I have never really gotten the hang of these two options? Siena, any one? Please?

ok, without being too technical, i'll just give u a brief rundown on how a normal engine works, then i'll tell u what the turbo does so u can do the maths urself,

here goes,

the standard 4-stroke engine works by

- Inlet valve opens, air and fuel mixture enters the engine
- Valve closes and piston compresses this mixture
- Spark Plug ignites mixture, the resulting explosion pushes down the piston(inturn moving the crank)
- exhaust valve opens and all the burnt gasses escape,

the situations repeats itself in every cylinder, giving ur engine power to take u from point A to B,
the pic below should help u out,


Now the trick is, the power output of an engine is determined by a few important factors,
one of which is the amount of air and fuel mixture u can get into the cylinder (during the first stroke),

the higher the fuel-air mixture, the higher the power output,

Now what the turbocharger does is to FORCE as much air as possible into the cylinder that by the time of the first stroke, the cylinder is already semi compressed that by the second stroke you'll get double the compression and hence MORE POWER, !!! ,

get it?

HMMMM, Now dat settles it, but does that mean a turbo charged engine uses up more fuel or less, and which is better as per cost of maintenance etc

Engines come in both N/A (naturally-aspirated) and forced induction (turbocharged or supercharged).

In either type, fuel is mixed with oxygen to burn, producing an explosion in the cylinders creating power.

In the naturally-aspirated engine however, the amount of oxygen that can be mixed with a given amount of fuel, is limited to what the engine can suck in. More oxygen means more fuel can be burnt to produce more power.

This is where the forced induction - turbocharger / supercharger comes in.

The principle behind both are the same - more air is forced into the engine, in addition to the amount the engine can physically suck in, allowing more fuel to be burnt.

The turbocharger is more widely used, so I'll concentrate more on this method of forced induction.

A turbocharger, in layman terms, consists of 2 fans / turbines mounted on a common shaft, upstream of the exhaust manifold.
It is designed to increase power, without adversely affecting fuel economy.
As exhaust gases exit the manifold, they push against the "hot side" of the turbo, rotating the shaft.
The second fan, in the "cold side" draws in air, and forces it into the intake plenum, via an intercooler.

Intercooler's are not always employed, the purpose is to cool the air, as it leaves the turbocharger.
The air is heated by two things - the hot exhaust gases, and compression.

The more you compress air, the temperature rises accordingly.
Hod air isn't the best thing to introduce into an engine, as it loses power.
The intercooler cools the charge air, increasing its density.
Fuelling is mapped accordingly - most modern cars have a MAP sensor, that senses boost levels, and adds fuel as required.

It also allows the ignition timing to retard about 6 degrees, if boost raises intake temperatures to a level considered unsafe.

Of course, boost is proportional to engine conpression ratio - naturally-aspirated engines are generally high compression, between 9.0:1 and 11.5:1.
Turbocharged engines are usually lower - between 7.5:1 and 10.0:1, with the higher CR found in more recent cars, where ignition control has reached the level the engine can cope with high CR and boost without the risk of detonation.

On average, a turbocharged engine can develop anywhere between 20 and 40% more power than a naturally-aspirated version of the same engine.
Driven sensibly, it can be more economical than the non-turbo version, as the greater torque developed means the engine requires less throttle opening to cruise at any given speed, whereas the non-turbo will require more throttle opening to achieve the same cruising ability.

The gear ratios of the turbo version are also generally higher than the non-turbo version, which also aids fuel economy.

On older cars, where turbos were quite large in relation to the engines they were fitted to, there was a lot of "lag."
Lag is the point at which the turbocharger is spinning, but not enough to generate any useful boost, so no power.
As I mentioned before, older turbocharged cars had lower compression ratios to cope with low octane fuels, and archaic ignition systems.

On a lower CR turbocharged engine, more boost can be run, but when off boost, the lower compression issue becomes an issue.
These engines will feel slower than a non-turbo of the same car model at low rpm, as the engine will make less power. These cars had to be revved quite high to generate enough boost to be quick from low speeds.

Modern turbocharged cars, with smaller turbos that spool up quicker, and combined with their higher compression, have virtually eliminated turbo lag. It's still there, but not as noticeable as it used to be.

A car can have single turbochargers, (common on I4 / I6 engines) or twin turbochargers, (common on I6, V6, V8, V10 or V12 engines).
Twin-turbos can be simultaneous, whereby they both boost at the same time, and boost drops at the same time too.

Or, they can be sequential - whereby one spools up from lower rpm, then as boost drops off, the other turbo takes over.
The result is an engine that feels like a large, naturally-aspirated motor, with virtually no lag, and a lot of power, even at high rpm, where turbochargers are usually out of puff.

Twin-turbo's are usually small, and low inertia. Or, one could be slightly larger than the other, to allow more top end power.

Another method of forced induction is the supercharger.
A similar principle to the turbocharger, it relies on crankshaft power to drive the turbine.
In this case, there's only one turbine involved, on the "cold side" - the shaft has a pulley, that's driven by a toothed belt, directly from the crankshaft.
Advantage over turbocharger is, it has no lag in terms of power delivery, since the crank is always rotating at a constant speed. The pulley is quite small, so the shaft rotation is much greater than that of the crank.

The result is, even at idle speed, the engine's still getting enough boost to make power.
It also runs cooler than a turbocharger, and often don't require intercoolers to increase charge air density.

However, a supercharger absorbs power, to create boost, unlike a turbocharger, where boost is generated by waste product in the form of exhaust gases.
Superchargers also make less specific power output than turbochargers - if both were fitted to similar engines, say an Audi 2,7 V6 30V, and were both set at 9 psi of boost, the turbocharger will still make more power than the supercharger.

Of course, both supercharger and turbocharger can be combined on one engine - compound charged.

This set-up has the advantage of making a small engine deliver power like a larger, naturally-aspirated one - supercharger for bottom-end power, tailing off midrange, and a small turbocharger taking over, boosting from mid-range to top-end.

An example of this engine is in the Volkswagen Golf V GT - it's a 1,4L 16V, and with compound-charging, develops 125kw (170 hp) which makes it the Volkswagen Group's 2nd most powerful 4-cylinder engine.

Siena baba ! u too much.
Thank God u came back to nairaland. Thanks for the lecture.