the set rpm when running, regardless if car is stopped , going uphill or down hill.>Marvin
Being a mechanic and working on small engines in the 65 to 240 hp range,Watercraft.Higher compression will give better throttle response,a bit more acceleration and actually slightly less to speed on two identical engines.The only problem with this formula is it is two stroke engines I am talking about.
Same basic rules apply for 4 stroke engines. Higher C:R > higher output, better throttle response, (better FE as well), and better volumetric efficiency. The only real hitch is (like I mentioned in an earlier post) the human factor. It's much harder to resist using HP you have than HP that you don't have. That alone makes it hard to quantify the benefits of increasing C:R in my opinion and even if the engine is more efficient, if the driver doesn't have the restraint to continue driving conservatively, or doesn't realize that they're driving more aggressively (just as common in my experience), then any benefits will be lost and then some.
I think I've shown that restraint is possible.
My car is putting out around 275HP and I've driven multiple tanks in a row without going WOT even once. Yes it's boring, but so is my commute.
Stomping on the 'GO!' pedal gives instant gratification, but restraint yeilds long term rewards.