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Old 06-25-2008, 11:05 PM   #1
Mr. Wyndham
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Turbocharge your way to efficiency.....and power!

I was shown this today...and was impressed, so I wanted to share.

I always wished GM had produced a Direct Injected NA version of the 2.0L Ecotec so that we could compare it's numbers to that of the Turbo version....but they don't. This is exactly what I've been looking for, though (minus the engine type, so here you have it (an Australian publication):

A turbo engine has the potential to be more economical than a naturally aspirated engine for two reasons.
  • Thermal Efficiency
A turbocharged engine is more thermally efficient than a naturally aspirated engine. This is easy to understand when you remember that the turbo uses energy normally wasted out of the exhaust.

If you stand behind a car on a dyno as it undergoes a full power run, you’ll be amazed at the force of the gas coming out of the tail-pipe – you can feel it from literally metres behind the car. As it exits the engine, the gas is also hot – up to 800 degrees C – and it is hot because it has energy that can do work.

Since a turbo makes use of both the heat and flow, in a turbo car a greater amount of fuel is converted to useful work.

(Another way of looking at this is to consider that there is always a temperature drop across a turbo. That is, the temperature of the exhaust gas measured in front of the turbo is always higher than the temperature of the exhaust gas measured after the turbo. This temp change is indicative of the energy being taken from the exhaust gas. The turbo also takes much of the pulsing out of the exhaust gas – the reason why a turbo car usually needs less muffling of its exhaust. Again, the turbo is drawing energy from the exhaust.)

So a turbocharged engine is more thermally efficient than a naturally aspirated engine.
  • Shape of Power Curve
The other reason that a turbo engine has the ability to more economical relates to the way in which it develops power.

In a given naturally aspirated engine, the power developed at each rpm depends on aspects such as valve timing and intake and exhaust manifold tuning. For example, the engine might be ‘tuned’ to develop a lot of power at low revs. However, even with variable valve timing and variable inlet manifolds, a naturally aspirated engine with good bottom-end power is very unlikely to have good top-end power – the compromise to get the good bottom-end power output is simply too great.

The reason that good bottom-end power will improve fuel economy is that the car will be able to use low engine revs more of the time. That is, the gearbox will not need to change down, so keeping revs lower. This is important for economy as the frictional losses inside the engine increase rapidly with higher revs – the throttled engine becomes less efficient as revs rise. But low engine revs can only be used if the engine has sufficient power to propel the car at those revs!

Of course, fitting a bigger engine will give good power at low revs – but a bigger engine will also have higher internal friction, so there’s no overall gain.

Therefore, to gain the best real world economy, what is needed is a small engine (so having low internal friction) running at low revs (again, small internal frictional losses) that develops lots of power at those low revs.
And the best way to efficiently give a small engine good low-down power is to turbo it.


So let’s take a step back. In the comparison shown above of the two 2-litre engines, the turbo engine has better fuel economy, better CO2 emissions, 50 per cent more bottom-end power and 34 per cent more top-end power.
In short, it’s better in every respect (except, it must be said, cost to build).

Having driven both engines on the road in recent times, the turbo engine is massively better – and in fact in freeway travel, we scored an even greater fuel economy win to the turbo engine than the above official test figures show.
Now all we need is for GM to pop out a Turbo V6....hmmmm.........
"Keep the faith." - Fbodfather
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