At least two thirds of the voters got it right.
Edited by Rasputin, 29 June 2013 - 16:09.
Posted 29 June 2013 - 14:42
Edited by Rasputin, 29 June 2013 - 16:09.
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Posted 01 July 2013 - 07:43
Edited by WhiteBlue, 01 July 2013 - 07:50.
Posted 01 July 2013 - 13:03
Posted 01 July 2013 - 17:30
Posted 01 July 2013 - 18:54
Edited by meb58, 01 July 2013 - 18:55.
Posted 01 July 2013 - 19:18
Not true for all engines/vehicles. See, for example, this graph.At any given road speed, the greatest available thrust is in the lowest gear that permits (accceleration at) that speed.
Posted 01 July 2013 - 19:49
Posted 01 July 2013 - 20:09
Edited by Rasputin, 01 July 2013 - 20:12.
Posted 02 July 2013 - 03:55
Nice graph what is it - 200 hp turbo thingy geared for 165 mph?Not true for all engines/vehicles. See, for example, this graph.
Edited by gruntguru, 02 July 2013 - 06:09.
Posted 02 July 2013 - 04:31
Posted 02 July 2013 - 05:03
Csquared- what is that graph supposed to be proving? Surely having a red line much higher than max power rpm is pretty much irrelevant to this discussion?
Posted 02 July 2013 - 06:33
Csquared- what is that graph supposed to be proving? Surely having a red line much higher than max power rpm is pretty much irrelevant to this discussion?
Perhaps you're missing the trick here. No one is debating that the greatest available thrust comes from the greatest torqe at the wheels - torque moves the pork. At any given road speed, the greatest available thrust is in the lowest gear that permits (accceleration at) that speed. That will invariably put the engine speed above the torque peak.
Clearly, if we were constraining with a fixed gear rather than fixed speed, the torque peak would provide the greatest acceleration.
Not true for all engines/vehicles. See, for example, this graph.
Posted 02 July 2013 - 06:46
That's exactly what I was trying to say. If my post or the graph were unclear, I apologize. I searched something like "torque gear curve" and picked the first image that came up where the lines crossed. From the image's URL, I'm assuming the car is a 3rd-gen RX-7.At least it does disprove Canuck's statement . . . . .
. . . . eg at 100 mph Canuck's statement suggests you should be in third gear when in fact there is better acceleratiopn in fourth.
Posted 02 July 2013 - 11:43
Posted 02 July 2013 - 23:36
Posted 03 July 2013 - 01:53
Posted 03 July 2013 - 06:01
I am OK with Csquared's post, the point I think needs to be made is - for max acceleration you should be in the gear that sees the highest engine power. Any other suggestions about "higher revs", "lowest gear", "torque peak" etc are irrelevent.People so busy looking to prove someone else wrong, they're happily missing the point.
Posted 03 July 2013 - 06:45
Finally some words of wisdom, thank you.I am OK with Csquared's post, the point I think needs to be made is - for max acceleration you should be in the gear that sees the highest engine power. Any other suggestions about "higher revs", "lowest gear", "torque peak" etc are irrelevent.
When the power has dropped off to the point where an upshift will produce identical power - its time to upshift (ie the intersection of lines on the thrust chart above.)
Posted 03 July 2013 - 22:41
Finally some words of wisdom, thank you.
Edited by rgsuspsa, 03 July 2013 - 23:07.
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Posted 04 July 2013 - 01:35
Seconded. People coming up with various rules of thumb is the #1 reason that this discussion can get so confusing. Those rules of thumb almost always correlate well with the correct things to do with the real engines we actually have, which is why they're hard to disprove, but they are not generally correct. Yes, in reality, engines that we all drive don't redline at RPM far above power peak, and in reality we don't have gearboxes with 100 gears, so waiting to upshift until you hit the rev limiter can't be that wrong, but that beats around the bush in a way that "always be in gear that gives you maximum power at that speed" doesn't.I am OK with Csquared's post, the point I think needs to be made is - for max acceleration you should be in the gear that sees the highest engine power. Any other suggestions about "higher revs", "lowest gear", "torque peak" etc are irrelevent.
When the power has dropped off to the point where an upshift will produce identical power - its time to upshift (ie the intersection of lines on the thrust chart above.)
Edited by Dmitriy_Guller, 04 July 2013 - 01:38.
Posted 04 July 2013 - 01:47
I guess you're not going to like what I'm about to say next then. However, I must. When you are at peak power at a given speed, the lowest possible gearing or highest possible gearing won't matter, you will be accelerating at the same rate.Urgh. I stand by what I meant if not what I typed. If you're being a technical pedant, then my statement is incorrect. Clearly if you're operating the engine at such a high (relative) RPM that the engine torque multiplied by drivetrain ratios produce less torque at the wheels than the next higher gear selection, you will find greater acceleration in the next gear at that road speed.
