Article: adding a turbo to improve efficiency

[Daox]

You seem to be looking more so at highway driving. The larger peak BSFC area will definitly increase fuel economy in city driving where you are doing a lot of acceleration at lower speeds.

You are correct for highway driving though. However, if you change your gearing you can take advantage of that low rpm torque on the highway to gain FE, or you use pulse and glide.

[JohnnyGrey]

The only thing your map shows is that you could possibly retain your stock FE while accelerating faster. That's why the turbo stretches those BSFE zones upwards. To get more fuel economy, you need to stretch them downwards. A smaller engine does just that. Besides, I doubt even the most feeble economy cars need boost to reach 25mph quicker.

My four banger can crank out 130lbft of torque quite efficiently. Problem is, 90% of the time, I can't use it without soon ending up at an aerodynamically inefficient speed. While accelerating in the city, I can easily keep up with traffic while in vacuum, under 2K rpm. I have no use for boost, and I wouldn't suggest lugging an engine under boost.

[Duffman]

Originally Posted by diesel_john

but the air fuel ratio on the diesel can be as high as 200 to 1. so at light loads i am pumping a lot of extra air for no reason.

what i want to do is reduce the effective displacement at light loads.

Sorry to jump in this late.
The lean nature of the diesel is one of the 3 reasons they return more MPG than gas engines. The only way that reducing displacement could help a diesel is physically removing the friction of the parts (ie taking pistons and bearings out).

Some of the other posts are implying to use a turbo to eliminate pumping losses in a gasser, this is really not possible. To eliminate pumping loss vacuum you need to fill the cylinder with something, either air or EGR. If you go the air route you have to lean it out, if you dont then you have effectively just pushed the gas pedal farther to the floor as you are adding fuel to keep 14.7:1 A/F ratio. You dont need a turbo to lean out the mixture.

One thing you could do is advance the cam for an earlier intake opening so that the vacuum is smaller for a shorter duration of the piston downstroke. But this is opposite of the atkinson cycle so you couldnt do both.

[JohnnyGrey]

Originally Posted by diesel_john View Post
but the air fuel ratio on the diesel can be as high as 200 to 1. so at light loads i am pumping a lot of extra air for no reason.

what i want to do is reduce the effective displacement at light loads.

Diesels don't really pump air, they move it. A diesel engine takes air at ~14.5pisa and evacuates it at ~14.5psia. A gasoline engine does pump air because it takes it from anywhere from ~4 to ~14.5 (best case WOT) and evacuates it at ~14.5 pisa.

[tasdrouille]

Originally Posted by JohnnyGrey

The only thing your map shows is that you could possibly retain your stock FE while accelerating faster. That's why the turbo stretches those BSFE zones upwards.

That will increase your overall FE.

To get more fuel economy, you need to stretch them downwards. A smaller engine does just that.

I agree, that's what I said in my previous post.

Besides, I doubt even the most feeble economy cars need boost to reach 25mph quicker.

Imagine a corolla with a say 1.3 liter engine? Impractical if N/A. Add a turbo and you've got a killer FE car.

[JohnnyGrey]

Some of the other posts are implying to use a turbo to eliminate pumping losses in a gasser, this is really not possible. To eliminate pumping loss vacuum you need to fill the cylinder with something, either air or EGR. If you go the air route you have to lean it out, if you dont then you have effectively just pushed the gas pedal farther to the floor as you are adding fuel to keep 14.7:1 A/F ratio. You dont need a turbo to lean out the mixture.

Turbos only eliminate pumping losses when employed on smaller engines in the same power range. For example, a 2.0T won't suffer from the pumping losses a naturally aspirated 3.0L engine will because the 2.0T's manifold is closer to atmospheric most of the time, than the 3.0L is. Slapping a turbo on a given engine won't do anything for pumping losses though.

[JohnnyGrey]

That will increase your overall FE.

No it won't! The turbo will decrease your 0-60 time, but once you hit 60mph, you'll have used at least as much fuel to get there as the N/A motor.

Imagine a corolla with a say 1.3 liter engine? Impractical if N/A. Add a turbo and you've got a killer FE car.

That was my original point. To get any FE benefit from a turbo, you have to use it on a smaller motor to push down the efficient BSFE regions. Slapping a turbo on the original 1.8L won't do you any favors in the FE department.

[tasdrouille]

Johnny, I'm sorry, but look at the bsfc map in post #60. You can get better FE with the turbo engine vs N/A if you operate the engine in the zone between the bsfc island of the N/A engine and the baseline. If you say otherwise, well you don't know how to read a bsfc map then.

[JohnnyGrey]

Post 60 is a BSFC map of a N/A Saturn DOHC motor, and your hypothesis of what would happen if it were turbocharged. For a fuel economy increase to occur, that island needs to be moved down, not left, right or up. Turbocharging stretches it upwards and only upwards. This is because until you start hitting boost, the turbo does absolutely NOTHING for your engine. Do you understand what torque is? My car does not need 130lbft of it to maintain a 55mph cruise on flat ground, therefore, I don't spend much time in that island unless i'm climbing a mountain. If I do operate in that high efficiency region, I'll soon be going so fast that any efficiency gains at the crank will be eaten by wind resistance.

We're talking apples to apples here, slapping a turbo on THE SAME motor. Doing so will not improve fuel economy, and will certainly not reduce cost per mile operation as premium fuel is often required.

[Duffman]

I would say because you have enlarged the size of the 0.42 island it has the potential to increase FE if you are smart enough to hold the engine in the 2000 +- range that the island falls in