How forced induction engines work

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Turbochargers are powered by exhaust gases passing through a turbine.

The quest for speed, or perhaps I should say available speed, has been part of man for as long as the internal combustion engine. We’ll leave the horse out of this! I say “available speed” because although many cars are capable of more than 250km/h, few drivers (thank heaven!) take them to that limit.

But, fortunately, some things, such as superchargers and turbochargers which were built primarily with power in mind, have practical advantages as well. For example, during the 1914-18 war Paul Daimler of the Daimler Company used Roots type blowers in aircraft engines. Because air pressure is lower at high altitude and normal aspiration would be impossible. Daimler used the blowers in a bid to maintain ground level pressure in the inlet pipe while at whatever altitude at which the engine operated.

Daimler later encouraged supercharging in post-war racing and sports cars.

The supercharger is well named because it boosts, or supercharges the induction pressure in an engine above atmospheric pressure. Higher performance requires more air and more fuel to enter the engine and this is determined by displacement and rpm.

Mind you, there were snags and the early supercharged engines proved unreliable, largely due to the unsophisticated spark plugs of the time. As we have seen before, the branches of technology often advance in unequal steps.

The main difference between the supercharger and the turbocharger is the method used to drive the blower (compressor). The supercharger is mechanically driven while the velocity of the exhaust gases drives the turbocharger via a finned wheel (turbine).

Naturally, mechanical drive will absorb some engine power and this was always a point to be held against the supercharger.

However, the turbo cannot truly be said to deliver power for free, because when exhaust gases drive a turbine this tends to create back-pressure in the exhaust and also saps engine power.

When talking of turbochargers, the subject of lag inevitably arises. This means there is a delay between actual acceleration and turbo boost. By using exhaust gases to drive the turbine there is an interruption of the exhaust gases; hence the back-pressure.

Smaller, lighter turbochargers have been built to keep up with engine acceleration and minimise low speed lag, but such units were inevitably unable to produce the required boost at high revs when more air was called for.

Conversely, a larger turbo would give plenty of top end oomph, but response at the bottom end suffered.

There are various clever ways of overcoming such hassles; one, the Variable Geometry Turbo, uses automatic adjustment of the turbine blades allowing a greater or lesser turbine surface when needed. Another approach takes the incoming exhaust gases through an adjustable nozzle and diverts them onto the turbine. As the revs increase the vanes open and the turbine speed stabilises.

Turbos are relatively trouble-free, but when they do go wrong they can be horrendously expensive to put right or renew. Are they worthwhile? Certainly. They give small capacity engines big engine performance and when not in operation allow normal fuel consumption.

Saying that turbochargers are generally trouble-free may be stretching the point a little. Far better to say they give little trouble if treated with respect and a little knowledge. In fact, I’d be inclined to suggest anyone who buys a turbo-equipped car more than a few years old should seek advice immediately from somebody experienced in the particular marque.

Handbooks/manuals are good guides in many ways, but vehicle manufacturers often tend to err on the bright side when it comes to service intervals, especially when a new model comes on the market and faults have failed to materialise.

For example, one manufacturer initially recommended that its turbo-engined model should have the cambelt, tensioner, roller and tensioner damper renewed at 180 000km intervals, but later this was modified to 120 000km. No doubt several unhappy drivers learned by this mistaken optimism. Always get the very latest figures from the manufacturer or dealer agent.

Oil and filter changes at the correct intervals are important for any engine, but even more so for those with a turbo. Do not be tempted to use normal mineral oil if fully synthetic is recommended. The few bucks you’ll save will cost you dearly in the long run.

One common cause of turbo failure is failing to let the engine idle sufficiently to let the turbo cool sufficiently after long, brisk motoring. On some engines a few minutes idling is advised. The turbo relies on engine oil pressure for lubrication and if the unit is revving at, say 30 000rpm and the engine is switched off, the turbo will continue running and perhaps be starved of oil.

Another possible cause of failure is neglecting air filter changes.

One problem is that if the turbo was designed to give the correct pressure in the middle engine speed range, the boost pressure would be too high at maximum engine speeds, raising the risk of detonation and a burst engine.

Three basic ways of preventing such dangers include the waste-gate, compressor blow-off valve and turbine outlet restriction.

The waste-gate allows gas to go through it, by-passing the turbine. As intake manifold pressure rises to a predetermined level a diaphragm valve opens and closes again as engine speed drops. That is the basic operation. More sophisticated versions are available.

The blow-off valve on the inlet manifold takes care of excess air delivery and the turbine outlet restriction is an orifice downstream of the turbo to lessen turbine output at high engine revs. The latter tends to cause higher exhaust back pressure than the other methods.

Typical symptoms of turbo trouble may be heavy smoking during hard acceleration, poor performance or even a kind of whistling noise from the turbo itself.

The basic message is to get an expert to check any car that you have, or are thinking of buying. The cost of renewal or neglecting faults can be horrendous. When a turbo blows up, it can cause all sorts of other damage, so if the car you hope to buy looks as if it has recently had a new one fitted, be extra suspicious - and careful.

Do not forget to inform your insurance company if an aftermarket turbocharger is fitted.


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boss, wrote

IOL Comments
12:13pm on 7 February 2011
IOL Comments

30000rpm? that's ALMOST 9000!?!?

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