Turbocharging - False Promises?

[dgs49]

The EPA has recently come to the conclusion that there is something fishy about these new turbocharged cars we are seeing.

All other things being equal, the displacement of an internal combustion engine determines how much fuel is consumed and how much power is generated. Thus, an engine with a bigger displacement will out-perform an identical engine with a smaller displacement, because more fuel is sucked into the larger cylinders. More fuel equals more power. (Air-fuel mixture is drawn into the cylinders by the vacuum created when the cylinder moves away from the combustion area of the cylinder, with the intake valve open).

A “turbocharger” is a complex device that harnesses the forces of the exhaust gases of an engine to turn an impeller; the impeller, through a mechanical connection, turns a compressor that forces air-fuel mixture into the intake of the engine. The compression of this air-fuel mixture supplements the vacuum that normally draws the mixture into the cylinder, getting more of that mixture into the combustion process. So, in effect, the turbocharger makes an engine of a given displacement act like a bigger engine – it burns more fuel, and makes more power. And it also stands to reason that the faster the engine is turning, the greater the pressure and velocity of the exhaust gases and the more effect the turbocharger will have.

So while a turbocharger will not have much effect when the engine is turning at low rpms (most driving circumstances), it can have a dramatic effect when the engine is “pushed,” that is to say, when you have your foot to the floor. And theoretically, if you have two identical engines, one turbocharged and one not, in normal driving they would have very similar power and fuel economy, but when stressed, the turbocharged engine will burn more fuel and make more power.

So to summarize, if you have a car with a turbocharged engine and you want to take full advantage of both the power of the turbocharger and the efficiency of the small-displacement engine, you have to drive it very sedately is most situations, “flogging” it only when you feel you want or need optimum performance. Driving this way, you can get, “the economy of a four-cylinder car with the performance of a V6 (when necessary).” It is noteworthy that the EPA City and Highway driving cycles employ driving models that are quite “sedate,” and they provide fuel economy numbers that are optimal for turbocharged cars.

But there is a lot of disappointment these days about the efficiency of the turbocharged cars that are being sold in the marketplace. People expect greater levels of performance with no penalty in fuel economy, just like the EPA says. Part of this is the manufacturer’s fault – they employ engines that are simply too small to power the weight of the vehicle – and partly stupidity on the part of the consumer. You can’t drive aggressively all the time and expect to get good fuel economy, even if your engine is 2 liters or less.

A good example of the problem is the Nissan Juke. The Juke is a unique little “niche” car that is sold as being sporty, functional (AWD), and yet economical. It has a tiny 1.6 Liter four-cylinder, turbocharged engine that puts out a maximum of 180 HP. This is enough to make it accelerate quite briskly, especially in this segment. 0-60 times are under 7 seconds. But the problem is that this puny engine is too small for the car. Driving it “sedately” results in little girls on bicycles passing you on the road. The only way to get even marginally acceptable performance under normal driving conditions is to push it fairly hard, all the time. And consumers report getting average fuel economy in the 14-16mpg range, a far cry from the EPA rating.

Would it make any difference if the salesmen explained that your fuel economy will depend on how you drive? Nah, too obvious.

[dales]

Car sales people are parasites and whores.

[Jarlaxle]

And yet...my wife's Grand National, a 3900+lb car (G-body Regals are heavy cars) with a fire-breathing turbocharged 272ci (about 4.5 litre) V6 making close to 900HP (put down 763 at the wheels, add 20% for driveline losses), will easily top 20MPG and has managed 25 on a long highway run. Also note that actual owners of another small-displacement turbo car (Chevy Cruze Eco, with a tiny 1400cc engine) are reporting BETTER than the EPA numbers! (On a highway run, topping 45MPG is the norm.)

I topped 30MPG many times in my old Shelby Charger...considering the car had the engine tuned much more for power than economy, along with deep gearing (60MPH was 2900RPM), wide, sticky performance tires, and me often driving it kind of hard (I was 20-something...), that's pretty good. In the turbo cars not optioned out as weekend racers (and thus, with taller gearing), mileage matched or beat the non-turbo version of the same engine, though I recall the turbos did require mid-grade fuel. (Mine, with the boost turned up, needed premium fuel.)

Also note that PLENTY of turbos work at low RPM's! Offhand, the VW/Audi 1.8 litre turbo engine has, because of the turbo, torque from 1800rpm! Volvo's old light-pressure turbocharged 5-cylinder from the mid-90's was the same way...torque peaked around 2000RPM. Heck, the turbo on my work truck starts spooling up at about 1000RPM!

[dgs49]

color me very skeptical.

[Jarlaxle]

Not at all complicated. Liz's GN doesn't need to spin the turbo much...it's a good-sized engine that would probably make ~300HP WITHOUT the boost. With the engine tuned for economy while in steady-state cruising (it has programmable EFI), a combination of a lean fuel mixture, lots of ignition timing, and overdrive makes it burn less fuel. The Charger was similar...the Direct Connection controller in the computer added boost and timing, and the aftercooler I added let it use even more timing. (Also, it's only a 2.2 litre engine and the car was ~2300lbs.)