[Ducati] exhaust energy, turbos and diesels

[Kerry and Brad Black]

when i was doing my 4th year project (1991) they'd just started compound turboing truck engines.  they were running a second, low pressure turbo after the main turbo (compressing inlet air) that was geared to the crank, directly adding to the power at the flywheel.  they were about 43% efficient, from memory up from 40% or so - a small, but worthwhile improvement.

i did another, small project on a buick/holden v6 race motor used on a national open wheeler class that was about 25% efficient at best.  diesels are much better than petrol motors for several reasons - higher comp ratios, leaner mixtures, able to take lots of boost with ease, lower specific power output, smaller rpm range, etc.  electronics have made them much better too.

also, they tend to run at a much higher % of their maximum output.  an engine running at 80% of its peak output for any given rpm is more efficient than one running at 20%.  diesels have much less of a variation (why they leave trucks idling for hours), but the variation in efficiency of a petrol engine from light load to heavy load is quite amazing.  it's why even a gently driven large capacity petrol engine is bad, but a hard worked small engine may not be.  egr is one way to get back some part throttle efficiency, but there's still a large variation.

which brings me on to my point.  if you wanted to make real use of the energy going out the exhaust you'd have either a wide ranging variable turbo (not sure if they exist) or a twin / triple / quad turbo set up.  the smallest turbo would be boosting while you were cruising around at 60 - 100km/h, because in reality that's where a lot of bikes spend a lot of time (mine does) and when they're also very inefficient.

most people don't worry about this sort of stuff tho, and no one turbos a bike to help the part throttle fuel economy.  but when you're running an interstate truck and spending $100,000 a year on fuel (probably more), 1% increase in economy is a decent chunk of cash.  10% makes owners take real notice.

brad