Turbocharged = Supercharged?

The concept behind forced induction is the same. The supercharger and the turbocharger force air into the engine to raise pressure above atmospheric pressures. This allows more air to be forced into the combustion chambers which allows more fuel to be added and potentially more power to be created.

The difference is the means; the turbo is fed by the exhaust gases that escape from the engine. The exhaust spins a turbine which is coupled to a turbine on the intake side, compressing the intake air. The supercharger is belt driven off the crankshaft.

-Bruce

Yes and no. Turbochargers are powered by the exhaust and are always spinning anytime the engine is running. Superchargers run off the crankshaft, possibly with some sort of "activate/deactivate" system similar to the electric clutch on your A/C compressor, and aren't necessarily spinning just because the engine is running.

If you've seen the movies, think about the scenes in either Mad Max or The Road Warrior - He was (at least supposedly) running a rig with a supercharger - when he wanted the extra "oomph", you saw him hit a gearshift-mounted switch, then you got the cut to the supercharger starting to spin up. The switch pulled in an electric clutch to start the supercharger doing its thing.

A turbo doesn't have a switch - it's always turning when the engine is running, but since the exhaust powers it, the speed (and therefore, the amount of air it pumps) is constantly going up and down in proportion to actual engine speed. When you hit the switch on the supercharger, it immediately (ignoring the details of inertia involved in bringing a spinning mass up to speed, which for this comparison aren't important) starts pumping more air than the engine is trying to suck, giving you a nearly instant jump in power. With a turbo, you stomp on the gas, and as the engine picks up speed, so does the turbo, which eventually gets up to a sufficient speed to pump in more air than what the engine would suck on its own. They end up doing the same thing in the end, but the power "ramps up" over a realatively long time with a turbo, while a supercharger goes from "off" to "on" (with a corresponding boost in power) almost instantly.

Basically, if you need lots of power *RIGHT NOW*, a supercharger is the way to go. If you need lots of power, but aren't worried about how long it takes to get it up to peak, a turbo will get the job done for you.

Why do they use turbochargers at all for passenger cars? Doesn't it make more sense for them to use only belt-driven supercharging instead because of the lower RPMs and lower temperatures?

Approximately 9/28/03 21:49, snipped-for-privacy@webtv.net uttered for posterity:

A turbocharged engine uses the exhaust gas to spin a turbine style compressor, which runs at very high speed.

A supercharged engine uses a belt, gear, etc. to spin a lower speed compressor.

All else being equal, it is easier to get the supercharged engine to produce boost at very low engine rpm all the way up. A turbocharger doesn't normally begin producing boost until higher in the rev range. There are exceptions to this of course.

A turbocharged engine will probably have slightly better mpg than the supercharged one, also. Also, you can probably expect higher maintenance costs for either one, depending on how you drive it.

basically, yes. Both do the same thing, force more air into your motor. A turbo uses your exhaust gases to spin up, the supercharger uses a belt connected to your motor to spin up. That's the simple version.

----------------- Alex __O _-\

You get free power from the exhaust gases that would just go out the tail pipe. A well designed turbo system will last a long time. I had a Dodge Omni GLH Turbo that had 153k miles on it when I sold it. It was still going strong at that point.

----------------- Alex __O _-\

Supercharged: A compressor delivers pressurized air to the intake system. Compressor is driven by a mechanical linkage.

Turbocharged: A compressor delivers pressurized air to the intake system. Compressor is driven by a turbine that is in turn driven by exhaust gas.

So while both methods are a way to increase induction air pressure, they are very different because of the way the compressor is driven.

Made-for-the-movies scenes from "Mad Max" notwithstanding, that is virtually never done in the real world. The blower spins whenever the engine is running.

Turbocharging is generally more efficient for a couple of reasons. At low speeds and anytime the throttle is substantially closed (for example steady-state cruising at 70 mph), the turbo is not delivering any boost nor is it drawing any power from the engine by increasing exhaust back pressure. Only when full power is demanded by opening the throttle does air flow through the turbine pick up to the point that exhaust back-pressure increases and the turbine spins fast enough to create boost on the intake side and increase engine power. Since a mechanical supercharger is always being turned by the engine, there is always a small parasitic loss even though it "unloads" substantially when you close the throttle and the blower spins in a partial vacuum.

Also, when booost is being required, a turbo increases the overall thermal efficiency of the engine because waste exhaust heat is partially recovered to do useful work.

The advantage to a mechanical supercharger is strictly the fact that it begins working right off-idle. A properly sized turbo will, however, begin delivering boost VERY quickly. Horror stories of "turbo lag" are greatly exaggerated.

Not *entirely* true. The presence of the turbo does increase back-pressure on the engine and thus does take power from the engine. The reasons that it is ultimately more efficient than a mechanical blower are 1) it only draws power when actually delivering boost- cruising steadily at 70 requires no power to drive the turbo, and 2) although it takes power to drive, it does make use of some of the waste heat in the exhaust, because the hot exhaust gasses do cool as they expand through the turbine.