In the context of the question - peak power or peak torque at a given road speed - the lowest possible gearing at peak power will produce the greatest instantaneous acceleration at the given speed. People so busy looking to prove someone else wrong, they're happily missing the point.
Posted 04 July 2013 - 02:05
This is The Technical Forum, son!If you're being a technical pedant ...
Posted 04 July 2013 - 02:11
If you wish to BE at peak power (I presume you mean engine rpm peak power)at any given speed,does not that automatically require you to be at a quite precise gearing !!! ? You appear to be saying acceleration in first gear has no potential to be superior to acceleration in top gear.I guess you're not going to like what I'm about to say next then. However, I must. When you are at peak power at a given speed, the lowest possible gearing or highest possible gearing won't matter, you will be accelerating at the same rate.
Edited by johnny yuma, 04 July 2013 - 02:11.
Posted 04 July 2013 - 02:21
Posted 04 July 2013 - 02:34
This is it really, gruntguru hit the nail on the head, as acc is Force over Mass and Force is Power over Speed, it's all about keeping the engine at its powerwise sweetspot.I am OK with Csquared's post, the point I think needs to be made is - for max acceleration you should be in the gear that sees the highest engine power. Any other suggestions about "higher revs", "lowest gear", "torque peak" etc are irrelevent.
When the power has dropped off to the point where an upshift will produce identical power - its time to upshift (ie the intersection of lines on the thrust chart above.)
Posted 04 July 2013 - 03:09
How can several different gears all put you in the power peak at a given speed ???I am saying that if several different gears all put you in peak power at the speed you're in, it doesn't matter which one of those gears you choose.
Posted 04 July 2013 - 03:27
You can have a flat power peak (or plateau, if you will). Many new turbo engines at least on paper have those.How can several different gears all put you in the power peak at a given speed ???
Posted 04 July 2013 - 03:57
Well good-oh , but you will still accelerate at a greater rate in a lower gear even if you do have a flat graph.Dare I add it's because ,mechanically,and in no way relating to what your flat graph engine might be doing,your Torque at the Axles is much improved by the lower gearing chosen..Thus,the Force at the tyre/road contact patch is greater,thus greater acceleration. In other words torque at the first motion shaft of the gearbox won't change with your flat graph engine,but it will change at exit shaft from gearbox in each gear,unless it's 1:1.You can have a flat power peak (or plateau, if you will). Many new turbo engines at least on paper have those.
Edited by johnny yuma, 04 July 2013 - 04:03.
Posted 04 July 2013 - 04:00
No. If you have a flat POWER curve, it doesn't matter which gear you are in. A flat power curve will have a torque curve that rises sharply as revs reduce.Well good-oh , but you will still accelerate at a greater rate in a lower gear even if you do have a flat graph.Dare I add it's because ,mechanically,and in no way relating to what your flat graph engine might be doing,your Torque at the Axles is much improved.Thus,the Force at the tyre/road contact patch is greater,thus greater acceleration.
Posted 04 July 2013 - 04:08
Yeah,only boffins and rocket scientists allowed.This is The Technical Forum, son!
Posted 04 July 2013 - 04:11
yeah right,Hey I gotta get me some of these fancy curves,dammit !...(no harm in feeding the chooks is there?)No. If you have a flat POWER curve, it doesn't matter which gear you are in. A flat power curve will have a torque curve that rises sharply as revs reduce.
Posted 04 July 2013 - 06:33
yeah right,Hey I gotta get me some of these fancy curves,dammit !...(no harm in feeding the chooks is there?)
Posted 04 July 2013 - 08:08
Edited by Rasputin, 04 July 2013 - 08:43.
Posted 04 July 2013 - 13:03
Though I can add that as Force is Power over Speed, Force goes to the sky at zero speed, why we have whee-spin.
Posted 04 July 2013 - 13:56
Oh, so that's why.
Once again I would like to congratulate myself for not participating in this discussion.
Posted 04 July 2013 - 18:25
This is an area where our simplifying assumption really break apart. It's impossible to accelerate from zero without a slip somewhere, and once you have a slip somewhere, these simple equations which are still very hard for some people to fathom don't apply anymore. Usually one of the hardest racing physics things to program in racing simulations is the tire behavior at low speeds.Happy o enlighten you, in theory you have as much force as you want at take-off, funny isn't it?
Posted 04 July 2013 - 22:52
Posted 04 July 2013 - 23:40
Oh, so that's why.
Once again I would like to congratulate myself for not participating in this discussion.
Edited by johnny yuma, 04 July 2013 - 23:41.
Posted 05 July 2013 - 02:04
Pedantic interlude...It's impossible to accelerate from zero without a slip somewhere
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Posted 05 July 2013 - 03:00
Point taken. In that case, at standstill, you're going to be dividing zero power by zero speed, which would reduce to some finite acceleration once you cancel out the terms that give you zeros at standstill.Pedantic interlude...
