|
This article was first published in 2005.
|
Forced aspiration can achieve incredible things. You can think of it as
making an engine variably sized, giving the power of (say) a 4-litre engine with
the economy potential of a 2-litre. But it’s even better than that, because the
power of the 4-litre comes with the weight and size of the smaller engine... so
you don’t need a huge car to fit a huge engine. A forced aspirated engine also
has an average power across its rev range that is higher than a naturally
aspirated engine. The translation is that your 2-litre that became a forced
inducted 4-litre goes even harder on the road than a typical 4-litre!
Sound like magic? Well, the outcome can sure be magical.
So how do you achieve these results? Leaving aside chemical supercharging,
there are two methods of forcing more air (and so allowing the addition of more
fuel) into your engine. The first is a turbo and the second a supercharger.
Turbos
A turbo uses the waste heat in the exhaust gases to spin a turbine. Mounted
on the same shaft as the turbine is a compressor, which forces air into the
engine. As the turbine spins faster, so then does the compressor. The result is
that once the engine is developing enough exhaust gas flow to spin-up the turbo,
the compressor can feed more air into the engine’s intake than it can normally
breathe, creating boost.
Both the compressor and turbine are like fans – the exhaust gases drive one
fan which in turn spins the other which blows air into the engine.
Turbos make use of the heat and flow of the exhaust gases, energy that is
usually wasted in the mufflers and as hot flow out of the tailpipe. This means
that theoretically, a turbocharged engine is more efficient than a naturally
aspirated engine. The practical outcome of this is improved fuel economy. That
might not be the case when compared with the non-turbo’d engine, but it will be
when compared with a naturally aspirated engine producing the same power. (In
normal road use, anyway. At constant full throttle, the answers change.)
A turbo has no physical driving connection to the engine – there’s no belt
drive, for example. Instead, the ‘drive’ is by fluid flow (ie the exhaust gases)
down the exhaust manifold.
Superchargers
Superchargers are also air pumps that push air into the engine. Like turbos,
they create boost by attempting to force more air into the engine than it can
readily breathe. However, a supercharger is directly driven by the engine by
means of a belt-drive from the crankshaft – in this respect it’s like an
alternator or a power steering pump.
Superchargers come in three major types.
Centrifugal superchargers use what looks like the compressor side of a turbo
(although it’s bigger!). Like turbos, they cannot develop lots of boost at low
engine speeds – instead, their airflow output rises rapidly with engine revs.
Roots superchargers use intermeshing "paddle-wheels". Usually each
"paddle-wheel" has two or three lobes which are curved so the intermeshing is as
airtight as it can be without the rotors touching. The lobes can be straight or
twisted along their length.
Screw superchargers compress the air between two intermeshing screws, taking
it in at one end and compressing the air as it travels along the screws.
Both the Roots and screw types of superchargers are called ‘positive
displacement’ designs, because they have about the same output volume of air for
each rotation. On the other hand, being much more like fans than pumps,
centrifugal superchargers increase dramatically in output airflow as they rotate
faster.
Picking and Choosing
So which is better? There’s not a lot of point in having a purely theoretical
discussion. In the real world, aspects like the availability of second-hand
units and the number of cars with factory forced induction have a decisive
impact on which way to go. For example, if you can buy a car complete with a
factory turbo installation, you’d be mad to instead buy the naturally aspirated
version and then fit a turbo yourself. Same with superchargers – never try to do
what the factory has done for you.
However, let’s say that you’re modifying an engine which is not available in
factory forced aspirated models. The next step is to look for off-the-shelf kits
– either supercharged or turbocharged. Again, if a company has spent hundreds of
hours developing a forced aspirated model of your car’s engine, why would you
want to re-invent the wheel? (That’s not to say you need to use the whole of
that kit. Instead, it’s often good to pick and choose – for example, use their
cast turbo exhaust manifold but go your own way with turbo matching and engine
management.)
But it gets harder if there’s no factory forced aspirated engine, and no
aftermarket turbo or supercharger kit. What then? The next step is to look at
what turbos and superchargers are available new and second-hand to suit the size
of the engine and the power you’re chasing.
Specifics
In this particular case I was looking at the best approach to forced
aspiration on an engine with a naturally aspirated 43kW. (Yep, 43kW!) The aim
was to pick a turbo or supercharger that would boost peak power by perhaps only
30 per cent, but would be effective at lifting torque across as much of the
engine operating range as possible. In other words, boost had to be available
early.
This is a much trickier matching exercise than increasing peak engine power
by (say) 200 per cent. That’s because unless the forced aspirated engine is very
carefully modified, it’s likely the boosted engine will drive pretty badly. For
example, it’s not at all unusual for a big turbo engine to have decent
performance over only half its rev range. It’s primarily for this reason that
the selection of a turbo or blower should always err on conservative side. In
road cars it is always better to have a lower peak power but a higher
average power across the usable rev range – something which people swapping
power figures at dyno comps completely forget.
With this particular engine having a starting peak power of 43kW, the maximum
power the turbo or blower would need to flow was only about 55-60kW.
