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Trail Savvy: Turbo talk

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A turbocharger is a turbine driven by exhaust gases. It increases the combustion efficiency of an engine by compressing intake air before the engine consumes it. Because the air is being compressed, the engine has more oxygen available for combustion, thus increasing output and efficiency.

Why boost?

In recent years, we’ve seen nearly every vehicle manufacturer drop large displacement naturally-aspirated petrol engines from their line-ups, and focus attention on the more efficient (turbocharged) diesels. Some markets still demand the old-school petrol engines (including ours, the United States and the Middle East), which is why Toyota still offers a 4.0-litre V6 in its Hilux, Prado and 70 Series Cruisers, for example, while Jeep is not sidelining its 3.6-litre Pentastar V6… and Nissan’s 5.6-litre V8 still shunts its Patrol Wagon. But these are becoming exceptions, rather than the rule.

The death-rattle of free breathing petrols (and large displacement diesels in light commercial vehicles, for that matter) has come quickly, because of the obvious gains that a turbocharger offers in terms of both power and economy. Modern turbo engines (generally) reach peak torque at around 2000 rpm, while their naturally-aspirated counterparts only reach peak torque a couple of thousand rpms later. Peak power is also reached far earlier in turbocharged engines, which makes them excellent power-plants for heavy vehicles like 4x4s, that often tow, or need tractable power, from a low rpm.

Diesel engines have used turbocharging for years to boost their low specific outputs, and these have become mainstream in the commercial and 4×4 world. So, it was just a matter of time before turbo petrol engines became mainstream, too. A key side-benefit of using compressed air is that the same (or more) power can be produced by a much lighter and smaller displacement engine. Less weight means more efficiency, in addition to a real-world fuel saving of around 8 -15% between a smaller turbocharged engine, and a larger naturally-aspirated engine making similar power.

But − and there is always a “but” when it comes to mechanical things − the “free” power offered by turbocharging brings a few complications.

The downsides

Firstly, turbos add expense. The engines themselves need to be more robustly constructed because of the higher compression ratios at play. In addition, the turbocharger itself is a precision-engineered piece of kit that requires extra pipework to plumb it, and usually also an intercooler, into the engine. Turbo engines run hotter unless they have an intercooler (essentially an extra radiator) to ensure that the air being force-fed into the combustion chamber is cooler, and therefore more oxygen-rich.

The extra expense doesn’t stop at the purchase price of your vehicle, either, as turbos are prone to failure when used outside optimum running conditions. Consider that a turbo impeller spins at north of 160 000 rpm on full boost, which explains why bearings and impeller shafts are known to give up well before the rest of the engine.

Replacing a turbo is expensive. A little digging shows that on a Toyota Hilux/Fortuner 2.8 turbodiesel (for example) a replacement turbo will cost R24 550, plus R3k for labour. That’s a relatively simple variable geometry unit. Many modern high-tech turbodiesels have twin-, triple- or even quad-turbo setups − the latter in the case of BMW’s potent but complicated M50D. And, when one turbo blows or starts timing out, chances are that the others will be replaced as part of the fix. And replacing the four turbos on that 3-litre straight six M50D quad turbo diesel engine, as found in some X5 models, will cost R109 000.

Because of their propensity to give up after the warranty period on a vehicle is over, buying a high-mileage multi-turbo diesel or petrol vehicle can be a risky proposition, so it makes sense that the more high-tech examples suffer relatively higher depreciation.

Taking care

Turbos spin at incredible speeds, asking a lot from the associated bearings, shafts and the oils used to keep them friction-free. This is not to say that a well-maintained turbo will not last. In fact, if it is taken care of correctly, you may well enjoy many hundreds of thousands of trouble-free kilometres on a single ‘snail’. Turbo designs have improved vastly over the years, ceramic bearings have overcome many heat-associated problems, and separate oil circuits pumped by independent electric motors (rather than being reliant on mechanical drive from the engine), have upped reliability. That said, lately we’ve also seen some turbos let go spectacularly on more than one modern vehicle, so a few turbo-care driving tips still apply….

1. Warm up

Because of the intricate, precision design of a turbo, temperature fluctuations can have a huge effect on its reliability as the materials expand and contract. Rather than booting your bakkie onto the highway only seconds after starting up in the morning, allow your engine to warm up slowly so that the viscous-when-cold oil can get to all the places it needs to. Ideally, it’s best to idle your vehicle for a short while before driving, and then try to keep the engine out of high-boost range for about ten minutes (around 5km) as the oil heats up.

2. For the long haul

Big trucks and long-distance tourers keep their turbos in tip-top shape, often for millions of kilometres, even though they spend much of their lives at wide-open throttle. The reason that they last so long is that the turbos are almost always at optimal operating temperature and benefiting from good oil circulation. When your driving mix frequently includes quick drives to the shop and back, a turbocharger never gets up to this temperature, and thus may not last.

3. Cool it down

Pulling a heavy trailer up a mountain pass in your diesel or petrol turbo vehicle, and turning the engine off when stopping at the top for a smoke break, is about the worst thing you can do. Although modern oil-circulation and cooling systems (such as air-to-water intercoolers) are much more efficient than they used to be, preventing a turbo from receiving proper oil flow while it is glowing white-hot will not do it any favours. After a hard drive, it makes mechanical sense to take the last kilometre to your destination nice and easy, to allow proper cooling. Either that, or let your car idle for between thirty and sixty seconds after arrival. The newest cars, with stop/start technologies, generally have systems in place to keep water and oil circulating while the turbo spools down, to prevent damage. Fact is, cooked oil causes sludge build-up and the potential of blocked oil passages.

4. Drop a notch

One of the great things about turbo motors is the lovely surge of low-down torque, making city driving and long hills effortless. Unfortunately, that surge comes at a price, and riding that torque wave in too-high a gear will mean that your engine is struggling and your turbo or turbos will be spooling at maximum rpm. By dropping a gear and letting your engine rev more freely, you’ll not only reduce stress on your gearbox and engine internals, but your turbo too, by reducing its boost pressure and rpms.

5. Keep it lubed

As with any engine, clean oil is vital. However, you’ve seen some YouTube video of an old Camry with 200 000 miles on it and never an oil change, so you think that it’s fine to ignore intervals. Wrong… clean oil is far more critical in keeping a turbo motor alive than it is with a less-stressed naturally-aspirated motor. In general, the oil specified to run in a turbo motor contains additives to reduce decomposition at the extremely high temperatures reached by turbos. They also have to continue to lubricate at high rpms, whether the engine is hot or cold. For this reason, it’s imperative to change your oil on schedule. Many careful owners change their oil twice as often as the manufacturer’s handbook suggests.

With any luck, using these tips will mean that you’ll be happily on boost for years to come. That old adage is true for turbos too: take care of them, and they’ll take care of you.
By Andrew Middleton

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