Everyone involved in the engine industry today must be completely familiar with the advantages of turbocharging both the C.I. and S.I. engines. Turbocharging potentially gives higher power density, from which flow the advantages of lower package bulk/weight, higher vehicle performance, and improved fuel economy and emissions profiles.

In the case of the diesel engine, these advantages are so overwhelming that it is difficult today to find an oil-burning vehicle which is not turbocharged, and it is hard to see any serious threat to its supremacy from other supercharging devices.

In the case of the gasoline engine however, and especially for ‘sporting’ vehicles, while the simple turbocharging concept has served us well for two decades, the advantages are far from overwhelming, and it is by no means clear that turbocharging will be the favoured method of increasing aspiration density in the future. The free-floating response characteristic is sometimes undesirable, especially in high power/weight ratio vehicles, and increasingly, the thermal mass of the exhaust-side components causes problems with cold-cycle emissions. While the former problem can be addressed very successfully by the application of good design principles, and the use of variable geometry, the problem of slow warm-up is rather more fundamental.

It is against this background that we have seen a resurgence of interest in supercharging in recent years. In summary, the principal advantages of supercharging compared to turbocharging would include:-

• More direct linkage between engine output and operator demand.
• Minimum exhaust-side thermal inertia.
• Minimum under-bonnet heat.
• Smaller size and easier packaging in many applications.
• Potentially lower cost through the avoidance of special materials.

The relative merits of these points will depend upon the application and the details of the vehicle design, and in great measure upon the type of supercharger.

Superchargers as a whole can be divided into three families, sharing a common principle of mechanical drive from the engine, but each having rather different characteristics. The basic family groupings would comprise:-

• Low specific-speed devices without internal compression, relying on ‘back-compression’ to raise the pressure of the pumped air. The commonest example is the Rootes blower, which has been in use for very many years, and is still the dominant type today. Its greatest strength is its comparative simplicity of construction, combined with reasonable performance at low speed. However, reliance on back-compression means that efficiency will always be poor, resulting in a high drive power requirement and elevated air outlet temperatures. The low-speed nature of the compression process means that this class is inherently bulky, leading to installation difficulties.
• Low specific-speed devices using internal compression, common examples being the Lysholm screw-type compressor and the G-Lader. The use of internal compression makes these types significantly more efficient, although they still suffer from substantial bulk. Their natural characteristics display good low-speed performance, although this depends in large measure upon maintaining very close clearances between the moving elements, presenting considerable manufacturing problems.
• High specific-speed devices, using a centrifugal compressor, characterised by high adiabatic efficiency. The impeller is similar to that of a turbocharger and requires to be run at 15 to 20 times crankshaft speed. A variety of types are in use, most commonly using a step-up pulley drive followed by a high ratio gear drive, to produce the required speed of perhaps 140,000 rev/min, depending upon impeller size. Because the compressor operates with high gas speeds, it is inherently compact, but as its speed is linked to the crankshaft, the natural characteristic is to deliver a rising boost pressure with engine speed, rather different from the previous types.

At Turbo Technics , we took the view that package size, weight, and fuel economy are of paramount importance in our market, and for these reasons we have chosen to develop the centrifugal supercharger. This is a route that has been followed by other manufacturers in the past using geared drives but was not considered a viable option in order to meet our goals. The principal drawbacks to the geared approach were considered to be :

• The impeller speed required means that the gear ratio will be high, resulting in a poor tooth contact ratio.
• Practical gear design forces the use of a relatively large compressor impeller with a smaller trim inducer, lowering efficiency, and increasing size and weight.
• Tooth contact velocities are high, making noise control difficult.

A viable alternative to gearing is to employ a planetary traction drive, relying on friction between rolling elements to provide the drive force, which does not impose the same speed and ratio constraints.

This approach offers the benefits of :

• Very compact packaging, with minimum length and profile.
• Low weight.
• Low noise.
• High efficiency, particularly at cruise conditions.

A considerable volume of investigative work has been carried out over the years on the subject of traction drives, and the behaviour of lubricated rolling elements at high speed is reasonably well understood. As applied to the TT machine, we have 3 rollers rotating around fixed axes, supporting a spindle at the centre. The outer annulus is a flexible ring, which is deformed elastically to provide a clamping force on the rollers and hence on the spindle. The compressor impeller is mounted on the spindle, which in turn is located axially by a collar and groove arrangement at the centre of the roller.

Because the annulus is flexible in order to provide the spring clamping, the drive connection from the pulley needs to allow the rotor to flex freely and this is accomplished by incorporating 12 drive pins within the end face of the annulus. These in turn engage with loosely fitting holes in the drive plate.

