Re: Advantages v disadvantages of a diesel!!!


"Albert T Cone" <me@xxxxxxxxx> wrote in message
news:3me4f2F15inieU1@xxxxxxxxxxxxxxxxx
>>> The absence of an electrical ignition system reduces the non-critical
>>> failure rate. They _are_ more reliable - warrenty claims and pretty
>>> much any mechanic can confirm.
>>
>> Not the ones I speak to - sorry but anecdotal evidence is meaningless....
> Perhaps, but warrenty claim records aren't anecdotal.
>
>>>>> A petrol engine with ideally flat torque curve produces >90% of
>>>>> peak power for les than 10% of rpm range.
>>
>> Absolute bollocks!
>
> No, it is true by definition. I said for an *ideally flat torque
> curve*, i.e. with exactly same torque at all revs. If that is the case,
> then power obviously scales completely linearly with revs, and 90% power
> occurs at 90% of revs of peak power,

There would never be any peak power point! Your idealism prevents you having
one!

and peak power occurs at the rev
> limit, *by definition*.

But engines are designed to be strong enough to have the rev limit slightly
past peak power. IE 13000 rpm bike engines etc. Your power curve is
straight and never ending so no peak...

Therefore you get >90% of peak power for 10%
> of the rev band.

Who says that a flat line is the ideal?
And of course that would imply an endless power supply as if the torque
curve always stayed flat the power would keep increasing with rpm for ever!
No gear box needed.

Torque curves of course can never be flat obviously as you cant flow much
gas at 200 rpm or at 30,000 rpm because of the mechanical pumping
limitations, and speed of combustion etc...



>> I also build and sell dynamometers, and have hundreds of curves from
>> stock cars and bikes, and other than a peaky rs 250 aprilia 2 stroke that
>> is quite simply stupid!.
> That is nice, but I think you missed my point a bit.
> I know that real engines don't have flat torque curves, but an ideal
> petrol engine would.

Err it cant! And it wouldnt be ideal and would feel featureless too, and
would make people want to keep rteving harder as torque wouldnt fall away as
revs rore high. So no flat isnt ideal.

>
>>> To have a wider power peak, the torque curve has to drop off at higher
>>> rpm. Like a diesel :-)

No. An engine that makes useful torque over a greater rpm range is always
more flexible and needs less gears to match loads and speeds The torque
curve on diesels drops off about 2000 rpm earlier than on petrol, and is
lower to start with!

>>
>> Why does a power curve have to fall off?
> Surely that's clear? To give a wider power peak, the slope of the power
> curve must drop off. Since torque is proprtional to the slope of the
> power curve, it must also drop off.

No sense... More at any rpm is always better.

>
>> If 2 engines mame same power (and torque obviously) at say 2000 revs
>> the one that falls off only loses compared to the one that climbs!
>> Please wake up!

> I never said otherwise - that is trivially obvious. This boils down the
> the semantics of what 'equivalent' engines are. We clearly have different
> definitions.

2 LITRE DIESEL
2 LITRE PETROL

2 LITRE DIESEL TURBO
2 LITRE PETROL TURBO

Anything else is not comparing either like with like or cost with cost,
since turbos are expensive.
And the point is to compare onre fuel with another.


>>>>> (&)because of (*) it's possible to be within a few % of peak power for
>>>>> a significantly larger fraction of the time than for a petrol lump.
>>
>> Diesels often need 6 gears or more (trucks) precisely because they lack
>> this very flexibility. They cannot compete here any way at all.
>> You really are va bit clueless here!
> It seems you are choosing to ignore facts to sustain your prejudice.


>>> Bah. I enjoy a good high rpm thrash as much as anyone - I'm a big fan
>>> of Vtec-alike engines, but I can see the advantages of high-torque, low
>>> rpm engines, especially in everyday, normal driving.

You mean like a similarly sized petrol engine already has even if you ignore
the top end?

>>
>> But a diesel is just like any normal engine but with the top end chopped
>> off! Thats why people think they are torquey! Its bull***.
> No it isn't. A diesel with the same peak power as a petrol engine _is_
> comparatively torquey. You know this.

Yes but to do this it needs to be much bigger or have a turbo which means
that it is cheating!
If the diesel can have one so can the petrol! Same turbo, same cost.


>>> Yes it is. You ask an average non car nut how often they hit the
>>> redline.

>> Peak power may be 5 thou redline is overev capability, so you drop back
>> into the power after a change, something else diesls cant do...
> You drop back to something below the peak power after a change. A diesel,
> properly driven, will still be _at_ the power peak after a change.
>
>>> Most petrol engines never produce more than 70% of thier peak
>>> power.
>>
>> Errr why?
> Because of the way that _most_ people drive, obviously. They simply don't
> use the full rev range.
>
>>>> Aturbo petrol will destroy your diesel!
>>>
>>> A turbo petrol of the same power will lose out to the diesel. A turbo
>>> petrol of the same peak torque will obviously destroy the diesel
>>
>> Aturbo engine of the same cost, weight, cc, torque, will always destroy
>> an oil burner.
> Yes. However a petrol of the same cost, weight, power willalways lose out
> the the oil burner. I would argue that peak power is a valid point on
> which to judge equivalence.

Why? To do so means you are simply using a bigger more expensive heavier
engine, or a turbo, which can also be applied to the petrol!


.

Loading