RED-HOT turbos, shattered turbine wheels and huge repair costs are enough to instil fear in any prospective buyer. This was especially true in the case of a certain Bavarian turbodiesel sedan sold locally in the early 2000s, which prompted ex-CAR technical editor Jake Venter’s feature Pop goes the turbo in the June 2003 issue. Back then, turbocharged vehicles were rare – and largely unknown entities.
Today, however, the picture is different; turbo engines are available in all categories of the motoring market. And the number of vehicles that employ forced-induction increase by the month. Eventually, turbos will feature on all internal-combustion engines.
On the following pages, find out everyhting you should know about turbocharging.
Engine power is limited by the amount of air that flows through the unit. The more oxygen that’s available for combustion, the more fuel can be added, leading to higher power outputs. This is especially true in petrol engines, which mostly run at the ideal air-to-fuel ratio of 14,7:1 (stoichiometric ratio). Diesel engines have excess air available, but fuelling can be added only up to a point without affecting other areas such as emissions. Enlarging capacity increases an engine’s ability to take in more air, but that also makes it larger and heavier. A different method is to raise the intake pressure of the air above that of ambient conditions, so as to force more air into the combustion chambers, i.e. pressure charging.
Why turbos are popular
Previously, turbochargers were ulitilised to produce more power; the objectives of lowering fuel consumption/emissions and improving reliability were not very important. Today’s turbo applications are concerned with efficiency, except on some performance cars. Globally, emissions legislation has become increasingly stringent (especially concerning CO2, NOx, hydrocarbons and particles), as per Euro 6 legislation.
Turbocharged engines are more efficient because:
• Turbos harness exhaust energy that would otherwise go to waste, with a small penalty of higher back pressure in the exhaust. This is also the reason why most turbo engines do not sound as good as NA units. Direct injection in petrol engines helps with charge cooling and prevents knocking. This allows modern turbopetrol engines to run at much higher compression ratios and more advanced ignition timing to further increase overall efficiency.
• Downsized turbo engines produce as much power and (even more) torque than larger-capacity NA engines while weighing much less. This reduces the overall mass of a vehicle and makes it easier for engineers to package smaller engines.
• The high torque characteristic of turbo engines at low engine speeds enables engineers to employ taller gearing to further reduce fuel consumption by running the engine in a lower (optimal) speed range. The high torque output also enhances driveability of the vehicle.
Turbo vs. natural aspiration
To illustrate the effect of a turbocharger on an engine, we chose Ford’s 2,0-litre, four-cylinder petrol engine. In the Focus 2,0 GDi, it’s naturally aspirated, while the Focus ST has a turbocharger bolter onto its version.
In terms of their hardware, the engines have the same capacity and even the bore and stroke dimensions are similar. Both feature direct injection, but the turbo engine has a lower compression ratio of 9,3 to 1, versus 12,1 to 1, to combat knock at high loads because of the higher in-cylinder pressures (due to the extra mass of air that is available for combustion in the turbo engine, boost runs up to 1,4 bar). The ST’s maximum torque figure of 360 N.m dwarfs the 202 N.m achieved by the NA engine (see graph above). The brake-mean-effective-pressure (BMEP; 22,8 bar versus 12,8 bar) calculation indicates that the turbo engine has to deal with much higher pressures and stresses than the NA unit. However, the extra torque and power results in a small fuel penalty of only 0,6 litres/100 km on the European drive cycle. If the turbo engine’s capacity was reduced to align the power figures with those of the NA unit, it would be lighter on fuel. That’s the essence of the engine downsizing phenomenon.
Are modern blowers reliable?
The latest turbochargers are a far cry from the units available 12 years ago when Pop goes the turbo was published. A major change, for example, was the addition of stop/start techno-logy; engines should be able to quickly stop when the system kicks in despite the fact that the turbocharger might still be spinning at extreme speeds (above 200 000 r/min). This would have been impossible on older-generation turbos.
Turbos on highly boosted engines now have watercooled centre housings. The coolant – taken from the engine cooling circuit – acts as a heat sink after engine shutdown to prevent the oil from coking around the bearings and oil galleries. The layouts of the cooling system and turbo are designed to promote natural circulation, or flow (creating a thermo-siphon effect), after the engine is switched off.
That said, often this process of natural cooling isn’t enough and an electric water pump is employed to provide forced cooling for a few minutes after the engine is switched off.
The engine-durability programmes of turbocharged and NA engines are almost the same, and therefore (theoretically) turbo engines are designed and tested to last as long as NA units. For example, turbo over-speeding at altitude is prevented by wastegate-control strategies to lower peak boost if needed. Statistically, however, there are more items that can fail on a turbo engine and it must endure higher internal stresses.
Turbos are here to stay because the positives outweigh the disadvantages. Without concrete proof that modern turbocharged engines are prone to frequent failure – as was the case 12 years ago – we should welcome the technology with open arms.
How a turbocharger works
1 The exhaust valve opens and the remaining combustion energy (heat and pressure) is released down the exhaust ports.
2 The expelled exhaust gas is then forced through the turbine side of the turbocharger that harnesses some of the energy by spinning the turbo shaft connected to the compressor side.
3 The compressor receives fresh atmospheric air from the air-intake system and raises the pressure to above atmospheric pressure in the intake manifold.
4 The intercooler stage increases the efficiency by cooling the fresh-air charge after the compressor stage (resulting in denser air) before it reaches the intake manifold.
5 The engine breathes air at the intake valve at a higher pressure than atmospheric, which results in more air filling the combustion chambers than in a naturally aspirated (NA) engine of similar capacity.
Turbo compounding and electric boost
Turbochargers run effectively and efficiently only in a narrow air-volume and pressure-ratio range. The differences in volume-flow rate of an engine at idle and at full load at maximum speed far exceed the capability of a single turbocharger.
Engineers use clever wastegate strategies to release some boost in the mid-engine speed range (resulting in a flat torque curve on turbocharged engines), as well as variable-vane technology to allow turbos to supply boost more effectively in the low engine-speed range.
A way to resolve the difference in airflow volume is to employ turbo-compounding, which uses a small turbo to supply boost in the low engine speed range and a larger unit to do the same in the higher range.
Engineers are currently working on electrical assistance for turbochargers. This will allow the use of a single large blower that spins up electrically at lower engine speeds to supply boost when sufficient exhaust energy is not yet available.