Turbos and Intercoolers… and your 4×4
Turbo chargers, or altitude compensators, as they are also known, are essential components on today’s diesel powered off-road vehicle engines. While many diesel engines were able to run reasonably efficiently without turbos in the past, these engines were dirty, slow, underpowered and required huge amounts of fuel to work. However, the demands of modern environmental legislation as it relates to engine design and emission control, have brought about the wide spread use of turbo chargers to help clean up exhaust emissions.
The tacking on of turbos to existing engines, with some minor modifications to the engine was the cheapest way for car manufacturers to obtain the compression ratios needed for more complete combustion of diesel, which results in cleaner exhaust emissions, more power, and vastly increased fuel economy, two aspects of turbo technology that are of cardinal importance to the 4×4 fraternity. These same effects could be realised in other ways, of course, but turbos were available, relatively cheap, and while they are not as dependable as they perhaps could be, the principles of their operation was well understood, but most importantly, they work very well for as long as they last. So, exactly what are turbo chargers and how do they work?
In purely technical terms, the turbo consists of a casing, sometimes referred to as the turbo assembly, usually made from cast iron because of its dimensional stability, and is placed as close to the engine as is practical to prevent pressure losses in the exhaust gas due to cooling and thus contraction of the gas. Through the casing, which is divided into two parts, passes a shaft with an impeller on each end, usually referred to as the turbine wheel and the compressor wheel.
On the exhaust side, hot, high velocity exhaust gas drives the turbine wheel, which in turn drives the compressor wheel to compress the intake air to pressures of between 0.5 of a bar and 1 bar, commonly referred to as the level of boost, on small capacity engines like those typically found in 4×4 vehicles. Incorporated into the casing is a wastegate, also known as a dump valve, through which excess pressure is dumped, or vented into the exhaust system. Dump valves can be controlled either by simple spring tension, or electronically by means of an actuator.
The internal wall through which the shaft passes also contains the support bearings for the shaft, oil and sometimes water seals, and a thrust plate to accommodate the thrust developed by the compressor wheel. Lubrication of the bearings is by means of pressurised engine lubricating oil. In some cases, provision is also made for water-cooling of the bearing assembly.
There is however one major drawback to turbos, and in particular as it relates to off-road driving. Since the turbo is driven by the exhaust gas coming from the cylinders, there is not much of it at low RPM’s, a problem that is compounded firstly by the fact that the exhaust gas has to overcome the inertia of the system, and secondly that gasses are highly compressible; the exhaust gas will first compress to a degree before it starts to act on the turbine wheel.
While under low-load situations a diesel vehicle will develop sufficient torque to take off smoothly, a sudden demand for full power could mean that there might not be sufficient power and torque available while the turbo increases its rotational speed to provide the required boost.
In off-road driving it is thus imperative that this phenomenon be kept in mind when approaching obstacles.
With a slower throttle response, care must be taken to ensure that the turbo is up to speed, usually from around 2400-3000 RPM, before approaching obstacles such as stretches of deep sand or mud, that require a constant speed and sufficient torque to negotiate successfully.
Why fit turbos?
The single most important factor that determines the efficiency of any diesel engine is the compression ratio; the difference between the pressure inside the cylinder just before detonation of the air/fuel mixture, and the ambient atmospheric pressure. Since diesel engines ignite this mixture by means of the adiabatic heating (heating through compression) of the mixture, it makes sense that the lower the compression ratio, the more incomplete combustion will be.
Older engines compensated for this by using very rich mixtures that generally caused excessive smoking, reduced performance, and very frequently, dilution of the engine oil, especially in engines with worn injectors.
Worn injectors can spray fuel onto the cylinder walls from where it mixes with and removes the oil film on the walls, and eventually ends up in the oil. Even worse, since the flame front never touches the cylinder walls in an efficient engine, fuel sprayed on the cylinder walls could potentially partially ignite with the result that the flame front is even further disrupted by burning erratically, or by burning for too long.
Turbos prevent most of these issues; having a continuous supply of compressed air in the inlet tract means that the engine is not dependent on the amount of air sucked in the cylinders during the inlet stroke. Instead, a huge volume of compressed air is forced into the cylinders for as long as the inlet valves are open, and in conjunction with a precisely controlled amount of fuel which when combined, provide the optimum level of enrichment of the mixture to burn efficiently and completely. However, other factors also determine combustion; injection pressure, timing and duration, as well as to a large degree, the temperature of the intake air, which is determined by the efficiency of the inter cooler, where one is fitted.
