I've been doing some research (OK, Googling to you folks) on this concept of the sweet spot of an engine. It gets mentioned a lot, especially in connection with fuel economy. People seem to know what they're talking about, but there's remarkably little that's objective. It's always something like "the engine's not labouring and not revving its head off - somewhere in between".
Is it anything to do with maximum torque? Are engines more efficient at a particular point on the rev counter?
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It's where there is efficient combustion (ie. high torque output) and low mechanical losses.
The most fuel efficient zone for an engine to operate is likely to be just below the peak torque but compounding factors like having forced induction and variable valve timing make it a rather grey area.
Summary - N_C can likely link some torque vs RPM graphs and explain what bit is the 'sweet spot'.
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Don't take it as gospel from my mouth but I would have thought the point on a power graph where the torque crosses or comes as close to the power (BHP) of an engine.
In terms of revs, yes the lower the better as there is less friction on the moving parts.
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>> I would have thought the point on a power graph where the torque crosses or comes as close to the power (BHP) of an engine.
In that case no diseasel engines have sweet-spots because this happens at roughly 5500rpm ;-)
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How would that work anyway, Diddy? It would depend on the units you measured the torque in and the y-axis scale you chose to plot it on. You could make the curves cross wherever you liked or - as Lygonos suggests - not at all.
Last edited by: WillDeBeest on Wed 23 May 12 at 22:07
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>>It would depend on the units you measured the torque in and the y-axis scale you chose to plot it on.
If the X-axis is RPM, and the Y-axis shows both (1)power in BHP and (2)torque in lb-ft, the power and torque lines will cross at 5252 rpm.
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And that's my point. If you plot the power in kW (larger units, curve moves down) and the torque in Nm (smaller units, curve moves up) instead, the curves appear in different places and probably don't intersect at all. The point of intersection is meaningless.
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Oh - you're talking about cars ;-)
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Its about efficient zones not erogenous zones CG.
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I think people assume the point where the engine is quietest and smoothest is the sweet spot. It's probably not the point it's most efficient but merely the point where the engineers have tuned out the NVH best
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"Its about efficient zones not erogenous zones CG."
Yes, it is. I carefully worded my post to avoid references to "other" spots and any lewd jokes, puns etc.
OK, I did consider it. Then I got a grip. ;-)
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On my Prelude, especially on a motorway there was a (sweet) "spot" between 70-80 when the car just seemed to (hard to describe!) "squat down and settle" it seemed to run quieter and smoother, and every little vibration and rattle stopped. Having said that, hitting the rev limiter at 135 was "disturbing" as the car made all kind of judders and lost acceleration!
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Yep, squatting down and settling makes sense.
Look at the speed limits around the world - 70-80 ties in with 120km/h, which is pretty common.
en.wikipedia.org/wiki/Speed_limits_by_country
So manufacturers will 'tune' their cars's components be most efficient and comfortable at that sort of speed.
Having said that, they also have to consider the engine, and optimum torque figures as well. I drove a new Civic the other day which was happy at 120 in 6th (~1750RPM) , but for any overtaking oomph above 120, one had to drop to 5th to get into the torque band, which only started at 2000RPM.
My Opel hits exactly 3000RPM in top at 120km/h.
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>> So manufacturers will 'tune' their cars's components be most efficient and comfortable at that sort
>> of speed.
That explains why my 320d feels completely happy, relaxed, and "just getting into its stride" at 100 mph. I have driven far quicker cars, but nothing that reels in a motorway so completely effortlessly, and makes license losing speeds feel so ordinary.
100 mph = 2500 RPM, and the claimed 43 mph in reserve is entirely believable. Pulls like a train past the ton if you ask it to.
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>> That explains why my 320d feels completely happy, relaxed, and "just getting into its stride"
>> at 100 mph. I have driven far quicker cars, but nothing that reels in a
>> motorway so completely effortlessly, and makes license losing speeds feel so ordinary.
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>> 100 mph = 2500 RPM, and the claimed 43 mph in reserve is entirely believable.
>> Pulls like a train past the ton if you ask it to.
100%. An Autobahn cruiser of note.
And if you drove it at a decent speed for a long trip, you'd find that the mpg was surprising as well.
With the Civic, I can believe their figures of 4-point-something litres per 100km on the open road.
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too late to edit.
320d
380 Nm (280 ft·lb) (38.8 kgm)
at 1750-2750 rpm
so right in the band there!
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This post shows an engine performance map
forums.tdiclub.com/showpost.php?s=18811588dc2fc4f80053de8a0b779c7c&p=2076501&postcount=14
The thick black line is the engine's maximum brake mean effective pressure. This is proportional to torque, so, the engine speed for maximum BMEP is the same as for maximum torque. The vertical axis on the graph is effectively a measure of engine load - maximum load at the thick black line, and very low loads near the lower edge of the graph.
The blue lines represent lines of constant power. As each car needs a certain amount of power to overcome drag at a specific speed, these lines can also be considered as lines of constant vehicle speed.
The thinner black lines are contours of equal brake specific fuel consumption, i.e., how quickly does the engine consume fuel per kW. In this respect, a goal of engine matching and CVT design is to keep the engine close to the "centre of the onion".
In this case, the best BSFC is obtained very close to the engine speed for maximum torque, and at high load. As mentioned in the post, for this engine, it's difficult to imagine conditions where it's not sensible to be in the highest gear the engine will tolerate without stalling.
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>> In this case, the best BSFC is obtained very close to the engine speed for
>> maximum torque, and at high load. As mentioned in the post, for this engine, it's
>> difficult to imagine conditions where it's not sensible to be in the highest gear the
>> engine will tolerate without stalling.
