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Originally Posted by NeilBlanchard
I have a couple of questions about piston ICE's:
How much pressure could be developed in the combustion chamber *just* from fuel burning? If there was no compression of the intake air (with only a turbocharger in place) would the resulting pressure from the burning fuel be enough to get decent torque?
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That's a kinda tricky question to attempt to answer, since the fuel needs to be mixed with the air to ignite--especially with a compression ignition (diesel) engine.
However, the atkinson cycle engines do attempt to address this with overlapping of valve openings. IE some of the intake charge comes from spent exhaust gases.
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Particularly, if you did not have to "do the work" of compressing the air with the piston -- which uses the momentum of the flywheel, then would the net pressure gain be the same?
Do diesels get all their additional efficiency from the higher compression? Or, are other factors contributing?
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Diesel efficiency compared to Otto cycle efficiency (gas engine) is due to higher compression ratio mainly, but also the ability to use extremely lean air:fuel ratios. IIRC, when given full power, the air:fuel ratio is still leaner than 18:1. At idle, it can drop even more to the neighborhood of 50:1.
There is also no throttle plate, which reduces the pumping losses drastically, and makes better use of a turbo. There is essentially no vacuum in the intake manifold while running.
Something else that comes into play is the use of a higher energy content fuel (diesel) that is sprayed at extremely high pressures, so the fuel is atomized much MUCH better.
More torque at low rpms also allows better highway gearing; I've heard of people putting the bumper of their diesel truck against an immovable object, putting the truck into gear, and slowly letting the clutch out, which instead of stalling the engine causes the drive wheels to break loose and start spinning in place. So for an efficient drivetrain you have lower rpms at cruise.
For the direct injected variety (which is nearly all modern diesels) they also enjoy more power and better efficiency. And the technology for the diesel is improving much quicker than for the gas engines, which have stayed nearly the same for years. The fuel efficiencies for particular models through the years have stayed good or gotten better, while the power available has continued to climb.
Generally speaking, a diesel powered vehicle will get nearly double the economy of a similarily powered gasser even if the same gearing is used.