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Old 05-08-2014, 02:55 PM   #97 (permalink)
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Jerry, A few posts back you asked if I had any input that might help others. There's no sliver bullet, but if I had to pick one thing, it would be to recommend that folks stick with a diesel and install a turbocharger. Both the Centurion and the new XR3 are naturally aspirated diesels. But diesels love turbochargers. A turbocharger will increase power output and reduce fuel consumption too (improve bsfc - make the engine more efficient).

But your focus on aerodynamic drag and mass, and reducing friction - and appropriate tall gearing, are the general keys. As for gearing, the idea is to get the engine rpm down at cruising speed. You could say it another way by saying that you want to get the engine loading up so the engine is operating in its range of minimum bsfc (brake specific fuel consumption, or the fuel consumed per unit of output). As rpm drops, the power output capability of the engine also drops. So engine loading goes up if the engine is operating at low rpm where it cannot develop maximum output. So engine loading is just a different way of saying the same thing.

Just reducing mass does not do much to improve fuel economy (some, but not much). What it does, however, is to reduce the road load (the necessary power). With reduced road load, you can use taller gearing (where the engine cannot produce as much power) which will then improve fuel economy by increasing engine loading. Or you can use less installed power (a smaller engine, because not as much power is needed). Or you can do both. But if you look at a fuel consumption map, you'll find that the engine has islands of varying bsfc. And somewhere there will be a sweet spot where the engine is delivering power at the lowest fuel consumption. So you want to get inside that island. And that island is not a single point. There's plenty of latitude for normal operation.

The fundamental problem with production cars is that they are heavy (high mass) and people want lots of acceleration. Acceleration is a factor of the power to weight ratio. So getting the mass down means that you can have good acceleration with a smaller engine. But one also needs to reconsider the amount of acceleration one really needs, and make a value judgment about what they're willing to pay for that lightening acceleration. The truth is that they are paying for it even when they are not using it. They are paying for it in the form of far too much installed horsepower, which translates into poor (low) engine loading at normal operating speeds.

In a way, that's like buying a pickup truck or a utility van for those two times a year when one needs to haul a sheet of plywood home. And then all the rest of the time, dragging around a fuel-hungry beast for those everyday trips to work or the market.

I used an analogy in my book, "Alternative Cars in the 21st Century" wherein I point out that most of a car's energy goes to getting itself to the destination, and the occupant come along for free. It's a cute analogy, but it's real.

Last edited by rqriley; 05-08-2014 at 03:05 PM..
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