Article 1 – Turbochargers – Basic Introduction.
So many enthusiasts and prospective tuners are aware that turbocharging is a great way of
increasing engine power, but not too many people know exactly how this happens. So with that in
mind we will be explaining the basics, and some ‘not so basics’ of turbocharging over the next few
issues, as well as explaining the do’s and don’ts of turbo related performance modifications. All this
information will be factual and based on actual data provided by TurboDirect S.A – the Garrett
Exclusive Performance and Piece Parts Distributors for South Africa.
Turbocharging is a form of forced induction. The turbocharger forces air into the engine cylinders at
a pressure greater than atmospheric pressure(the air you breath is under pressure) So how does that
make more power?
In order to fully understand how turbocharging works we must first touch on the basic operation of
an internal combustion piston engine.
Normally aspirated engines use the rotation of the engine to ‘suck’ in air from the atmosphere and
mix the incoming air with an appropriate amount of fuel, either supplied via carburettor or fuel
injector. The engine compresses it in the cylinder and then ignites this mixture to produce the power
stroke. Independent tests have shown that when ignited, the mixture will expand up to 98 times its
uncompressed volume, depending on mixture, cam & ignition timing and compression ratio.
Basically what this means is that a 2 litre, 4 cylinder motor has a cylinder volume of 500ml each,
when the compressed air-fuel mixture is ignited in this cylinder the resultant force is like trying to
squeeze 49 litres of gas into a cylinder only big enough for a half a litre. This sudden pressure gain
forces the piston down away from the cylinder head and results in the engines power stroke which
rotates the crank that the piston and conrod is attached to. I should however say that although
theoretically a 2 litre engine has a cylinder displacement of 500ml per cylinder (multiplied by 4
cylinders equals 2.0 litres) it may only draw 350 to 400ml of clean air fuel mixture on its intake cycle.
This is because there is some resistance in the inlet and exhaust systems of all engines, (hence
headwork like flowing and porting aswell as bigger valves etc always make a large improvement) but
for the sake of this exercise we’ll ignore these factors.
With forced induction we are in essence forcing more air and fuel into the engine before we set fire
to it. Lets discuss pressure – you have a few assumed variations of pressure (Gauge Pressure, and
Atmospheric pressure) and you have different methods of reading this pressure (PSI, BAR, KPA) as
follows:a) Gauge pressure ignores atmospheric pressure and at the coast or reef the gauge will
indicate zero PSI/Bar/KPA when the engine is off.
b) Atmospheric pressure on the other hand takes into account the outside atmospheric
pressure at your position. Did you know – at the coast the atmospheric pressure, which
is the outside air that you breath - is 1.0bar or 14.5PSI, and at the reef (Johannesburg
for example) the pressure is 0.84bar or 12.18PSI.
Now lets say you have a turbocharged car boosting 1.0bar or 14.5PSI – the atmospheric pressure
gauge will indicate 2.0BAR or 29PSI – on a typical boost gauge found in most turbocharged vehicles
you will read 1.0bar or 14.5PSI.
At a gauge pressure of 0.5bar we are increasing the cylinder pressure by 50% more than normal
atmospheric pressure at the coast, and as pressure is directly proportional to volume the 2 litre
motor we spoke about before now has a theoretical displacement of 50% greater than its true
capacity.
Basically your 2 litre engine just became a 3 litre engine. And so the theoretical size and
performance of your motor would increase with the level of boost you apply. Although true figures
aren’t 100% accurate to this theory as the formulas and calculations don’t account for differences in
air density, flow rate of charge air, temperature, fuel mixture, or flow rates of exhaust and inlet
manifolds – however the basic principals are what we need to understand.
We will tackle more technical issues of turbocharger compressor sizes, flow rates and volumes
versus pressures in later issues but for now let’s keep it simple. Get it? Got it? GOOD!
So how do Turbochargers themselves work?
A turbocharger is an air pump – simple as that! The waste exhaust gas, instead of flowing through
the exhaust system, is instead used to spin a turbine wheel, which in-turn spins a compressor wheel
attached to other end of the assembly, enabling more air to be forced into the engine
OK – lets get your terminology correct from the start - There is a turbine side of the turbocharger
and a compressor side of the turbocharger. The turbine housing bolts onto the engines exhaust
manifold and exhaust gases and heat energy flow through the turbine housing spooling (or
spinning/accelerating – known as blade excitation) the turbine wheel as it passes through the
housing and blade surface area and out the turbine housing outlet – through the downpipe and
exhaust system. The more exhaust gas and heat flowing through the turbine section, the faster the
turbine wheel spins.
–
High temperature/high pressure exhaust gas passes from engine cylinders,
through manifold into turbine housing
The turbine wheel has a shaft which is friction welded onto it and runs inside the turbochargers
bearing housing through to the compressor wheel on the opposite end which attaches to this shaft –
but that will be discussed later on.
Many people believe that the turbine wheel is spooled by the air flow through the manifold alone,
this is untrue and not possible without energy. In this case the energy is heat which is the primary
driving force of the turbine wheel. Did you know that in a 250Kw motor roughly 80Kw of power is
released as energy through the exhaust.
–
A turbine wheel is driven by the 'waste' exhaust gases.
–
The gas expands through the wheel and releases energy into it, ccausing high speed
rotation
Some interesting facts about the speeds at which a turbocharger rotates at –
•
Turbine Facts;
–
GT12
•
–
38mm wheel up to 280,000rpm
GT70
•
165mm wheel up to 70,000rpm
Next issue we will be discussing the effects of using different turbine housing ratios aswell as the
bearing systems available including ball bearing and journal bearing centre sections.
See you next time!