Quote:
Originally Posted by JamesNoBrakes
The compressor does some of the compressing, which is why the turbo engine is relatively (usually not excessively) low compression, it's a combined effect. You can't compare NA engine compression ratios because all of the compression occurs in the cylinder in those cases. Otherwise, why would you need to use high octane fuel in a turbo?
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From an efficiency standpoint, it doesn't matter if the air is already partially pressurized before entering the cylinder as long as the volume ratio between TDC and BDC is reduced. While you may achieve similar initial pressure and temperature at ignition by having the same total mass of fuel and air, just in two difference sized cylinders with differing CR, during the power storke, the expanding gases will expand more in the engine with the higher CR, and less in the engine with lower CR. Thus in the lower CR (Turbo) engine, the same amount of fuel burned does less work.
By doing some of the compression externally, turbo engines in a sense have a reverse Atkinson cycle. Efficiency is being traded for power.
Quote:
Originally Posted by JamesNoBrakes
I don't think anyone is suggesting anything of the sort, they are suggesting replacing that big inefficient V6 with a smaller engine that has the same displacement capability (or even better) via a turbo. This is what improves efficiency, you can also get some amazingly flat torque "curves" these days as well. Those 4cyl are adequate and I don't really see why we'd need to get a "V6", but if you HAVE to have more power, a turbo-4 would be a better way than a 6cyl V6, technology and especially design are to the point where it can be done reliably over thousands and thousands of miles.
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I'd argue that those V6s really aren't that inefficient. Most of today's V6 cars exceed 30 mpg on the highway. Smaller fours boosted to the point of similar power of larger V6s just aren't putting up real world numbers better than that.
I completely agree on the torque curve benefit of a turbo engine. If you're looking for a nice flat torque curve, a turbo is the way to go, particulalry as today's NA engines seem to be getting less and less low-end torque for their power with every new upgrade. All I'm arguing is that if you're doing it purely for efficiency, you are wasting your money.
Since turbos usually require premium fuel anyway, instead of downsizing and turbo charging, more car makers should be doing what Mazda is doing with their SkyActive engines....Direct Injection on a NA engine, and take the compression ratio up to about 14:1...or 12.5:1 to run on regular. That'll make power and increase efficiency, but for less cost than turbocharging.