Doh

Sucking his keyboard for inspiration, Pete Fisher typed:

In a sense, yes. LR engines are notoriously inefficient, so anything which
reduces the losses due to engine speed (as an o/d does) will have a
proportionately greater effect than with a super-efficient modern unit.
I've seen those figures before, and from what I hear they are never achieved
in the real world.

 

Why would you do that? The whole point is that when the full power is
not needed, you can run the engine at a much more fuel-efficient
speed. In general lower revs = lower number of fuel-burning strokes =
less energy losses = less fuel burned. When you need the extra power,
however, it will generally be more efficient to get it by running at a
more power-efficient engine speed, so you'd simply change to a lower
gear.

Hopefully the above will answer your question.

Anyway, it's very easy to test this empirically if you have an
instantaneous mpg readout on your car - simply drive at the redline in
first gear and see what it's reading, then change into top on the same
road and see the difference.

I promise you will see a huge difference.
--
_______
..'_/_|_\_'. Ace (bdotrogers a.t compaqnet.fr)
\`\ | /`/ DS#8 BOTAFOT#3 SbS#2 UKRMMA#13 DFV#8 SKA#2 IBB#10
`\\ | //'
`\|/`
`

 

Sucking his keyboard for inspiration, Pete Fisher typed:

Perhaps, but the fuel consumption I have achieved with a variety of LRs from
petrol series vehicles up to the Td5 Discoveries has been pretty poor on
average. Overdrives are popular on utility LRs, but mainly because they
make for quieter cruising (a real issue with the older Landies) rather than
better economy.

 

Not completely answered, but are you saying that statement is incorrect?
Seems, to me, a pretty good starting point for considering the complex
interaction of factors which determine the specific fuel consumption.

Fair cop, wrong word, particularly round here. How about, in my
experience, gained over years of driving/riding vehicles with IC
engines, the high gear strategy is *usually* successful in reducing fuel
consumption given steady state conditions (constant speed, flat road).
Ever driven one of those Hondas with an 'economy warning light'?

You will now say that much of my experience was in SOCs and SOBs without
modern ECU assistance. There you have a valid point. In terms of sports
bikes you may have an argument. Even your 70 in first, versus 70 in top,
statement might hold some water. Substitute a more appropriate gear
though and I remain unconvinced.

As I said earlier, when I get a chance I will test my hypothesis in a 6
speed 2.0 TDi car. I will attempt to reduce the number of variables to a
minimum, but in the interests of keeping my licence conduct the
experiment at a legal speed. This will naturally render it of no use to
anyone here. If I remember, I will conduct it again in France where a
higher steady road speed will be easier to hold for a variety of
reasons.

To repeat, it depends on the particular vehicle and the chosen speed.
You can't make a sweeping statement that the gear won't ever matter, any
more than I can say it always will.

I still maintain that, on an economy run, drivers would generally be
trying to get in to a high gear at low revs at reasonably high road
speed (subject to aerodynamic limitations) when neither ascending nor
descending gradients. If you read about such events, the techniques for
climbing gradients in particular can be very sophisticated.


--

+-------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 |
+-------------------------------------------------------------+

 

Usually, yes.

The stuff that makes an engine less than 100% efficient. Without them
all the energy in the fuel would be converted to power, which is
clearly not the case. In general the higher the engine is revving the
greater the losses, as a proportion of energy consumed.

Can I suggest that you do the experiment I suggested before arguing
the point any more? I'm absolutely certain that the results will
convince you, but I'm far from sure that my ramblings will be as
effective ;-)

--
_______
..'_/_|_\_'. Ace (bdotrogers a.t compaqnet.fr)
\`\ | /`/ DS#8 BOTAFOT#3 SbS#2 UKRMMA#13 DFV#8 SKA#2 IBB#10
`\\ | //'
`\|/`
`

 

if you were riding on the motorway, doing whatever your average speed
in such situations, which gear would you be in, and why?

 

My scorpio has instantaneous fuel display on the computer.

It's better on fuel at 80mph than it is at 70mph, but 90mph is much much
worse than 80.


--
Pete M - OMF#9
"Save your breath for cooling your porridge!
W&P Range Rover V8 Turbo
Scorpio Ultima 24v
Tatra 805

 

you're right, but you are only taking into account rolling resistance
(1st order velocity related). There are also 2nd-order (pumping) and
3rd-order (drag) components of the equation that go to make up the
equation that specifies power required per road speed. It's not a huge
difference, the 2nd-order pumping components mean that a faster
spinning engine is sapping more power for a given road speed than a
slower spinning one.

