These days few automotive engineers can resist the temptation to add a turbocharger or a supercharger to one of their engines. Such devices have not only given diesel engines a new life, but have also been utilised by petrol engine designers to increase the power developed per litre in engines ranging in size from two to six litres.
Air-chargers or blowers increase an engine’s output by forcing more air into the engine than would otherwise be ingested, so that more fuel can be added, resulting in more power. However, any particular application has to be carefully crafted and exhaustively tested to make sure that the final result meets with engineering and customer expectations. This means that one should be very careful when having an after-market turbo fitted.
The choice between a turbo- or super-charger is not only based on engineering considerations but also takes fashion and customer acceptance into account, so it’s seldom possible to state categorically that any particular layout is better than another. Currently, both these types are fitted to a variety of production vehicles.
Differences between the two types
The most important difference is the fact that a supercharger, commonly known as a blower, is driven by the engine via a suitable belt, chain, or gears, so that it cannot over-speed, and does not need a valve to relieve the pressure. The pressure increase, as well as the maximum pressure, is determined by the engine speed in combination with the impeller shape. Most blowers are designed to work on the principle of positive displacement, so that when rotated slowly they will displace a calculable volume of air. This is achieved by having vanes or lobes that fit closely inside a casing.
Blowers have been constructed in a number of different layouts, but the most popular type is called a Roots blower, named after two brothers who built the first one. Blowers tend to boost the intake manifold pressure, and hence the torque, from just above idling speed.
In contrast, a turbo will only pump air if it is rotated very fast, because it is driven by the exhaust gas, and the vanes are shaped to scoop the air inside a casing that is not close fitting. The turbo consists of two impellers. The first, called the turbine, is situated inside the exhaust manifold, and connected by means of a rotating shaft to the second impeller, called the compressor, inside the intake manifold. These two impellers have totally different shapes, because the first has to absorb energy from the outwards-rushing gas, and the second has to give off energy to the intake air.
The air has inertia, so it doesn’t change speed instantaneously, giving rise to so-called turbo-lag, resulting in a slight hesitation when a turbocharged engine is accelerated. These days, lag is almost non-existent, because the turbo is made so small that most impellers can easily fit into the palm of your hand. This reduces the turbo’s inertia, but as a result, most turbos have to spin up to more than 200 000 r/min to deliver any useful boost.
The boost, or excess pressure above atmospheric, delivered to the manifold, is controlled by a valve, called the wastegate, that allows some exhaust gas to by-pass the turbo unit and rejoin the gas that has flowed past the turbo further along the exhaust system. Turbos have very little effect below about 1 500 or 2 000 r/min but, when the boost starts to multiply, the smooth increase in acceleration has been likened to the car being pulled forward by a giant elastic band.
Both types of air-chargers tend to heat the air, because the pressure, volume and temperature of a gas are closely related. This means that you cannot change one without affecting the other two and, if the volume is fixed, then a rise in pressure must result in a rise in temperature. Heated air is less dense, so it contains less oxygen, and can be mixed with less fuel, resulting in a drop in power. This explains why many blown engines have intercoolers, which are small radiators that use an air-to-air or air-to-water interface to cool the incoming air, resulting in an increase in power. Blowers have a long history and are still being enhanced and modified to improve their characteristics, as we shall see.
Superchargers
1. Petrol engines
Supercharging petrol engines started in the early ’20s when the lessons learnt in designing aero engines were being applied to production engines. Mercedes produced one of the first, if not the first, production supercharged cars in 1922, and went on to use supercharging not only in its bigger cars, but also on its legendary racing cars of the ’30s. Many other manufacturers followed, and supercharging enjoyed a golden era from about 1925 to the beginning of WW2.
After the war, supercharging was to all intents and purposes dead, except in the USA where the initial availability of cheap Roots aircraft cabin blowers led to a huge industry devoted to supercharging any suitable or unsuitable engine. On production cars, supercharging has recently been revived by Jaguar, the new Mini and Mercedes-Benz. The latter uses blowers for two different applications. In the AMG-tuned V8 models the blowers are used to improve the output of big (5-litre) engines, but in the C-Class kompressor models the blowers are designed to deliver three different boost levels to give three different outputs from the same 1,8- litre engine. In this way the same cubic capacity engine does duty in the C180, C200 and C230 Sports Coupé.
Downsizing in this way, ie using a boosted smaller engine to deliver as much power as an unboosted bigger one, is a growing trend, because the engine becomes more efficient, and will use less fuel. In fact, the next step, which is just around the corner, is to use an engine so small that it can only deliver enough power to cruise at reasonable speeds, but will be too weak to accelerate at a reasonable rate. This is theoretically viable because a car uses more power to accelerate briskly than to cruise at (say) 140 km/h. The extra power needed to accelerate will then be provided by an electrically-driven supercharger that can be engaged at will by the driver or an electronic control.
The major disadvantage of blowers is the fact that the maximum boost occurs at maximum engine revs, where it is least needed, at least for most applications. This may change, because a company called Integral Powertrain is working on a mechanically-driven centrifugal blower, ie with an impeller not unlike the one found inside the intake manifold on a turbo. This unit has an electrically activated traction drive that provides a gear ratio from 0 to 150 times the crankshaft speed to run the blower up to 225 000 r/min. This allows the compressor shaft to accelerate from rest to 150 000 r/min in less than 300 milliseconds. Coupled to an engine, it will give a 1,4- litre the performance of a 2,0-litre.
