save the bunnies by shifting at 2500rpm
Discussion
it was ages since i read about this through a load of corporate b
ks at work, but didn't some scientists work out the most efficient/economical way of driving was to drive WOT and shift at 2500rpm?
what i don't get with that is:
if oil pressure is directly linked to rpm, then surely it's putting more wear on the oil to keep film strength up over bearing faces at low rpm/pressure, WOT... in petrol cars atleast, diesels have to put up with quite a torque surge after 1800-2000rpm
and, in the case of fixed cam engines, surely peak efficiency is going to be closer to halfway between idle + redline - a balance between power output at x rpm and valve lift
i'm not a scientist, so presumably have missed the boat somewhat
discuss?!
ks at work, but didn't some scientists work out the most efficient/economical way of driving was to drive WOT and shift at 2500rpm?what i don't get with that is:
if oil pressure is directly linked to rpm, then surely it's putting more wear on the oil to keep film strength up over bearing faces at low rpm/pressure, WOT... in petrol cars atleast, diesels have to put up with quite a torque surge after 1800-2000rpm
and, in the case of fixed cam engines, surely peak efficiency is going to be closer to halfway between idle + redline - a balance between power output at x rpm and valve lift
i'm not a scientist, so presumably have missed the boat somewhat
discuss?!
Peak thermodynamic efficiency, i.e. the petrol explosion efficiency, is most effective at lower speeds (more time for complete combustion). While power is down at those revs, power per gallon of petrol is at its highest.
If you cam the engine in such a way as to maximise the HP at lower revs (like all cars used to have when they didn't need to cheat to get high HP and also good fuel economy on the crappy EU scale) you can quite happily pootle around between 1750 and 2500 rpm at full throttle. Older cars may be a different thing because oil technology wasn't so great.
If you cam the engine in such a way as to maximise the HP at lower revs (like all cars used to have when they didn't need to cheat to get high HP and also good fuel economy on the crappy EU scale) you can quite happily pootle around between 1750 and 2500 rpm at full throttle. Older cars may be a different thing because oil technology wasn't so great.
Hmm.
Normally aspirated diesel, redlines at 2500rpm.
RX8 redlines at 9000 rpm.
CBR250 redlines at 18,000rpm.
2500 rpm is the most efficient for ALL of those engines? My cars VANOS doesn't kick in until 3k rpm!
[edit] Fire99 Great minds and that...[edit]
Normally aspirated diesel, redlines at 2500rpm.
RX8 redlines at 9000 rpm.
CBR250 redlines at 18,000rpm.
2500 rpm is the most efficient for ALL of those engines? My cars VANOS doesn't kick in until 3k rpm!
[edit] Fire99 Great minds and that...[edit]
Edited by MarJay on Thursday 11th February 23:13
davepoth said:
Peak thermodynamic efficiency, i.e. the petrol explosion efficiency, is most effective at lower speeds (more time for complete combustion). While power is down at those revs, power per gallon of petrol is at its highest.
If you cam the engine in such a way as to maximise the HP at lower revs (like all cars used to have when they didn't need to cheat to get high HP and also good fuel economy on the crappy EU scale) you can quite happily pootle around between 1750 and 2500 rpm at full throttle. Older cars may be a different thing because oil technology wasn't so great.
ahhhh, i see, i did indeed step off the pierIf you cam the engine in such a way as to maximise the HP at lower revs (like all cars used to have when they didn't need to cheat to get high HP and also good fuel economy on the crappy EU scale) you can quite happily pootle around between 1750 and 2500 rpm at full throttle. Older cars may be a different thing because oil technology wasn't so great.
i'm guessing that would explain all those curious engines bmw made for the 3? series back in the late 80's for south african? markets - the ones with weak valve springs, less bearings, lower rev limit etc. x or E type or something
i still don't see though, that manufacturers have designed the actual production engines themselves to operate that efficiently at such low revs - over fueling, valve open/overlap i get that they could be designed that way, but wouldn't that need, long term, a higher compression - diesel?
With the S2000 (and no doubt the RX8 etc.), they spin up the revs so quick, it's actually hard to keep the revs below 2500. There would prob be a fair amount of load at that low revs making it not the best way to drive.
That said, for most average cars (which is what these things are aimed at), changing at 2.5k is prob ok.
That said, for most average cars (which is what these things are aimed at), changing at 2.5k is prob ok.
