leolo007

IS IT POSSIBLE?? My question is based on a comparison I did of and Audi A4 with my Sentra. Both have the same engine displacement-wise (1.8 liter), both have about the same MPG rating from fueleconomy.org but the Audi is considerably heavier, but its turbo. So who thinks its possible. Most people I have talked to say its not possible but thats because they know of cars made turbo for racing purposes only, Im not planning on revving past 3krpm, in fact with a small turbo that spools up quick and gives me some good torque probably wont need to rev past 2.75k or 2.5krpm. What do you guys think?

Fabrik8

If you add power, you will consume more fuel. Most of what dictates an engine's fuel economy is the basic engine efficiency. You can't just magically get more power out and consume less fuel, unless you alter the engine's internal efficiency. This means reducing weight, reducing friction, making flow more efficient, making combustion more complete, etc.

It's kind of like riding a mountain bike on the road. Going to to gym and doing leg workouts might give you more power, but it won't make your slow, knobby tired bike more efficient. Switching to a light, skinny tired road bike and some spandex would. That's pretty stupid sounding but I hope you're getting my point.

Using a turbo for fuel economy is a little backwards, unless you're looking to start with a small displacement engine and then turbo it to get the available power up to the same level as a larger engine. This is pretty popular for small cars in Europe. You can get good economy if you keep your foot out of the gas, but more power is there if you need it. The smaller engine you have, the less fuel it will consume if you don't need the available power. For example, if you need 20HP to cruise along at around town speeds, the extra 140HP you have on tap (making up numbers here) from a Sentra isn't needed, and you're loosing efficiency from the extra fuel, rotating mass, and from the friction. If you start with a little 1.2L to 1.4L engine (which was/is very common in Europe), that 20 HP you need comes with less wasted energy because the engine is smaller. Along the same lines, this is the reason that some of the GM V8s (and to a much more sophisticated degree, the new Accord V6) will turn disable cylinders under light engine loads when the power isn't needed.

If I understand what you're saying, you want to gain fuel economy but adding a turbo. Start with a smaller engine, add a turbo, and then drive economically most of the time. Otherwise you're not gaining anything.

leolo007

Hey Fabric8, I understand what your saying but I think there's more to it. I'd be running a small turbo at around 5psi. I've read thats low enough to not have to increase gasoline. Air would be forced into the engine instead of the engine wasting energy pulling the air in. This would be sorta like making flow more efficient. Why do you want to make flow more efficient? So the engine doesnt work hard pulling that air in. The turbo would take care of that.

I dont think the mountain bike example really relates to this unless you could tell me that a sknny guy burns the same amount of calories riding a bike as a person that works out under the same conditions. I really cant say yes or no.

Let me see if I can give a good example. Instead of acceleration lets assume a constant speed up a hill. Lets suppose I go 3000 rpm to maintain 35mph in 3rd gear. (I have no idea if those numbers are accurate but for this example it doesnt matter) With the N/A engine I have 75hp and 45% throttle, where as the turbo engine would have more horsepower at that rpm therefore throttle is at 30%. So less gas is consumed. This makes some sense to me, not sure I made a good example.

lolfordlol

The audi came with factory turbo so its tuned for the turbo. More power = more fuel sucked in. u cant push more in and expect better gas mileage.

leolo007

So tune the sentra engine for the turbo...

Fabrik8

I think you're confused. You might be confusing the general rule of thumb that you can use the stock injectors at 5 PSI. This only means that the stock injectors still have enough flow capability to be used with a little more power than stock. You need that extra flow for that extra power.
You're trying to bend the laws of physics; you can't get more power out of an engine by adding more air without adding more fuel.

If you're putting out the same power you'll need the same amount of fuel, unless one engine is fundamentally more efficient than the other, which doesn't have much bearing on whether there is a turbo or not. Frictional losses and pumping losses are generally the same whether there is a turbo or not. And again, you still need a certain amount of fuel for a certain amount of power for a given engine efficiency.

You're looking for free lunch, you want more power and less fuel consumption by adding more air. Physics (and thermodynamics) says no. The only free lunch is if you can improve how well the engine turns any amount of air and fuel into output power. This really means losing less power, not adding more.


My mountain bike analogy is all about efficiency. No matter who rides the bike, there are the same losses because of the weight of the bike, friction of the tires, air drag from the riding position, etc. Those things are constant no matter whether you are skinny or a bodybuilder. A body builder may be able to overcome them better, but they are still wasting power.
A road bike is lighter, lower friction, more streamlined, and because of that it makes much better use of the power that you put into it, no matter whether you're skinny or a bodybuilder. That was my point. It was a bad example I guess, it didn't work out the right way apparently.

99CIVICFTW

more air going into your motor means more fuel. thats how a motor works.

sircrx

Ok here you go. Lets put it this way. If you get a turbo and never use it. Therefore never going under a boost load you will not get any better gas mileage nor will you considerably hurt your gas mileage. If you don't use the boost you don't use more fuel. The second you push air into the motor (not having the motor suck it in) you are under boost and have more air in the motor. With more air inherently you will get more fuel. If not you have a problem. If you want to add more air to your engine without adding fuel just to get maybe 2mpg more out of your car then say goodbye to it's reliability.

You sound like there is not really a way any one of us is going to convince you otherwise since you say that a couple of people have told you already that it is impossible. It IS impossible but I am just another one of those people and you will have to try and fail before you realize it is true. More air = More fuel. there is no way around that. Yes even at 5psi that is still + 5 pounds per square inch of air that the motor is not used to seeing and will inherently send more fuel to compesate.

Yes your little uphill comparison makes sense but you also have to realize that with the NA at 45% it is sending a certain amount of fuel because the air to fuel mixture is at it's best. Now the turbo engine at 30% going up the same hill and same RPM is ALSO sending the same amount of fuel because fuel is not governed precisely off of the gas pedal and how far you are pushing it down. The computer is what actually sends the fuel after calculating how much air to fuel mixture it is reading from various areas. With that added 5psi and more power would come more fuel at 30% throttle. There really is NO way of adding more power to your car without consuming more fuel. But you don't have to believe me. Go ahead and spend the money on making a turbo kit work on the car and hopefully spend enough to where it is reliable and try to make sense of spending a couple grand to possibly save 2mpg which I still say is impossible.

Fabrik8

Yep, my point exactly.

You're not buying anything by adding a turbo to the same engine and not using it, then when you use it you'll consume more fuel from the extra power.

Fabrik8

Physics/thermodynamics says:

Rule #1: For every pound of air, you need X pounds of fuel to have proper combustion.

Here's what you're trying to do:

Add more pounds of air, and not add X more pounds of fuel, because you want better fuel economy.
You've just violated Rule #1. I don't need any more rules than that, because that's the one that matters.

I love making dumb analogies.
It takes a gallon of paint to cover a room. If you use less than a gallon, you can't cover the room all the way, and the walls patchy and bare.

I give you 3 gallons of paint so you can paint 3 rooms.

You want to paint 5 rooms. You would have to ask me for more paint, right?