Quote:
Originally Posted by sgtlethargic
In other words, what percentage of a diesel's efficiency is due to no throttling?
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Its pretty significant. Look at a P-V diagram of the Diesel Cycle and the Otto Cycle side by side (especially at part throttle). The lower "loop" of the Otto cycle is pumping losses.
This is the reason for the Prius using an "Atkinson cycle" engine (not an Atkinson engine). It uses a delayed intake valve closing to allow air to be pushed back out before compression begins. This means it isn't pulling vacuum on the intake side of the engine, increasing efficiency. Toyota also programs the hybrid system to maintain engine load in a sweet spot while charging to maximize efficiency.
This is also the reasoning behind a Hot Air intake (among a few other factors). Hot air is lower in density, which means more is required for a given HP demand. Therefore at a given HP demand, the throttle opening is larger (less pumping loss) than without a hot air intake.
Edit, some more thoughts while I'm still @ lunch...
This is a great argument for a small ICE engine that could operate around 90% power at 80mph - lets say 35hp. Supplement it with a relatively large EV system that can regenbrake and provide acceleration force/hill-climb power and you have a pretty efficient system.
When looking for non-car/motorcycle engines in the small power output range, its common for the BSFC to be pretty crappy by automotive standards (also for them to be dirty). Diesel gensets, lawn equipment, etc. generally use quite low tech carbs and are designed for maximum reliability (neglect), not maximum fuel economy. The also rarely have emissions controls.