Turbos and injection
• Mechanical injectors:
This type of injector works because a small amount of fuel is contained within the injector assembly that is kept under pressure by means of a tapered needle valve that is kept closed by a heavy spring tension. Since diesel fuel is near incompressible, diesel fed into the injector increases the pressure inside the injector body, overcoming the spring tension and a small, metered amount of fuel is sprayed into cylinder in an inverted funnel shape. However, while these injectors work reasonably well, the typical injection pressure is insufficient to effectively atomise the fuel, resulting in relatively large droplets that are difficult to combust cleanly, resulting in excessive smoking under acceleration and other heavy loads.
• Monorail injection:
In contrast to the purely mechanical system in which injectors are individually fed from the pump, all the injectors in a monorail system , also known as common rail systems are connected to a reservoir, called a rail, which is kept under pressure from the pump via a single feed line. The advantages of such an arrangement are that higher injection pressures can be obtained by increasing the positive displacement capacity of the pump, as well as by controlling the operation of the injectors by a microprocessor.
By increasing the pressure in the system it has become possible to use smaller orifices in the injectors, with the result that fuel is atomised, or vaporised more effectively, giving much smaller droplets in the spray pattern. By manipulating the airflow pattern into the cylinder, swirling it around as it were, more efficient mixing of the air and fuel can be achieved, ultimately resulting in more effective combustion and higher levels of fuel economy than was ever possible before.
While a purely mechanical diesel injection system can work without a turbo, a monorail system cannot. Monorail diesel systems depend on the high volume of compressed intake air supplied by the turbo to work efficiently. But not only that, a computer controlled system adapts to changing and /or variable conditions in ways that no mechanical system can; by being “certain” of a continuous, adequate supply of compressed intake air, the microprocessor can adapt the injection timing and duration to ensure optimum engine performance under all operating conditions.
Intercoolers are nothing more than heat exchangers, or radiators, that are meant to cool down the very hot intake air coming from the turbo. Since hot air expands greatly and less of it can thus be fitted into any given volume, by cooling down the air more of it can be compressed into the same volume, a fact that has a potentially positive bearing on the overall efficiency and fuel consumption of turbo-equipped engines.
However, not all turbo-equipped off-road vehicles are fitted with intercoolers as standard equipment; space considerations sometimes preclude the fitment of intercoolers while on the other hand, they are sometimes fitted to vehicles within the same model range but with higher spec and trim levels.
Nevertheless, the main factors that determine the efficiency of any inter cooler are location and thus the rate of airflow through its core, its volume and rate of which air flows through it, and last but not least, length and diameter of the ducting from the turbo to the inlet manifold.
Cooling of the intercooler
Similar to the radiator of the engine cooling system, the intercooler also needs a positive flow of atmospheric air through its core, to cool down the air passing through it. To achieve this, manufacturers usually locate inter coolers directly in front and where possible, slightly below the radiator, but since the core of an intercooler is very much more open than that of a radiator, this placement does not greatly interfere with airflow through the radiator, while at the same time it benefits from the action of the radiator fan to increase air flow.
However, there is a drawback to all this because in the closely related fields of off-road driving and energy conversion, nothing is free; in this case the penalty being the vastly increased length of the ducting. When compared to a direct connection from turbo to inlet manifold, an intercooler placed in front of the radiator could mean that several times the volume of air that would have filled the original ducting could be required to fill the intercooler and associated ducting, which means that turbo-lag can increase significantly if the ducting is of the wrong diameter in the case of a DIY installation. Off-road vehicle manufacturers have for the most part eliminated this issue in factory installations but the increased ducting length means that the cooled air can and does pick up heat on the way to the engine, partly negating the cooling effect of the intercooler.
• Fan assisted Intercoolers:
Very few manufacturers have fitted intercoolers with dedicated fans; even though a fan-assisted intercooler can be much more compact than one that relies on an airflow induced by movement through the atmosphere, they still take up space in the (generally crowded) engine compartment, a fact that can cause issues during routine maintenance and repairs. While these fans usually draw cool atmospheric air from outside the engine compartment, many experienced 4×4 drivers consider these systems to be somewhat less than effective because of the small size of the intercooler, its low capacity fan, and the high temperatures in modern engine compartments.
