Asperated or Not Asperated

Since the dawn of internal combustion people have looked for ways to get more power from their engines. In crude terms, there are three ways to do this: increase the size of the cylinders, add more cylinders, or force more of the gas-and-air mixture, (the fuel,) into the cylinder on each stroke. The first two make the engine bigger and heavier. The third avoids those pitfalls, but adds complexity.

Naturally aspirated engines

Aspiration, or breathing, describes how an engine draws in the fuel charge. In a naturally-aspirated engine suction created by the downward motion of the pistons draws in the mixture. The quantity of fuel is limited by cylinder volume and the difference between the vacuum produced in the cylinder and atmospheric pressure. This is why engines produce less power at altitude where the air is less dense.

Forced induction

Getting a larger charge of fuel into the cylinder means increasing the pressure differential, effectively pressurizing the inlet side of the engine as if it was below sea level. This entails using a pump to push more fuel into the cylinder.

Two methods exist: supercharging and turbocharging.

Supercharging

A supercharger is an engine-driven pump mounted above the inlet manifold. Typically it looks like two coarse-threaded shafts or scrolls meshed together. A belt turns these shafts, which suck in air, compressing and delivering it to the cylinders.

A supercharger runs whenever the engine is turning, so it’s always delivering an increased volume of air. The downside is that due to internal friction, it consumes an appreciable amount of power.

Turbocharging

The turbocharger is a pump driven by exhaust gases. As these leave the engine they can be diverted to spin a small turbine. The other end of the turbine shaft has an impeller, which pushes air into the inlet side of the engine.

The beauty of a turbocharger is that it doesn’t run until needed, so there are no parasitic losses. The downside is that it doesn’t start pumping the moment the driver demands more power. Instead there’s a lag while the exhaust gas pressure builds up and starts to spin the turbo.

Injection and carburetion

In modern engines, gasoline is injected directly into the cylinder after the inlet valves close. This means the injection system has to work against the denser charge in the cylinder, which has led to dramatic increases in fuel rail pressures.

Older engines with throttle body injection can also use forced induction but don’t need such high fuel pressures. Carburated engines can be turbo or supercharged too, but the mechanical arrangement gets complicated. One method is to put the carb in a plenum chamber, where it adds fuel to already pressurized air.

Future trends

Use of forced induction is growing. Companies like Ford and BMW are making extensive use of turbocharging while Chevy is supercharging its Corvette Z06. Some companies are even combining both on one engine – something described as “twin charging” – as Volvo does on its T6 E-Drive.

Also in the pipeline: superchargers with clutches and electric turbo and superchargers. Intended to increase efficiency, these are likely to become common in the years ahead.

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