S. Muralidhar

Turbochargers have their roots in racing. But when incorporated into streetcars, they have the power to convert them from `feet-draggers' to fleet-footed predators.

Turbochargers are becoming popular among diesel car-owners for their ability to boost the engine's peak power output. The Tata Indica V2 and the Indigo have them, and so do the Mahindra Scorpio CRDe and Mercedes Benz C220 CDI.

While increasing the engine's power is the most visible effect of a turbocharger, there are also other benefits compared to non-turbocharged engines.

They include a reduction in emissions and a huge boost to the torque characteristics of the engine.

So, while it is a combination of these benefits that drives carmakers and customers to go for turbocharging, often it is the increased output that overpowers all others as the primary objective.

Since diesel cars tend to be heavier than and are also underpowered compared to their petrol-driven versions in the same engine-size class, turbochargers are opted for the increased power-to-weight ratio they offer.

Inside a turbocharger

Turbochargers, like many other automobile performance enhancing devices, have their roots in racing. But when incorporated into streetcars, they have the power to convert the cars from `feet-draggers' to fleet-footed predators.

All internal combustion engines need a specific mixture of air and fuel to burn which produces the energy and the driving force. The simplicity of turbocharger technology is in the way it achieves its purpose, by just boosting the flow of one of the two constituents — air — in the combustion process. This, in turn, leads to enhanced engine power output after other systems work in tandem following the increased airflow.

Though variants of the technology are used for specialised applications or for high-end petrol-driven cars, turbochargers are more often used in diesel engines to boost power and performance. In addition to the fuel delivery equipment, all naturally aspirated engines have an air intake mechanism built in to kick-start the combustion process.

So, as part of the combustion cycle, when the intake valves open in the engine, the piston's downward movement creates a vacuum thereby forcing air into the combustion chamber.

With simultaneous delivery and mixing of fuel into the chamber, and sustained sparking from the plugs, combustion takes place. For engines that are not fitted with turbochargers, a lower-than-optimum flow of air into the combustion chamber under normal conditions becomes inevitable due to the restrictions imposed by under-optimal intake manifold design, camshaft design and a relatively-less-than perfect valve timing.

How it works

The turbocharger consists of a turbine, which is connected to the engine exhaust line, and a compressor, powered by the turbine.

The engine's exhaust gases are routed to and caught by the turbocharger's turbine blades spinning it to high speeds sometimes to as much as 150,000 rpm.

As a result, the turbine cranks and powers the shaft on which it is mounted. In turn, the spinning shaft powers the air compressor at the other end of the system.

The compressor is located on the engine air intake path. This device actually speeds up the process of airflow into the engine. This is also the reason why turbocharged engines are also called forced air-inducted engines.

So, instead of the usual flow of air into the chamber due to the vacuum created by the pistons, air is actually force fed into the engine by the turbocharger's compressor.

This leads to an excess of air inside the engine's combustion chamber, up to a level which in is excess of what is required for combustion under the regular (non-turbo) cycle.

Here, a bit of jargon has to be introduced. The Stoichiometric combustion ratio is the right level of air and fuel mix needed for ideal combustion in the engine's chamber.

Most modern cars depend on on-board electronics to fine-tune the fuel injection process to match this preset ratio.

So, in a turbocharged engine, to balance the air-fuel mixture, sensors on board the car pump more fuel into each of the engine's combustion cycles, even as more air gets force fed into the engine. This leads to an increase in the amount of fuel burnt during each injection.

The pumped up combustion leads to an increase in the total power output of the engine. Turbos are now widely used to boost the power of smaller-size diesel engines that may be available in vehicles that range from the entry-level to the most luxurious.

A 5-15 per cent increase in rated peak power output is achievable by introducing a turbocharger, a key factor for attracting performance-oriented buyers in most size segments.

What is more, turbochargers recycle some of the exhaust gases and since they enable better combustion, such engines are also able to lower the vehicle's emissions.

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