Quote:
Originally Posted by Ecky
Sorry for a bit of a threadjack, but,
In a modern car engine, at optimal load, how much does efficiency change across the RPM band and how much room for improvement is there?
It would make decent sense from a simplicity (and thus cost) standpoint to use a transmission/drive like Old Mechanic has and keep an engine at constant load, varying the RPM when you change driving conditions, if the RPM range of high-efficiency (at high load) is wide enough. I've always thought CVTs were a wonderful idea, with current implementations only falling short in that they're sometimes lossy due to friction, and have limited range of ratio.
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No problem on the threadjack -- it's plenty related enough, I think.
It's a great question. A modern CVT (such as the Nissan unit) provides the full range of useful ratios from a low-enough-low to break traction on a dry road, to a high-enough-high to arrive at a top speed that is HP limited. A lower low would only be useful for lawn mowing, or pushing a snow plow, if one had the traction, and a higher high would reduce top speed and acceleration near top speed.
Neither a CVT nor an IVT keep an engine at either an optimum or constant load. IVT's have, so far, been very lossy. A Cub Cadet has a fan blowing on it's hydrostatic because there are larger losses in an IVT than in a geared transmission (in most systems waste goes off as heat).
About one or two decades ago, Audi offered the same car with CVT*, conventional automatic, and manual. At that time, the conventional automatic was slightly less efficient than the CVT and manual, which were equal. Now, conventional automatics are just as efficient as manuals too. Dual clutch manumatics can be slightly more efficient than the others, especially when coupled to a highly-tuned engine, (that can benefit from the very close gear spacing) but otherwise the difference is slight, because of things like variable valve timing that have provided broader power bands than in the past.
If a car requires 15 HP to travel at 60 mph, it requires 15 HP whether in second, third, fourth, fifth, or sixth gear. A 200 HP engine does not produce 15 hp efficiently, regardless of the rpm. It will produce 15 hp more efficiently if the rpm is low -- in many engines, very low. That means the torque required is relatively high, and the throttle is further open than it would otherwise be, so there are fewer pumping losses. So in general lugging an engine produces better mileage than letting it rev more freely.
This (pumping losses) is the main reason a V6 Honda Accord gets significantly worse mileage than a 4 cylinder version. In both, assuming automatic transmissions, the optimum gear is selected, but in one engine you are operating with the throttle 90% closed, (10% power) and in the other 94% closed (6% power). In the first case, the engine might be 10% efficient, and in the second it might be 8%.
Throw away the big engines, and install a 20 hp engine running at it's torque peak, where it is producing 15 hp. It will run at 30% efficiency. So it will get about three times the mpg of the 4 cylinder. Flat out, the car will have a speed of 65mph, and the 0-60 time will be about 1 minute. To make the car driveable, you'd need a source of burst power, like an electric motor with batteries, or a hydraulic motor and an accumulator.
BSFC maps will tell you the efficiency of an engine at any load and rpm. They have one or more islands (like the colorful chart above for an electric motor). You want the engine to operate in the island with the lowest BSFC. But the driver demands all sorts of HP requirements, so the engine cannot run at the constant torque and rpm define that island.
Thus the idea for a series hybrid (and to a lesser extent a parallel hybrid, and to a lesser extent yet, a light hybrid). In a series hybrid, you can operate at peak efficiency (or very close) all the time. When the batteries (or hydraulic accumulator, or flywheel) are not able to absorb full load power, turn the engine off.
The engine operates at peak efficiency or not at all. The advantage of a plug-in hybrid over a hydraulic or flywheel hybrid is the longer cycles: less time spent warming up, fewer heat shocks, etc -- a big portion of wear is from startups.
On the other hand, a hydraulic hybrid can have other advantages, such as simplicity. So it will be interesting to see what OldMechanic comes up with -- his idea goes beyond just the IVT.
* Now CVTs often have discrete speeds available, because many people find they like the more conventional sound of and engine going through the gears. The difference in efficiency is negligible, because engines are now so flexible.