Lubrication is by cooled engine oil fed through the centre of the drive shaft/ drive plate assembly to a connecting hole through the centre of the impeller spindle. Radial oil holes through the collar at the centre of the spindle then distribute the oil around the roller and the annulus before being drained through a spiral groove from the interior surface of the annulus. The spindle surface is lubricated by oil carried around by the rollers, and the bearings are lubricated by oil mist resulting from the churning effect of the rollers on the oil. The oil then returns to the engine sump.

The action of the oil is to both lubricate and cool the rolling elements, but at high speed develops a "plastic" characteristic which aids the transmission of friction torque between the elements. In this respect, traction oil has superior properties to engine oil, but practical considerations dictate that an oil supply from the engine system is used.

One of our lead programmes in the development of the supercharger has been the Rover K series engine as fitted to both the Lotus Elise and MGF. Both vehicles use a common installation, with the supercharger mounted above the alternator using a common serpentine drive belt. The pulley drive ratio is 1.8, giving an impeller speed of approximately 125,000 revs/min at maximum engine speed.

The natural characteristic of a directly driven centrifugal supercharger is, of course, to give a rising boost pressure with speed with the pressure rise equating approximately to the square of the speed. Obviously this would be a very undesirable characteristic for a road vehicle, and must be modified. A variety of means are available to achieve this, the simplest being to rely on pressure loss around the inlet system to restrict the boost pressure at higher speeds.

MATCHING - EFFECT OF THROTTLE POSITION

The compressor characteristic shows the superimposed Wide Open Throttle matching line for the 1.8i engine, and also shows the effect on the matching line of running at part throttle. Unlike a turbo charger, with a fixed ratio supercharger when the throttle is closed the supercharger continues to turn at the same speed and the compressor therefore operates along the same speed line. Examples of operating points are shown at 50% and 25% load at 5,000 revs/min and show the effect of positioning the throttle either in the conventional position after the compressor or the alternative of positioning the throttle before the compressor. In the former case, the engine swallowing points are to the left of the surge line and the compressor can only be operated at this condition by adding a re-circulation valve to artificially increase the compressor airflow. This also has the effect of increasing the part throttle supercharger power requirement.

The alternative is to mount the throttle in front of the compressor, thus operating the compressor in a lower pressure condition, and as the compressor characteristic is based on corrected flow the operating points now lie within the useable area of the compressor map without the need for any additional control system.

The forward throttle position was chosen after considerable testing with the 1.8 engine, and while it does complicate the installation (charge cooling is used and the charge cooler has to operate across the full manifold pressure range), the overall driving characteristic is significantly improved. The throttle is deliberately undersized giving a relatively high compressor inlet depression at higher speeds, with little effect at lower speeds, resulting in a flattening of the manifold pressure curve.

COMPRESSOR INLET DEPRESSION - ROVER 1.8 K-Series

MANIFOLD BOOST PRESSURE - ROVER K-Series

The resulting performance curves show an increase in power from 118 bhp to 191 bhp with a considerable broadening of the usable power band and show an increase in torque of 33% at 4,000 revs/min. The resulting performance characteristic suits a high power lightweight vehicle such as the Elise extremely well.

The associated supercharger drive power requirement peaks at the maximum engine speed - wide open throttle, but falls rapidly as the engine speed is reduced and also (not shown) as the throttle opening is reduced.

COMPRESSOR DRIVE POWER

TT’s other lead programme was the application of the supercharger to the American Rotorway Executive helicopter engine, to provide altitude compensation. The helicopter uses a vertical shaft 4 cylinder boxer engine of 2.4 litres and the design brief was to maintain sea level power to 7,000 ft altitude. The vertical shaft engine dictated the use of a vertical supercharger, and the engine layout dictated the mounting of the compressor at the lower end which in turn presented some interesting oil sealing requirements. Two oil drains are provided to cater for nose down and nose up attitudes and a PTFE-lined lip seal of similar construction to those used on Formula One crankshafts seals the spindle. Drive is by a single V belt and a stepper motor- controlled variable restrictor is mounted on the compressor inlet, controlled by the ECU to regulate delivery pressure according to atmospheric pressure.

The supercharger is deliberately matched with a large compressor so that, in the event of a supercharger failure, the maximum naturally aspirated airflow would be maintained. A priority valve is also incorporated.

Installation constraints meant that the supercharger had to be compact and light and a total unit weight of slightly over 4 kgs has been achieved.

Current development work is aimed at reducing both the size and the weight of the supercharger and body envelope packaging dimensions of approximately F 132 x 105 long can be achieved with a unit suitable for engines up to approximately 250 BHP, with a weight of approximately 3.7 kg.