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Supercharger: How It Works And Why It Exists (Simply Explained)

Bev Roberts 13. February 2015 Dirty hands, Magazine No Comments

In a non-charged or `normally aspirated` four-stroke engine the vacuum generated by the induction stroke sucks air from the filter into the inlet manifold and on into the cylinder. In a carburettor engine the passage of air over the main-jet orifice in the carburettor sucks the correct amount of petrol into the air-flow and mixes with it. This mixture is called `the charge`. The piston then rises on the compression stroke and the charge is ignited by the spark plug, firing at the appropriate moment. In a modern fuel-injected engine electronic sensors read the flow-rate of the air and inject the correct amount of fuel into that air-flow.

The vacuum in the inlet manifold is the driving force for the carburettor, sucking in ambient air. The pressure of ambient air at sea-level is 14.7 lbs per square inch (psi). A charger replaces the vacuum with forced air and increases the amount of fuel/air mixture going into the cylinders. The engine power available increases pro-rata. However, there are critical limitations. Drag-racing cars use superchargers set to extreme levels of out-put to produce maximum power, even to the point of melt-down.

The fuel/air ratio is critical at 14 parts air to one part fuel. If the mixture is `rich`, (too much fuel) the engine will choke and not burn the charge efficiently, wasting fuel. If the ratio is `weak`, (too much air) the mixture will burn too hot and will cause the collapse of the piston-crowns. Exact tuning of the mixture is critical. Electronic Fuel-injection systems are designed to control this ratio automatically.

However, as the altitude increases ,the ambient air decreases in density and oxygen content, depriving the engine of power. What is needed is a method of increasing the volume of dense air whatever the altitude. The emergence of the charger was brought about by a need for more power although the purpose of them on aero-engines is self-evident. At 30,000 feet there is very little oxygen. Even at the Everest summit an engine would not run because the air is too `thin`, not that there`s anywhere to drive except down and gravity would make an engine quite unnecessary! Superchargers appeared in racing cars in the 1920s, notably in the Mercedes-Benz sports-car. Before long they were in general use.

The charger`s purpose is to supply a much greater volume of air to the engine which when mixed with fuel produces a larger `charge` of the fuel/air mixture. The charger is driven by the crankshaft by a drive-belt or, as in the case of the 1920s and 1930s cars like Bentley and Alfa-Romeo by direct coupling to the end of the crankshaft itself, through a gearbox.

Chargers rotate at huge speeds, anything from 50,000 rpm to 80,000 rpm depending on engine revolutions. A plentiful supply of engine oil is vital for lubrication and cooling. The engine would usually be equipped with an oil-cooler. How does it work?

Supercharger by akitarescueoftulsa

Supercharger by akitarescueoftulsa

The engine crankshaft drives the charger through a gearbox. The shaft running through the charger has a turbine fan (or impeller) bonded to that shaft. The turbine runs in a compression-chamber and pumps the pressurised air into the engine intake system (through the carburettors if fitted) and on into the intake manifold. With fuel injection the air is measured by the sensors and the correct amount of fuel is supplied. At sea-level (14.7 psi) the air pressure at the engine will be around 22 psi at maximum engine revolutions. This represents a nett gain of around 40% on the bhp rating and torque output. Fuel consumption? Don`t ask!!

The charger story is not finished yet, however. There is another problem to overcome. As air is compressed it gets very hot and loses density, losing a lot of it`s potency. How can we cool the air to restore it`s density? Water-cooled engines have radiators, air-cooled engines have cooling fins, what can you do with hot air apart from fly a balloon? The answer is an intercooler or charge-cooler.

The intercooler can be either fitted in the cold-water flow from the bottom of the radiator or out in the ambient air-flow. It is a simple heat-exchanger or radiator. The high-pressure hot air from the charger to the engine is routed through the intercooler which drops the temperature and raises the density of the air. An air-cooled intercooler is usually made from aluminium which is naturally cooler than iron or brass. As the air passes through it, a relative `refrigeration` takes place. Down goes the temperature of the charge and up goes the air density, bhp and torque. Oh! AND the fuel consumption!

Important note: when bringing the car to rest it is advisable to allow the engine to idle for a few minutes. This is because as the engine stops, the oil supply stops. Without lubrication and cooling the charger will wear very quickly at 50,000 rpm! We hope this article has broadened your understanding.

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About Bev Roberts

Speed, the smell of hot oil, the roar of a straight-through exhaust and the scream of an engine at max revolutions. They have all been a large part of my life for almost 50 years. It is time to share my experiences with you, dear reader. Do you want to know more? Read on through my `Full Bio` and many articles.

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I was born in Hereford UK in 1948 and brought up in Gloucester UK. I played Rugby football internationally as a schoolboy. At the age of 17, a new and wet driving license in my paw, I entered motor racing. I was supported and financed by my parents and so my journey began.

In 1965 I bought a 1293cc Mini-Cooper `S` and campaigned it for a season. Having quickly made some good friends in the racing fraternity, several interesting opportunities came my way. I joined a sports-car team and raced in the Le Mans 24 hours in 1968 and 1969 in a Lola T70. Mechanical failure defeated both efforts. During that period I owned and raced a `D type` Jaguar and an AC Cobra. In those days cars like that were available and not too expensive, now they reside in museums and private collections. I had a chain of interesting cars through my youth including Jaguars, Minis, Mustangs and Lotus-Cortinas.

As a young driver I had my share of accidents too. Often the car would only be worth scrap-metal value by the time I got it home! I worked for an Aston-Martin/Jaguar dealership for a while, which enhanced my experience and gave me the opportunity to sample some very exotic machines, Ferrari, Facel-Vega, Iso and Maserati to recall a few of them.

At the end of 1969 I moved to South Africa to work on my uncle`s farm but the S.A. government had other ideas and drafted me into the army. After five years had passed I was thanked and released from the service. While I was there I bought a beige Cadillac Eldorado, previously the property of Marilyn Monroe. While I was away on a patrol my girlfriend had it re-sprayed pink! I was unimpressed by both the joke and the bill for the work!

When I returned to UK in 1974 I left it behind. On my return I found that the once-mighty British motor industry was in decline and was headed for oblivion. Motor racing was now very expensive so I turned to commercial transport. Driving large trucks gave me freedom and a chance to see some of the world. I don`t remember ever making a career choice but for the next thirty years a truck was my home. For about ten years in that period I owned two trucks of my own.

I also owned a famous MGB-GT, known as `Lucky`. If you`d like to read `The Story of Lucky` there is an article in Inopian`s archive. I finally retired, due to ill-health, in 2008. Since I had varied knowledge and many experiences on our subject I decided to share the stories of the cars I enjoyed (and hated) with the new generation.

Enjoy Inopian… it is constructed and written for you.

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