When one reads the specifications for a vehicle, they always give the maximum power and torque figures - eg 110 bhp at 5800 rpm, 110 lb ft at 4000 rpm.
These figures are very useful for giving one an idea of how how the vehicle is going to drive. But it struck me that because of the way I drive, I'm probably more interested in how much power and torque a car have at 2500 or 3000 rpm. And of course, those figures are never given (though one can sometimes find graphs which will give you the information you want).
Anyone agree with me that it would be useful to publish figures for power and torque at, say, 3000 rpm?
|
Anyone agree with me that it would be useful to publish figures for power and torque at, say, 3000 rpm?
Defintely, although like you said, the graphs were what I use. That sort of info would be hard to quantify as a list of figures, and probably harder to understand. Also more open to 'fiddling' than a graph I would think.
|
If it is a TD the max torque is likely to be at around 1800 - 2000rpm, i.e. just where you want it.
Although I enjoy a crisp free revving petrol engine IMO a modern TD has ideal characteristics for road driving, loads of torque from 1500 and as the torque tails off the power starts to build giving a useful power band from under 1500 through to 4500.
|
|
|
|
There is a way to eke out a little more information from the torque and power figures usually quoted by manufacturers which isn't widely known. The manufacturers usually publish something of this form;
Max Torque: T (Nm) at MAXT rpm
Max Power: P (kW) at MAXP rpm
Using the fact that at maximum torque, the torque curve is flat, one can calculate the gradient of the power curve, as;
Power (W) = Torque (Nm) x angular velocity (rad/s)
differentiating with respect to engine angular speed gives:
Rate of change of power (W per rad/s) = MAXT
This idea can be used in the reverse way, in order to calculate the gradient of the torque curve at the maximum power speed.
With this extra info, it is possible to fit simple power and torque curves over the rev range. These usually provide quite a good fit to data derive from engine dynamometers, except for engines which are strongly ram tuned.
This method was developed by Prof. Gordon Lucas of Loughborough University during his research work in the 60's.
A snippet of code which implements the curve fit, and draws the approximate curves is included below. (Apologies for the length of the post). The code gives some odd answers at speeds below tickover, but these aren't relevant anyway.
% Engcurv Engine date curve fit program
% Program to implement Gordon Lucas's perfomance prediction work.
% For a description of which, see
% Author Fenton,John,1931-
% Title Handbook of vehicle design analysis /by John
% Fenton
% Publisher London:MEP,1996
% Notes Includes bibliographical references and index
% Ctrl.no 0852989636
% The coeefficients to implement a cubic polynomial curve fit are found.
% A matrix method is used to solve the simultaneous equations.
% This is the imperial version of the code.
% Inputs are in r.p.m. , b.h.p. , and lb. ft.
% The output is poor below 1000 rpm, but this sacrifice
% produces better torque figures for the real rev. range
% Written by XXXXXX XXXXX, Loughborough University 1997
clc, clear
if (1==0)
% Takes user input
ntmax=input('Please input the engine speed [r.p.m.] at which max. torque occurs :- ');
tmax=input('Please input the value of max. torque [lb. ft.] :- ');
npmax=input('Please input the engine speed [r.p.m.] at which max. power occurs :- ');
pmax=input('Please input the value of max. power [b.h.p.] :- ');
maxspeed=input('Please input the max. rated speed of the engine [r.p.m.] :- ');
else
% uses stored data
ntmax=3750;
tmax=116;
npmax=5750;
pmax=113;
maxspeed=6500;
end
% Solves the simultaneous equations using a matrix method
mata=zeros(4,4); % Sets up a blank matrix, ready to be filled
mata(1,1:4)=[(ntmax^3) (ntmax^2) ntmax 1 ];
mata(2,1:4)=[3*(ntmax^2) 2*ntmax 1 0 ];
mata(3,1:4)=[(npmax^3) (npmax^2) npmax 1 ];
mata(4,1:4)=[3*(npmax^2) 2*npmax 1 0 ];
matb=zeros(4,1);
matb(1,1)=tmax*ntmax/5252;
matb(2,1)=tmax/5252;
matb(3,1)=pmax;
matb(4,1)=0;
% The matrix of coefficients of the cubic polynomial are now found
coeffs=mata\matb;
for revs = 1:maxspeed;
if revs < ntmax % linear curve fit
power(revs)=tmax*revs/5252;
else % cubic polynomial curve fit
power(revs)=(coeffs(1,1)*(revs^3))+(coeffs(2,1)*((revs)^2));
power(revs)=power(revs)+(coeffs(3,1)*(revs))+coeffs(4,1);
end
% Torque is then calculated knowing both power and speed
torque(revs)=(5252/revs)*power(revs);
end
% Re-arranges torque data for ploting
for counter=1:(maxspeed-ntmax);
torque(ntmax-counter)=torque(ntmax+counter);
end
revsm=[1:maxspeed];
power2=torque.*revsm;
plot(power),hold on
plot(torque)
Number_Cruncher
|
|
That's just showing off!!
|
No - It's Gordon Lucas who is the smart one for coming up with the idea!
