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What Is Forced Induction?

Forced induction is the name given to products which can enhance an engine’s performance by improving the flow of combustible fuel to the engine. The most common and widely known example of a forced induction system on a motor car or other vehicle is the turbocharger and a similar device known as a “supercharger”.

Forced induction devices deliver more air and fuel mixture to the combustion chamber than would be possible using just natural aspiration methods, i.e. a traditional carburettor based system. This increased air flow, mixed in the correct proportion with the fuel, delivers a significant power boost to the vehicle. This improvement in performance is widely used because it gives an uplift to the vehicle’s horsepower, but it does not significantly increase the weight of the vehicle, as would be the case if a larger engine were to be fitted to the vehicle in order to increase the horsepower.

How Does a Forced Induction System Work?

To explain the principle of forced induction we will use the example of a turbocharger because this is the most widely use form of forced induction in relation to motor vehicles. Turbochargers are frequently used on petrol driven vehicle engines, but they are also commonly available for diesel engines.

The objective of a forced induction device is to increase the power of the engine without unduly increasing the weight. The metric that is being measured is the “Power to Weight Ratio.” It is usually possible to fit a larger engine to a vehicle in order to increase the available horsepower but doing so will also increase the weight of the vehicle and the power to weight ratio will be adversely affected by such a modification. This is also a much more expensive option in most cases when compared to fitting a turbocharger to the vehicle. It is also not possible to fit larger engines into most vehicles without major restructuring of the chassis and other components, so this is usually not a realistic option unless the vehicle is designed at the outset to offer different engine size options.

The main purpose of a turbocharger is to increase the flow of air into the engine. It does this by compressing the air and literally “forcing” it into the engine’s fuel combustion system. When the extra supply of air is mixed with the correct amount of combustible fuel, the engine is provided with what it needs in order to produce more power.

Turbochargers are very clever in this respect in that they are designed to use the engines exhaust fumes, a waste product, to spin the turbine in the turbocharger. The benefits of this are obvious in that a waste product, which would normally be expelled into the atmosphere, is being used to improve the performance of the engine and the vehicle.

Once the turbocharger’s turbine is spinning at the correct speed, (up to 150,000 rpm), it can then drive the air pump to compress the air and drive it into the engine where it is mixed with the combustible fuel at the correct proportion. Forced induction systems like this can increase the power of an engine by up to 50% if they are setup correctly. It is not difficult to see why they are so popular since turbochargers can often be retro-fitted to engines that were not factory fitted with them thus giving a new lease of life to older and less powerful vehicles without the need to replace them.

The power increase provided by a forced induction system like a turbocharger is not just a matter of increasing the power, and therefore the speed, of the engine. There are also other very good reasons, other than speed, for the fitting of a forced induction system such as a turbocharger. The most common of these reasons is to deliver more power to the engine so that it can perform certain tasks. This is often the case with off road vehicles. Off road vehicle drivers are not usually too bothered about speed as such but they do need power in order to traverse rough terrain, climb steep inclines or to tow other vehicles or machinery and a turbo charger gives the extra power to perform those tasks at a reasonable cost.

Another example of where a turbocharger can help is when vehicles are driven at high altitudes. As your height above sea level increases, so the Oxygen content of the air decreases – a problem only too familiar to climbers and others who frequent high altitudes on foot. As the altitude increases, breathing becomes more difficult and people get tired more quickly, losing energy and sometimes having serious health consequences. It is quite common for climbers to use oxygen masks when climbing at high altitudes for exactly this reason. Without the oxygen, the climb would be near impossible to achieve.

What affects the human body and respiratory system also affects the respiratory system of a motor vehicle. At high altitudes It becomes more difficult to deliver enough air into the engine to mix with the fuel at the right level, with a resultant loss of power in the engine. Modern fuel injection system can automatically adjust the amount of fuel being delivered to the engine, but they alone cannot increase the level of air available to mix with that fuel – that is where the forced induction system, or turbocharger, comes into its own.

Although a turbocharger does not completely fix this problem, (if the air is thin even a turbocharger cannot use air that just isn’t there in sufficient quantities), it does make matters much better and often enables the vehicle to keep going whereas without one being fitted the vehicle may have had to come to a halt.

A Short History of Forced Air Induction Systems

You might be forgiven for thinking the forced air induction system, especially turbochargers, are a relatively new invention but you would be wrong in thinking that. You are right to think, however, that they have only recently become a viable option in motor cars although they have been around for longer than many people think.

Turbochargers have been fitted to heavy duty commercial vehicles such as lorries and buses for many years, since the turn of the last century in fact, thanks to a Swiss engineer called Dr Alfred Buchi who filed a patent for one as early as 1904. Buchi was able to implement his invention as early as 1911 and they became commonplace in heavy vehicles including buses, trucks and even some aircraft. It was, however, a long time before turbochargers were available as an option to be fitted to cars and other non-commercial vehicles.

In fact, it was the release of the Saab 99 in 1976 that a production road car was available with a turbocharged petrol engine. Since then, and certainly from the 1980s, most car manufacturers have offered turbochargers as options in some of their vehicles. Today, they are either fitted as standard into many vehicles or available as an option to be fitted at the factory.

Superchargers, which are a variation of the turbocharger that does not use exhaust gases to power the turbine, were devised by Gottlieb Daimler who introduced the concept of pressure charging in his original patent for his ‘car’ in 1889. Only a few years later he had made a piston driven device, a “supercharger”. Turbochargers and supercharges are both regarded as forced induction devices.

Is There A Downside to Forced Induction Systems and Turbochargers?

There are rumors in circulation that a turbocharger can actually reduce the lifespan of an engine but there is little or no evidence of this being true. One well known negative aspect of the turbocharger that is true, however, is the notorious “lag” that is evident with many turbochargers, especially when fitted to older engines. This phenomenon is easy to spot – the driver put his foot down on the throttle to cause the turbo to kick in and there is a short lag, (which can seem like forever), before it does so. In more modern engines this is a less frequent occurrence and not really considered to be a problem so much as it was before.

Another slight negative aspect to the turbocharger is that the exhaust gases needed to spin the turbocharger’s turbine are not a totally “unrestricted” or freely available resource. Interrupting the vehicles exhaust flow in this way does inevitably impact on the overall efficiency of the engine as it has to work harder to force the exhaust gases out of the system. However, this is easily outweighed by the benefit the turbocharger brings to the vehicle in terms of increased horsepower and speed.

Another forced air induction device is the Supercharger which avoids this problem since its turbine is not driven by exhaust gases, as is the case with the turbocharger, but instead it uses a belt drive or other direct drive system, from the engine to do this thereby removing the impact on the exhaust system caused by the turbocharger.

Source:

  1. What Is Forced Induction And How Does It Work? – Practical Motoring
  2. A breif look at what forced induction (FI) is and how it works – Engine Basics

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