treyZ28

During the intake stroke, a pressurized intake manifold will be forcing the piston down as opposed to an NA engine where the vacuum is acting against the piston's down stroke. In essence, with a turbocharger, you would be using the exhaust gasses to directly power the engine via hydraulic pressure.

How significant is this? I'm guessing that it isn't very significant or turbocharged cars would get better gas milage. Then again, a turbodiesel which is almost always under positive pressure (TDI for example) gets great mpg.

TT632

iTrader: (3)

This is a small advantages of a supercharged/turbo engine. From 0 to 180 deg after top dead center on the intake event, the piston will see a force due to the pressurized intake charge equal to the area of the piston times the pressure in that cylinder. Times 8 for an 8 cylinder
ex: 10 psig of boost times the area of a piston with a 4" bore equals 125.7 lbs of force on one piston
Of coarse, this would not apply during int/ex overlap and the end of the intake stroke when the piston is on the way up. But these account for a minor portion of the intake event.
Is it significant? compared with a 1000 psig or higher combustion event it would appear to be very minor, but it all adds up.

andereck

Well maybe you should look at the other side of the turbo's operation as well. When a turbo is producing boost there is backpressure in the exhaust manifold. This translates to higher pressure on the piston crown. So on the exhaust stroke there is more negative work due to working against the exhaust pressure.

My90Iroc

iTrader: (26)

Hydrualic refers to fluid power but it sounds like the point you're making is that the pressurized cylinder helps push down on the piston. Clearly the pressurized manifold helps fill the cylinder with air better than N/A sucking air in. The significance is clear by the extra power produced.

When talking fuel mileage the factors go beyond how efficiently you fill the cylinder. First off, more air in the cylinder needs more fuel, meaning worse fuel mileage.
Another factor is given rpm when measuring fuel mileage. Generally, more rpm means worse fuel mileage.
To go a step further you need to look at torque output of the engine. Each vehicle takes a given amount of torque to move it down the road. If the necessary torque to move occurs at 3000 rpms then that's where you'll cruise. If you improve the efficiency of the engine and the necessary torque now occurs at 2000 rpm you can cruise at 2000 rpm. That's where your comparison to diesels comes in but gas vs. diesel engines is really apples and oranges.
I'm not exactly sure what type of answer you're looking for I think what you want is that if you can fill the cylinder more efficiently to the point where you can lower your rpm and maintain the necessary torque then it will be significant.

treyZ28

Well, you cant compare making 200hp vs making 400hp. Thats just silly. Clearly, I'm talking about similar driving conditions. Whether the vehicle is turbocharged or NA, its going to need to make the same rwhp to get down the same road at the same speed.

BrianMSmith

the difference between turbo charged and supercharged is that turbo charged uses "wasted" exhaust gas, supercharged uses crank power. Either one raises engine brake mean effective pressure (bmep), which is directly proportional to power/work.

Nobody really analyzes actual pressure on the piston during intake because it's too complex and transient of an analysis, so you just look at overal engine pressure drop from intake to exhaust, and draw your correlation to power from that.

turbos have the potential to be more energy efficient because they utilize wasted energy in the exhaust. But that's not how they're used. They are used to boost specific power (per liter). The engine manufacturer can more cost effectively build a smaller engine at lower power if the objective is fuel consumption (or reduce the weight of the vehicle).