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

In summary, the conversation discusses the design of an intake manifold for a hypothetical engine with a 1:1 bore to stroke ratio, 117.89mm bore diameter, and 7.8l volume. The engine was intended to produce 450 bhp at 6500 rpm with a rev limit at 7500 rpm. The individual is unsure of the maximum torque and how to find it, considering only one data point of 492.8Nm at 6500 rpm is available. The conversation also touches on the possibility of plotting a graph between power and torque, the challenges of generalizing data from similar engines, and the unusual combinations of parameters in the hypothetical engine design.
  • #1
Sai Bhagavan
9
0
TL;DR Summary
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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  • #2
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
 
  • #3
Hello thanks for the reply and the welcome. I wanted to know the maximum torque number and at which RPM it is produced.
 
  • #4
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.
 
  • #5
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?
 
  • #6
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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  • #7
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.
 
  • #8
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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  • #9
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.
 
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  • #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?
 

1. How do I gather data for a power and torque graph?

To plot a power and torque graph for an engine, you will need to gather data from the engine's performance tests. This can include measurements of the engine's speed, torque, and power output at various points. You can also use mathematical equations to calculate these values based on the engine's specifications.

2. What is the best way to display a power and torque graph?

The most common way to display a power and torque graph is by using a line graph with the engine's speed on the x-axis and the power and torque values on the y-axis. This allows for a clear visualization of the relationship between speed, power, and torque.

3. How do I determine the engine's maximum power and torque from the graph?

The maximum power and torque can be determined by finding the peak points on the graph. The highest point on the power curve represents the engine's maximum power output, while the highest point on the torque curve represents the engine's maximum torque output.

4. What factors can affect the shape of a power and torque graph?

The shape of a power and torque graph can be affected by various factors such as the engine's design, fuel type, air intake, and exhaust system. Other factors like temperature, altitude, and engine condition can also impact the engine's performance and therefore, the shape of the graph.

5. How can I use a power and torque graph to compare different engines?

A power and torque graph can be a useful tool for comparing the performance of different engines. By plotting the data for each engine on the same graph, you can easily see the differences in power and torque outputs at various speeds. This can help you determine which engine is more suitable for your needs.

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