There are two main ways to get more power from a car’s engine . the primary (and until recently the most popular) is to extend the capacity of the engine. The second is to extend the amount of fuel /air mixture going into the cylinder.
Generally, the more fuel/air mixture going into the cylinders, the more power the engine will produce. So part of the answer is to tune the carburetor , plate and manifolds to permit the engine to `breathe’ more freely, but there are limits to how much power are often extracted from an engine by these means while at the same time maintaining the engine’s reliability and adaptability. An alternative way of getting more fuel/air mixture into the cylinders is with a turbocharger.
What is a turbo?
A turbocharger is essentially a pump driven by the exhaust gases passing out of the exhaust manifold. The unit consists of a wheel with vanes – the turbine – that fits inside a housing within the exhaust. From this turbine a brief central drive shaft runs to a similar veined wheel called the compressor that feeds into the engine’s air intake.
When the exhaust gases flow
From the engine, they spin the turbine, which in turn spins the drive shaft to show the compressor. So, when the engine is running, the exhaust gases drive the turbine which makes the compressor pump air into the engine.
A fixed amount of fuel is automatically sucked in with the air if the engine features a carburetor. If the engine has fuel injection , the pc control unit is programmed to suit the boost pressures. The faster the engine is running, or the larger the throttle opening or both, the faster the turbocharger will spin. The faster the turbo spins, the more pressure, or boost it develops and therefore the more air it forces into the engine to make more power.
Engine at idle
- When the engine is idling it does not generate enough exhaust flow to spin the turbo fast enough to produce any real boost. The air passing through the compressor side of the turbo housing is being sucked through by the engine, rather than pumped through by the compressor. All the exhaust gases have to go through the turbocharger because the waste gate is shut.
- When the accelerator is depressed to feed in more fuel and air, the engine speed increases. This results in a greater exhaust flow which spins the turbine wheel faster. The turbine drives the compressor which compresses the air passing through its housing and sucks in more. It forces the pressurized air into the inlet tract.
- A small turbine gives excellent response but the back pressure limits the maximum power and also tends to over boost from mid-speed range upwards. To overcome this, when a small turbine is used, it is fitted with a waste gate, which limits boost from the turbocharger by diverting the exhaust gas from the main turbine once the preset boost has been achieved.
Although the turbo is designed to pressurize the mixture going into the engine, too much pressure would be dangerous because it can cause ‘knocking’ (per-ignition) and put an excessive amount of strain on the interior components of the engine. Therefore the maximum boost pressure that the turbocharger can produce has got to be limited by a valve referred to as a waste gate.
The waste gate may be a safety valve , located within the turbocharger, that opens to let a number of the exhaust gases bypass the turbine and flow directly into the exhaust . If the boost pressure is getting too high, the waste gate is activated by a pressure-sensitive actuator which senses the pressure being produced by the compressor.
Compressing the air causes problems of its own. When the air is compressed it heats up, which tends to form it expand. Because the aim of the turbo is to urge as much fuel/air mixture into the cylinder as possible, this hot air must be cooled down.
To do this, most turbocharged cars are fitted with an inter cooler. This seems like a small radiator, and cools the compressed gas that leaves the turbocharger. as the air cools down, its volume shrinks, therefore the amount of fuel/air mixture fed to the engine – and hence the power output – increases.
Installing a turbocharger
The turbo unit is plumbed in to the exhaust as near to the engine as possible. This helps to stay it compact and also helps prevent turbo lag. If there was an extended length of pipe between the engine and therefore the turbo, there would be a time delay between the accelerator being pressed down, the engine speed increasing, and therefore the turbo accelerating. The effect would be like having an elastic throttle cable.
Therefore, the turbo is usually bolted directly on to the manifold. The exhaust outlet is within the center of the turbine housing and leads off to the exhaust pipe. On the inlet side, the pressurized air leaves the compressor housing via a large-bore pipe. This runs through the inter cooler (if fitted), then to the manifold , or occasionally plenum chamber, where the fuel is added by injection before the air enters the engine.
The high speeds at which the turbine can spin create lubrication and cooling problems. In some turbochargers the turbine can spin at up to 200,000rpm, and therefore the hottest parts of the turbo will be at or near the temperature of the exhaust gas about 900°C.
Most turbo units have the central drive shaft bearing fed with oil from the engine. The turbocharger’s lubrication system is specially designed to deal with high temperatures. The oil drain pipe is of huge diameter to make sure that the oil, which develops a creamy consistency after browsing the turbocharger, will drain back to the sump under gravity. If there have been a restricted flow in this pipe, it might cause a build-up of pressure round the bearing within the center housing that would end in oil leaks on the turbocharger.
Some turbos have a water-cooled center bearing to reduce heat still further. The advantage is that, because the water remains being warmed by the engine, it continues to circulate and take heat faraway from the bearing for a couple of minutes after the engine has been stopped.
Early criticisms of turbo engines were their poor performance off-boost – when the engine wasn’t turning fast enough to spin the turbine quickly – and therefore the amount of your time it took for the turbocharger to start boosting once the accelerator was pressed. The poor off-boost performance was because road-going turbo engines don’t usually have a really high compression ratio . Forcing a lot of pressure into the cylinders is like raising the compression ratio so, if the engine started with high compression, at high boost the pressures inside the engine could promote detonation problems, or ‘ knock ‘, which might end in serious engine damage.
As a rough guide, every three pounds of boost are like increasing the compression ratio by an element of 1 . So if an engine with a compression ratio of 8:1 had a turbo which could deliver nine pounds of boost, the effective compression ratio would be about 11:1. a mean family car features a compression ratio of 9:1. Better engine and turbo control is that the answer – most turbo systems now use some sort of engine management which takes care of the electronic ignition and fuel injection system systems, retarding the ignition slightly if the engine starts to knock. Saab’s APC (Automatic
Performance Control) system goes one step further. Not only does it reduce boost pressure to a secure level, it also allows the engine to be run on any grade of fuel because the management system automatically compensates – although you get the simplest performance only with the highest grade. Early turbo engines suffered turbo lag, partly through poor engine management and partly because the lack of suitable turbo units often meant that the engines and turbos weren’t ideally matched to every other – an outsized turbo on a little engine will give good top-end power but will lack flexibility. Lag is nearly inevitable because a little engine would take time to `spin up’ an outsized turbo unit. a little turbo on an outsized engine gives good mid-range power with little or no lag, but ultimate power is compromised. These problems are minimized by better matching of the turbo and engine sizes, and by using lighter materials like ceramics and new designs like variable flow nozzles (see sideline overleaf).
The obvious benefit from a turbocharged engine is that of increased performance combined with economy – a turbocharged two-liter engine gives similar performance to an turbocharged three-liter one, without burning far more fuel than a two-liter.
It’s often simpler for a manufacturer to turbocharge an existing engine than to design and develop a new, larger one. Adding a turbo to an engine doesn’t usually significantly increase fuel consumption unless the improved performance is used to the full.