Don’t let turbo lag be a drag
Its big disadvantage, however, is that it takes time for the system to ‘spool up’ when acceleration is called for — a characteristic well known as ‘ turbo lag.’
It has become almost dogma now that a combination of turbocharging and downsizing is the preferred route to maintaining desired engine performance levels while also reducing fuel consumption.
But are there other ways of achieving the same ends? Specifically, could a supercharger be used instead of a turbocharger?
Both devices serve the same purpose of delivering pressurized air to the engine’s intake system. The big difference is in how they pressurize the intake air.
Turbos make use of exhaust flow to turn a turbine wheel that’s connected to a second turbine wheel. That second turbine pressurizes the intake air that feeds the engine.
In theory, it’s a simple concept and it offers the big advantage of making use of wasted energy in the exhaust gas, without adding any load to the engine. Its big disadvantage, however, is that it takes time for the system to ‘spool up’ when acceleration is called for — a characteristic well known as ‘turbo lag.’
Automakers have taken a broad range of approaches to overcome that trait — ranging from variable turbine geometry to dual turbos of different size to auxiliary electric drive.
Most of those approaches have been successful, in varying degrees, but all have the common downside of adding complexity and cost.
So, why not use a supercharger instead? Superchargers are typically mechanically driven by the engine itself so their output is directly proportional to engine speed and response is instantaneous.
That characteristic is readily apparent to drivers, giving them an immediate sense of power when they tip into the throttle.
Because they’re independent of the exhaust system, superchargers also offer more latitude in underhood location and are not as heat intensive in operation, which provides ancillary benefits.
The big downside is that driving the supercharger uses some of the power the engine develops, even when high boost levels are not required — which is the case in most normal driving. Those parasitic losses can reduce the overall efficiency of the engine in typical use.
There’s also the challenge of gearing the supercharger drive. If it’s set up to provide the desired response at low speeds, pressures could become excessive at higher speeds, necessitating some form of blow-off valve, which would reduce efficiency.
Conversely, if set up to optimize efficiency at high speed, it would lack low-speed response, similar to a turbocharger.
But there are solutions.
One alternative, incorporated in the Eaton supercharger used on Volvo’s T6 engine, is an electronically controlled clutch drive. During idle, light throttle, and cruising situations, the supercharger is disengaged — but it hooks up instantly when additional power is called for.
Another approach is a variable-speed drive system that allows the supercharger’s output to be altered electronically, independent of engine speed or exhaust energy, to match the engine’s requirement. The British-based supplier, Torotrak is currently developing such a system.
Both superchargers and turbochargers have advantages and disadvantages — and in both cases, there are ways of mitigating the disadvantages. If superchargers are able to compete or offer an advantage on price, they may yet prove to be practical alternatives to turbochargers.