2. Diesel engines
Very few modern road-going diesel engines are fitted with superchargers, most likely because a turbo is more suited to diesel engine characteristics, as we shall see. However, many large diesel engines, built for marine or rail use, are designed as two-strokes, and these engines are not just scaled-up motorbike engines. Instead, they have exhaust valves in the cylinder head, but get fresh intake air from ports in the cylinder wall that are fed with air under pressure from a supercharger. This layout combines well with the diesel cycle because the blower feeds air only, and for some of the time the exhaust valves are open, so that the clean air is able to scavenge the combustion chamber. In the past, a number of manufacturers have produced two-stroke diesel trucks, but we’re not aware of any currently in production.
Turbochargers
1. Petrol engines
The petrol-engined turbo has had a chequered career in the hands of both amateur and professional engineers. American hot-rodders started fitting diesel turbos to petrol engines after WW2, and this soon led to the production of turbos for petrol engines. The 1962 Chevrolet Corvair was one of the first production turbocharged cars, and in Europe Saab also produced some early turbocharged models.
Problems soon arose, because the combustion pressure in a petrol engine is limited by the onset of detonation. This occurs because a petrol engine inhales a mixture of fuel and air, and once combustion starts, the advancing flame front compresses and heats the remaining mixture to the point that, unless conditions are perfect, the so-called end gas detonates without the need for a spark.
The meeting of the two flame fronts produces shock waves that cause engine damage and can sometimes be heard as a pinging noise. The main causes of detonation are a very high combustion pressure, too far advanced ignition timing and fuel with too low an octane rating.
So, though compression ratios on unblown engines have an upper limit of about 12:1, in blown engines this has to be reduced to about 8,5:1, to ward off piston damage. Knock sensors weren’t available in the early days of turbocharging, so damaged engines soon limited the spread of turbo-fever, in spite of the relatively low boost of not more than 0,5 bar.
Nowadays, most engines are fitted with knock sensors, so boost pressures of up to one or even 1,5 bar are the order of the day. Further development of turbocharging on petrol engines is limited because of the detonation threshold, but twin turbos are often used in parallel on V8s, to keep the turbine wheels small so that lag is minimised. So, for more sophisticated turbo layouts, one has to look to diesel engines.
2. Diesel Engines
The first turbo was designed by Alfred Büchi in Switzerland before WW1, and the concept has been applied with great success on large marine and rail diesel engines. Diesel engines do not control the amount of power developed by means of a throttle butterfly in the intake manifold as petrol engines do. Instead, these engines always inhale as much intake air as the engine speed will allow. The power demand is controlled by the amount of fuel injected into the combustion chamber. This changes the nature of the combustion process so that detonation is not a problem. The individual fuel droplets issuing from the injector have to first evaporate and then find some oxygen-bearing air before they can burn. This means that the flames are surrounded by air, and not a mixture, so that the end-gas is not combustable, and detonation cannot occur. In addition, at maximum power delivery, a diesel engine has at least 20 percent excess air inside the combustion chamber, while at lower power levels the percentage of excess air is even greater.
The lack of a butterfly valve in the intake manifold means that there is no sudden change in intake manifold pressure. This makes diesel engines particularly suited to this form of boosting, because there is less variation in turbo speed. Furthermore, the combustion process shows that blower boost pressures are not a limiting factor in diesel engine development. Instead, the physical strength of the engine becomes a limiting factor.
The first modification to the traditional turbo was the development of variable nozzle turbo-chargers (see figure 4). These utilise a narrow vane opening at low engine speed to boost torque, and a wide vane opening at high engine speeds to optimise the torque for the different airflow in the new conditions.
The latest turbo developments are even more exciting. BMW, Opel and Mercedes-Benz are selling or soon will be selling diesel engines fitted with two-stage turbocharging. The BMW layout uses one small and one bigger turbo working in such a way that at low engine speed the intake air flows through the larger turbo without picking up much of a boost, but is compressed by the smaller turbo. This unit is small enough to react fast even at idling speed, and BMW claims that the six-cylinder, three-litre unit develops 530 N.m at 1 500 r/min. As the engine speed rises, the larger turbo supplies more of a boost, but the small unit also supplies a useful boost, so that the maximum torque of 560 N.m is developed at 2 000 r/min. The flow of air between the two units is controlled by an electronically activated control valve so, at high engine speeds, the large turbocharger does most of the boosting. By this means a boost of at least two bar can be obtained without the lag that a single bigger turbo will cause. The maximum output is 200 kW at 4 400 r/min.
The Mercedes-Benz triturbo SLK shown at the recent Geneva motor show is fitted with two small parallel-acting turbos, working in series with a bigger turbo to give a 3,0-litre V6 diesel engine a torque maximum of over 600 N.m.
Turbo problems
South Africa is not the ideal country for these high-tech engines, because we do not have enough mechanics that have been trained to service them properly. In addition, most buyers of these vehicles are not told how to drive them, and what combination of conditions to avoid. Last year we were criticised by a number of people for warning the public against the high percentage of turbo failures, and although the position has improved, we’re still getting complaints.
One of the problems is that at Gauteng altitudes some turbo designs overspeed to try and compensate for the lack of intake pressure, and this leads to overheating. Another reason for failure is bad servicing, especially on diesel engines. These units need an over-supply of air, and if the aircleaner and associated piping gets blocked then the engine will overheat. A common scenario is to have fun in the veld with an offroad vehicle, and then drive home with a wet or mud-covered intake system. Soon after that the engine will be damaged by overheating.
Lugging an engine, especially a turbodiesel, is also harmful. This refers to the habit of driving slowly in a high gear at a large throttle opening. This cannot be done if the vehicle has an automatic transmission, because it will change down. Perhaps all turbodiesels should be fitted with autoboxes.
