Turbocharging? Vacuum? Too much thinking. Smart people unite

[ar5boosted]

Originally Posted by Fingie

i own a GT-FOUR. It's 20 mpg at best for me lol

Yeah, but which one.

The ST185 with TVIS [mine] could do 900km/600miles on a 60-litre tank.

Then the fuel consumption went up profoundly on the ST-205 when the TVIS was removed.

Now, mine has the ST215W engine, and fuel consumption is expected to be 20mpg for sure. I think the ST185H was specced at 33mpg iirc and I was certainly able to get that.

[rwhyde]

Sorry, I am late to this party/question. Stepping back to the original question of identical engines, one with turbo and one without, which gets the best mileage (also assuming same gearing), the answer to me lies in which one puts the LEAST amount of air in the cylinders, thus requiring less fuel. If your turbo puts more sir in the cylinder, then more fuel is required to attain a perfect A/F ratio. This is why manufacturers are moving to smaller displacement along with the turbos to get as much if not more power and then equal or better economy. Of course some direct injection and such help as well. When you can reliably get 400hp from 2 liters, then who needs mega displacement. But i digress, back to fuel economy. Certainly upping compression, along with direct injection to avoid detonation and variable cam timing, all are key elements in optimizing fuel economy and still offering performance when required to keep things fun. One of my earliest mentors told me that the best fuel economy was attained at the peak torque (not hp) of any engine. OF course that was long before variable valve-timing was in everything.

[ar5boosted]

Originally Posted by rwhyde

Certainly upping compression, along with direct injection to avoid detonation and variable cam timing, all are key elements in optimizing fuel economy and still offering performance when required to keep things fun. One of my earliest mentors told me that the best fuel economy was attained at the peak torque (not hp) of any engine. OF course that was long before variable valve-timing was in everything.

There's other things that they do with Direct-Injection like doing separate sprays with the injectors to cool the cylinders.

Then they can change the AFR in a way that's not possible with port-injection so they can run both lean and rich whenever needed.

[TurnOne]

2004/5 Mazdaspeed Miata 6 speed vs NA 6 speed show 2 mpg (combined, city and highway) advantage for the NA model. 9.5 compression ratio compared to the NA's 10.0.

Only way I can see a normal factory turbo car achieving better highway mpg is to use a smaller displacement engine. Smaller motor means less friction, right? Assuming both are putting out the same HP at (say 60 mph for example) to make the car overcome the friction of going down the road.

Adding a turbo would only add power, and any theoretical gains would be wiped out as soon as you boost one time more than you expected. Everyone mashes the accelerator to the floor sometimes right? I have a bunch of vehicles. Some turbo, some diesel turbo, some NA. Most with manuals, one with TC and one with CVT. Everyone of them gets the full throttle treatment sometimes.

[ar5boosted]

Originally Posted by TurnOne

Adding a turbo would only add power, and any theoretical gains would be wiped out as soon as you boost one time more than you expected.

Turbochargers increase the overall thermal efficiency of the engine.

What that actually means is technical and debatable.

Without changing the AFR you won't be getting better fuel consumption, that's agreed.

There's two interesting aspects of turbo's. The first is running speed. If you are in Europe and you want to drive 130kmh everywhere then a turbo engine is your friend.

The other aspect is with Turbo-Diesels.

Every Diesel engine is made more efficient by having a turbocharger.

The main issue with turbocharging on petrol engines is probably the thermal management. I wouldn't say I'm an expert. VW have lots of engineers that really seem to have cracked the fuel efficiency thing in an admirable way.

[Mustang Matt]

I'm really confused here.

When I think of cruising, I am imagining cruise control set at 60 mph on a road trip.

The thing about turbo cars is that while the wastegate is near 0 vacuum it's open and not building boost

[ar5boosted]

Originally Posted by Mustang Matt

When I think of cruising, I am imagining cruise control set at 60 mph on a road trip.

The thing about turbo cars is that while the wastegate is near 0 vacuum it's open and not building boost

I'm pretty sure that the graduation of the wastegate on small-efficient-turbo cars is done a little different than on traditional n/a cars.

I'm sure that some level of boost to help air into the combustion chambers is available well before 0 vacuum. I think that the Europeans have done a bit of tuning on the wastegate controllers to make them better/more-fuel-efficient for 60-80mph cruising.

I'm just requoting some common industry discussion here so that I don't say the wrong thing :

"The Bottom Line
So just how much fuel can a turbocharged gas engine save? Well, that depends, of course, on what you're comparing it to and whom you ask. It's safe to say, however, that a small, modern, gasoline-turbo engine will save about 8 to 10 percent over a larger engine that makes similar power and torque. There are side benefits to turbocharging small engines too--such as reduced weight. Ford's Brett Hinds says that the upcoming EcoBoost V6 is 30 pounds lighter than a V8. That may not sound like much, but every bit counts. And that's especially true, as automakers will continue to hunt for even more fuel efficiency in the coming years."

Top 5 Turbocharger Tech Innovations: The Truth about Fuel-Sipping Turbos

[benbenben]

This one is straight forward and fairly simple to answer. For identical cars with the exception of one running a turbo, when cruising at some set speed over the same conditions, the non-turbo car will return better fuel efficiency.
.
This can be seen if the throttle position is considered. There will always be more throttling/pumping losses with the turbo vehicle since the throttle needs to be more restrictive. The non-turbo engine gets to operate with less throttling to make the same power.
The non-turbo also has less back pressure and also weigh slightly less, though that is a very minor effect.
.
If the non-turbo car is optimized with a higher compression ratio, it will get even better fuel economy.

[ecocruze]

Been peaking in on this thread time to time. I have never truly understood why the turbos have lower compression. Why not keep the compression the same? Probably a dumb question but there it is lol

[ar5boosted]

Originally Posted by 14'ecocruze

Been peaking in on this thread time to time. I have never truly understood why the turbos have lower compression. Why not keep the compression the same? Probably a dumb question but there it is lol

Well you are right in what you are thinking. That's because they were configured for more power.

Let's say you want to increase a 2L engine from 125hp to 250hp. To do that implies that you need space in the combustion chamber for twice as much air.

So you'd lower compression to allow for twice as much volume of air. Because you must otherwise the engine will explode.

Lot's of these new fuel-efficient engines don't really run lower compression engines at all although they are turbo-charged. In addition, the turbo doesn't really add a lot of power. The amount of power they actually add would be traditionally considered as pitiful. Typically it's only 30-50%.

However, the turbocharger is to be technically accurate, adding another two cylinders to the engine. [The inlet and outlet chambers of the turbo are technically speaking a cylinder each].

So a typical 3-cylinder 1.0T engine is actually a 5-cylinder 1.0 engine. A four-cylinder 2.0T is actually a 6-cylinder 2.0 litre engine.

[Doing my best to follow what the Europeans are up to]