100% agreed on that point! A good turbo installation (and the Chrysler turbine installations, despite being in ugly K-cars, were among the best-engineered ever sold to the masses) will outlive the piston engine its attached to. All that is required is a) good oil flow, but not excessive, 2) adequate cooling, 3) a driver who remembers to idle the car for a minute after running under boost before shutting it down so that the heat soaked into the turbine wheel can be carried away by flowing oil and exhaust, rather than cooking the oil trapped in the turbo bearing when the engine gets shut down.

Same effect, different method of getting there.

Technically, a "turbo-charger" is more correctly a Turbo-supercharger. In either case, a compressor forces additional air into the cylinders allowing more fuel to be burned resulting in more power. In a turbocharger, an exhaust driven turbine drives the compressor wheel. This effectively reuses waste heat that would otherwise go out the tailpipe. It is therefor possible to either create more power out of the same size engine, or the same power with a smaller engine. The downside is that since you are recovering waste heat, the turbo charger gets HOT. Really hot, as in glow in the dark hot. Tough on bearings and oil. Also the realtionship between throttle openning and power produced is inherently non-linear - it takes a while for the engine to produce enough exhaust flow that then takes a while to accellerate the turbocharger to the point where it produces pressure. This is what is termed "turbo lag". Modern applications have pretty well eliminated\masked this though through various electronic and mechanical methods. But it can't be eliminated 100%.

A supercharger is mechanically driven by belts, gears or chains. It is therefor possible to have it produce pressure at very low engine speeds. No lag, the boost is always there. BUT because the engine is driving the compressor directly, a large portion of the additional power produced by the engine is absorbed by turning the compressor to produce the additional power. To the detriment of fuel economy - no such thing as a free lunch! But throttle responce is nice and linear, and the system can be setup to greatly enhance low rpm power. Much less heat, but you have an additional drive system to maintain.

Some BIG diesel engines (ship and locomotive) have a combination of the two. The compressor is so big that it would take an eternity for exhaust gas alone to spin it up. So there is a mechanical drive that spins it up until there is enough exhaust flow for the turbine to take over. Sounds like a mechanical nightmare to me! I have been in the engine room of a 600ft ocean going car ferry (M.V. Scotia Prince) the turbo compressor wheel was about 8 feet across.... :-)

Kevin Rhodes Westbrook, Maine Quite a few turbo cars over the years, gas and diesel

turbocharger to

I saw on speedvision that turbo lag has been eliminated. Rally cars have been using a system that spools up the turbo by causing combustion on the exhaust side of the turbo rather than waiting for exhaust gases to reach a certain velocity.

-Bruce

Toyota used an electromagnetic clutch with the MR2's supercharger which enabled it at a certain RPM. That's the only example I can think of though.

-Tim

Granted...

However, the point I was trying to make remains valid: Turbos are always spinning when the engine is running. Superchargers might or might not be spinning when the engine is running.

How frequently they are or aren't spinning in real-world applications was a non-consideration. If, in even one case, the supercharger isn't spinning while the engine is running (barring malfunctions), then what I stated remains applicable. And although I do realize that the sequences from Mad Max/Road Warrior that I mentioned were "designed for the movies", and as such, should be taken with a large grain of salt, the point remains that a supercharger *CAN* be turned on and off, while a turbo can't, which is a point of difference that can be used to distinguish between the two types of unit.

I ought to look up the guy I've seen running around town in the blown '69 Camaro (tags read "BLWN69", so I'm assuming it's a '69...) if only to get a close-up look at how he's running his setup. I've seen him sitting at lights with the (it's exposed for all the world to admire) blower pulley clearly not moving, and other times, seen him sitting there with it obviously spinning, so I really am kinda curious about exactly how he's got it rigged.

I'm torn two ways on that particular car... It somehow manages to simultaneously look like both the sorriest hick-created pile of junk ever to roll down the street, and the most bad-ass piece of rolling iron ever to come off an assembly line. Go figure!

Basically, yes. However, there are differences. While hardly bolt-on accessories, mass produced turbos are cheap enough to put into econoboxes like the Dodge Omni and Colt. All other things being equal (displacement, compression ratio, rpm, and amount of boost), the turbocharged engine either uses less fuel or produces more power than a supercharged engine (it converts more of the fuel's energy to mechanical power).

Robbie Coltrane's Planes and Automobiles had an episode about supercharging, and it mentioned that the system that Mercedes introduced in their race cars was only part time, to increase engine longevity. It proved to be a false economy, as they were forced to keep the supercharger going to keep up with the supercharged Bentley (although, the first year, they swept the field). Ironically, the race was won by a normally aspirated Bentley, as the supercharged Bentley was, also, unable to finish the race.

[Who'd a thunk that Haggrid was a gearhead]

Not street legal, and will ruin your exhaust manifold in short order