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Boosting our way into the future

12.06.2014

IN AN era where manufacturers are producing increasingly more powerful and efficient cars, there’s been an increase in the amount of turbocharged cars on the market. What are these strange snail-like creatures that lurk under the bonnet of many new vehicles and why are manufacturers using turbos for fuel efficiency? They are there for performance - right?

These are some of the questions I’m frequently asked in letters and thought it an apt time to address the public about the wonderful world of forced induction. I’ll focus on turbochargers, as the majority of manufacturers prefer these to superchargers from a cost and efficiency perspective.

You see, a regular engine without turbo makes use of ambient air (the air around us) which flows into the engine and mixes with fuel to create the mini explosion required to spin an engine and give the car drive to its wheels. This creates exhaust gas which we see coming out of our exhausts.

What a turbo does, is place a turbine in the way of the exhaust gas as it comes out of the engine. This is called an exhaust-driven compressor.  Another turbine on the opposite side of the turbo charger then sucks in ambient air. The two airflows spin each turbine and force very hot air through the channel between the two turbines, through piping. The amount of air being forced through this channel is called, ‘boost’ by many.

You’ll often hear people talking about how much they’re boosting in terms of the bar unit of measurement. This is how much air pressure is being fed through the engine. The boost is controlled by a wastegate which detects when the desired amount of boost - which is set by the Engine Control Unit (ECU) - has collected within the system. It then removes the excess air and either expels it into the atmosphere or pushes it back into the exhaust where it exits the vehicle

Back to the air that comprises the boost. This hot air enters an intercooler which uses coils, or in some cases liquid, to cool the air. This is required because when any gas is compressed it creates heat and the hotter the intake charge air temperature is, the more chance there is of pre-detonation (knock) of the fuel/air charge prior to timed spark ignition, which means the death of your engine.The cooler air is then fed through more piping into the vehicle’s intake manifold and into the engine.

You’ll probably have heard the punctured tyre sound a turbo makes; this sound, or hiss occurs when the driver lifts off the throttle which causes the throttle body to close leaving the air trapped. This air has to go somewhere, so between the intercooler and throttle body there’s a blow-off valve (also known as a dump valve) which releases air and gives off that distinctive noise. Some dump valves recirculate the air and others - as its name suggests - dump air into the atmosphere.

Once that highly pressurised air enters the intake manifold, the engine’s computer has been programmed to add the correct fuel ratio with this air, which allows for increased power and torque.

So what is turbo lag? As you’ve read, the above process requires the air entering the engine to go on quite a journey through the engine. Those turbine impellers need to spool up to speed and this also takes time; so while the air passes through them, they speed up and build the boost until the desired level, which is when the power comes in. There are designs out that reduce lag such as light-weight impellers, variable geometry and twin-scroll turbochargers or Anti Lag Systems (ALS).

I’m sure you can tell that more fuel and air leads to more power; it’s simple to understand, but how would a turbo make a car efficient?

Well, firstly, it has a greater thermal efficiency than a naturally aspirated car, meaning it uses gasses from the exhaust that would normally be wasted, to help produce power. This also means that a turbo makes better use of both heat and flow, denoting the same amount of power can be made out of a 1.4-litre turbo as what can be made from a 2.0-litre naturally aspirated engine, for example. Therefore, a turbo requires less capacity to create the same - if not more - energy.

The way in which a turbo vehicle makes power, also helps with fuel economy. Take any naturally aspirated engine and check where it produces its peak power and you’ll normally see that it comes in close to the vehicle red line; if not, then the earlier peak power will mean a drop in top-end power in any case, which is not desirable.

This is where a turbo comes in handy; it can produce power in the lower rev range. Decent bottom-end power means the car will be able to use low engine revs more of the time. Frictional losses inside the engine increase as revs build, meaning the engine becomes less efficient as revs rise. This is why many performance cars have big engines because these give good power at low revs.  But a bigger engine will also have higher internal friction, so there’s no gain in consumption. That’s why so many manufacturers use turbocharged engines in all cars now as it helps with both performance and fuel economy.

Article written by Sean Nurse
12.06.2014
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