On May 8, 9:48=A0am, Dyno <d...@[EMAIL PROTECTED]
> wrote:
> Mortimer wrote:
> > "Nate Nagel" <njna...@[EMAIL PROTECTED]
> wrote in message
> >news:fvuo59119s3@[EMAIL PROTECTED]
> >>> If you believe in those calculations, then why
> >>> not put diesel in your car? That would give you the best mileage.
Righ=
t?
> >> Oddly enough, it does.
>
> > I've always wondered: is it the higher compression ratio or the
differen=
t
> > fuel which gives diesels their greater fuel economy and their greater
> > low-end torque. Or a combination of the two?
>
> Diesels benefits from three main factors:
>
> 1: Higher mechanical compression ratio. Cycle efficiency is a function
> of expansion ratio. This is one of the biggest reasons Diesel have
> higher torque.
>
> 2: Unthrottled operation. In the Diesel cycle load is controlled by fuel
> alone. This reduces gas pumping losses incurred during the
> intake/exhaust strokes. In future Diesels this may fade somewhat as
> throttling is being used to facilitate achieving the EGR levels needed
> to meet oxides of nitrogen regulations.
>
> 3: Fuel heating value. Diesel fuel contains around 10% more energy per
> gallon than gasoline. Diesel fuel is formulated to promote autoignition,
> the Diesel cycle ignition method. The metric for this is called the
> cetane rating. The higher the cetane rating the lower the octane rating.
> For Diesel engines high cetane fuel is desirable. This is obviously
> undesirable in a spark ignition engine.
What he said. I'm not sure how much 2) contributes, but sort of as an
aside, this is the reason why you see sewer-pipe sized exhaust pipes
on Diesel vehicles - they are essentially pumping the full
displacement of the engine (or more, if supercharged) of air through
the engine every two revolutions, unlike a gasoline engine where this
only happens at WOT.
nate
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