How to plot a power and torque graph for an engine?

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Discussion Overview

The discussion centers around plotting a power and torque graph for a hypothetical boxer-6 naturally aspirated petrol engine designed for a master's project. Participants explore how to determine maximum torque and its corresponding RPM, as well as the feasibility of plotting power and torque based on limited data.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant seeks to plot a power and torque graph using only the maximum power value and the known torque at 6500 RPM.
  • Another participant explains the relationship between power, torque, and RPM, providing a calculation for power based on the given torque value.
  • Several participants emphasize the need for more data to accurately plot torque against RPM and suggest looking at similar engines for reference.
  • Concerns are raised about generalizing torque trends from other engines to the participant's design, given the unique specifications of the hypothetical engine.
  • Some participants question the feasibility of the engine's design parameters, particularly the high RPM and low power output relative to engine size.
  • Discussion includes the importance of engine design factors such as head design, valve area, and cam profile in determining torque curves.
  • Participants suggest that a dynamometer could provide accurate torque and power curves, but acknowledge that the engine is still in the design phase.
  • One participant notes the unusual combination of engine specifications and the implications for expected performance.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to proceed with the power and torque graph due to the limited data available. There are multiple competing views regarding the feasibility and implications of the engine's design parameters.

Contextual Notes

Participants express uncertainty about the maximum torque and its RPM, as well as the validity of extrapolating data from similar engines. The discussion highlights limitations in the current design phase and the need for further empirical data.

Sai Bhagavan
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TL;DR
Plot power and torque graph of an engine using peak power value at an RPM.
Hello all,
I have designed an intake manifold for boxer-6 naturally aspirated petrol engine with 1:1 bore to stroke ratio and has a bore dia of 117.89mm and a volume of 7.8l as part of my master's project. The engine was to produce 450 bhp @ 6500 rpm and I did set a rev limit @7500 rpm. The problem is I don't know at what rpm the engine produces it's maximum torque and how to find that. The amount of torque it produces at 6500 is 492.8Nm. Can I plot a graph between power and torque just by using the maximum power value and if how? Thanks in advance.
 
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Welcome to PF.
Sai Bhagavan said:
Can I plot a graph between power and torque just by using the maximum power value and if how?
Power = torque * RPM.

Sai Bhagavan said:
The amount of torque it produces at 6500 is 492.8Nm.
6500 RPM = 6500*2*Pi/60 = 680.7 radian per second.
Power = 492.8Nm * 680.7 = 335438. watt = 335.438 kW = 449.8 HP
 
Hello thanks for the reply and the welcome. I wanted to know the maximum torque number and at which RPM it is produced.
 
I think that you will need more data. You should start by plotting torque against RPM.
Then compute the power from that and plot it on the same graph.

With the data you now have you can only plot one torque point = 492.8Nm at 6500 RPM.

To guess more points you must look at other plots for similar motors to see how the shape of the torque to RPM curve changes.
 
I have some doubdts though. Even if I find at other plots for a similar configuration engine, can we generalize the same trend for my project also?
 
No, you'll need the data from your engine. If you only know power at one RPM value, you only know torque at that same RPM. How did you obtain the power/torque figures for 6500RPM?
 
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Well I have designed the intake on my own for the specified parameters. It is my own project. So am stuck with single values of power and torque each.
 
Sai Bhagavan said:
I wanted to know the maximum torque number and at which RPM it is produced.

Sai Bhagavan said:
So am stuck with single values of power and torque each.

Those two things don't go together.
 

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Sai Bhagavan said:
Well I have designed the intake on my own for the specified parameters. It is my own project. So am stuck with single values of power and torque each.
How do you know the engine will make that power though? If this is just a hypothetical, sure, you can reasonably estimate based on similar engines.

Also, just as a side note, 7500RPM seems very high for a 7.8l six cylinder engine. Are you expecting it to be hugely oversquare?
 
  • #10
Well it is a hypothetical engine. Exists only in my dreams. I just wanted to assume how that engine would be as most of the engines are not square so I just wanted to design a road going square engine as part of my project.
 
Last edited:
  • #11
Ahh. In that case, you can assume any torque curve you want. In reality, it'll obviously depend on head design, valve area, cam profile, etc.

Also, a lot of road going engines are square, or close to it. Some modern efficiency-focused designs are going a bit more in the undersquare direction, and performance engines can be a bit oversquare (since it enables higher peak RPM), but on the whole, most engines are fairly square(ish).
 
  • #12
Well I don't have the head design yet but I have the valve opening and closing timigs for both intake and outlet.Cam profiles are yet to be designed. But my professor asked a tentative power torque graph so had to do this.
 
  • #13
You can make a decent guess. You've got some weird combinations going on there though - 7.8L flat 6 with a 7500RPM redline and a 117.89mm stroke gives a mean piston speed of around 29.5m/s. This is extremely aggressive even for a racing engine - even Nascar and F1 usually run 2-3m/s slower than this. However, you're only making 450hp, which is a very mild level of output for a 7.8 liter engine making peak power at 6500RPM, and you're only making 360 foot pounds which is also very mild for a 7.8. Granted, you'll make more torque lower down, but still, a 7.8 with that kind of piston speed should be making upwards of 800hp.
 
  • #14
cjl said:
You can make a decent guess. You've got some weird combinations going on there though - 7.8L flat 6 with a 7500RPM redline and a 117.89mm stroke gives a mean piston speed of around 29.5m/s. This is extremely aggressive even for a racing engine - even Nascar and F1 usually run 2-3m/s slower than this. However, you're only making 450hp, which is a very mild level of output for a 7.8 liter engine making peak power at 6500RPM, and you're only making 360 foot pounds which is also very mild for a 7.8. Granted, you'll make more torque lower down, but still, a 7.8 with that kind of piston speed should be making upwards of 800hp.
I wanted the engine to be naturally aspirated. I was free to choose the parameters and I solely designed the whole engine around the power it is intended to make and the configuration. Yeah it seems a bit bizzare.
 
  • #15
Yeah, but even naturally aspirated, a 7.8l spinning 7500rpm should be making 800+ horsepower. Why such a large engine? For a 6 cylinder revving that high, you realistically wouldn't want any more than 4.5l or maybe 5l, and you could even keep the horsepower about the same at that engine size.
 
  • #16
You should put the engine on a test brake/dynomometer. It will give you a very accurate Torque/rpm graph and I imagine most modern ones will give a power curve along with it.
 
  • #17
Joe591 said:
You should put the engine on a test brake/dynomometer. It will give you a very accurate Torque/rpm graph and I imagine most modern ones will give a power curve along with it.
Sir the problem here is am trying to design the engine from ground up. So engine is till in design phase.
 
  • #18
cjl said:
Yeah, but even naturally aspirated, a 7.8l spinning 7500rpm should be making 800+ horsepower. Why such a large engine? For a 6 cylinder revving that high, you realistically wouldn't want any more than 4.5l or maybe 5l, and you could even keep the horsepower about the same at that engine size.
The compression ratio I used is a more traditional 9.5 so that the engine will run on normal road going fuel and not on higher octane fuels. My calculations landed me at 7.8l capacity.
 
  • #19
What calculations did you do?
 

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