MoTeC Global Forum

Some of this probably applies to endurance events, where fuel can be limited.

I read things like this a lot: http://www.classiccarsforsale.co.uk/news/general/800022866/is-turbocharging-the-way-forward/ which have no technical information and seem to be repeated everywhere.

Do turbo charged engines really have better BSFC than NA?
I thought it was mainly down to the compression ratio and unless the engine was direct injection, then lowering the C/R to avoid detonation meant lowing BSFC.

There are some turbo desiel engines that have high BSFC, but they do not have a lowered the C/R.
And I could see if the lambda was lean then maybe turbo BSFC could be better.

Compression ratio as expressed is a geometric compression ratio and does not tell the full picture regarding compression pressures.

In an engine at 9:1 compression that delivers compression pressures in the region of 12Bar will deliver considerably higher compression pressures if it is fed with 1 Bar of boost pressure.

stevesingo wrote:Compression ratio as expressed is a geometric compression ratio and does not tell the full picture regarding compression pressures.

In an engine at 9:1 compression that delivers compression pressures in the region of 12Bar will deliver considerably higher compression pressures if it is fed with 1 Bar of boost pressure.

Yes but how does that translate into BSFC?

I suppose there will be a point where the energy spent generating the boost is greater the benefits and there will be an optimum somewhere. This will also be effected by the geometric compression ratio and the dynamic compression ratio, which will be greatly influenced by the variable cam timing.

Then if we have a turbo engine with a higher specific output, we can downsize for lower parasitic losses and/or run at lower engine speeds for a given specific output which further reduces parasitic losses.

I imagine there are many many factors effecting the use of turbo charging modern engines for better BSFC. There must be something in it as the OEMs are pushing this in a big way.

looking at current road cars, the lower BSFC for turbo cars happen when little to no boost is produced. IE when cruising the highway. They tend to be smaller engines than the equivalent NA version. It appears that they are able to run further up the efficiency curve at said load, while still able to produce large Hp under boost to satisfy accleration duties. The biggest factor that I have been able to deduce is dynamic CR, while VE still plays a big part.

Real world example of NA
Ford 302W in V8SC(2012) uses around 85L/100km while producing 600+Hp
My previous 302W only produced 350Hp and still used around 75L/100km when doing so.

The static CR was almost the same(9.5 VS 10.0:1 in V8SC) but the dynamic was drastically different. The V8SC would also have a VE over 110% at points.

For Turbo examples, look at current and previous BMW and Merc models. The europeans have been chasing low fuel usage for many a year before us Aussies even bothered.

In my opinion, modern diesels are very trick inside already and BSFC's are pretty lean. And all major diesel makers have BSFC below .300. So actually, there really is very little difference in engine efficiency despite all the truck stop BS out there.

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