Mike,

You wrote about turbos and superchargers in the past (http://www.greencarcongress.com/2005/04/another_approac.html) and their potential to help automakers downsize their engines, but what I am wondering about is what is the effect of turbos and supercharger on fuel economy on their own (everything else being equal).

If we take the same engine with and without turbo/supercharger, what kind of fuel economy hit could be expected?

Well, it’s a bit tough to talk about them on their own, absent injection systems etc., but...if you turbocharge the non-turbo engine in your car, depending on your driving habits, you could lose 5%–10% fuel economy or more.

Turbocharging works as a tool for improving fuel economy when it is combined with engine downsizing.

In common steady-state cruising conditions, the power required by a vehicle is usually less than 25% of what the engine can provide, according to Honeywell. Automakers size their engines for acceleration and driveability performance, not just for cruising. This results in those engines operating far away from the optimum fuel economy range most of the time, hence resulting in poor overall fuel economy.

Turbocharging boosts the power yield for a given displacement compared to a non-turbocharged engine. GM estimates/targets a power yield of 60hp/liter for naturally aspirated engines, 100 hp/liter for turbocharged engines, for example.

The principle of downsizing, then, is to use a smaller engine that is closer to the “road load” cruise line of power need, but that delivers power and torque needed for acceptable acceleration and driveability via the turbo.

So it’s not that turbocharging makes an engine of the same displacement more fuel efficient than a naturally-aspirated engine of the same displacement. It’s that a smaller, downsized, turbocharged engine can replace a larger, non-turbocharged engine. It’s through the use of the smaller engine that the fuel economy savings come.

Thanks for the explanation!

I suspected that the point was that if the fuel economy loss was relatively less than the power gain (ie. 10% loss for a 20% power gain), you could downsize the engine and win that way.

I hadn't considered what you say about cruising vs. peak power/driveability. It is indeed a big dilemma faced by automakers (which can, in part, be lessed by displacement on demand technologies - but you still have to lug the big engine around and it adds mechanical complexity).

I don't want to make you work more than you already do, but if you wrote a general post about engine downsizing & turbochargers/superchargers explaining what you said to me above, I'd certainly feature it on Treehugger. I think that this area of potential fuel economy gains is under-reported and I'm sure that you'd do a better job than I at giving an overview of the possibilities.

Good explanation mike. Turbocharging can be very helpful in fuel consumption, take the VW passat for example which uses a turbocharged 2.0L 4cylinder engine providing similar acceleration to a V6 accord,and provides similar economy to an accord V6 hybrid. Now thats technology thats overlooked, The potential is great.