Furthermore, the engine is a 1.5 litre but (critically!) it revs to only 4000
rpm.
First up, let’s look second-hand. The Japanese Kei class cars are available
in turbocharged and supercharged forms and those turbos and blowers can be
obtained from Japanese importing wreckers. The legal maximum power of these cars
(in 660cc form – the engine capacity is another legal limit) is 47kW. Typically
these engines rev to well over 8000 rpm.
With the 1.5 litre guinea pig engine reving to only 4000 rpm, in rough terms
it will have an airflow requirement of about a 750cc engine that revs to 8000
rpm.
So the airflow requirement can be approximated in both power terms (original
supercharged/turbo engine power = 47kW, desired engine power = 55-60kW), or in
revs/capacity terms (original supercharged/turbo engine = 660cc and 8000 rpm,
new engine = 1500cc and 4000 rpm).
And buying new? The smallest ball-bearing Garrett turbo is the GT12. This
turbo has a recommended power application range of 37 to 97kW and is said to
suit engines from 0.4 to 1.2 litres. From Eaton is available the MP45 positive
displacement Roots-type blower. It’s suitable for engines in the 1-2.4 litre
range – so is a bit large for this engine.
So as can be seen, in this particular application there are available both
new and second-hand turbos, and also second-hand superchargers. So which way is
better – a blower or turbo?
Comparisons
There’s a very wide variety of factors that can be looked at when weighing-up
the pros and cons of turbos versus superchargers.
1. Mounting
A supercharger is mounted on the engine where it can be driven from the
crankshaft. In most cases this means the pulley of the blower is in-line with
the crankshaft damper and main drive pulley. Heavy brackets hold the
supercharger in place so that the drive loads don’t cause flexing, which would
throw the belt off. Most superchargers have their own oil supply – no connection
to the engine oiling system is needed.
A turbo is mounted on the exhaust side of the engine, normally on a
custom-made exhaust manifold. A high pressure oil supply is derived from the
engine (usually via a T-fitting on the oil pressure sender) and a return line is
plumbed back to the sump – normally necessitating removal of the sump to do this
work. In addition, water cooling lines need to be connected to the engine
coolant system.
In most cases mounting a turbo on an engine is more work than mounting a
supercharger.
2. Efficiency
As already indicated, a turbo has the potential to boost overall engine
efficiency to higher levels than will be achieved with a supercharger.
However, efficiency also has another meaning – it can be applied to the
efficiency of the forced aspiration device itself. In this context, the lower
the efficiency, the higher the temperature of the boosted air. Turbos, screw
blowers and centrifugal blowers have high efficiencies, while Roots blowers have
lower efficiencies. High intake air temps place a greater load on intercoolers,
reduce potential power and increase the likelihood of detonation.
Most factory and aftermarket superchargers use Roots designs, so unless you
pick the supercharger carefully, a turbo will have a higher efficiency.
3. Boost Characteristics
There are two aspects to consider: where boost is developed in the engine
load and rev range, and how boost can be controlled. Turbochargers and
centrifugal superchargers do not generally develop boost at very low engine
loads. Instead, full boost is available from about one-third of redline revs (eg
from 2000 rpm in a 6000 rpm engine). (Note: this applies only to well-matched
turbos and blowers!) On the other hand, positive displacement superchargers
(screw and Roots) can develop more or less the same level of boost from idle to redline.
Most turbos have built-in exhaust gas wastegates, allowing the very fine
control of boost when it is actually occurring. (At revs and load less than peak
boost, the turbo wastegate is normally fully shut, trying to bring boost up as
fast as possible.) Superchargers may have a bypass valve that can be used to
control boost levels, but in general the peak boost is determined by the gearing
relationship between engine and blower speed – something alterable only be
changing pulley sizes.
Therefore, turbos have much better versatility in the control of boost levels
– but only once the engine is producing enough exhaust gas to spin the turbo
fast enough to produce boost. If boost is wanted at low engine loads and revs, a
positive displacement supercharger is better than a turbo.
Conclusion
Deciding between a turbo and supercharger can be a very difficult decision to
make. In the case of the engine being discussed here, access within the engine
bay to the exhaust side of the engine is very tight (making fitting a turbo
harder) and boost is wanted at very low revs (requiring a positive displacement
supercharger). That seems to spell ‘blower’ as the answer, but it’s also likely
that fine control over boost will be needed (turbo), and fuel economy is very
important (again turbo).
If you’re looking at forced aspiration and are unsure which way to go, answer
these questions.
- Is there a factory forced aspirated version of the motor already
available?
- Is there a well developed forced aspiration kit for the motor already
available?
- What is the range of turbos and superchargers (both new and second-hand)
that will suit the application?
- Will it be easier to fit a turbo or supercharger in the engine bay?
- What efficiencies are the turbos and blowers likely to have?
- What are the required boost development characteristics?
Think through the answers to those questions and you’ll be much better placed
to make a decision.
Email a friend
Print article
|