Superchargers Vs. Turbochargers

At a 'family' celebration the other Saturday (they are the ones that you just have to go to!), I overhead a couple of young guys arguing the toss about the differences between a 'blower' and a 'turbo'. After listening for a while, I just wandered off because I didn't want to get mixed up in that one. Afterwards though, I kept thinking things like; "Well, both of them are pumps, the same as the engine, when you get right down to it. The 'suck' and 'squeeze' part of the four strokes are, anyway."

For this issue of Quality Parts, I thought we could explore the basic differences between these two devices, both of which have been designed to boost engine performance. In the early days, turbochargers were quite large and were only used on the larger diesel engines that ran at a fairly constant and relatively low speed (compared to a spark ignition engine that is). Around these times, the supercharger was all the go for petrol engines, either fitted as 'after market' to smaller engines used in the UK (MGs and Morries) or the big bangers from the US where the 'build 'em big and they have to go harder' philosophy was rampant before the energy crisis of the 70s.

In those good old days of carbies, they were usually 'upstream' from the supercharger, because they rely on air passing through the venturi to draw the fuel up the discharge tube and mix with the intake air.

Pressure relief is also needed in these early systems, because it is a fuel/air combination being compressed that can get out of control if it starts to burn before it gets into the combustion chamber!

These days though, with fuel injection, only air is being compressed and the point when the bypass valve actuator opens is controlled electronically by means of a solenoid controlling a boost control valve.

In this next view, the solenoid is turned 'off', allowing the valve's internal spring to move the valve across to the right. When this happens, the high pressure air in the intake manifold, works against the spring on the other side of the bypass valve actuator, opening the by-pass valve.

This effectively drops the boost pressure to a point where the amount of air recirculated through the actuator valve is balanced by the pressure in the intake manifold. In other words, the air just keeps going around and around.

A generally held opinion about superchargers is that they take power from the engine to drive them, so they are not really all that efficient. Without getting into comparative graphs and so on, just let me say that the increased power output from an engine that is supercharged (even lightly), is far and away superior to the same engine without it!

Some claimed advantages of using a supercharger over a turbocharger are that, as this is belt driven from the engine crankshaft, the boost response is immediate and the supercharger operates in an environment where high exhaust temperatures are not in close proximity. On that point then, let's now turn our attention to the exhaust driven.

Not only does the heat around the exhaust cause problems with bearing lubrication but the speeds the turbine spins at, add to the situation. When you consider that the shaft can easily be spinning at speeds over 100,000 rpm when the thing is really singing, then I'm sure you can understand that the bearings have to be something special.

In fact, they are simple, plain bushes, but they are fully floating, with oil lubricating both sides.

To stop the hot gases from turning the oil to carbon, not only is there a constant flow of engine oil but the turbine housings are usually water cooled and there is an effective seal between the bearing and the turbine wheel.

So, why doesn't the pressure just keep building with an increase in engine speed? Have another look at the schematic in Figure 3. Notice the "Wastegate Control Valve". As the air pressure developed by the compressor wheel builds up with engine speed, the intake manifold pressure is also fed to the underside of the diaphragm in the wastegate valve.

When this pressure builds to the design point, the diaphragm moves against spring force and opens a valve in the exhaust stream. When this happens, can you see that some of the exhaust gas from the engine will bypass the turbine wheel and go straight down the exhaust pipe? I hope so.

This effectively reduces turbine speed and the pressure built up by the compressor. All very simple really.

What this funny word means is that something that is stopped tries to stay that way and something that is moving, also tries to keep right on moving, despite the outside forces being applied.

The problem for the turbocharger then, how to get it spinning quickly when the throttle is opened quickly and how do we stop it from spinning at a million km/h, when we come to a stop?

Design is the answer to the first part, by using a split pulse, entry duct on the exhaust side. This allows the exhaust pulses at low engine speeds to build on one another and this helps the turbine to spin up at low engine speeds. Exhaust manifold design too, plays an important part in this whole process.

The second part of the speed question, is the reason that an engine should be left to idle for a minute or two, after a high speed run, to allow engine oil to continue to be pumped through those bearings, while the shaft speed slows down.

There's another aspect about compressing the intake air that I haven't touched on yet and it's an important one too, as it also relates to temperature.

Whenever air is compressed, it gets hot -- witness to that is the bicycle pump after pumping up a tyre. As the air temperature increases, the density of that air (at that pressure) is less dense, meaning that the air molecules are further apart. This also means that there will be less oxygen in that sample of air that will be around to help with the combustion process. Enter the Intercooler.

No, this is not just a marketing ploy to sell more vehicles (we've all seen the phrase, Turbocharger with Intercooler). It really does help. There are usually two types; Air to Air and Air to Water.

Figure 4: Turbocharger.

The example in Figure 3 is obviously an Air to Air type. The idea of course, is to extract as much heat as possible from the compressed air before it enters the combustion chamber. Not only does this mean that the air is more dense and has more oxygen in the same volume, but it will also be heavier! Remember what I said about inertia before? Once that cooler, heavier air is moving, it will want to continue doing that, even with the pressure behind it. The result is a 'ram air' effect to go with the pressured charge. A double whammy!

Mind you I don't think you would notice the difference from the seat of your pants while driving, but tests do show a marked improvement when intercooling is carried out.

I hope you can see now why I didn't buy into that animated discussion I mentioned at the start. I'm not about to say that one of these two forms of charge boosters is better than the other. Just let me say that either one, correctly engineered for the engine, will certainly boost the output and, depending on how it's set up, the torque curve will also flatten out.

Well, that's it from me for now, so cheers!