It is not impossible, even in a normal car. Switch engine off. Engage first gear. Release clutch. Crank engine.
Now, that is a completely useless exercise 9 times out of 10, but it is SOP when starting up a steep slope in a 4wd diesel. If you slip the tires you will skid to the bottom of the hill, on typical clay dams for example.
Posted 05 July 2013 - 05:09
Electric motor has max torque from 0 rpm.Will spin wheels if insufficient traction just the same,low range or high.Is SOP in petrol 4WDs too(14 years with Parks and Wildlife Service)for starting movement on steep slippy slopes.But it can't extract you if you're bogged !! Best to keep off muddy roads in wet weather,farmers justifiably hate you when you rut their roads.Pedantic interlude...
It is not impossible, even in a normal car. Switch engine off. Engage first gear. Release clutch. Crank engine.
Now, that is a completely useless exercise 9 times out of 10, but it is SOP when starting up a steep slope in a 4wd diesel. If you slip the tires you will skid to the bottom of the hill, on typical clay dams for example.
Posted 06 July 2013 - 09:49
Isn't that called an asymptote or something?If only we could divide by zero.
Posted 07 July 2013 - 23:50
Posted 29 July 2017 - 23:49
4 If the final drives are truly optimised for top speed then the petrol engine has more axle torque at a given roadspeed (checked at 3 rpms) than the diesel so will generally accelerate more quickly (absent vagaries in the trans schedule) and it will have a slightly higher top speed..
The quiz setter is out of his depth.The least wrong answer is correct in his opinion.
Posted 31 July 2017 - 12:04
Hello all.
Here is a graph made with the DOS/QuicBasic RoadLoad program (at http://www.pattakon....attakonEduc.htm near the bottom of the web page):
The power data were taken by the link of Saudoso.
The cyan curves are for the Gasoline Mercedes E-250, the white curves are for the Diesel Mercedes E-250.
Both cars have the same 7G-Tronic gearbox (the difference is in the final ratio). The seven gear ratios (used for the graph) of the 7G-Tronic were found in the Internet.
With slightly higher peak power and slightly lower weight, the gasoline version will have a slightly higher top speed (provided the final gear ratios are optimized for the top speed).
However, in the intermediate speeds is where things are really interesting and tricky.
For simplicity, in the graph, the final transmission ratio was set to give 250Km/h top speed in both cases (Diesel and Gasoline).
According the graph:
Until 40Km/h (where the two curves intersect) the Diesel (white curve) has a slightly stronger accelerating force.
At 42Km/h the Diesel has to shift to 2nd, while the Gasoline uses its 1st gear till 45Km/h (which gives some 15% higher accelerating force).
At 45 the Gasoline needs to shift to 2nd, which reduces the accelerating force slightly below that of the Diesel.
Till 58Km/h (both cars with 2nd gear) the accelerating force on the Diesel is higher (but not too much).
From 58Km/h to 63Km/h the accelerating force on the Gasoline car is higher.
At 63km/g the Diesel has to shift to 3rd.
From 63Km/h to 70Km/h (wherein the Gasoline has to shift to 3rd) the accelerating force on the Gasoline car becomes again substantially stronger (some 10-15%).
Above 70Km/h, with both cars using necessarily the 3rd gear in the gearbox, the Diesel has, again, a slight advantage (slightly higher accelerating force).
At 88Km/h things change (the accelerating force on the Gasoline gets higher), and at 95Km/h (wherein the Diesel has to shift to 4th) the Gasoline has a substantially higher accelerating force which is maintained till 103Km/h wherein the Gasoline has to shift to 4th, too.
And so on.
The torque curves are quite significant for such a comparison.
For instance, in case the gasoline torque and power curves keep their peak values but with a significant drop in the intermediate revs, then the gasoline would be substantially slower than the Diesel in the intermediate speeds.
Thanks
Manolis Pattakos
Posted 31 July 2017 - 23:48
Petrol wins. (just)
Lower weight.
Higher peak power.
Wider band >200hp - 17% vs 16%
Wider band >150hp - 51% vs 50%
Posted 02 August 2017 - 02:42
Hello Gruntguru.
Reasonable question:
How the "150hp" was chosen?
Differently speaking: why not to check the "Wider bad > 170hp"?
Based on the RoadLoad graph, the 170hp seems a better choice for comparisons.
Thanks
Manolis Pattakos.
Posted 02 August 2017 - 05:28
Ha Ha - the graph shown in the link has lines at 200 and 150 hp to make life easier.
Posted 04 August 2017 - 03:33
Hello Gruntguru.
170hp is the power either engine can provide at most just after each gear shift.
So, excluding the fisrt gear, at all other gear ratios both engines run permanetly above 170hp.
Thanks
Manolis Pattakos