Interest in supercharging

Press, Volume CXV, Issue 33750, 24 January 1975, Page 10

Interest in supercharging

World-wide concern about fuel supplies and pollution from cars is encouraging car manufacturers to take further interest in supercharging—not just as a way of getting more power from an engine, but because supercharging can cause greater engine efficiency with more power from a given amount of fuel, and fewer unburned pollutants.

Already more than one major manufacturer has produced experimental supercharged models, although none have yet placed such a model on the market.

Supercharging has been known for years, and it was frequently applied to high-performance and racing cars in the 19305. But at that time, most superchargers were driven from the engine’s crankshaft, whereas today most of the interest is in turbochargers, which are driven by the engine’s exhaust gases.

Makers who produced supercharged models in the past included Bentley, M.G. and Mercedes, to name just three, and some American manufacturers also produced cars with superchargers. Basically, a supercharger is just an air pump, designed to force an extra charge of fuel-air mixture into an engine’s cylinders. Pressure charging, as it Is known, makes an engine more powerful and more flexible, and although the engine’s exhaust gases are cleaner because of more complete combustion, any fuel saving depends entirely on the way the car is driven. If full use is made of the extra performance, fuel consumption naturally rises. A supercharged engine usually requires a lower compression ratio than a normal engine — which can allow the use of unleaded petrol — but it also requires an efficient cooling system, as engines often run hotter when pressure charged.

In an engine, the amount of power produced is directly related to the weight of fuel consumed. In an unsupercharged engine, the power is limited by the amount of air that can be sucked into the cylinders by the piston.

As the air is drawn through the carburettor to the engine, it takes with it the correct amount of fuel, as metered out by the carburettor. Because there is only a fraction of a second for this to take place, the cylinders are, at best, only partly filled with petrol-air mixture.

Pressure charging at the rate of about 51b per sq. in. gives a marked improvement. Air is drawn through the carburettor by the “pump” and the inlet manifold is, in effect, filled with air-fuel mixture under pressure, and engine power is often increased by up to a third. The pressure charger, used, whether belt-driven

or a turbocharger, has to be designed to give good boost at low speeds, without giving too much as the engine runs faster: too much boost at speed can cause burned valves and blown cylinder-head gaskets.

The charger must also run quietly: earlier models were frequently notable for the high-pitched scream they emitted at speed.

For several years one British firm, Shorrock, has produced “bolt-on” superchargers as a performance accessory, although at relatively high cost.

Such a unit fitted to an 850 c.c. Mini will increase top speed from 70 m.p.h. to 85 m.p.h., and lower the 0 to 60 m.p.h. acceleration time from 28.7 sec. to 15.4 sec.

On an Escort GT, a supercharger has been- recorded as increasing top speed from 95 m.p.h. to 105 m.p.h., and lowering 0 to 60 m.p.h. acceleration time from 13.7 sec. to 9.2 sec.

In the past, supercharger installations have been designed primarily for performance, but this emphasis has now changed. Supercharger and turbocharger experiments overseas are continuing, in the hopes of getting more power from smaller engines, with less pollution.

Meanwhile several firms are taking an interest in the turbocharging of small diesel engines to make them more suitable for use in cars.

For car use, diesels have the disadvantages of high weight for their power, and less smoothness and quietness than equivalent petrol units. Turbocharging, however, raises the power out put considerably, and gives smoother and quieter combustion.

One. of the leading firms in this field is Mercedes. Already 40 per cent of the company’s car output is diesel-engined, and it feels that this proportion will rise as fuel costs increase.