Which also explains why in ECO PRO mode, my BMW asks for a gear change at about 1700 RPM.
Very informative post, as always N_C. Thanks.
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>> Which also explains why in ECO PRO mode, my BMW asks for a gear change
>> at about 1700 RPM.
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>> Very informative post, as always N_C. Thanks.
>>
Yep, that big, lazy diesel engine will tick along all day at ~2000. It never fails to amaze me how people rev the nuts (and bolts) off of diesel engines. Why? Can't you understand the concepts of torque?
In many large pick-ups/SUVs I drive, I go 1-3-5-6 on the gears. No point in the intermediates, as the torque will propel you along quite nicely to get up to 'cruising' speed.
Sadly, due to the poor quality of 'derv' in this country, many manufacturers can't bring in their latest engines.
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>
>> fails to amaze me how people rev the nuts (and bolts) off of diesel engines.
>> Why? Can't you understand the concepts of torque?
Yes, but don't you understand the concept of a gummed up EGR valve? or a full up DPF?
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>> >
>> >> fails to amaze me how people rev the nuts (and bolts) off of diesel
>> engines.
>> >> Why? Can't you understand the concepts of torque?
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>> Yes, but don't you understand the concept of a gummed up EGR valve? or a
>> full up DPF?
>>
Zero, i'm not talking plod-plod-plod motoring - yes, every vehicle needs an 'Italian tune up' occasionally. But constant traffic-light grand prixs? Or delivery van races?
makes no sense. Twonk it by all means, but do it on the open road and let the engine breathe a bit, not in 50m dahes round the suburbs.
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While the efficiency sweet spot is related to N_C's 'onion' - as for the driveability sweet spot, I think that's simply where the engine is from just before its torque peak to just after maximum power (assumin NVH is well managed).
For my old Civic it was from ~5800-8200rpm, for the Forester 2500-6500rpm, Swift 4000-7000rpm.
Shogun 2000-3500rpm but NVH wasn't that great!
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>> For my old Civic it was from ~5800-8200rpm, for the Forester 2500-6500rpm, Swift 4000-7000rpm.
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Civic! VTec Yo! I love those Honda engines. They spin nicely!
As do most Japnaese engines, for that matter - torque peak very high up, but can get a tad tedious on the long runs. I drove a 1.5 shed of a daihatsu last year which revved to 4000 at 120km/h. Hideous in terms of NVH, and I couldn't imagine it on the long road, it would have driven me crazy.
>> Shogun 2000-3500rpm but NVH wasn't that great!
>>
I'm in a Pajero (Shogun) 3.5 turbodiesel this week. Will let you know how that behaves, with some intermittent acceleration tests (80-100, 100-120, 120-140 etc) as well. Always important stuff for the buyer to know.
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>> Very informative post, as always N_C. Thanks
+1
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>> the highest gear the engine will tolerate without stalling.
I wonder if this is strictly true from the viewpoint of engine longevity anyway. It's possible to drive along gently at quite low speeds in high gears, but any incline or attempt at strong acceleration may make the engine a bit unhappy, even these days with managed engines that don't pink loudly.
Of course the point is taken that that engine is most efficient at low revs and high load/throttle opening. But isn't there perhaps a crankshaft speed below which damage can be done N_C?
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>> difficult to imagine conditions where it's not sensible to be in the highest gear the
>> engine will tolerate without stalling.
I won't be there, thats also the place where you will be knocking seven bells out of your DMF. Not at all a sensible place to be.
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I'm yet to be convinced of what exactly kills a DMF - plenty of Taxi drivers chunter around at 1200rpm and seem to be ok.
My friend finished his off towing an Audi out of snow (Quattro too - hahahaha) with his Mondeo TD - he said he was slipping the clutch a bit rather than lugging at very low rpm.
I'd love to see some good scientific study reports into the causes of DMF failure if anyone has some!
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>> I won't be there, thats also the place where you will be knocking seven bells
>> out of your DMF. Not at all a sensible place to be.
>>
Is there a DMF on your Mitsubishi Zero?
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>> Is there a DMF on your Mitsubishi Zero?
On a 1.6 petrol with less than a hundred horsepower? do me a favour. They were hardly worried about too much torque when they dropped this sewing machine motor in.
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>> OK, I did consider it. Then I got a grip. ;-)
>>
On what?
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Isn't the most efficient spot where the inlet vacuum is highest?
Remember when people fitted vacuum gauges to improve economical driving? I'd have said it's the point where you need least pressure on the accelerator - the point where the car feels as if it is driving itself, and all you need to do is rest a carpet-slippered foot gently on the accelerator pedal.
At slow speed it's when you don't need any accelerator at all - the idling control does the work.
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>>Isn't the most efficient spot where the inlet vacuum is highest?
No, it isn't - see the BMEP / BSFC map above.
With a petrol engine particularly, light throttle is where pumping losses are highest.
Vacuum gauges were popular because they were simple.
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Sweet spot on my vintage car is hard to judge as the perception is of the engine and transmission, but it's happiest at an indicated speed of about 45-47 miles an hour. Before you say, that's slow, add the speedo error of most moderns, and I'm sure most say more than 50. My speedo is pretty accurate as I recalibrated it. (Jolly easy with old style mechanical speedos, albeit boring to do).
And the car will go a heck of a lot faster but I bottle out at 65.
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"OK, I did consider it. Then I got a grip. ;-)
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On what?"
Oooh, you ARE awful!
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The "sweet spot" - what is it? Where is it? - Lygonos