In an S2000 it won't work too well because the engine is quite compromised (i.e. it's more akin to a racecar engine torque curve) and an RX8 doesn't have a normal ICE so it's not the same.
A slow engine speed allows fuller burning of fuel. But valve timings and injection needs to be optimised for this to work, as do turbochargers if fitted.
A slow engine speed allows fuller burning of fuel. But valve timings and injection needs to be optimised for this to work, as do turbochargers if fitted.
Fire99 said:
MarJay said:
CBR250 redlines at 18,000rpm.
I forgot about those little pocket-rockets. I thought the old CBR400 revved high. But the 250... marvelous!It probably doesn't idle much below 2.5k.
Best BSFC point in most engines is just shy of peak torque and below WOT- say 85-90% load.
However it's all too easy to get caught up in the best efficiency for the engine and ignore the comparatively greater effect of the vehicle roadloads, vehicle drag and transmission. Vehicle and gearing changes have a much higher effect than engine changes.
In a study carried out in 1997 For instance a 10% reduction in rolling resistance will increase highway fuel economy by about 1%. On a manual transmission a reduction in final drive ratio by 10% will benefit fuel economy by 6%. Compare that to eeking out the odd 2-3% on base engine changes via adding valvetronic or another 2-3% by upping the CR a ratio, or going to a twin plug design to benefit fuel economy by a whopping 1%, adding a high swirl combustion chamber torun more EGR and seeing a 1-1.5% fuel economy benefit.
With all this in mind- its best to have the engine running in as high a gear as possible at the lowest engine speed- DESPITE there being higher heat loss at low rpm and the engine not running close to its best BSFC point- the engine will be running less throttled at this higher load- and this will predominate along with the higher gearing (gearing effects things much more- remember?)- and the engine turning lower rpm is better from a friction point of view.
for those of you still focused on the thermodynamic efficiency- if you run your engine close to your peak torque- at say 4500 rpm, you're likely to be in a lower gear and also running at a lower load-hence running higher pumping losses (more throttled)- I can assure you that these two effects will predominate significantly when it comes to vehicle fuel economy.
However it's all too easy to get caught up in the best efficiency for the engine and ignore the comparatively greater effect of the vehicle roadloads, vehicle drag and transmission. Vehicle and gearing changes have a much higher effect than engine changes.
In a study carried out in 1997 For instance a 10% reduction in rolling resistance will increase highway fuel economy by about 1%. On a manual transmission a reduction in final drive ratio by 10% will benefit fuel economy by 6%. Compare that to eeking out the odd 2-3% on base engine changes via adding valvetronic or another 2-3% by upping the CR a ratio, or going to a twin plug design to benefit fuel economy by a whopping 1%, adding a high swirl combustion chamber torun more EGR and seeing a 1-1.5% fuel economy benefit.
With all this in mind- its best to have the engine running in as high a gear as possible at the lowest engine speed- DESPITE there being higher heat loss at low rpm and the engine not running close to its best BSFC point- the engine will be running less throttled at this higher load- and this will predominate along with the higher gearing (gearing effects things much more- remember?)- and the engine turning lower rpm is better from a friction point of view.
for those of you still focused on the thermodynamic efficiency- if you run your engine close to your peak torque- at say 4500 rpm, you're likely to be in a lower gear and also running at a lower load-hence running higher pumping losses (more throttled)- I can assure you that these two effects will predominate significantly when it comes to vehicle fuel economy.
dylan0451 said:
if oil pressure is directly linked to rpm, then surely it's putting more wear on the oil to keep film strength up over bearing faces at low rpm/pressure, WOT... in petrol cars atleast, diesels have to put up with quite a torque surge after 1800-2000rpm
engine friction of crankshaft, bearings, pistons, rings all go up with load and engine rpm, valvetrain (if not rollers) goes down with engine speed slightly- but valvetrain friction as a proportion of the whole engine friction is very low. Oil pump friction comes under anciliary friction or parasitic losses and this is also quite low.PS For all of those who think im nasty on these boards- or 'take no prisoners' or am rude/arrogant, witness if someone genuinely doesnt know something and has a question or wants to learn- I have no problem, when someone talks utter b
kss and tries to pass it off as fact- all too common on these boards, still not backing down- then I get nasty 
. SOmeone has to moderate the technical BS being spouted.Gassing Station | General Gassing | Top of Page | What's New | My Stuff