Re: Dirty Diesel...

["Al" <alpout@optusnet.com.au>]

"Bradley Torr" <truenorth@one.net.au.SPAMTRAP> wrote in message
news:01c0f793$edf0ac40$34c28ec6@humenwy...
>
> Al <alpout@optusnet.com.au> wrote in article
> <3b2c29c4$0$25479$7f31c96c@news01.syd.optusnet.com.au>...
>
> > The compression ratio is the difference in volume in the cylinder between
> when
> > the connecting rod (conrod) is at its furthermost position, and its
> nearest
> > position (ie when the conrod is at, say, 12 o'clock and 6 o'clock).
> That's a
> > bit simplistic, but it should give you an idea.
>
> Yup, great, thanks Al, I understand what 'compression ratio' is now. So
> with the higher compression ratios one finds in Diesel engines, it means
> that for each revolution, the ratio of maximum cylinder volume over minimum
> cylinder volume is around double that of petrol engines. How exactly is
> this more 'efficient'? Does it mean that, for every revolution of that
> cylinder, more power is produced?

Ah, now you've got me without my thermodynamics book.  I'll have to look that
one up, might take a couple of days.

But here goes anyway:

The thermal efficiency is the relationship between the input energy to the
output energy.  A tonne of oil, or coal, or gas or any kind of fuel has a
certain amount of energy contained in it (IIRC around 40MJ/tonne for
hydrocarbon fuels, eg coal, oil etc), but only a certain amount of this is
actually usable for the desired purpose (such as propelling a train, car or
generating electricity).  The rest is waste, put out the radiator and exhaust.
Raising the compression ratio increases the thermal efficiency of the engine,
but I can't remember why at the moment.  Most petrol engines are around 25-28%
efficient (yup, 3/4 of the fuel you put in your tank is wasted), most diesels
are IIRC around 38-40%, and a coal/oil burning power station is at best 36%.

If you want some more details, I've got some reports I've done for uni about
various kinds of engines and fuels that I can email to you.

> > 2 reasons.  Firstly, petrol engines are cheaper to make (by economies of
> > scale).  A fully assembled Family II engine out of Holden in Port
> Melbourne is
> > worth about $2000.  A similar sized Diesel would be worth about twice as
> much,
> > due to larger castings being needed for the larger stresses imposed on
> the
> > engine (thermal and mechanical), the injectors themselves, etc.
>
> OK, this would be a good reason why Diesel isn't widely used in passenger
> cars! :-) However, London taxis (and presumably many taxis elsewhere) run
> on Diesel, and Diesel cars are also more popular in Europe. Is this because
> petrol is so expensive for our European counterparts, and because taxis are
> in use so much more than the average car, the relative cheapness of diesel
> fuel outweighs the increased costs of purchasing cars with Diesel engines?
> Seeing how most London taxis are stuck in London traffic for their working
> lives, concerns over acceleration and straight-line performance probably
> don't count for much in any case.

Not too sure here, but I think it's to do with taxation over there.  I think
diesel is viewed as their LPG, hence why London cabs are all Dies-on-hills.
Similar thing in the country here with farmers buying diesel cars for use on
the farms because of tax exemptions.

I'll go one step further and say that the only reason diesel engines are more
expensive is because of the economies of scale.  Think of diesel vs petrol as
similar to VHS vs Beta.

Al