BTW, forget all the stuff people have been spouting about intake
cycles and fuel flow - that's all bollocks.

 

And I suspect that there is (for every engine/gearbox combo) a
'sweet-spot' where the whole thing is operating most efficiently
(intake/exhaust flows, power losses in transmission, power losses in
the engine itself all being at their optimum figure).

Phil.

 

top of the range lycra?

 

Well I'm not going to attempt to explain it. Particularly in these days
of lean-burn, ECU controlled stoichiometric combustion and VTEC.
Empiricism will do for me.

I only just about got my head round boiler (external) combustion
calculations when I did my RSH Diploma in Air Pollution Control. That
was all strictly simple excess air territory, at least in those days.


--

+-------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 |
+-------------------------------------------------------------+

 

This debate is more about engine speed than road speed though. So is it
better on fuel at 70mph in a gear lower?

--

+-------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 |
+-------------------------------------------------------------+

 

Ah. Max torque rpm, perhaps?

It used to be the case that, for petrol cars anyway, the gearing and
suchlike was arranged so that at 75mph in top gear, the engine was turning
over at approximately maximum torque rpm. This was in the days when fuel
consumption at 75mph was one of the figures quoted in adverts etc.

 

It takes a lot of fuel to push a vehicle (any vehicle) up to cruising
speed, but not much, relatively speaking, to keep it there, as inertia
plays a big, big part. However, in theory, yeah, it shouldn't make too
much difference whether the engine is turning at (say) 5k revs or 4k
revs to maintain that speed. But it does, nonetheless.

(I remember someone from Iveco or Mercedes trucks, I forget which
maker, telling me that no matter *what* the engine and chassis guys
did, it *still* took a litre of diesel to get any 38-tonne rig from a
standing start up to 50mph.

 

To be honest, 80mph seems to be the economy cut-off point for most
cars and bikes I've ridden or driven. Especially naked bikes, for some
reason.

 

Could be. I'm not an expert..

Before 75mph marked you out as the spawn of satan?

Phil.

 

No or probably not. A modern petrol/ignition internal combustion engine
is at its most efficient when working against a lights load at about 1/2
of its red line and at it's most fuel efficient when slowing from low
revs to even lower under no load at all. Thus economy test
drivers/riders will accelerate moderately to as high a speed as is
suitable given shit like rolling resistance, wind drag factors and stuff
and then spend as long as possible slowing down to nearly stopping
before starting all over again.

 

Aero drag. I'm *always* suspicious of claims that any vehicle gets better
fuel economy at 80 mph than it does at 70. Unless there is an obvious
gearing mismatch, it makes no sense because air drag is significantly
higher at 80 than 70.

The reason gasoline engine vehicles do better in a higher gear at a given
speed is lower pumping loss due to having the throttle opened wider.

 

Accompanied by the sound of a chisel on slate Pete

Oh yeah, so it is (about engine speed)...

It seems to use the least fuel in overdrive top provided there are at least
2500 rpms on offer, it's auto, so faffing about with the selector is not
worth the hassle.


--
Pete M - OMF#9
"Save your breath for cooling your porridge!
W&P Range Rover V8 Turbo
Scorpio Ultima 24v
Tatra 805

 

I understand why lorry driver get pissed off being forced to brake
unnecessarily, so it can give a great sense of satisfaction, when
someone's being a twat, to force them to do just that. Coming through
the motorway underpasses into Basel this morning I came across one
trucker who was determined not to let me in at a
two-lanes-merge-into-one point, to the extent that he closed the gap
between him and the car in front, despite me sitting there indicating
in the outside lane waiting to get into it. Eventually I forced my way
in, to the sound of hooting from the irate trucker, who I then
proceeded to piss off even more by touching my brakes, just slightly
more than enough to flash the brake lights, and having the
satisfaction of seeing his cab dip as he suddenly shaved off about
10kph, while I proceeded happily along, TYVM.

--
_______
..'_/_|_\_'. Ace (bdotrogers a.t compaqnet.fr)
\`\ | /`/ DS#8 BOTAFOT#3 SbS#2 UKRMMA#13 DFV#8 SKA#2 IBB#10
`\\ | //'
`\|/`
`