• Water-cooled Intercoolers:
At first glance, the idea of fitting a water-cooled intercooler might sound like the solution to all intercooling issues. However, in the 4×4 context, they may very well create more problems than they solve, because they require extra piping, wiring, a dedicated water pump, a reservoir, and last but not least, a dedicated heat exchanger to cool down the water that was heated up while cooling down the intake air, not forgetting that for the system to be effective, this extra heat exchanger must have an airflow similar to that required by the main radiator.
Considering the fact that having a reliable and leak proof vehicle on an Africa overland expedition is much more important than the few extra Nm of torque an intercooler may or may not deliver, it becomes clear that the potential for leaks, burst pipes and other disasters far outweigh the uncertain benefits of such a system.
Based on the principle that water (which is pumped through the intercooler to absorb heat from the intake air) is a better coolant than air, this type of aftermarket system may be suitable for drag- or street racing but not so much for off-road applications where fewer pipes, pipe connections, pumps, brackets, clamps, and other assorted aftermarket hardware on a 4×4 vehicle means less chance of anything breaking down.
Intercooler… or not?
Intercoolers are not bad in themselves; any vehicle that had one of whatever type fitted in the factory has a potentially better fuel consumption and slightly higher torque rating than a similar vehicle that does not sport an intercooler. The benefits are however not huge and in most cases the presence of a benefit can only be determined by either long usage or dynamometer testing.
In the area of DIY intercooler installations on off-road vehicles, the potential for mistakes and disappointment increase in direct proportion the cost of the project. Factory built 4WD vehicles are already powerful enough and their degree of reliability in standard form is generally sufficient to survive almost anything, and in most conditions.
Most, if not all, experienced off-road drivers will much rather take a standard, unmodified 4×4 vehicle on an African overland expedition than one on which several, or any, untried and untested modifications had been performed. However, this is not to say that everybody should avoid fitting an intercooler of one type or another. It only means that the modification should be approached with a large measure of common sense and good judgement since off-road conditions in Africa in particular, are extremely tough and any breakdown of a component that was of dubious value to begin with, could easily turn a well-planned expedition into a costly disaster.
Given the fact that some turbos rotate at up to 300 000 RPM’s and that the bearings that must handle these speeds, loads and temperatures are really, really small, it stands to reason that they must be provided with the best possible lubrication. While it is true that service intervals on modern diesels have nearly doubled in recent years, it must be borne in mind that turbos depend on pressurised engine oil for their lubrication.
Mineral-based engine oil however, is susceptible to all manner of contamination and while modern oil filters are reasonably efficient, the fact remains that the oil that must lubricate a turbo can contain anything from solid particulate matter to sulphuric acid. Since turbos do not have their own cool and filtered oil supply, skimping on engine servicing is inviting trouble; dirty and contaminated engine oil is the single biggest cause of turbo failures and there are no spares or repair facilities in the hinterland of Africa.
While synthetic oils are better able to withstand the conditions inside modern engines, they are expensive but on the other hand, how expensive is a ruined turbo? Especially if that turbo disintegrated on the N4; not the N4 running through South Africa, but the trans-continental dirt road running through Africa, roughly following the equator.
Protect your turbo:
This may be preaching to the converted, however, turbos fail for very few reasons and chief among them are dirty oil, and lack of any oil.
Many devices go under the collective name of “turbo protectors”. Since turbos depend on pressurised engine oil for lubrication, it follows that if an engine is switched off while the turbo is still spinning at a couple of hundred thousand RPM’s, the turbo will be starved of lubrication. Over time, this lack of sufficient lubrication causes wear of the bearings, seals and the thrust plate, which requires lubrication not to seize since it rotates at the same rate as the shaft.
Fitting a device that either allows the engine to run at idle for a minute or so before shutting off while the turbo reduces its speed, or a device that supplies a quantity of pressurised oil to the turbo for a while after stopping the engine, will go a very long way towards increasing the longevity of any turbo.
From the off-road perspective, any reliable method to make a 4×4 vehicle more durable is worth investigating. Overland expeditions through, for instance Africa, are extremely difficult and demanding and there are no repair facilities on call except in a few places, like perhaps some of the larger cities.
Which of course, does not mean anything if you are stuck with a blown turbo in the middle of the Congolese rain forest.