Turbo and improved economy

[fatman57]

ok - i know a turbo will only add to the load on my engine but hear me out...........

Correct me if I am wrong:

Valve overlap is there for 'scavenging' - this means there is a certain area of the revs of an engine in which the inlet manifold, exhaust manifold and degree of scavenging all come into effect and allows the engine to be really efficient (apparently acting like a 'supercharger' sucking old air out of and fresh air into the chamber).......

only thing is in normal driving it is very hard to keep the engine one speed so the tuning of the exhaust manifold and inlet manifold + scavenging are not used to their full effect.

A low application turbo I have been told can increase fuel economy @ partially open throttle (due to there being less pumping losses).

So my plan is to remove the scavenging on my engine altogether and use the turbo to do everything else. A small amount of exhaust gases should remain having a little Atkinson effect (plus decreasing the warm up time). I Will hopefully have much fewer bits of unburnt fuel being sent out the exhaust (also because since there is now no scavenging there is no chance of it going out the exhaust at the inlet phase as I am sure some of it does at the moment).

I can tune the turbo to do many cool things and will hopefully be able to tune it to idle well as well as giving me as much power as I can get when I need/want it.

Is this all BS or is there something worth working with here.........much obliged for any comments/advice.

[mechman600]

I suspect that a turbo will not increase your economy. Under very light part throttle driving, aka hypermiling, your manifold is under the same exact amount of vacuum as if there was no turbo. Think about your manifold pressure not in terms of vacuum, but in terms of barametric (absolute) pressure, where barametric pressure is approx 30"HG. At light throttle low revs you may have 5-10"HG (PSI = "HG/2) pressure. At WOT you may have 25"HG, or close to barametric pressure. Sort of backwards to a vacuum guage. Add a turbo, and you increase this pressure on top of what you already have. 10 PSI of boost gives you a manifold pressure (absolute) of 50" HG. This will only happen under heavy throttle, a state that we all try to avoid when we hypermile.

However, you may be on to something with messing around with valve overlap. You could possibly increase manifold pressure (ie, reduce vacuum) with an atkinson cycle and a turbo more than you could without a turbo. However, would it idle? In this situation, I suspect that a belt driven supercharger would be a better choice, because it would increase manifold pressure as soon as the engine started to turn over. I believe Mazda had a Miller cycle V6 that relied on a supercharger.
Miller cycle - Wikipedia, the free encyclopedia
Unfortunately, if this engine would have been a MPG wonder, we would all know about it and everyone else would have copied it. But they didn't.

The only time a turbocharged engine has proven to be an asset is when a much smaller turbocharged engine has been used with similar power of a larger NA engine that the car would normally be equipped with. Maybe this could be an option for you.

[gone-ot]

...an analogy: sucking air thru a straw doesn't get any better when you put a propellor at the other end of that straw.

...however, a turbo *will* let you *use* all the octane value of ethanol-laced fuels (E85, etc.) although you might have to "double-gasket" the engine heads to drop the static compression.

[tjts1]

A lot of turbo cars today are more fuel efficient than their NA counterparts.
http://autospeed.com/cms/A_109931/article.html

[KITT222]

The turbo gives you more power to the engine (Star Trek joke hiding in there) without extensive work. Its kinda like cheating. The new Cruze will have 1.4L turbo and a 1.8L turbo to give it more power in tiny engines. In some cases more power is better, so less effort is exerted to get the car moving. But somewhere there is a line where it becomes too much power, and its just excess. The Aveo had 104HP and got 30 MPG HWY. Too little power. Increase by 4, (other modification probably as well) and its boosted to 35MPG HWY, both in a 1.6L engine. Maybe a turbo will be good, but maybe it wont. You just need to know that fine line where MPG peaks, where after MPG decreases.

[Piwoslaw]

As KITT222 mentioned, a turbo gives you more power from a smaller engine. If you just slap on a turbo, then at best you won't notice a difference in fuel consumption (mostly since hypermiler stay in low rpms while turbo kicks in at higher rpm), but if you replace your engine with a smaller turboed unit, then you'll notice an improvement. This is called downsizing and auto companies are starting to get the hang off it. A smaller engine uses less fuel than a larger one (in most cases), plus it's lighter, requires a smaller starter motor, and so slightly smaller battery, etc. Downsizing from a 2.0l to 1.6l turbo you may save as much as 50kg (110lbs) in weight, and maybe 5%-10% in fuel consuption.

[mechman600]

I have often dreamed (a bit twisted I guess) of putting a sport bike engine into a very small car, like a 'Busa engine (1300cc, 170 HP) into an original Mini.

Now I am wondering if you could go cheaper and find a old 1000cc bike - the years when they didn't rev high yet, like in the 70's with 7000-8000 rpm redlines and 75 HP instead of the litre bikes now with 11000+ redlines and 150 HP. Then add a turbo to get it up to the "normal" 100-120 HP that most compact cars "need", EFI/electronicalize it, and see what happens. Maybe stuff it into an older Corolla. Or maybe a turbo'd 70's 750 with 90-100 HP, electronicalized and stuffed into an Echo. I know, I know, it would require much wizardry and probably some alchemy, but if the auto manufacturers are going with small and forced induction, surely we can for cheaper than buying a new one.

I suggest older as opposed to the already high tech newer, because the new bike engines are tuned for too high of revs, and trying to get "cold" camshafts and parts for a sport bike would be 100% impossible. Just a thought. First one to try it wins the prize....

[fatman57]

ye - i am aware fo the smaller engine turbo thing and the miller cycle (which I was also looking into).

I would use it on a four cylinder inline, maybe a 1.6 or a 1.8.

The big thing i need some feedback on is the cam setup - do you think it will work?

A big issue is the idle..........by my laymans logic the pistons will still be sucking in air on the downstroke and since so little goes in for idle the turbo could simply add a little if needed but i doubt it will need to do much for idle, mostly for getting it going........

[Piwoslaw]

Take also into account that just slapping on a turbo may do more harm than good. Engines with fuel injection should have their computer remapped to account for the higher intake pressure (=more oxygen). Also, an engine not built for turbo may not be able to handle greater pressures and torque. This goes for manifolds, valves, pistons, transmissions, etc.

[comptiger5000]

Piwoslaw is right about the computer tuning. However, as long as you keep the boost low (under 5 - 6 psi on most engines), the stock internals should be just fine.