Number_Cruncher
|
There is a way to eke out a little more information from the torque and power figures usually quoted by manufacturers which isn't widely known.<<
NC - your posts are generally impressive, but that is brilliant - including the acknowledgement to Gordon Lucas also.
I liked the way that MB used to quote engine power - by saying that 90percent of max torque was available over a specified speed range.
|
Unfortunately not too many ordinary motorists understand the significance of engine torque and power figures, and most understand even less about their effect on vehicle performance. Even a goodly percentage of motoring journalists don't understand the significance! They glibly quote engine torque figures in isolation with no reference whatsoever to gear ratios. The thing which is important is torque at the driving wheels and this depends also on the ratio of wheel speed to engine speed. It's not much good directly comparing the engine torque of a high geared diesel to that of a low geared petrol. Nevertheless many motoring journalists do exactly that and would have you believe that the acceleration will be proportional to the quoted engine torque.
What I'd like to see are acceleration times in various gears for speed ranges such as 10-30, 30-50, 50-70 etc. An even better criterion would be distance travelled in a certain time. The popularly quoted 0-60 time isn't of too much value because the ordinary motorist would never treat their car in the brutal manner that is used to get these standing start times.
--
L\'escargot.
|
the ordinary motorist would never treat their car in the brutal manner that is used to get these standing start times.
Bless - I think my friend's cars would think they'd gone to the big scrapyard in the sky if they were owned by you!
(From cold most of the time, as well)
|
Bless - I think my friend's cars would think they'd gone to the big scrapyard in the sky if they were owned by you! (From cold most of the time, as well)
I can only assume that you've never witnessed a 0-60 test. I have, and I wouldn't want to own a car that had been through it.
--
L\'escargot.
|
I can only assume that you've never witnessed a 0-60 test. I have, and I wouldn't want to own a car that had been through it.
With them, every journey is a 0 - 60 test!
|
With them, every journey is a 0 - 60 test!
I doubt it. The clutch and gearbox don't survive many 0-60 tests ~ usually just enough to obtain an average that is worth publishing.
--
L\'escargot.
|
I doubt it. The clutch and gearbox don't survive many 0-60 tests ~ usually just enough to obtain an average that is worth publishing.
I remember reading in Autocar some years ago about how they tried and tried to match Chevrolet's 0-60 time for the then new Corvette, without sucess. Enter stage left, Mr. Chevrolet test driver to show how it was done - accelerator hard down, dump the clutch, don't lift off the accelerator and don't use the clutch to get from 1st gear into 2nd (60mph came soon after). Easy!
S6 1SW
|
|
It'snot much good directly comparing the engine torque of a high geared diesel to that of a low geared petrol.
I don't think gearing is the most important. What I want to compare is the engines, I can usually
change the gears myself (but not the engine..!)
Actually I tend to look for the reverse effect: whenever a car has good 0-60 time I like to check if this
has been achieved via low gearing (i.e. 'cheating').
Also I like to know what happens if I put it in top gear from idle revs with max throttle - how smooth it will be...?
Curiously I have noticed that from some of the chip-tuning data for TDs there is no visible impact on the bottom part of the power curve from the dyno readings. But this is exactly where I would like to see some improvement as this would expand the power band.
|
Curiously I have noticed that from some of the chip-tuning data for TDs there is no visible impact on the bottom part of the power curve from the dyno readings. But this is exactly where I would like to see some improvement as this would expand the power band.
I assume this is because chip tuning relies mainly on incresing the turbo boost pressure, so at low revs where the turbo is off-boost, you hardly get any more power than from an un-chipped model..
|
|
Oops, I meant to add that in practice, my car responds a lot better from idle with a tuning box fitted, than without the tuning box. My car was quite laggy at low revs and its a lot better with the box fitted, and the boost comes in a lot stronger. Whether they're all like this I dont know. I've not seen a torque curve for the particular box I have got.
|
|
|
|
It's not much good directly comparing the engine torque of a high geared diesel to that of a low geared petrol. >>
We have been down this road before, it is a very legitimate comparsion because the difference in torque output and the difference in revs at which max torque is produced are both far too large for the small difference in gearing to obviate them.
A 2.0 petrol geared to 25mph / 1000 in top will be doing 2000 at 50, some 1500 short of it's torque peak (iro 150lb/ft). Putting your foot down will result in a rather lethargic response.
a 2.0TD geared to 30mph / 1000 will be doing about 1650rpm at 50, just building up to it's torque peak at around 1800 (iro 250lb/ft). Put your foot down and it will stomp away.
|
|
|
|
|
Very few modern torque curves are flat-particularly on petrol engines-many have two or more peaks.The figures quoted by manufacturers are from officially-observed tests to a set procedure.
|
|
|
|
|
