Making a car more efficient by adding a turbo??

[leolo007]

Hello, OK my theory comes from comparing gas mileage of an Audi A4 which weighs considerably more than my Sentra and has the same engine, displacement-wise. They give about the same mileage city and highway. Just about everyone I ask say its not possible, but thats because all the turbo cars they've dealt with were made turbo for making them faster and of course faster means more power means more gas. But I want to do something different. Has anyone heard of someone trying this or do you have any comments, ideas, criticism? thanks, looking forward to your replies! opcorn:

Leo

[UncleBob]

no matter what you do, the turbo acts as a restriction in the exhaust which will reduce your efficiency. Not much, mind you. Usually around 5-10%

The two cars you are comparing, two different engines. You can't compare them directly

[KiwiBacon]

Yes it can be done, but by downsizing the engine.
A smaller turbocharged engine running at higher load can be more efficient than a larger engine running at part load.

The smart car is a good example. It uses a 550cc turbo engine to push itself around.

If you turbo your existing engine, you'll lose economy. As Bob said it's adding a restriction as the power from the turbo isn't needed at cruising loads.

[leolo007]

would it be correct to say then that if you remove the turbo from the audi and do the proper tunning, (or de-tunning) it would be more fuel efficient?

[curtis73]

In effect, yes. A turbo is an on-demand boost device. If you had two identical audis, one with a turbo and one without, and drove them both like a grandma, they would probably return the same mileage. If you drove them like a 19-year-old, the turbo car would spend more time in a boost situation, therefore it would spend more time injecting additional fuel to burn with the additional air, and would probably have lower MPGs.

[MagicRat]

The turbo will recover some wasted thermal energy in diesel engines.

Gasoline engines have throttle blades which effectively eliminates the ability for the turbo to recover the lost thermal energy......... unless the intake manifold actually achieves positive (ie above atmospheric) pressure.

Usually this occurs only under heavy throttle use, where the gas engine is developing big power and consuming lots of fuel, eliminating the efficiency advantage.

So, the answer is...... no, simply adding a turbo is not going to increase fuel economy. However, it is possible to design a gas engine to take advantage of the turbo. Such an engine would have to be sufficiently small in displacement and power that it would normally operate at full throttle all the time. Such an engine would result in an unreasonably underpowered car for normal driving.

[curtis73]

true... a 4L non-turbo engine and a 2L pushing 15psi could be very similar in peak power output, but the 80% of the time that the smaller engine spends not in a boost situation could save a lot of fuel over the 4L

[MagicRat]

Originally Posted by curtis73

true... a 4L non-turbo engine and a 2L pushing 15psi could be very similar in peak power output, but the 80% of the time that the smaller engine spends not in a boost situation could save a lot of fuel over the 4L

I would agree, but my actual point is to take advantage fully of the energy released by the thermal expansion of exhaust gases (which is reclaimed by the turbo and re - introduced in to the reciprocating mechanism by the pressurizing of the intake manifold, exactly the way turbo diesels increase efficiency. )

Since this happens most efficiently at full throttle, I was thinking of just enough power to maintain a good highway cruising speed, via a very small 20 hp gas engine.

Obviously this is immensly impractical, unless it was combined with a hybrid powertrain, where an electric motor would provide extra acceleration when needed.

[UncleBob]

as far as gas engines, you can play with compression ratio and cc's, but that isn't really answering the question. He was talking about two identical engines, one with a turbo and one without

the one without the turbo, even at off-boost, cruising speeds/RPM's will have less back pressure (NA cars rate backpressure in tenths of a psi at cruise, unlike turbo engines). There is going to be effeciency discrepencies either way

the NA engine is going to run higher CR most likely, which automatically makes it more effecient most likely

[KiwiBacon]

Originally Posted by MagicRat

However, it is possible to design a gas engine to take advantage of the turbo. Such an engine would have to be sufficiently small in displacement and power that it would normally operate at full throttle all the time. Such an engine would result in an unreasonably underpowered car for normal driving.

AKA Smart Car.

[beef_bourito]

couldn't you use alcohol/water injection to cool the intake charge on the turbo car, allowing you to run leaner than usual and save some fuel?

[KiwiBacon]

Originally Posted by beef_bourito

couldn't you use alcohol/water injection to cool the intake charge on the turbo car, allowing you to run leaner than usual and save some fuel?

An engine at cruise doesn't need any more air. In fact it always needs less than it can consume, that's why they use a throttle to restrict the air consumption.

Boosting an engine is these conditions does nothing good. It costs economy in a big way.